Lung ultrasound in nephrology: Basics, applications, limitations, and future directions

Table 1 Commonly used methods to assess hemodynamic status in nephrology practice

Method	Strengths	Limitations
Physical exam	Standard bedside evaluation: Does not require additional training for physicians; Positive findings are typically clinically meaningful	Very limited sensitivity; may fail to detect a substantial number of patients with volume overload
Body weight	Short-term weight changes can indicate fluid accumulation or loss; Can be self-monitored by the patient at home	Inaccurate readings may result from improper calibration, use of different scales, or inconsistent techniques such as wearing varying amounts of clothing during each measurement; Weight changes may not capture congestion related to fluid redistribution
Intake-output documentation	Provides an overview of the patient’s fluid balance	Documentation errors are common, especially outside the intensive care setting; Does not capture congestion resulting from fluid redistribution
Continuous hematocrit monitoring	Delivers real-time insights into relative changes in intravascular blood volume, enabling adjustment of ultrafiltration rate and volume accordingly	Use is limited to patients receiving hemodialysis; Typically operated by nurses or technicians, requiring dedicated staff training; Does not evaluate tissue congestion, extravascular lung water or cumulative fluid burden
Bioimpedance	Offers information on total body, extracellular, and intracellular water, allowing for calculation of both absolute and relative fluid overload	Unable to distinguish between compartmentalized edema (such as ascites, pericardial, or extravascular lung water) and overall increased total body water; Does not provide information on intravascular volume
Right heart catheterization	Offers detailed assessment of hemodynamic parameters including right atrial pressure, pulmonary artery pressure, pulmonary capillary wedge pressure, pulmonary vascular resistance, and cardiac output	Invasive modality, generally limited to specialized intensive care settings; Does not assess the presence or absence of extravascular lung water, as elevated pressures do not always correlate with volume overload; Lacks information on the degree of venous congestion; Susceptible to errors from incorrect transducer calibration, leveling, zeroing, or improper balloon inflation; Most nephrologists are not adequately trained to interpret waveforms or recognize measurement errors
IVC POCUS	Estimates right atrial pressure; Relatively simple to perform and can be learned with brief training	Estimating right atrial pressure using IVC POCUS is unreliable in mechanically ventilated patients; A plethoric IVC is not specific to volume overload and may be seen in conditions such as cardiac tamponade, pulmonary embolism, tricuspid regurgitation, or pulmonary hypertension; A small, collapsible IVC cannot distinguish between hypovolemia, euvolemia, or high cardiac output states; IVC may appear small and collapsed despite elevated right atrial pressure in cases of intra-abdominal hypertension; IVC collapsibility is influenced by the strength of respiratory effort, which varies significantly among patients, limiting the real-world applicability of standardized cutoffs from studies
Internal jugular vein POCUS	Estimates right atrial pressure; Especially helpful when the IVC is difficult to visualize or yields unreliable information, such as in patients with cirrhosis	Susceptible to errors from improper bed positioning, excessive transducer pressure, and off-axis imaging; The assumption that right atrial depth is consistently 5 cm from the sternal angle has been shown to be inaccurate - often requires concurrent focused cardiac ultrasound to determine this; Scanning protocols vary across the literature, limiting standardization
Lung ultrasound	Identifies and quantifies extravascular lung water; More sensitive than chest X-ray for detecting cardiogenic pulmonary edema; Can be performed using basic, lower-cost ultrasound equipment	B-lines are not specific to pulmonary edema and may also appear in conditions such as lung fibrosis, infections, or contusions; In certain cases, distinguishing between cardiogenic and non-cardiogenic pulmonary edema requires concurrent assessment of left ventricular filling pressures using cardiac Doppler ultrasound (an advanced POCUS skill)
Venous Doppler/VExUS (hepatic, portal, intrarenal, and femoral veins)	Detects and quantifies systemic venous congestion; Enables monitoring the response to decongestive therapy through repeat assessments	It is an advanced skill that requires competence in Doppler ultrasound; Lack of simultaneous ECG may limit interpretation, particularly the hepatic vein waveform; Does not differentiate pressure and volume overload; Requires mid- to high-end ultrasound equipment with ECG capability
Focused cardiac ultrasound	Offers insights into cardiac function, chamber size, pericardial effusion, and major valvular abnormalities; Experienced users can also estimate stroke volume, pulmonary artery pressure, and left ventricular filling pressures	Considered an advanced skill; nephrologists performing Doppler assessments typically require formal certification in critical care echocardiography; Requires mid- to high-end ultrasound equipment; Accuracy depends on adequate acoustic windows, which are affected by factors such as patient body habitus, positioning, and operator expertise

IVC: Inferior vena cava; ECG: Electrocardiogram; POCUS: Point-of-care ultrasonography.