Mitra, S.; (2007) The pathophysiology of fluid removal during haemodialysis: implications for blood volume monitoring and determination of dry weight. Doctoral thesis , University of London.

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Abstract

The aim of the research was to characterise the pathophysiology of ultrafiltration (UF) during haemodialysis (HD) using a range of assessment tools. The hypothesis was that objective monitoring would facilitate accurate estimation of dry weight (DW) and help optimise UF. Measurement timing, white-coat effect, UF prescription and volume status influenced blood pressure (BP) readings, particularly predialysis and immediate post- dialysis measurements. The 20-min post-dialysis BP best reflected interdialytic control. Extracellular volume measurements (ECF) using Bioimpedance spectroscopy (BIS) were reproducible, accurate, and confirmed the tight link between nutritional and volume status. The late increase in ECF and decline in nutritional status as end-stage approached, were only partially corrected by dialysis. Relative blood volume (RBV) characteristics were investigated using short UF pulses (perturbation analysis), at different hydration states, to identify predictors of hypotension and approaching DW. RBV change was greater as predicted DW approached. The critical level of RBV reduction leading to hypotension showed a wide inter-individual variation. The approach to linearity of RBV decay curve was a surrogate for plasma refill (PR) and predicted impending hypotension. The role of heat exchange on cardiovascular stability was also examined. Indocyanine green emerged as a suitable tracer for determining PV repeatedly during HD. The method was employed simultaneously with BIS to demonstrate abnormal proportional compartmental fluid distribution and assess fluid shifts and PR during UF. A significant relationship was demonstrated between ECF change, PR and haemodynamic stability. The dissociation between simultaneous absolute and RBV measurements was explained by a changing Fcell ratio, suggesting significant intravascular shifts within the microcirculation during UF. Objective monitoring has improved understanding of the pathophysiology of UF during HD. However, the main factors limiting fluid removal during HD are patient-specific responses to UF. Individualised prescriptions will require use of systems to characterise patient responses, and prompt appropriate adjustments. These studies may contribute.

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