The relationship between chronic volume overload and elevated blood pressure in hemodialysis patients: use of bioimpedance provides a different perspective from echocardiography and biomarker methodologies

Abstract

Chronic volume overload is very frequent in hemodialysis (HD) patients and is directly associated with hypertension, increased arterial stiffness, left ventricular hypertrophy (LVH), heart failure and ultimately with higher mortality and morbidity. One major issue is that presently there are very few comparative studies of the various methods (clinical, bioimpedance, inferior cava vein diameter (ICV) and Brain Natriuretic Peptide (NT-proBNP)) for volume status evaluation and their correlation with cardiovascular disease. In 160 patients treated by chronic HD in our center, euvolemic according to clinical assessment, we performed evaluation of volume status through bioimpedance spectroscopy (BIS), ICV and NT-proBNP, as well as echocardiography, to estimate the left ventricle structure and function. Despite appearing clinically euvolemic, severe fluid overload, as defined by a relative tissue hydration (RTH)--i.e. fluid overload over extracellular water ratio (FO/ECW)--above 15% was found in 25.6% of patients. Four categories of patients were considered according to pre-HD BP and BIS values. Forty-five percent of patients (group A) had a reasonable control of BP and volume (SBP < 150 mmHg and RTH < 15%), 29.3% (group B) were classified as hypertensive (SBP > 150 mmHg and RTH < 15%), 16.7% (group C) had high blood pressure and marked volume expansion, (SBP > 150 mmHg and RTH > 15%), while 9% (group D) had SBP < 150 mmHg despite RTH > 15%. Assuming that BIS is the most accurate and validated method to assess hydration status, we calculated the positive predictive value for ICV-based evaluation--18%, with a sensitivity of 67% and an important proportion of false negative cases (45%). NT-proBNP was even less accurate: PPV of only 26%, with a sensitivity of 60% and a specificity of only 45% and an extremely high proportion of false positive cases (73%). Group A patients had the best cardio-vascular profile: lowest LV mass and NT-proBNP levels. Using multi-frequency body impedance spectroscopy, we found a large group of hypertensive and/or fluid-overloaded patients despite apparently being at "dry weight" on clinical evaluation and a marked discrepancy between clinical appearance and fluid status. Of the 4 different methods, assuming BCM "gold standard", there were major disagreements and discrepancies between the other three methodologies. BCM is a valuable and simple bed-side tool for the correct management of BP and risk stratification in HD patients as it allows for excellent discriminators of more abnormal cardiac and vascular profiles.