Modern tools for optimizing fluid management in dialysis patients: a comprehensive review

Abstract

Maintaining optimal fluid balance is crucial for patients with end-stage renal disease on dialysis, as both fluid overload and excess removal can lead to poor outcomes. Traditional approaches such as physical exam and chest X-ray have limitations when assessing volume status. This review carefully examines the tools that provide more precise options, including lung ultrasound, echocardiography, Venous Excess Ultrasound (VEXUS), bioimpedance analysis (BIA), and passive leg raise (PLR). We discuss the principles, supporting evidence, and practical uses of these techniques differentiating between static and dynamic methods to evaluate ultrafiltration tolerance. By integrating these modern techniques with clinical judgment, nephrologists can optimize fluid management in dialysis patients. While these tools show promise, further research is needed to establish standardized protocols and evaluate their impact on patient-centered outcomes.

Introduction

Cardiovascular disease remains the leading cause of death in patients with ESRD on dialysis [1]. Fluid overload contributes to hypertension, left ventricular hypertrophy, heart failure, and increased mortality in this population [2,3,4]. Conversely, excessive ultrafiltration can lead to intradialytic hypotension, myocardial stunning, and a decline in residual renal function [5]. Accurate fluid status assessment and careful ultrafiltration management are crucial for optimal care of patients on dialysis.

Volume overload is an excess of extracellular fluid beyond the body's homeostatic capacity which manifests as increased total body water leading to venous and pulmonary congestion. Traditionally, nephrologists have relied on clinical parameters such as blood pressure, physical examination findings, and changes in body weight to evaluate the patient’s volume status. However, these methods have limitations and can often be misleading [3, 6, 7]. Findings such as edema and pulmonary rales have poor sensitivity and specificity for detecting volume overload in dialysis patients [8]. Chest X-ray, another commonly used tool, can only detect pulmonary edema when a significant degree of volume overload is already present [9]. Even the use of dry weight, a fundamental concept in dialysis management, has its limitations [10]. There is a need for more objective ways to assess volume status. This review focuses on emerging techniques that enable direct visualization of organ congestion or objective measurement of fluid status, complementing established methods like relative blood volume monitoring and biomarkers.

The physiological principles and diagnostic tools described in this review are applicable across all dialysis modalities. Studies cited include evidence from both acute and chronic settings, as the challenges of assessing volume status, detecting organ congestion, and predicting ultrafiltration tolerance are common to all forms of renal replacement therapy. Where relevant, we highlight specific considerations for different dialysis populations.

Several modern diagnostic tools have recently shown great promise as alternatives to traditional methods of assessing volume. This review aims to provide a thorough overview of these advanced fluid management techniques and their use in dialysis with a focus on ultrasound-based methods (lung ultrasound [LUS], echocardiography, Venous Excess Ultrasound [VEXUS]) and non-ultrasound methods (bioimpedance analysis [BIA], passive leg raise [PLR]). Each method's principles, advantages, and limitations will be discussed while critically examining the evidence supporting them. While there is no gold standard for fluid assessment, and technology cannot replace thorough clinical evaluation, these tools may enhance our understanding of volume status. We will also compare static methods, that assess current fluid status and organ congestion, with dynamic methods, that predict response to fluid removal. Additionally, we will suggest ways to incorporate these tools into clinical practice and areas for future research.

Ultrasound-based methods

Lung ultrasound

LUS has emerged as a valuable tool for assessing volume status by detecting extravascular lung water (EVLW) and pulmonary congestion [11]. The basic principle of LUS is the detection of B-lines (Fig. 1A), which are vertical hyperechoic artifacts arising from the pleural line and extending to the bottom of the screen [12]. The number and distribution of B-lines correlate with the degree of EVLW and can be used to semi-quantitatively assess the severity of pulmonary congestion [13]. In contrast, an A-line pattern is a reverberation artifact that consists of multiple horizontal hyperechoic lines at constant intervals that are seen in patients with minimal EVLW (Fig. 1B).

Fig. 1

A Lung Ultrasound showing B-line pattern. B Lung ultrasound showing A-line pattern

Several studies have demonstrated the superiority of LUS over traditional methods for detecting pulmonary congestion in dialysis patients [8, 14]. In the LUST study, Torino et al. found that LUS had a significantly higher sensitivity (97%) and specificity (79%) for diagnosing pulmonary congestion compared to clinical assessment alone [8]. Another study by Zoccali et al. showed that a B-line score ≥ 15 which indicates pulmonary edema before dialysis strongly predicted cardiovascular events and mortality, independent of New York Heart Association class and left ventricular ejection fraction [14].

LUS is advantageous to the nephrologist as it is non-invasive, radiation-free, and can be performed quickly at the bedside. It detects subclinical pulmonary congestion, which may not be evident on physical examination or chest X-ray [15]. Additionally, LUS can be used to monitor the response to interventions, such as ultrafiltration, in real-time [11]. This enables nephrologists to adjust fluid removal targets based on the patient's evolving volume status, potentially reducing the risk of excessive fluid removal and intradialytic hypotension.

LUS is operator-dependent and requires adequate training to ensure accurate and reproducible results. It is important to recognize that the presence of certain lung pathologies, such as emphysema or fibrosis, can affect the interpretation of B-lines [12]. Moreover, LUS provides limited information on the underlying cause of pulmonary congestion, which may be multifactorial in dialysis patients. Despite these limitations, the growing evidence supports the incorporation of LUS into fluid management protocols to improve outcomes in dialysis patients [16, 17].

Echocardiography

Echocardiography provides valuable information on cardiac structure and function, allowing for the assessment of volume-related abnormalities and monitoring the response to interventions. One key parameter in the echocardiographic assessment of fluid status is the inferior vena cava (IVC) diameter and collapsibility [18]. A dilated IVC with reduced collapsibility suggests an elevated Central Venous Pressure (CVP) with consequent renal congestion (Fig. 2), while a small IVC with exaggerated collapsibility indicates a low CVP. However, IVC measurements can be influenced by factors such as cardiac function, intrathoracic pressure, and intra-abdominal pressure and should be interpreted in the context of the patient's clinical scenario [19].

Fig. 2

IVC ultrasound image showing a plethoric IVC with minimal collapse suggestive of volume overload

In a study by Brennan et al., a dilated IVC (diameter > 2.1 cm) was found in 50% of hemodialysis patients and was associated with higher left ventricular mass index and lower left ventricular ejection fraction [19]. The authors concluded that IVC dilatation is a common finding in hemodialysis patients and may reflect chronic volume overload and its consequences on cardiac structure and function.

Doppler analysis of mitral inflow and tissue Doppler imaging (TDI) can provide additional information on left ventricular filling pressures and diastolic function [20]. An increased ratio of early mitral inflow velocity to early mitral annular velocity (E/e' ratio) suggests elevated filling pressures and volume overload [21]. In a study by Le et al., an E/e' ratio > 15 before hemodialysis was associated with significantly higher extracellular water and inferior vena cava diameter compared to patients with an E/e' ratio < 15 [20]. The authors suggested that the E/e' ratio could be used to identify hemodialysis patients with volume overload and guide fluid removal strategies. Images of the mitral inflow Doppler (Fig. 3A) and TDI (Fig. 3B) are shown demonstrating an increased E/e’.

Fig. 3

A Mitral inflow Doppler. B Mitral TDI showing increased E/e' ratio

However, echocardiographic parameters can be influenced by factors such as heart rate, rhythm, and the presence of mitral valve disease and should be interpreted cautiously [21]. Moreover, echocardiography techniques require a significant amount of training before proficiency is achieved. Despite these limitations, several studies have demonstrated the utility of echocardiography in guiding fluid management in dialysis patients. Combining echocardiography with other methods of volume measurement could potentially result in better blood pressure control and reduced left ventricular mass [22].

Venous Excess Ultrasound (VEXUS)

VEXUS is an emerging tool for assessing venous congestion using hepatic, portal, and renal Doppler patterns in conjunction with IVC assessment [23]. The VEXUS grading system categorizes venous congestion into four grades based on the severity of abnormalities in these Doppler patterns and IVC size (Table 1).

Table 1 VEXUS grading criteria

While still an experimental approach, VEXUS is a promising technique for evaluating venous congestion and its impact on renal function in dialysis patients. In the setting of volume overload, venous congestion can lead to increased renal interstitial pressure, reduced renal perfusion, and ultimately may lead to a decline in residual renal function [23]. By detecting venous congestion early, VEXUS may allow for more targeted fluid management interventions to prevent or mitigate these adverse effects. For example, in a study by Beaubien-Souligny et al., a higher VEXUS grade was associated with a more rapid decline in residual renal function (−0.17 vs. −0.05 mL/min/1.73m² per month, p = 0.03) and a higher risk of dialysis initiation (hazard ratio 2.44, 95% CI 1.16–5.13) [25.] The authors concluded that VEXUS could help identify patients with subclinical venous congestion who may benefit from more aggressive fluid removal strategies to preserve residual renal function. Figure 4A demonstrates hepatic vein doppler with S wave reversal, a severely abnormal finding. Figure 4B shows portal vein doppler with > 50% pulsatility index, a severely abnormal finding. The combination of these findings would result in a VEXUS of grade 3 which indicates severe congestion.

Fig. 4

A Hepatic vein doppler waveform with referred flow pattern suggesting venous congestion. B Portal vein with pulsatility with > 50% suggesting venous congestion

It's important to note that while VEXUS shows promise, its utility in patients without residual renal function requires further investigation. In patients with advanced renal fibrosis and poor renal blood flow, the interpretation of renal venous flow patterns may be less reliable [23]. This relates to the fundamental concept that venous congestion's impact on renal function may be different in patients with end-stage disease compared to those with acute kidney injury [23]. This limitation highlights the need for validation studies specifically in the chronic hemodialysis population with varying levels of residual renal function.

Incorporating VEXUS into fluid assessment protocols may be useful in dialysis patients with persistent volume overload despite adequate ultrafiltration by differentiating between true fluid overload and fluid redistribution from the venous system. However, the optimal VEXUS cut-offs for predicting renal outcomes and guiding interventions in dialysis patients have yet to be established. Additionally, this technique may be speculative for dialysis patients as it has been primarily studied in acute kidney injury rather than to manage chronic dialysis patients. The reproducibility of VEXUS measurements and the impact of operator experience on its diagnostic accuracy require further investigation. Overall, VEXUS is still a relatively new technique and further research is needed to validate its utility in the dialysis population.

Non-ultrasound based methods

Bioimpedance Analysis (BIA)

BIA is a non-invasive method for assessing fluid status and body composition in dialysis patients [24]. It measures the resistance and reactance of body tissues to a low-amplitude electrical current [25]. Different BIA techniques and devices may yield varying results, with measurements being particularly sensitive to fluid changes in the limbs rather than the trunk [24, 26]. Various parameters can be derived from these measurements such as total body water (TBW), extracellular water (ECW), intracellular water (ICW), and overhydration (OH) [27]. BIA provides an objective and quantitative assessment of TBW and its distribution, which can help guide ultrafiltration goals [28]. BIA can also detect subclinical fluid overload, which may not be evident on physical examination or by monitoring changes in body weight [28]. Additionally, BIA can be used to monitor the response to interventions and assess the adequacy of fluid removal over time [29].

In a randomized controlled trial by Onofriescu et al., bioimpedance-guided fluid management resulted in better blood pressure control, reduced arterial stiffness, and a trend toward longer survival compared to standard clinical assessment [30]. Another study by Huan-Sheng et al. found that a BIA-guided dry weight reduction program significantly improved left ventricular mass index and ejection fraction [31]. These findings suggest that BIA can be a valuable tool for optimizing fluid status and improving cardiovascular outcomes in dialysis patients.

However, BIA also has some limitations. The accuracy of BIA measurements can be affected by factors such as body position, skin temperature, and the timing of measurements relative to dialysis [26]. BIA equations that estimate fluid compartments and body composition may not be valid in all dialysis populations, particularly those with extreme obesity or malnutrition [26]. Additionally, the cost and availability of BIA devices may limit their widespread use in some dialysis units.

In recent years, newer BIA techniques, such as bioimpedance spectroscopy (BIS) and vector bioimpedance analysis (BIVA) have emerged as promising methods for assessing fluid status. BIS uses a multi-frequency electrical current to provide a more detailed assessment of fluid compartments, while BIVA uses a graphical analysis of bioimpedance vectors to classify hydration status [32, 33]. These techniques may offer additional advantages over traditional BIA, but more data is needed to establish their role in clinical practice.

Passive Leg Raise (PLR)

PLR is a simple, noninvasive bedside maneuver that transiently increases venous return to predict fluid responsiveness in critically ill patients (Fig. 5) [34]. It requires an accurate method to measure a pre and post PLR cardiac output. This can be achieved via echocardiography, pulmonary artery catheter, or non-invasive cardiac output methods such as biompedence. Traditionally the PLR has been used to evaluate volume responsiveness for patients in shock. Recent evidence suggests that PLR may also be valuable in guiding fluid removal during dialysis [35].

Fig. 5

PLR protocol

In a study by Monnet et al., the PLR test was used to predict hemodynamic intolerance to fluid removal in 39 critically ill patients receiving intermittent renal replacement therapy (RRT) [35]. The authors found that patients who exhibited a PLR-induced increase in cardiac index (CI) > 9% before RRT were more likely to develop intradialytic hypotension (IDH), with a sensitivity of 77% and specificity of 96%. The physiologic rationale behind this observation is that if a patient's cardiac output increases with PLR, suggesting that they are preload responsive, they are also more likely to experience a decrease in cardiac output with fluid removal and a reduction in preload. Conversely, if the patient is not fluid responsive there is a higher probability that the patient will not decrease cardiac output with fluid removal.

While these findings are promising, it's important to note that validation studies of PLR have primarily been conducted in critically ill patients receiving acute renal replacement therapy [35]. The applicability across different dialysis settings requires further investigation, as hemodynamic parameters and cardiovascular adaptation may vary significantly between acute and chronic settings [5]. Given that cardiac injury and adaptation patterns differ across dialysis populations [5], future studies should evaluate PLR's predictive value in both outpatient and acute settings, with particular attention to how timing and frequency of assessments might need to be modified for different clinical scenarios.

While PLR provides a means to evaluate ultrafiltration tolerance that can be incorporated into clinical practice, it is important to acknowledge the limitations of this single-center study in a small cohort of critically ill patients. Larger, prospective studies are necessary to validate these findings and determine the optimal PLR cut-off for predicting IDH in the broader dialysis population.

Discussion

The evidence presented in this review highlights the potential of modern fluid management methods to improve outcomes in dialysis patients. However, incorporating these tools into clinical practice requires a thoughtful and individualized approach. Here, we provide some recommendations for using these methods in different clinical scenarios.

For patients with suspected fluid overload, a combination of LUS (to detect pulmonary congestion) and echocardiography (to assess IVC diameter and collapsibility) can provide a comprehensive assessment of volume status. BIA can be used to quantify the degree of overhydration and guide ultrafiltration goals. VEXUS can be employed to assess venous congestion and guide targeted interventions in patients with persistent volume overload or worsening renal function, while PLR can help predict the risk of intradialytic hypotension and inform ultrafiltration profiling strategies. In this paper, we provide a physiologically guided framework to apply these principles.

For patients with recurrent intradialytic hypotension, a thorough evaluation of fluid status using LUS, echocardiography, or BIA should be considered. For example a patient with B-lines and a severe VEXUS grading may provide evidence for a patient that should have their dry weight probed. If fluid status is found to be adequate, other strategies such as ultrafiltration profiling, dialysate cooling, or Midodrine administration may be considered [36]. For patients with persistent hypertension despite adequate ultrafiltration, BIA can be used to assess for occult fluid overload. If present, a gradual dry weight reduction program, guided by serial BIA measurements, can be implemented. If BIA suggests euvolemia, other causes of hypertension, such as arterial stiffness or sympathetic overactivity, should be considered [37].

Static methods such as the E/e' ratio and VEXUS provide a snapshot of the patient's current fluid status and degree of organ congestion which can indicate that a patient is not at their ideal dry weight. In contrast, dynamic methods like PLR focus on predicting a patient's response to fluid removal and may provide a better understanding if a patient can tolerate a high UF rate. By combining them, nephrologists can gain a comprehensive understanding of a patient's fluid status and tailor their management accordingly. For example, in a patient with a high E/e' ratio and VEXUS grade indicating significant fluid overload and organ congestion, a PLR test showing volume de-responsiveness may support the decision to pursue more aggressive ultrafiltration. On the other hand, in a patient with moderate fluid overload but a PLR test suggesting preload responsiveness, a more cautious ultrafiltration prescription may be warranted with longer treatment times or additional dialysis sessions. We describe this approach to guiding ultrafiltration in a flowchart (Fig. 6). This flowchart combines static assessments to determine congestion level with dynamic assessment to evaluate volume responsiveness in patients undergoing hemodialysis. Based on these results, the algorithm suggests whether to consider no ultrafiltration or to pursue a conservative or aggressive ultrafiltration strategy.

Fig. 6

Integrated Fluid Removal Guidance (IFRG) algorithm

For practical implementation of these methods in the dialysis unit, consideration must be given to logistics and staff training. Lung ultrasound is particularly accessible, with competency achievable after just 10–15 supervised examinations following standardized protocols [12, 13]. In contrast, echocardiographic assessments require more extensive training [18], and VEXUS examination needs advanced ultrasound skills [23]. BIA measurements require standardized pre-dialysis positioning [24, 26], while PLR testing can be readily integrated into pre-dialysis assessments [34, 35]. The frequency of these assessments should be individualized, with increased monitoring for patients with recurrent volume-related complications [6].

Fluid management decisions should not be based on any single parameter or method but rather on a comprehensive assessment of clinical and imaging data. The choice of which fluid management tool to use will depend on the specific clinical scenario, the available resources, and the expertise of the dialysis team. The interpretation of these tools should always be done in the context of the patient's overall clinical picture, considering factors such as cardiovascular comorbidities, residual renal function, hemodynamic stability, and conditions that affect fluid distribution like hypoalbuminemia, where optimal volume status often requires balancing peripheral edema against organ perfusion [3, 4].

Future directions and research needs

The widespread adoption of these modern fluid assessment tools requires consideration of both their economic impact and patient-centered outcomes. Initial costs include equipment acquisition and staff training, but evidence suggests these may be offset by reduced healthcare utilization [38]. Data from bioimpedance monitoring studies suggest significant cost savings through reduced hospitalizations and improved cardiovascular outcomes [29]. The increasing availability of portable ultrasound devices and the relatively low cost of training make these approaches particularly attractive, especially considering that lung ultrasound findings strongly predict mortality and cardiovascular events [14].

Key research priorities include developing standardized protocols for integrating these tools into clinical practice and evaluating their impact beyond traditional clinical parameters. Future trials should emphasize patient-centered outcomes such as quality of life and functional status, while considering patient preferences in fluid removal strategies. Robust cost-effectiveness analyses across different healthcare settings are also needed to definitively demonstrate both the economic and patient-centered benefits of these approaches.

Conclusion

This review highlights the limitations of traditional methods for assessing fluid status in dialysis patients and the potential of modern fluid management techniques to improve outcomes. Ultrasound-based methods such as LUS, echocardiography, and VEXUS, as well as non-ultrasound methods like BIA and PLR, each provide unique insights into fluid status and can help guide individualized management decisions (Table 2). The advantages and disadvantages of each method are provided in Table 3. Static methods like E/e' and VEXUS quantify current fluid status and organ congestion, while dynamic methods such as PLR predict response to fluid removal. By using these tools in conjunction with clinical judgment, the nephrology community can work towards achieving the goal of euvolemia and reducing the burden of cardiovascular morbidity and mortality in this high-risk population.

Table 2 Summary of each technique

Table 3 Advantages and disadvantages of each technique