Anatomical backgrounds on gas exchange parameters in the lung

Abstract

Many problems regarding structure-function relationships have remained unsolved in the field of respiratory physiology. In the present review, we highlighted these uncertain issues from a variety of anatomical and physiological viewpoints. Model A of Weibel in which dichotomously branching airways are incorporated should be used for analyzing gas mixing in conducting and acinar airways. Acinus of Loeschcke is taken as an anatomical gas-exchange unit. Although it is difficult to define functional gas-exchange unit in a way entirely consistent with anatomical structures, acinus of Aschoff may serve as a functional gas-exchange unit in a first approximation. Based on anatomical and physiological perspectives, the multiple inert-gas elimination technique is thought to be highly effective for predicting ventilation-perfusion heterogeneity between acini of Aschoff under steady-state condition. Changes in effective alveolar P_O2, the most important parameter in classical gas-exchange theory, are coherent with those in mixed alveolar P_O2 decided from the multiple inert-gas elimination technique. Therefore, effective alveolar-arterial P_O2 difference is considered useful for assessing gas-exchange abnormalities in lung periphery. However, one should be aware that although alveolar-arterial P_O2 difference sensitively detects moderately low ventilation-perfusion regions causing hypoxemia, it is insensitive to abnormal gas exchange evoked by very low and high ventilation-perfusion regions. Pulmonary diffusing capacity for CO (D_LCO) and the value corrected for alveolar volume (V_AV), i.e., D_LCO/V_AV (K_CO), are thought to be crucial for diagnosing alveolar-wall damages. D_LCO-related parameters have higher sensitivity to detecting abnormalities in pulmonary microcirculation than those in the alveolocapillary membrane. We would like to recommend four categories derived from combining behaviors of D_LCO with those of K_CO for differential diagnosis on anatomically morbid states in alveolar walls: type-1 abnormality defined by decrease in both D_LCO and K_CO; type-2 abnormality by decrease in D_LCO but increase in K_CO; type-3 abnormality by decrease in D_LCO but restricted rise in K_CO; and type-4 abnormality by increase in both D_LCO and K_CO.

Key Words: Secondary lobule of miller; Acinus of Loeschcke; Acinus of Aschoff; Convection; Gas-phase diffusion (stratification); Aqueous-phase diffusion; Ventilation-perfusion heterogeneity

Core tip: The anatomical gas-exchange unit is organized into the acinus of Loeschcke, while the functional gas-exchange unit is given by the acinus of Aschoff. The ventilation-perfusion distribution in acinar regions is representatively predicted from the inert-gas elimination technique. The effective alveolar-arterial P_O2 difference plays a vital role in detecting moderately low ventilation-perfusion regions eliciting hypoxemia but not very low and high ventilation-perfusion regions. Pulmonary diffusing capacity for CO and the value corrected for alveolar volume estimate the impediment of alveolar walls and are more sensitive to detecting the abnormality of pulmonary microcirculation than that of alveolocapillary membrane.

INTRODUCTION

The morphological and physiological aspects of normal and diseased lungs were most actively studied in the era of the 20^th century from the 1950’s to the 1980’s. The morphological and physiological approaches are both equally important for raising the quality of basic science and of clinical medicine targeting the human lung. Although the lung morphology and the respiratory physiology have evolved in parallel, it is hard to say that the efforts to unite them in a variety of directions by morphologists, physiologists, and clinicians, i.e., the establishment of structure-function relationships, were successful enough. It is of clinical value to understand the anatomical basis of various functional parameters during pathophysiological decision-making in patients with various types of lung disease. Based on these historical facts, we try to shed light on the elucidation of anatomical backgrounds of various functional parameters that have been used for estimating gas maxing in conducting airways and gas exchange in the lung periphery. To accomplish this purpose, the following five matters were comprehensively addressed in the present review: (1) What is the appropriate anatomical design for understanding gas mixing in conducting airways and gas exchange in lung periphery? (2) Aspects of convective and diffusive gas mixing along peripheral airways in a lung model in which dichotomously branching airways are certainly considered. (3) Is the ventilation-perfusion (V_A/Q) distribution in the lung, which is quantitated in terms of the multiple inert-gas elimination technique (MIGET), supported by the anatomical and physiological facts? (4) Is the effective alveolar-arterial P_O2 difference (AaDO₂) estimated from the classical gas-exchange theory also supported by the anatomical and physiological facts? (5) What are the anatomical and physiological determinants on the pulmonary diffusing capacity for carbon monoxide (D_LCO)?

Anatomical design for understanding gas mixing and gas exchange in the lung

Branching of conducting airways: The pattern of airway branching in a normal lung is best studied on resin or silicon rubber casts[1-8] (Figure 1). The lower conducting airways end in clusters that are surrounded by the fibrous connective tissue septa. The cluster is defined as the secondary lobule of Miller[9,10], which contains lobular (preterminal) bronchioles and terminal bronchioles that supply the gas to or from the acini of Loeschcke[10].

Figure 1 Resin cast of human lung. Complicated dichotomously branching tree of conducting airways is exhibited. Adopted from ref[1].

Although conducting airways are most often described as a symmetric tree that branches off by regular dichotomy, in which all pathways have exactly the same length and each generation of branching is composed of airways uniform in size[2,3,11], this type of model is in fundamental conflict with the anatomical facts[2,12,13]. A more realistic representation is a tree of irregular, asymmetric dichotomy, where the dimension of conjugate daughter branches differs from that of their parents. The irregular dichotomy includes different branching angle and varied length-to-diameter ratio at a certain airway generation. Weibel[2,3] demonstrated that the pathway length in airways with 1.5-mm diameter varies between 1.5 mm and 11 mm, with the highest frequency at 3.5-4.0 mm. Similarly, airways of 2.0 mm in diameter (i.e., small airways) are spread out between the 4^th and 14^th generations of the branching tree, with a maximum at the 8^th generation. The valuable message emanating from the studies of Weibel is that although conducting airways are correctly modeled by an irregular, asymmetric dichotomous branching tree, the dimensional variation of airway diameters and path lengths assigned to each generation of the tree nearly follows the normal distribution, and this variation decreases as the generation progresses. Based on these findings, a simple design called “model A of Weibel”[2,3] or “typical path lung model”[4,5,14] has been developed. This model assumes that in an initial approximation, the asymmetric branching tree can be replaced by the symmetric branching tree using the mean (representative) value estimated from the normal distribution of airway diameters as well as that of path lengths at a given generation (Figure 2), which results in that one can assess gas mixing in the conducting airways with asymmetric branches from the model A of Weibel with simple geometry.

Figure 2 Schematic presentation of effective diameter and cross-sectional area along lower conducting airways and acinar airways (trumpet or thumbtack model). Generations of branching tree and diameter as well as cross-sectional area in each generation are obtained from model A of Weibel[2]. Lower conducting airways are classified into two routes; i.e., central airways [generation from zero (trachea) to 9^th] and peripheral airways (generation from 10^th to 16^th, in which 16^th generation corresponds to terminal bronchiole). Peripheral airways are characterized by membrane bronchioles, whereas small airways are defined by their diameters of less than 2.0 mm originating in the 8^th generation. Airways from the 17^th generation (respiratory bronchiole) to the 23^th generation (alveolar sac) are not included in peripheral airways. Instead, they are defined as acinar (alveolated) airways (acinus indicates that defined by Loeschcke). This is because acinar airways function by carrying gas and exchanging gas. Alveolar sacs end in alveoli formed by alveolar walls containing alveolocapillary membranes and capillary networks. Thickness of the alveolocapillary membrane is about 1.0 μm and total cross-sectional area of the alveolocapillary membrane and capillary network is about 130 m².

Anatomical definition of lobule and acinus: Different terms have been used for defining the lobule and the acinus throughout several decades. In 1947, Miller proposed the anatomical concept that is valuable for understanding the secondary and primary lobules[9]. As described above, the secondary lobule of Miller is the unit supplied by a lobular bronchiole having a diameter of about 1.0 mm (corresponding to the 12^th generation) and surrounded by the fibrous connective tissue septa (i.e., the interlobular septa) (Figure 3). The size of secondary lobules of Miller varies from 1.0 cm to 2.5 cm, each of which is comprised of varying numbers of acini defined by Loeschcke, ranging from three to twenty-four depending on the region selected[15,16]. On the other hand, the primary lobule of Miller is expressed by the area governed by an alveolar duct. The secondary lobule proposed by Miller has various advantages from anatomical points of view. Therefore, unless otherwise specified, the simple term of “secondary lobule” is used in the sense of “secondary lobule of Miller” throughout the current review.

Figure 3 Secondary lobule and acinus. Secondary lobule of Miller is defined by the area supplied by a lobular bronchiole while the acinus of Loeschcke is defined by the area supplied by a terminal bronchiole. Acinus of Aschoff is the area supplied by a first-order respiratory bronchiole. Secondary lobule of Miller is surrounded by fibrous connective tissue forming interlobular septum. In acini of Loeschcke and Aschoff, there are three important interstitial tissues, including peripheral interstitium, alveolar interstitium forming alveolar septum, and axial interstitium spirally surrounding airways, which begins at the alveolar duct extending to the hilum. Axial interstitium and alveolar interstitium are joined via intra-parenchymal interstitium and all interstitial tissues are in a serial linkage. Modified from ref[2].

In 1958, based on the bronchogram of peripheral airways, Reid[17,18] presented a different definition with regard to the secondary lobule. The secondary lobule of Reid is the unit formed by the structure distal to any bronchiole having a mm-branching pattern. The size of the secondary lobule of Reid is fairly constant (approximately 1.0 cm) in comparison with that of Miller and contains three to five acini of Loeschcke. The Reid’s definition is more useful when interpreting the bronchogram.

The acinus of Loeschcke is the unit distal to a terminal bronchiole (Figure 3), in which all alveolated airways engage in gas exchange. The size of the acinus of Loeschcke averages 7 mm to 10 mm in diameter and roughly 80000 acini configure the lung. An acinus contains alveoli ranging from 3000 to 4000[8,19], resulting in the lung consisting of 300-million alveoli.

The acinus of Aschoff is defined as the area supplied by a first-order respiratory bronchiole (Figure 3), the size of which is half of the acinus of Loeschcke[20]. All alveolated airways in the acinus of Aschoff participate in gas exchange. The acinus of Loeschcke is important when defining the anatomical gas-exchange unit, while the acinus of Aschoff is important when the functional gas exchange is defined.

Similar to conducting airways, intra-acinar, alveolated airways (respiratory bronchioles, alveolar ducts, and alveolar sacs) also show irregular dichotomous branching over an average of nine generations, terminating at between the 6^th to 12^th generations[8,21,22]. Large acini have more generations and smaller acini have correspondingly fewer. Although the issue of whether the variation of airway diameters and path lengths of each generation in the acinus (Loeschcke or Aschoff) follows the normal distribution has not been reliably concluded, Weibel and colleagues[2,3,8] presumed that the geometrically-simple model A of Weibel could be applied for integrating the branches of conducting airways and acinar airways. The features of model A of Weibel (Figure 2), which is also called the “trumpet model” or “thumbtack model”, are summarized as follows[2,23,24]: (1) the model consists of the average number of generations over which the airways must terminate in the alveolar sacs corresponding to the 23^th generation; (2) the 16^th generation defines the terminal bronchiole; (3) the 17^th-22^th generations correspond to the acinar airways, including the respiratory bronchioles and alveolar ducts; and (4) the number of airways in a particular generation increases tremendously as the generation increases, leading to the exponential increase in the generation-specific total cross-sectional area. Weibel[2] demonstrated that the average diameter of terminal bronchiole, respiratory bronchiole, or alveolar duct is 0.50 mm, 0.39 mm, and 0.33 mm, respectively, while the average total cross-sectional area of these airways is 230 cm², 420 cm², and 2630 cm² (equal to 0.263 m²), respectively. The total cross-sectional area including all alveoli amounts to about 130 m² in association with the harmonic mean thickness of alveolocapillary membrane of 1.1 μm[25].

Anatomical aspects of collateral ventilation: Three pathways exist for collateral ventilation in the acinus, i.e., alveolar pores (pores of Kohn), channels of Lambert, and channels of Martin. Although the typical size of alveolar pores ranges from 2 μm to 30 μm, there have been many anatomical, physiological, and theoretical studies leading to the conclusion that the alveolar pores are not potent in air-filled lungs[26,27]. These studies indicate that the alveolar pores take no part in maintaining alveolar ventilation.

In 1955, Lambert[28] described accessory communications between bronchioles and alveoli in the normal human lung. The diameter of Lambert channels varies from 0 μm (practically closed) to 30 μm, and they are considered to serve as collateral ventilation to adjacent alveoli when airway obstruction occurs at the level of terminal bronchioles[27]. Supporting this consideration, the important role of Lambert channels was identified in patients with pneumoconiosis, in which a striking accumulation of macrophages was found in the Lambert channels and their adjacent alveoli[29].

In 1966, Martin[30] reported intra-acinar communications at the level of respiratory bronchioles or alveolar ducts in the canine lung. The Martin channels were found in human lungs as well[9,31]. As the Martin channels have sufficiently low resistance to gas flow, they act as the primary pathway for inducing collateral ventilation in the acinus[27]. Therefore, one can conceive that only the Martin channels but neither the alveolar pores nor the Lambert channels serve to equalize the distribution of alveolar ventilation and the concentration of a certain gas in a particular acinus, though the extent of equalization elicited by collateral ventilation has not been conclusively quantified.

Definition of anatomical gas-exchange unit – importance of acinus of Loeschcke: The alveolus cannot be taken as a ventilation unit mostly because each alveolus is not independent of other alveoli. Two adjoining alveoli share the alveolar wall and each alveolus adjoins several alveoli, some of which are connected to different alveolar ducts. Furthermore, there are communications at the level of respiratory bronchioles or alveolar ducts (Martin channels). On the other hand, the acinus of Loeschcke can be taken as an anatomical ventilation unit because it is supplied only by a single conducting airway with no alveoli, i.e., the terminal bronchiole (Figure 3).

There is a serious problem concerning a perfusion unit formed by the pulmonary microcirculation. The dense, intertwined capillary network embedded in the alveolar wall forms a continuum of blood flow that is not partitioned into independent perfusion units. This disproves that the gas-exchange unit is organized into an alveolus associated with a separated capillary network in the alveolar wall. The capillary network is supplied by the arteriole that penetrates into the acinus along the acinar airways and is drained into the venule in the acinar periphery. The distance between an arteriole and a venule is of an order between 0.5 mm and 1.0 mm[32], suggesting that the perfusion unit given by the area surrounded by an arteriole and a venule extends over several alveoli, but it does not cover the whole area of an acinus of Loeschcke whose size ranges from 7 mm to 10 mm[25]. These facts are in accord with the idea that the perfusion unit is not congruent with the ventilation unit so that it gives rise to difficulty in anatomically defining the gas-exchange unit in which the ventilation and perfusion units should be matched. However, König et al[33] found that the whole acinar region of Loeschcke is perfused almost homogeneously by colloid gold used as a plasma tracer even at rest, indicating that all capillary networks existing in the acinus of Loeschcke are connected to each other. Based on the study of König et al[33], we considered that the anatomical gas-exchange unit is organized approximately into the acinus of Loeschcke.

Convective and diffusive gas mixing in conducting and acinar airways

In addition to the convective mixing elicited by ventilation, the diffusive mixing plays an important role in maintaining gas transport along peripheral and acinar airways with large cross-sectional areas (Figure 2). Numerous mathematical attempts have been made to estimate the effect of convection, diffusion, or interactions of both on gas transport in conducting and acinar airways using a model with symmetrically or asymmetrically dichotomous branching of airways[24]. The term of convection is defined as a mass transport resulting from the unidirectional displacement of molecules included in a certain volume. Convective displacement is identical for all molecular species and is not different for gases with different diffusivity. This indicates that convection (laminar or turbulent flows) occurs along gradients of total pressure but not along those of partial pressure.

On the other hand, the term of diffusion is defined as a result from the random motion of molecules due to their thermal energies, depending on their concentration (partial pressure) gradients and diffusivity. Hence, a separation of two gases with different diffusivity occurs during gas transport when diffusive transport constitutes a main limiting process, whereas a separation of two gases is not investigated when gas transport is mainly limited by convection. Diffusive transport in airways and alveoli occurs in axial and radial directions; yet the mechanism of these two directions is the same. Furthermore, the coupling of convection and diffusion occurs under a condition where the contribution of convection and diffusion to gas transport is of similar magnitude, which may occur predominantly in peripheral airways at generations between the 8^th and 12^th[34]. The magnitude of convection-diffusion coupling under a condition of laminar or turbulent flow is estimated using the dispersion coefficient proposed by Taylor and others[35-38]. However, the Taylor dispersion has been identified to be insignificant for gas transport during normal breathing[39-43], leading to the conclusion that it is enough to introduce only the convection and diffusion (particularly, axial diffusion), but not the dispersion, as the basic mechanism when assessing the gas transport along conducting and acinar airways.

Introducing both convection and diffusion as the gas mixing mechanism into model A of Weibel that has a simple geometry with trumpet or thumbtack shape, several groups of investigators analyzed the time-dependent change in gas concentration profile of an insoluble gas that is not absorbed into the blood (i.e., no gas exchange) along peripheral conducting airways and acinar airways during normal breathing[39,44-50]. Paiva[46] assumed: (1) no dispersion in airways; (2) no radial diffusion in airways; (3) no axial diffusion in alveolar regions; (4) no convective flow in alveolar regions (i.e., alveoli simply act as a sink for diluting a gas during inspiration while as a reservoir for expelling a gas during expiration); and (5) no collateral ventilation in acinar regions. As the theoretical calculation by Paiva[46] was made under a situation where the tidal volume was set at 500 mL and the functional residual capacity at 3000 mL (i.e., the representative lung volume during normal breathing), the end-inspiratory gas concentration in the alveolar region is reduced to 17% of that at the beginning of inspiration. Paiva[46] analyzed gas mixing kinetics along airways constituting the last 13^th generations, which correspond closely to the secondary lobule of Miller (Figures 2 and 3). The results emphasized by Paiva[46] are summarized below (Figure 4):

Figure 4 Concentration profiles along peripheral conducting airways and acinar airways during the respiratory cycle in thumbtack model. Upper abscissa: generation of airway branching defined in model A of Weibel. Lower abscissa: axial distance from alveolar region. Ordinate: fractional concentration of the test gas. 100% O₂ is inhaled (single breath) into and then exhaled from initially O₂-free lung with constant flow (tidal volume: 500 mL) and constant lung volume (functional residual capacity: 3000 mL). O₂ is assumed to not be absorbed into capillary blood (i.e., no gas exchange). Solid curves (black) indicate concentration profiles at successive equally timed periods during inspiration (1-5) and expiration (6-10). Dashed dotted lines (1-5) are tidal volume fronts (i.e., concentration fronts of non-diffusible tracer gas) during inspiration. Dashed line (6’) is a concentration profile calculated on assumption of no axial diffusion during expiratory phase corresponding to time point of solid line (6). Red solid line denotes “imaginary” boundary between common dead space and alveolar region conceptualized in classical gas-exchange theory. Boundary is defined by stationary gas front converting diffusional effect into equivalent convectional effect, which is formed at entrance of acinus of Aschoff (17^th generation of branching tree). See text for detailed explanation. Modified from ref[46].

(1) On inspiration, the front of a gas, which is expressed by the axial distance at which the concentration of a gas is reduced to 50% of the value at the beginning of inspiration, lags appreciably behind the front of tidal volume. The front of tidal volume attains to the 22^th generation of airways (i.e., the alveolar duct), whereas the front of a gas stays at the 17^th generation (i.e., the first-order respiratory bronchiole defining the acinus of Aschoff). The gas front is practically stationary, suggesting that the convective advancement of a gas into the periphery is counterbalanced by the diffusion-elicited proximal retreat of a gas. The volume from the airway entrance to the gas front is about 150 mL, which corresponds to the common dead space volume conceptualized in the classical gas-exchange theory. This indicates that the classical, common dead space can be defined as the inspiratory volume decided by the gas front, but not by the front of tidal volume. Furthermore, the classical gas-exchange theory assumes that the front of common dead space has a sharp, all-or-nothing transition boundary as if it is formed only by the ventilation-induced convective flow, implying that the effect of diffusion is converted to the equivalent effect of convection. The region distal to the boundary is the alveolar region in which gas exchange takes place. The definition of dead space specified by Paiva[46] issues an important message as to the way of how to define the functional gas exchange unit in the steady-state V_A/Q analysis and classical gas exchange theory, both of which will be argued in the following sections.

(2) On expiration, all intrapulmonary concentration gradients of a gas are rapidly faded out, which is mainly caused by the thoracic shrinkage-induced convective flow during expiration. The diffusive process in acinar gas phase imposes a negative impact on gas transport in an early stage, but not in a later stage, of expiration. The aforementioned findings during one respiratory cycle indicate that the stratification (i.e., the concentration gradient of a gas due to diffusion) is evident along acinar airways during inspiration but is minimal during expiration. Differing from inspiratory phase, the common dead space formed by the non-alveolated, conducting airways exerts nearly no impact on gas transport in conducting airways during expiratory phase. Instead, the common dead space literally functions as a simple reservoir for diluting the gas expelled from all alveolar regions during expiration.

Quantification of continuous distribution of V_A/Q

Definition of functional gas-exchange unit – importance of acinus of Aschoff: In general, the continuous distribution of V_A/Q in the lung has been quantitated by means of the MIGET. The most important matter in the MIGET is how to define the functional gas-exchange unit in which concentration of a gas should show uniform value. Although the acinus of Loeschcke approximates the anatomical gas-exchange unit, it is uncertain as to the issue of whether the acinus of Loeschcke concurrently serves as the functional gas-exchange unit. In the acinus of Loeschcke, an alveolus adjoins several alveoli and there are communications at the level of respiratory bronchioles or alveolar ducts (Martin channels), both of which tend to equalize gas-concentration difference over the acinus of Loeschcke. However, the point of whether the equalization of gas concentration in the acinus of Loeschcke is complete has not been confirmed. Therefore, it may be fraught with the problem to blindly adopt the acinus of Loeschcke as the functional gas-exchange unit.

In fact, Paiva et al[51] suggested that the regions distal to the alveolar duct, which correspond to the primary lobules of Miller, have different gas concentrations at the end of inspiration after a single-breath of a non-absorbable gas. However, they[51] also demonstrated that the gas-concentration difference between respective primary lobules of Miller formed during inspiration diminishes rapidly during expiration. Furthermore, caution should be paid to the fact that Paiva et al[51] disregarded the effect of Martin channels-induced collateral ventilation on gas-concentration equalization between the primary lobules of Miller. If the collateral ventilation-elicited equalization effect is taken into consideration, the gas-concentration difference between the primary lobules of Miller is expected to become much smaller. The importance of the acinus of Aschoff given by the region distal to the first-order respiratory bronchiole as the functional gas-exchange unit was implicitly indicated by Paiva[46]. He showed that the “imaginary” boundary separating the alveolar gas-exchange region from the dead space exists in the vicinity of the first-order respiratory bronchiole corresponding to the entrance of the acinus of Aschoff (Figures 3 and 4). As such, the study of Paiva[46] suggests that the functional gas-exchange unit under a steady-state condition is close to the anatomically defined acinus of Aschoff. This is because the size of the acinus of Aschoff is about half of the acinus of Loeschcke such that the gas-concentration variation, if any, is much smaller between the acini of Aschoff than that between the acini of Loeschcke. This results in that the acinus of Aschoff satisfies the essential requirement for the functional gas-exchange unit, namely the uniformity of gas concentration over the unit, more precisely than the acinus of Loeschcke does. Hence, we considered that the functional gas-exchange unit should be defined by the acinus of Aschoff whose area is smaller than that of the anatomical gas-exchange unit formed by the acinus of Loeschcke, leading to the conclusion that the MIGET argued in this section should be thought to predict the V_A/Q heterogeneity between the acini of Aschoff.

Basic rationale for determining V_A/Q distribution: Under a steady-state condition in which there is no change in the partial pressure of a gas (P) against the elapsed time (t) at any portion of the lung (i.e., ΔP/Δt = zero), the partial pressure of an inert gas in gas phase (P_A) and that in blood phase (P_c) of a given functional gas-exchange unit are described by the simple equation in accordance with the law of mass balance[19,52-54].

P_A/P_v = P_c/P_v = λ/(λ + V_A/Q) ∙∙∙∙∙ eq. 1

Where P_v denotes the partial pressure of a given inert gas in mixed venous blood, whereas λ is the blood-gas partition coefficient of a gas, which is decided by its blood solubility (α)[19,55]. The eq. 1 is approved only in a condition where the inspired air does not contain any trace of an inert gas. This holds true when an inert gas is infused through the peripheral vein but not when it is inhaled through the mouth. The eq.1 implies that the partial pressure of an inert gas in the functional gas-exchange unit is simply determined by the ratio of convective flow of ventilation (V_A) and that of perfusion (Q) distributed to each functional gas-exchange unit. There are many physiological assumptions that should be addressed for the establishment of eq. 1[19,54,56-59]. They are listed below.

(1) The functional gas-exchange unit is in a steady state such that the net transfer rate of an inert gas from capillary blood to alveoli in the functional gas-exchange unit exactly equals the net rate of elimination through expiration. Thus, the amount of an inert gas stored in blood, lung tissue, or alveoli is constant. The condition in which the MIGET is applied meets the steady-state definition.

(2) The tidal nature of ventilation is ignored. Namely, the rebreathing of a gas fulfilling the common dead space in previous expiration is not considered. The MIGET assumed the specific, personal dead space connecting to the individual functional gas-exchange unit. Therefore, in the next inspiration, each functional gas-exchange unit inhales the gas from each personal dead space that has the same gas composition as that expelled from each functional gas-exchange unit in the previous expiration. This assumption is highly dissociated from the anatomical and physiological facts. However, the volume of common dead space configured by upper and lower conducting airways amounts to 150 mL, which corresponds to only 5% of the lung volume represented by the functional residual capacity (3000 mL), suggesting that the gas contained in the common dead space is diluted largely when it reaches the gas-exchange units at an early stage of the next inspiration. Furthermore, the inspiratory tidal volume is 500 mL so that after inhaling the gas remaining in the dead space (150 mL), which is the averaged mixed alveolar gas exhaled from all functional gas-exchange units during previous expiration, an inspired gas with no inert gas refreshes the gas-exchange units in a later stage of inspiration (350 mL). These considerations suggest that the averaged inert-gas concentration at the entrance of functional gas-exchange unit during inspiration is almost zero under a condition where the MIGET is applied.

(3) The pulsatile nature of blood is disregarded so that capillary blood flow is taken to be continuous. Bidani et al[60] analyzed the effect of pulsatile flow (Q_C) and pulsatile capillary blood volume (V_C) on equilibration of O₂ between capillary blood and alveolar gas. They found little difference in O₂ equilibration profile between the condition where both Q_C and V_C are constant and the condition where both Q_C and V_C have pulsatile natures. Therefore, the assumption of constant blood flow may derive no discernable distortion regarding the steady-state gas exchange[61].

(4) The functional gas-exchange unit is assumed to receive capillary blood flow with the same hematocrit. As first suggested by Brisco[62], the gas solubility in erythrocytes is different from that in plasma, indicating that variation in hematocrit between capillaries may affect inert-gas exchange to some extent. However, Young and Wagner[63] demonstrated that although the inert-gas solubility is influenced appreciably by misdistribution of hematocrit, its effect on inert-gas exchange is small.

(5) The inert-gas solubility is taken to be constant irrespective of oxygen saturation with hemoglobin (Hb) (SO₂) and blood pH. Yamaguchi et al[64] experimentally demonstrated that SO₂ has no impact, while blood pH has a substantial impact, on the inert-gas solubility, including ethane and cyclopropane, both of which are confined to the protein fraction of blood. However, in the same paper, Yamaguchi et al[64] demonstrated that the effect of pH-dependent change in blood solubility of either ethane or cyclopropane on recovered V_A/Q distribution is small.

(6) Diffusion equilibration is assumed to be complete. This assumption applies both to the diffusion between capillary blood and gas phase through alveolocapillary membrane (aqueous-phase diffusion) and to the diffusion within gas phase [gas-phase diffusion (stratification)]. The blood-gas equilibration is valid for any inert gas that does not combine with Hb[52]. However, this assumption is invalid while analyzing the gas combining with Hb, especially carbon monoxide (CO) and nitric oxide (NO)[65].

As argued in the previous section, the stratification is evident in inspiration but not in expiration (Figure 4). Since the MIGET analyzes the gas only collected during expiration, the assumption that the stratification has little impact on gas transfer may be approved in the MIGET condition. Furthermore, Yamaguchi et al[66] demonstrated that excepting severe acute respiratory distress syndrome, the stratification plays no role during inert-gas elimination under the condition similar to that of the MIGET. The detailed discussions expanded above identified that most of the physiological assumptions introduced by the MIGET have correct grounds.

Determination of continuous distribution of V_A/Q: Saline containing a small quantity of six foreign inert gases with a wide variety of λ, including sulfur hexafluoride (SF₆) (λ = 0.009 mL/mL at 37 °C), ethane (λ = 0.09), cyclopropane (λ = 0.58), halothane (λ = 2.82), diethyl ether (λ = 14.4), and acetone (λ = 304) was infused through the peripheral vein at a constant rate of 2 mL/min[54,56]. After steady state was established (about 30 min after initiating infusion), expired gas, arterial blood, and mixed venous blood were simultaneously sampled and the concentrations of six inert gases in the samples were measured by gas chromatography equipped with flame ionization for hydrocarbons and electron capture for SF₆[67]. Using the data measured for six inert gases, Wagner and colleagues[56,58,59] established a novel method allowing for predicting a continuous distribution of V_A/Q in a representative manner. In this method, they assumed the 50 functional gas-exchange units with a variety of V_A/Q values, including the right-to-left shunt with V_A/Q of zero (the number of functional gas-exchange unit: 1), the dead space with infinite V_A/Q (the number: 50) composed of series (common) and parallel (alveolar) dead space, and the 48 units with finite values of V_A/Q ranging from 0.005 to 100, all of which are arranged in parallel and are equally spaced on a logarithmic scale. Applying the least-squares minimization coupled with enforced smoothing under non-negativity constraints for V_A and Q, they determined the distribution of V_A and/or Q along the logarithmic V_A/Q axis. However, it should be noted that the lung model adopted by Wagner et al[56,58,59] consists of unknown variables whose number (n = 50) greatly exceeds the number of measured variables (n = 6) so that no unique solution is generally obtainable[68,69]. In principle, the greater the number of gases, the more detailed and precise is the resultant estimate of the V_A/Q distribution. To solve the problem of a non-unique solution, Evans and Wagner[69-73] introduced the linear programing approach, in which the upper limit of V_A and/or Q distributed to a certain V_A/Q unit was examined based on the error of each inert-gas measurement. The linear programming can reliably decide the issue of whether the V_A/Q distribution is unimodal or contains more than one mode, i.e., bimodal or trimodal. The mode of V_A/Q distribution representatively determined by the least-squares minimization was shown to be qualitatively coincident with that by the linear programming[70] (Figure 5). Furthermore, Wagner and West[59,74-76] analyzed the matching between the measured arterial P_O2 (P_aO2) and that predicted from the representative V_A/Q distribution. They found a satisfactory agreement between the two values in patients with chronic obstructive pulmonary disease (COPD) and interstitial pneumonias at rest (Figure 6). However, Wagner[76] found that the P_aO2 predicted from the MIGET is appreciably higher than the measured P_aO2 in patients with interstitial pneumonias under an excise condition. This indicates that in addition to the V_A/Q heterogeneity, the aqueous-phase diffusion limitation through alveolocapillary membrane plays some role in worsening hypoxemia during exercise in patients with interstitial pneumonias.

Figure 5 Range of solution for ventilation-perfusion distribution recovered from multiple inert-gas elimination technique. Comparison between representative perfusion distribution obtained from multiple inert-gas elimination technique in terms of least-squares minimization and maximum perfusion distribution decided by linear programming. Adopted from ref[70].

Figure 6 Comparison of predicted P_aO2 from multiple inert-gas elimination technique and measured P_aO2. A: Chronic obstructive pulmonary disease at rest or during exercise; B: Interstitial pneumonias at rest while inspiring gas with 21% O₂ (room air) or 50% O₂. Adopted from ref[76]. COPD: Chronic obstructive pulmonary disease.

The representative distributions of V_A/Q in normal subjects as well as patients with interstitial pneumonia and COPD are depicted in Figure 7[59]. A normal subject revealed the unimodal distribution of V_A and Q along the V_A/Q axis, which resembles the sharp normal distribution. A patient with interstitial pneumonia showed the bimodal V_A/Q distribution with low V_A/Q regions associated with augmented right-to-left shunt. The formation of low V_A/Q regions and shunt may be ascribed to the interstitial fibrosis-elicited destruction of acinar structures. A COPD patient with predominant emphysema showed the bimodal V_A/Q heterogeneity with high V_A/Q regions, which may be attributed to the regional hyperventilation caused by the destruction of alveoli. A patient with predominant bronchiolar involvements revealed the bimodal V_A/Q heterogeneity with low V_A/Q regions, which may be caused by the destruction and/or obliteration of acinar bronchioles.

Figure 7 Representative ventilation-perfusion distributions predicted from multiple inert-gas elimination technique in various types of lung diseases. A: Young normal subject; B: Patient with interstitial pneumonia; C: Chronic obstructive pulmonary disease patient with predominant emphysema; D: Chronic obstructive pulmonary disease patient with predominant bronchiolitis. See text for detailed explanation. Adopted from ref[59]. V_A/Q: Ventilation-perfusion; COPD: Chronic obstructive pulmonary disease.

Based on the V_A/Q-D/Q concept developed by Piiper[77], in which D denotes the aqueous-phase diffusing capacity for O₂, CO₂, or CO through alveolocapillary membrane, Yamaguchi et al[66,78-80] elaborated a novel method that allows for predicting the continuous distribution of V_A/Q and D/Q under a steady-state condition. They[66,78-80] found that in addition to the V_A/Q heterogeneity, the D/Q heterogeneity is expanded in patients with interstitial lung diseases but less evident in patients with COPD (Figure 8). Therefore, it was conceived that the heterogeneity of aqueous-phase diffusion plays some role in impeding gas exchange, particularly in lungs with destruction of alveolocapillary membrane or pulmonary microcirculation.

Figure 8 Ventilation-perfusion (V_A/Q)-Diffusing capacity-perfusion (D/Q) distributions in normal and diseased lungs. A: V_A/Q and D/Q distribution in normal lung; B: That in lung with desquamative interstitial pneumonia; C: That in lung with chronic obstructive pulmonary disease characterized by emphysema. Patient with desquamative interstitial pneumonia (B) revealed minimal V_A/Q heterogeneity but very low D/Q regions that may yield “diffusion shunt” contributing to hypoxemia. Very low D/Q regions may be caused by decreased surface area and/or increased thickness of alveolocapillary membrane. Patient with chronic obstructive pulmonary disease (C) showed V_A/Q heterogeneity with high V_A/Q regions but minimal regions with very low D/Q. Modified from ref[80].

AaDO₂ in classical gas-exchange theory

Determination of alveolar gas composition: About 100 years ago, Krogh et al[81] first described the important principle in the field of respiratory physiology; namely, the gas exchange taking place in any lung region is determined by neither the ventilation nor the blood flow but by the ratio of one another. Shortly after this, Haldane[82] recognized that ventilation-perfusion heterogeneity could cause hypoxemia. These pioneering works led Fenn et al[83], Riley and Cournand[84], and others into the establishment of classical gas-exchange theory in a more precise fashion. Rahn[85] proposed the concept of “mean alveolar gas” while Riley et al[84,86] proposed the concept of “ideal alveolar gas”. These two terms are physiologically identical and defined as the intersection of blood and gas R lines, in which R is the respiratory gas-exchange ratio given by the ratio of CO₂ production (V_CO2) to O₂ consumption (V_O2) of the whole body (Figure 9). As the R is identical at any portion of the body under a steady-state condition, the R values at the level of capillary blood (blood R line) and alveolar gas (gas R line) are the same as that of mixed expired gas. It was theoretically identified that the intersection of the blood R line and gas R line defined the single V_A/Q value and the corresponding unique pair of P_O2 and P_CO2 in alveolar gas and capillary blood of a certain lung (the O₂-CO₂ diagram or the V_A/Q line[61,87])[84-86]. Although the P_O2 and P_CO2 values thus determined satisfactorily meet the metabolic state of the whole body, it does not secure that these P_O2 and P_CO2 can work as the representative indicator while analyzing the gas-exchange behavior in the lung. This is because the concept of mean (ideal) alveolar P_O2 and P_CO2 is largely lacking in the considerations on complicatedly intertwined lung anatomy and intricate physiological gas-transfer mechanism along the airways and in the lung periphery. Furthermore, to calculate the mean (ideal) alveolar gas, it is necessary to measure P_O2 and P_CO2 in mixed venous blood. As this procedure is cumbersome in a clinical setting, Riley et al[86] proposed a new concept of “effective alveolar gas,” in which the mean (ideal) alveolar P_CO2 is taken to equal the arterial P_CO2 (P_aCO2). Because of the relative flatness of blood CO₂ dissociation curve under a physiological condition, the difference between mean (ideal) P_CO2 and P_aCO2 is remarkably small (Figure 9), which indicates that it is not unreasonable to use P_aCO2 in place of mean (ideal) alveolar P_CO2. Therefore, P_aCO2 is defined as “effective alveolar P_CO2” and alveolar P_O2 estimated therefrom is called “effective alveolar P_O2”. As it is not necessary to measure the gas pressures in mixed venous blood, the concept of effective alveolar P_O2 has received a great deal of support from physicians, though the anatomical and/or physiological validations on effective alveolar P_O2 have been lacking. Based on the simplified form of alveolar gas equation, the effective alveolar P_O2 (effective P_AO2) is calculated as

Figure 9 O₂-CO₂ diagram and ventilation-perfusion line. A: O₂-CO₂ diagram, in which solid and dotted lines indicate gas R lines and blood R lines, respectively. Intersection of these two lines denoted by red circle provides a single ventilation-perfusion value and corresponding unique pair of P_O2 and P_CO2; B: Ventilation-perfusion line constructed by connecting each intersection of gas R lines and blood R lines. I: Inspired point; a: Arterial point; v: Mixed venous point. See text for further explanation. Adopted from ref[87].

Effective P_AO2 = P_IO2 – P_aCO2/R ∙∙∙∙∙ eq. 2

Where P_IO2, P_aCO2, and R denote inspiratory P_O2, arterial P_CO2, and respiratory gas-exchange ratio, respectively. The R value is taken to be 0.83 in general.

Using eq. 2, one can easily predict the effective alveolar-arterial P_O2 difference (effective AaDO₂) at the bed side as:

Effective AaDO₂ = effective P_AO2 – measured P_aO2 ∙∙∙∙∙ eq. 3

Unless otherwise specified, P_AO2 and AaDO₂ indicate effective P_AO2 and effective AaDO₂, respectively in this review.

Relationships between AaDO₂ and V_A/Q distribution decided by MIGET: Differing from the V_A/Q theory developed on the basis of the MIGET, the classical gas-exchange theory is not sufficiently warranted by the anatomical and physiological facts. Hence, it is necessary for certifying the issue of whether the effective P_AO2 indeed functions as the representative P_O2 in mixed gas exhaled from all functional gas-exchange units. For answering this question, it is indispensable at least to compare the results obtained from the classical gas-exchange theory with those from the MIGET.

We first examined what kinds of V_A/Q units are sensitively detected by O₂. We calculated P_AO2 in each V_A/Q unit on the assumption that P_O2 equilibration between alveolar gas and capillary blood is complete (Figure 10). We found that P_AO2 changes sharply in the units having V_A/Q ranging from 0.1 to 1.0, (i.e., the sensitivity of O₂ to V_A/Q detection is equivalent to that of cyclopropane) but not in other V_A/Q units, suggesting that AaDO₂ reflects gas-exchange behavior in regions with moderately low V_A/Q values but is insensitive to gas exchange in regions with very low and high V_A/Q values. The finding indicated that the ability of AaDO₂ detecting the V_A/Q heterogeneity was substantially limited and does not cover the whole range of V_A/Q heterogeneity as the MIGET does.

Figure 10 Ventilation-perfusion (V_A/Q) regions detected from gas exchange behaviors of O₂, CO₂, and inert gases. Left ordinate: excretion (E) of inert gas defined as P_A/P_v, where P_A is partial pressure of inert gas in each functional gas-exchange unit, while P_v is that in mixed venous blood. Right ordinate: P_AO2 and P_ACO2 in each functional gas-exchange unit. Abscissa: logarithmic V_A/Q value. E of each inert gas sharply changes in a certain range of V_A/Q depending on its blood-gas partition coefficient (λ). For instance, sharp change in E for SF₆ is found at V_A/Q ranging between 0.001 and 0.01. As such, SF₆ has high sensitivity to detecting gas exchange in regions with very low V_A/Q. On the other hand, E for acetone sharply changes at V_A/Q ranging from 10 to 100, indicating that acetone is susceptible to gas exchange in regions with very high V_A/Q. P_AO2 changes greatly at V_A/Q ranging between 0.1 and 1.0, suggesting that O₂ has sensitivity to detecting gas exchange in regions with moderately low V_A/Q (like cyclopropane). Meanwhile, CO₂ is susceptible to gas exchange in regions with moderately high V_A/Q ranging from 1.0 to 10 (like halothane).

Next, we investigated the coincidence between mixed P_AO2 estimated from V_A/Q distribution recovered from the MIGET and effective P_AO2 calculated from eq. 2 in patients with interstitial pneumonia or COPD (Figure 11). The coincidence between the two P_AO2 was satisfactory, indicating that the effective P_AO2 and effective AaDO₂ can accordingly change reflecting the formation of moderately low V_A/Q regions that is related to the emergence of hypoxemia. Hence, we concluded that effective P_AO2 and effective AaDO₂ are practically useful in a clinical setting as far as the gas-exchange abnormality elicited by moderately low V_A/Q regions is considered.

Figure 11 Comparison of effective P_AO2 and P_AO2 in mixed alveolar gas estimated from ventilation-perfusion distribution decided by multiple inert-gas elimination technique. Effective P_AO2 and P_AO2 estimated from multiple inert-gas elimination technique were compared [patients with interstitial pneumonia (n = 14) and chronic obstructive pulmonary disease (n = 11)]. Coincidence of two P_AO2 is satisfactory. See text for further explanation.

Pulmonary diffusing capacity for CO (D_LCO)

General survey on D_LCO: The most terminal organization in the lung is alveolocapillary membrane and capillary network embedded in the alveolar wall (septum) (Figure 12). The main mechanism of gas transfer through alveolocapillary membrane and capillary blood is the aqueous-phase diffusion so that the impaired gas transfer there is estimated by measuring the pulmonary diffusing capacity using diffusion-limited gases such as CO and NO, i.e., D_LCO and D_LNO (Figure 13). About 30 years ago, Guénard et al[88] and Borland and Higginbottom[89] independently elaborated a method for measuring the D_LNO. NO has an extremely high affinity with Hb, i.e., 400000 times higher than that of O₂ and 1800 times higher than that of CO[65], which indicates that the chemical reaction of NO with Hb has virtually no obstacle to NO uptake by erythrocytes (i.e., diffusion limitation but no reaction limitation). Therefore, the partial pressure of NO in pulmonary capillary network is assumed to be practically zero, suggesting that D_LNO may be predominantly decided by the diffusive process across alveolocapillary membrane and erythrocytes. However, this does not hold true in the case of D_LCO, in which besides the diffusive process across alveolocapillary membrane and erythrocytes, the replacement reaction of CO with HbO₂ causes a substantial resistance to CO transfer, i.e., diffusion limitation plus reaction limitation. In the current review, we will confine ourselves to describe D_LCO only. This is simply because D_LNO is not presently measured worldwide. If readers want to know a more comprehensive discussion on D_LCO and D_LNO, we refer them to a more extensive review reported by us[65].

Figure 12 Scanning electron microscopic image of perfusion-fixed alveolar walls. PA: Pulmonary arteriole; A: Alveolar walls, C: Capillary networks embedded in alveolar wall. Adopted from ref[25].

Figure 13 Partial-pressure equilibration of gases between alveolar gas and capillary blood. Inert gases that are not combined with erythrocyte hemoglobin (Hb) show instantaneous equilibration between alveolar gas and capillary blood. O₂ and CO₂ that have relatively low affinity with Hb need some time for completing equilibration between alveolar gas and capillary blood. CO and NO (red lines) that have very high affinity with Hb prevent rise in their partial pressures in capillary blood. Hence, CO and NO do not reach partial-pressure equilibration between alveolar gas and capillary blood any more even in normal lung. Affinity of NO with Hb is extremely high and chemical reaction of NO with Hb is very rapid such that partial pressure of NO in capillary blood is maintained at negligible level. Therefore, there is no “back-pressure” effect during measurement of D_LNO. However, affinity of CO with Hb is lower than that of NO (i.e., 1/1800 times that of NO). Therefore, partial pressure of CO in capillary blood rises gradually such that “back-pressure” of CO from capillary blood should not be ignored while measuring the pulmonary diffusing capacity for CO.

It has been over 100 years since Krogh[90] developed the method to measure the single-breath CO uptake through alveolocapillary membrane. Since then, the single-breath D_LCO has gained a position as the most clinically useful pulmonary function test after spirometry and lung volume examination. The practical method to examine the single-breath D_LCO was proposed by Ogilvie et al[91]. Concurrently, Roughton et al[92] proposed an important model describing the gas transfer through alveolocapillary membrane and erythrocytes. They assumed that the CO transfer process from alveolocapillary membrane to Hb condensed within erythrocytes could be simplified into the two processes; i.e., (1) the membrane diffusing capacity for CO (D_MCO) that reflects the diffusion limitation across alveolocapillary membrane and plasma layer surrounding erythrocytes, and (2) the blood diffusing capacity for CO (D_BCO) that is defined as the product of capillary blood volume (V_C) and specific gas conductance for CO in the blood (θ_CO). The θ_CO signifies the diffusive process across erythrocyte membrane and its interior incorporated with the competitive replacement reaction of CO with HbO₂. Since the reciprocals of D_MCO and D_BCO are the gas-transfer resistances that are connected in series, the total resistance for CO transfer, 1/D_LCO, is expressed as

1/D_LCO = 1/D_MCO + 1/(θ_CO∙V_C) ∙∙∙∙∙ eq. 4

In addition to the D_LCO, the D_LCO/V_AV, in which V_AV denotes the alveolar volume during a single-breath maneuver, is similarly important. This parameter expresses the D_LCO per unit alveolar volume, which is equal to the Krogh factor (K_CO), i.e., the rate constant of alveolar gas uptake per unit pressure. The relative contribution of 1/D_MCO and that of 1/D_BCO to overall resistance of CO transfer (1/D_LCO) were found to be 23% and 77%, respectively (Figure 14)[65,93]. The finding indicated that D_LCO is primarily weighted by D_BCO, suggesting that D_LCO-associated parameters are more susceptible to the damage of pulmonary microcirculation than that of alveolocapillary membrane.

Figure 14 Relative contribution of 1/D_M and 1/D_B to overall resistance of 1/D_L. Difference in membrane diffusing capacity for CO (D_MCO) and that for NO (D_MNO) is simply attributed to difference in their Krogh diffusion constants defined as (α∙d), where α is Bunsen solubility coefficient of the gas (mL/mL/atm) and d is gas diffusivity (cm²/s) in alveolocapillary membrane and plasma layer. D_MNO/D_MCO is assumed to be 1.97[65]. On the other hand, blood-diffusing capacity for CO (D_BCO) or NO (D_BNO) is defined as alveolar capillary blood volume (V_C) multiplied by specific gas conductance of each gas (θ_CO, θ_NO). θ_CO signifies the diffusive process across erythrocyte membrane and its interior incorporated with the competitive, replacement reaction of CO with hemoglobin O₂. Therefore, θ_CO changes significantly depending on surrounding P_O2. On the other hand, θ_NO is not influence by chemical reaction with hemoglobin and is predominant by diffusion across erythrocytes, thus resulting in constant θ_NO [4.5 mL/min/mmHg/(mL×blood)[65,93]]. The pulmonary diffusing capacity for CO is primarily governed by both diffusion through alveolocapillary membrane and erythrocytes as well as reaction in erythrocytes (limitation by both diffusion and reaction), whereas the pulmonary diffusing capacity for NO is almost evenly prescribed by diffusion through alveolocapillary membrane and that inside erythrocytes (diffusion limitation only). Important message drawn from this analysis is that the pulmonary diffusing capacity for NO is evenly sensitive to morphological abnormality in alveolocapillary membrane as well as that in pulmonary microcirculation. On the other hand, the pulmonary diffusing capacity for CO is more sensitive to microcirculatory abnormality. Adopted from ref[65].

Factors affecting D_LCO and K_CO: The D_LCO and K_CO are the physiological parameters that are useful for diagnosing the pathological changes occurring in alveolocapillary membrane and pulmonary microcirculation. However, there are many matters that should be argued with regard to the D_LCO-related parameters as listed below.

Influence of back-pressure of CO from pulmonary capillary blood: The CO back-pressure during D_LCO measurement is estimated at about 3% of the alveolar P_CO in a nonsmoking subject[65]. Therefore, the effect of CO back-pressure on D_LCO can be ignored in a subject with no habitual smoking. However, heavy smokers or repeated measurements of D_LCO produced the CO back-pressure that is not disregarded because the CO back-pressure reaches about 16% of the alveolar P_CO, which induces a significant error while estimating the overall D_LCO.

Influence of functional heterogeneities: The D_LCO is influenced by at least three heterogeneities[65]; (1) the distribution of alveolar tidal volume to alveolar volume (V_AT/V_AV), (2) that of diffusing capacity to alveolar volume (D_LCO/V_AV, i.e., K_CO), and (3) that of alveolar tidal volume in each region to total alveolar tidal volume. Of these, the heterogeneity of K_CO, which may be caused by the regional difference in rate constant of CO uptake due to local variations in the thickness of alveolocapillary membrane and plasma layer as well as the density of erythrocytes in microcirculation[94], is the most important, while other two heterogeneities may be considerably degraded during a breath-holding maneuver. Piiper and Sikand[95,96] theoretically and experimentally analyzed the significance of K_CO heterogeneity on overall D_LCO, resulting in a larger D_LCO value for a shorter breath-hold time. Furthermore, their analysis[95,96] identified that the overall D_LCO may be underestimated by 37% in a condition where a serious K_CO heterogeneity exists. However, it is impossible to universally apply the value reported by Piiper and Sikand[95,96] to clinical cases having various types of lung diseases because the extent of K_CO heterogeneity differs substantially in respective patients. Therefore, the measured D_LCO should be regarded as the “apparent or effective” value that is always underestimated by the K_CO heterogeneity to an uncertain extent.

To note, if the diffusing capacity (D) for a certain gas is measured under a steady-state condition, the apparent D value is significantly reduced due to the V_A/Q and D/Q heterogeneities. Indeed, Yamaguchi et al[80] identified an appreciable effect of these heterogeneities on apparent D value, particularly in patients with interstitial lung diseases (Figure 8).

Influence of alveolar volume: Declining alveolar volume (V_AV) without loss of acini (for instance, inspiratory muscle insufficiency) accompanies[65,97]: (1) slight decrease in D_MCO/V_AV but large increase in V_C/V_AV, (2) decrease in D_LCO, and (3) increase in K_CO, which indicates the “facilitated” CO transfer per unit lung volume in conditions with declining V_AV with no loss of acini. Overall V_C remains constant due to the stability of cardiac output (Q), which reinforces the redistribution of pulmonary perfusion during lung volume changes; thus V_C/V_AV increases with decreasing V_AV. Reduced V_AV decreases D_MCO but does not change V_C, leading to decreased D_LCO. Hence, the characteristic of decreased V_AV without loss of acini is decreased D_LCO in association with increased K_CO.

The situation is different in the diminished expansion caused by loss of acini such as post-pnemonectomy. Declining V_AV with losing acini elicits[65,97]: (1) decrease in both D_M and V_C associated with large decrease in D_LCO, (2) small increase in V_C/V_AV accompanied with increased D_MCO/V_AV (this is attributed to the redistribution of Q), and (3) small change in K_CO. The trend of D_MCO/V_AV is opposite to that without loss of acini. The extent of increased V_C/V_AV in lungs with loss of acini is less than that without loss of acini, leading to a restricted increase in K_CO (i.e., a lesser improvement of CO uptake per unit lung volume) in lungs with losing acini.

Influence of cardiac output: The increased Q (for instance, left-to-right shunt) elicits capillary distension and/or recruitment, leading to the increased surface area of alveolocapillary membrane (rise in D_MCO and D_MCO/V_AV) and the augmented capillary blood volume (rise in V_C and V_C/V_AV), which increase both D_LCO and K_CO. On the other hand, decreased Q shows the opposite trend[98,99].

Influence of alveolar hemorrhage: The behaviors of D_LCO and K_CO in patients with alveolar hemorrhage (for instance, ANCA-associated microscopic polyangiitis) are qualitatively similar to those observed in patients with increased Q. This is because hemorrhage-derived Hb released into the alveolar space absorbs CO before it reaches the capillary blood such that both D_LCO and K_CO are elevated even under a condition where alveolar walls are significantly injured.

Differential diagnosis based on behaviors of D_LCO and K_CO: The decrease in both D_LCO and K_CO is the general feature in lungs with various injuries of alveolocapillary membrane and/or pulmonary microcirculation because they reduce D_MCO, D_BCO, or both. We want to define these general behaviors of D_LCO and K_CO as the type-1 abnormality. However, there are many pathological conditions that do not follow the principle of type-1 abnormality. As discussed above, declining alveolar volume without loss of acini leads to decreased D_LCO in association with increased K_CO. We define these behaviors of D_LCO and K_CO as the type-2 abnormality. On the other hand, declining alveolar volume with losing acini elicits decreased D_LCO and restricted rise in K_CO. We define these aspects of D_LCO and K_CO as the type-3 abnormality. Increased Q and alveolar hemorrhage increase both D_LCO and K_CO. We define these increasing aspects of D_LCO and K_CO as the type-4 abnormality.

In 1951, Austrian et al[100] first proposed the disease concept of “alveolar-capillary (A-C) block.” This classic syndrome originated to describe the interference of gas transfer through alveolocapillary membrane by aqueous-phase diffusion in patients with various kinds of interstitial lung disease[101,102]. However, many physiologists and clinicians have questioned the role of impaired aqueous-phase diffusion as the factor inducing hypoxemia. This is because in addition to alveolar interstitium, acinar airways and pulmonary microcirculation are simultaneously injured in most of the interstitial lung diseases. The impediment of acinar airways and/or microcirculation evokes the V_A/Q heterogeneity leading to hypoxemia. In relation to this problem, there has been no direct study credibly addressing the issue of whether the impaired aqueous-phase diffusion across alveolocapillary membrane can indeed evoke hypoxemia. To answer this question, all gas-exchange parameters, including V_A/Q distribution, AaDO₂, D_LCO, and K_CO, should be simultaneously examined in patients with pathologically-confirmed pure lesion of impaired alveolocapillary membrane but no lesion of acinar airways or pulmonary microcirculation.

CONCLUSION

To answer the questions that have remained unsolved in the field of respiratory physiology, we analyzed the structure-function relationships in the lung from a variety of anatomical and physiological points of view. The important results found therefrom are as follows: (1) The model A of Weibel is useful for estimating gas mixing in peripheral conducting airways; (2) The anatomical gas-exchange unit is organized by the acinus of Loeschcke; (3) Although it is difficult to define the functional gas-exchange unit correctly, the acinus of Aschoff may act as the functional gas-exchange unit in an initial approximation; (4) The MIGET is supported by the anatomical and physiological backgrounds in many respects. Therefore, the MIGET is clinically valuable for predicting the heterogeneous V_A/Q distribution between the acini of Aschoff under steady-state conditions; (5) The effective AaDO₂ sensitively detects moderately low V_A/Q regions causing hypoxemia. Hence, the AaDO₂ is taken as a useful, clinical indicator for predicting impaired gas exchange. However, one should be aware that the AaDO₂ is insensitive to impaired gas exchange caused by very low V_A/Q regions and/or high V_A/Q regions; (6) The D_LCO and K_CO (D_LCO/V_AV) are useful for diagnosing the impediment of alveolar walls, including alveolocapillary membrane and pulmonary microcirculation. However, one should recognize the fact that the effect of regional variations in K_CO on overall D_LCO could not be totally eliminated; and (7) The clinically useful categorization with regard to behaviors of D_LCO and K_CO is the type-1 abnormality (decrease in both D_LCO and K_CO), the type-2 abnormality (decrease in D_LCO but increase in K_CO), the type-3 abnormality (decrease in D_LCO but restricted rise in K_CO), or the type-4 abnormality (increase in both D_LCO and K_CO). These categories of D_LCO and K_CO abnormalities allow for precise differential diagnosis concerning the damage in alveolocapillary membrane and that in pulmonary microcirculation.