Estimation of ideal volume status is a constant challenge in nephrology. This is usually based on clinical evaluation, along with other diagnostic tools such as natriuretic peptides and bioimpedance spectroscopy (BIS). Several methods have been developed to accurately assess fluid status in dialysis patients, including BIS, measurement of inferior vena cava (IVC) diameter by ultrasound, analysis of biomarkers such as natriuretic peptides, and, more recently, the observation of lung comets using chest ultrasound [1].

In 1996, Dr. Daniel Lichtenstein first described the comet tail artifact detected in lung ultrasound in a cohort of patients with both cardiogenic and noncardiogenic pulmonary edema [2]. This artifact is caused by micro-reflections at the subpleural interface, which the ultrasound interprets as distance, caused by the presence of extravascular lung water or small air-fluid interfaces, resulting in a narrow, laser-like ray on the screen. The artifact, now known as the B-line, has contributed to our ability to detect both clinical and, especially, subclinical pulmonary edema, offering a more precise and accessible alternative to traditional methods such as lung auscultation or chest radiography [3]. One of its main limitations is the lack of specificity. B-lines are a sonographic sign of lung interstitial syndrome, but they cannot differentiate between thickening of the interlobular septa caused by excess lung water and fibrotic thickening, as seen in pulmonary fibrosis (dry B-lines). Therefore, caution must be exercised when interpreting LUS findings in patients with known or suspected pulmonary fibrosis [4].

Many studies have demonstrated the clinical use of LUS. Studies such as LUS-HF and CLUSTER-HF have shown the use of LUS in guiding more intensive diuretic treatment in heart failure patients, leading to a reduction in B-lines and improved management of fluid overload[5][6]. While there are few studies on subclinical hypervolemia in chronic kidney disease (CKD) patients, one study by Baki et al. demonstrated that the combination of elevated natriuretic peptide levels and decreased inferior vena cava collapsibility index (IVCCI) increased the specificity and positive predictive value for detecting subclinical hypervolemia [7].

It is well known that the prevalence of hypervolemia in PD patients is considerable and is associated with increased mortality [8]. Persistent volume overload increases arterial stiffness, causes higher systolic BP, lower diastolic BP, and increased pulse pressure. This increases afterload to the left ventricle, resulting in left ventricular hypertrophy (LVH). All of these effects have been identified as independent risk factors for cardiovascular morbidity and mortality in end-stage kidney disease (ESKD) [9][10].

Asymptomatic lung congestion has been described in a significant proportion of dialysis patients [11]. In a multicenter study by Zoccali et al., moderate to severe lung congestion was present in 45% of hemodialysis patients, with 71% of these patients being asymptomatic or presenting only slight symptoms of heart failure [12]. Another study by Enia et al. reported that 58% of hemodialysis patients exhibited moderate to severe lung congestion, 38% of whom were asymptomatic (NYHA class I) [13]. Additionally, Mallamaci et al. demonstrated that 57% of asymptomatic dialysis patients exhibited moderate to severe lung congestion [14]. Thus, some dialysis patients are erroneously considered euvolemic.

Increased precision in volemia evaluation is an important unmet need in dialysis patients, particularly PD patients. Some studies suggest that PD patients may be more prone to overhydration compared to those undergoing hemodialysis (HD). Achieving euvolemia in PD patients is especially challenging because PD depends on self-treatment, daily ultrafiltration, and residual renal function, all of which can vary widely among individuals [8][15][16].

LUS has been validated as an effective method for evaluating extracellular lung water, particularly in heart failure and hemodialysis patients [1][16]. Although its role in PD patients remains uncertain, it may play an important role in evaluating volume status in PD patients. In hemodialysis, LUS has demonstrated a strong prognostic value, as the number of B-lines correlates with adverse cardiovascular outcomes and mortality, and LUS-guided ultrafiltration can safely reduce pulmonary congestion without increasing intradialytic complications. In peritoneal dialysis, LUS findings do not consistently correlate with bioimpedance or clinical signs. This suggests that LUS and other assessment methods evaluate different fluid compartments and are therefore complementary [1][11][15][17].

The technique itself is cost-effective and widely accessible, requiring only 3-5 minutes to perform. It can be efficiently carried out by trained technicians and demonstrates low interobserver variability [18]. The 8-point scoring system has shown excellent diagnostic performance in the evaluation of ESKD patients [19][20]. Measurement of IVCCI adds information about intravascular volume status [21].

Given these considerations, combining different methods and technologies may improve the detection of subclinical hypervolemia in PD patients. As such, we aimed to estimate its prevalence using LUS and to determine the concordance between LUS findings, IVCCI, and other clinical parameters and tools, such as BIS.

Twenty-two patients were enrolled. The mean age was 61 ± 11 years, and 59% were male. The median time on PD was 30 months (Table II). The prevalence of diabetes mellitus was similar across the groups. Regarding technique-related variables, 27% of the total cohort were on automated peritoneal dialysis (APD), with a slightly higher proportion (40%) in the higher B-line group. Notably, only five patients had a B-line score ≥4.

Residual renal function (RRF) tended to be lower in patients with higher B-line scores (mean of 1,334 ± 887 mL in the total cohort vs. 1,000 mL in the higher B-line group). Patients with higher B-line scores also had significantly higher mean diastolic blood pressure (75 ± 11 vs. 89 ± 15 mmHg, p = 0.029) and lower serum albumin levels (4.1 ± 0.3 vs. 3.7 ± 0.5 g/dL, p = 0.032), as detailed in Table III

Patients with higher B-line scores tended to have elevated hydration status, as assessed by BIA, and a lower IVCCI. Echocardiographic findings revealed no significant differences between patients with higher and lower B-line scores. Key parameters, such as left ventricular ejection fraction (LVEF), were preserved across groups, with values consistently within the normal range. No notable variations in cardiac chamber sizes or valvular function were observed between groups. Additionally, radiographic findings revealed that patients with a higher B-line score had a higher prevalence of pulmonary congestion, with chest X-rays showing signs of interstitial edema and mild pleural effusions.

All patients in the higher B-line score group were on diuretics, had higher ultrafiltration rates, and lower residual renal function, further underscoring the challenge in fluid management.

In this study, LUS proved to be a valuable tool for detecting subclinical hypervolemia in PD patients. Higher B-line scores (≥4) were associated with increased hydration status, elevated NT-proBNP levels, and reduced residual renal function, despite all patients being on diuretics. These findings highlight the need for more aggressive fluid management strategies, such as icodextrin and hypertonic solutions, in patients with diuretic resistance and declining RRF.

Strong correlations between B-line scores and BIA overhydration, as well as IVCCI, reinforce the sensitivity of LUS in detecting fluid overload. LUS and BIA provide complementary insights: LUS assesses extravascular lung water, while BIA evaluates total body water, extracellular water (ECW), and intracellular water excess [16]. Given its noninvasive nature and bedside applicability, LUS is particularly beneficial for patients with borderline cardiac function, recurrent heart failure episodes, or increased ECW due to hypoalbuminemia.

However, a critical limitation of LUS is the specificity of B-lines, particularly in patients with chronic lung diseases, where "dry" B-lines may appear independently of fluid overload, potentially confounding interpretation [4]. In our cohort, none of the patients with a B-line score ≥4 had underlying chronic lung disease, which reduced this confounding factor in our findings. Nevertheless, cautious clinical correlation remains essential when applying LUS in populations with pulmonary comorbidities.

While our study was not designed for direct comparisons between LUS and other methods, the results support integrating B-line score assessments alongside BIA and NT-proBNP. The inverse correlation between B-line scores and IVCCI (r = -0.722) further confirms the relationship between pulmonary congestion and reduced venous collapsibility. Conversely, the low and nonsignificant correlation between B-line scores and NT-proBNP (r = 0.334, p = 0.129) emphasizes the limited discriminatory value of NT-proBNP alone in this context, likely due to multiple influencing factors, including residual renal function and comorbidities. Therefore, NT-proBNP should be interpreted cautiously and preferably in combination with other assessment tools [22] [23].

The clinical relevance of these findings lies in the early identification of fluid overload, particularly in patients with declining RRF (r = -0.628). LUS can provide real-time insights, enhancing patient awareness and compliance with fluid management strategies. Detection of subclinical hypervolemia could imply proactive changes in clinical practice, such as adjustments in ultrafiltration rates, initiation or intensification of icodextrin use, optimization of diuretic therapy, or reinforced patient education to improve adherence to fluid restrictions. Although some therapeutic adjustments were made based on the findings, these interventions were not standardized and, thus, were not systematically analyzed or included in this study. Future research should focus on implementing protocolized treatment modifications guided by a holistic approach, which should include LUS findings, to better assess their impact on renal survival, overall outcomes, and healthcare utilization in PD patients.

Importantly, this study is the first to evaluate subclinical hypervolemia in PD patients using LUS, emphasizing its feasibility, short learning curve, and high reproducibility. If an ultrasound device is available in the unit, its use should be maximized, as LUS protocols are simple and easy to implement in routine practice.

Despite its advantages, our study has limitations. The small sample size (n = 22) reduces statistical power and generalizability. Additionally, differences in clinical practices between centers and potential selection bias must be considered. The cost and availability of ultrasound equipment in some settings may also pose challenges. Moreover, while IVC measurements provide valuable volume status information, they are influenced by individual variability, tricuspid insufficiency, and intra-abdominal pressure changes due to PD [15]. Therefore, integrating with LUS with other markers is essential.

A common concern is the time required for additional assessments during routine visits. However, with adequate training, LUS can be performed quickly, especially using the 8-zone protocol, which significantly reduces procedure time compared to traditional methods. Given its practicality and clinical relevance, LUS should be incorporated into routine fluid management in PD patients to optimize care and improve outcomes.

Lung ultrasound is a promising, noninvasive, and highly reproducible tool for the early detection of subclinical fluid overload in PD patients. It effectively complements other methods, such as bioimpedance analysis and NT-proBNP levels, enabling a more comprehensive assessment of fluid status, particularly in patients with declining residual renal function. The findings of this study should be interpreted with caution, taking into account its small sample size, single-center design, and criteria used to define subclinical hypervolemia. All in all, LUS shows potential for broader clinical use. Future perspectives include conducting multicenter randomized studies to confirm the clinical impact of routinely integrating LUS into the follow-up of PD patients and to establish standardized protocols for its use in fluid management.

Contributions des auteurs

Joana Dias and Vitória Paes de Faria contributed to the conceptualization and design of the study. They collected the data, conducted the clinical assessments, performed the data analysis and contributed to the manuscript writing.

Ana Marta Gomes contributed to the conceptualization and data interpretation.

The manuscript was reviewed/interpreted and approved by all the other authors: Maria Beatriz Bessa; Susana Pereira, Daniela Lopes, Rute Carmo, João Carlos Fernandes, Maria Clara Almeida, and Ana Marta Gomes.

Ethical statement

This study was conducted in accordance with the principles of the Declaration of Helsinki. Ethical approval was obtained from the Ethics Committee of the Unidade Local de Saúde de Vila Nova de Gaia Espinho. All participants provided written informed consent prior to inclusion in the study.

Financial disclosure

The authors declare that they received no financial support or funding for the conduct of this study.

Conflits d’intérêts

The authors declare no conflicts of interest related to this work.