Venous Doppler Ultrasound to Guide Ultrafiltration in Hemodialysis: Practical Insights for Nephrologists

In a prospective, single-center observational study, Gudiño-Bravo et al. [1] investigated the relationship between sonographic markers of venous congestion and ultrafiltration volume in maintenance hemodialysis patients using an abbreviated version of the venous excess ultrasound (VExUS) score. Their findings indicated that ultrafiltration volume correlated with changes in inferior vena cava (IVC) diameter but not with hepatic or portal vein Doppler waveforms. This commentary aims to highlight some key considerations for point-of-care ultrasound (POCUS) research in hemodialysis patients. It is not specifically intended as a critique of the study, but rather to use this study as an opportunity to guide future research in this emerging field and offer insights for practicing clinicians.First, this study is commendable for shedding light on an unmet need in the field, as objective methods for volume management in patients undergoing maintenance dialysis remain a persistent challenge for nephrologists. Incorporating central venous pressure (CVP) measurements to validate the sonographic findings is particularly noteworthy, as this is rarely feasible in real-world settings due to time and equipment constraints. That said, the study’s small sample size and the relatively young, otherwise healthy cohort limit its generalizability to broader patient populations. With a mean age of 38 years, only 4 out of 22 patients on antihypertensive medications, and predictable interdialytic weight gain alongside only moderately elevated CVP, this group does not reflect the typical hemodialysis population that would most benefit from the additional effort required for these advanced sonographic assessments. This distinction is further supported by the cohort’s sonographic features, including small IVC diameters (<2 cm), a very low prevalence of abnormal Doppler patterns, and minimal pulmonary congestion on lung ultrasound. The study’s inclusion of CVP measurements via hemodialysis catheters is a strength as mentioned; however, it also suggests that these patients did not have arteriovenous fistulas. This is significant, as arteriovenous fistulas are associated with right ventricular dysfunction and pulmonary hypertension, both of which are key contributors to venous congestion that can be assessed using VExUS [2, 3].It is also worth noting that the sonographic measurements in this study were performed by cardiologists rather than nephrologists. In routine clinical practice, it is impractical to refer hemodialysis patients to a cardiologist or echocardiography laboratory for pre- and post-dialysis assessments. On the flipside, these sonographic evaluations fall within the scope of appropriately trained nephrologists, as per current expert recommendations [4]. However, they involve advanced applications that require a deep understanding of Doppler principles and awareness of technical pitfalls. Performing such assessments, whether in clinical or research settings, without adequate expertise risks erroneous interpretations and inappropriate clinical decisions. One significant pitfall is the absence of simultaneous electrocardiogram (ECG) tracing during hepatic vein waveform interpretation. Without ECG, accurately distinguishing systolic and diastolic phases of the Doppler waveform is highly error-prone, even when the waveform appears classically normal [5]. Interestingly, the authors of this study do not mention using ECG, which could compromise the reliability of the waveform analysis.A common misconception is equating VExUS with a direct measure of volume overload that can be managed solely by diuresis or ultrafiltration. While VExUS quantifies venous congestion, it primarily reflects right atrial pressure and venous compliance, neither of which reliably correlates with intravascular volume [6]. For example, primary pressure abnormalities such as pericardial effusion or pulmonary hypertension require tailored treatment strategies beyond volume removal. Overzealous ultrafiltration in a patient with pericardial effusion could precipitate tamponade physiology by disproportionately lowering intracardiac pressures relative to pericardial pressure. Similarly, aggressive volume removal in severe pulmonary hypertension could impair forward flow and exacerbate hemodynamic instability. Tricuspid regurgitation further complicates hepatic vein waveform interpretation, as it can cause chronic waveform abnormalities. Management strategies depend on the underlying cause – whether there is a structural valvular pathology, or the regurgitation is primarily functional due to volume overload. Appropriate interventions might include volume removal, cardiac surgery, or ligation of a high-flow arteriovenous fistula contributing to right ventricular dilation and functional regurgitation. Furthermore, several cardiac abnormalities unrelated to volume status can significantly impact the hepatic vein waveform, as illustrated in Figure 1. Consequently, interpreting these waveforms without concurrent focused echocardiography and ECG can result in diagnostic errors and inappropriate clinical decisions [7, 8].Portal vein Doppler is gaining traction due to its perceived simplicity, with some considering simultaneous ECG unnecessary. Encouragingly, a study of 41 hospitalized hemodialysis patients found that portal vein pulsatility improved with ultrafiltration, outperforming IVC diameter and hepatic vein Doppler in reflecting volume changes [9]. However, placing undue emphasis on a single parameter risks misinterpretation and defeats the core purpose of the VExUS score. The strength of VExUS lies in its integration of multiple Doppler waveforms, each compensating for the limitations of the others. The absence of ECG, often overlooked, can make it difficult to differentiate cardiac from respiratory flow variations in the portal vein. Additionally, variability in sampling sites may further complicate interpretation [5]. As shown in Figure 2, pulsatility patterns can differ between the main portal vein and its branches, a variation observed in our experience but not well-documented in the VExUS literature.While intrarenal venous Doppler has limited utility in end-stage kidney disease (ESKD) due to local vascular changes and imaging challenges, femoral vein Doppler is emerging as a promising alternative to complement traditional VExUS components [8]. Although relatively easier to perform, it still requires ECG for accurate interpretation [5]. Future research should focus on incorporating femoral vein Doppler into VExUS scoring systems, especially as a substitute for intrarenal Doppler in patients with ESKD.When evaluating these waveforms in relation to ultrafiltration, particularly in outcome-based studies, it is essential to acknowledge that venous congestion can reemerge as intravascular volume refills from the interstitial compartment following dialysis. Figure 3a demonstrates a case where portal vein pulsatility normalizes immediately after ultrafiltration but reverts to an abnormal pattern within 5 h post-dialysis. Therefore, serial scans are necessary to confirm sustained decongestion, and cross-sectional studies offer limited practical value in this regard.Lastly, while IVC ultrasound is often considered a straightforward application, it is not without challenges. Using a two-dimensional ultrasound beam to image a three-dimensional vessel can result in falsely smaller anteroposterior measurements if the beam intersects the vessel peripherally rather than centrally –a phenomenon known as the ‘cylinder effect’ (Fig. 3b). Therefore, caution is warranted in placing too much confidence in the correlation between IVC diameter changes and ultrafiltration reported in this study and previous work by the same research group [10]. These findings may be challenging to replicate in real-world settings, where minor measurement errors are more common. Unlike the study’s controlled environment, which relied on expert echocardiographers performing multiple readings, routine POCUS by nephrologists may not consistently adhere to such rigorous standards. Furthermore, the imaging window matters significantly. The authors note using either subcostal or lateral views depending on image quality; however, diameters obtained from these views are not interchangeable, particularly in the longitudinal plane [11]. Another noteworthy point, also highlighted by the authors in their discussion, is that the IVC diameter cutoff of 2 cm is not an absolute threshold to proceed with Doppler assessment. Failing to obtain Doppler parameters could overlook venous congestion in patients with smaller body surface areas, as they may naturally have a smaller baseline IVC diameter. For instance, an IVC measuring 1.8 cm could still be plethoric in such individuals.In summary, assessing venous congestion using sonography is inherently complex and requires the integration of focused echocardiography and ECG for accurate interpretation. Adequate operator training is essential to ensure any favorable results observed in research settings can be effectively translated into real-world clinical practice. Unlike research evaluating a drug’s efficacy, where the clinician’s role is largely limited to prescribing, application of POCUS requires a high level of expertise in image acquisition, interpretation, and the clinical integration of the information obtained. Insufficient knowledge can lead to errors in clinical decision-making and potential harm to patients. On the other hand, conducting research without properly trained investigators often yields poor-quality studies with limited utility. While involving expert sonographers may address technical aspects of imaging to some extent, the ultimate responsibility lies with the nephrologist to interpret and apply the data within a nephrology-specific context, ensuring its optimal integration into patient care. It is imperative for nephrology professional societies to prioritize POCUS training and establish competency guidelines. Patients with ESKD face significant cardiovascular risks and stand to benefit greatly from bedside tools like POCUS to optimize their hemodynamic status. Future POCUS research in this population should focus on high-risk subsets and incorporate multi-organ assessments using practical, standardized protocols to ensure meaningful and actionable conclusions.The authors have no conflicts of interest to disclose.No funding was received for this work.Abhilash Koratala: conceptualization and writing the first draft and data collection. Claudio Ronco and Amir Kazory: reviewing the article critically and revising for important intellectual content.