During renal replacement therapy hypovolaemia due to ultrafiltration (UF) may, when not sufficiently counteracted by refill from the interstitium, result in hypotension. Combining two recently developed methods the haemodynamic process of refill was studied in order to find characteristics featuring hypotension. Relative blood volume (BV) and extracellular fluid volume (EFV) were measured continuously in 40 stable haemodialysis patients by means of an optical and a conductivity technique respectively. Regarding their postdialytic (PD) EFV the patients were divided into three groups: normohydrated (N, n = 20), dehydrated (D, n = 11) and overhydrated (O, n = 9). Significant differences between the groups were assessed in BV decrease (after 2 h until the end of treatment P < 0.05 and after 3 h P < 0.01), EFV decrease (after 3 h P < 0.05) and occurrence of hypotensive episodes (N,5; D,7; O,none; P < 0.01). During the entire session the speed of BV decrease was significantly higher in hypotensive patients (H) than in non-hypotensive patients (non-H). At the moment of hypotension (after 141 +/- 49 min) residual BV was less (P < 0.0005) in H (87.7 +/- 5.2%) than at the corresponding moment in non-H patients (96.5 +/- 4.0%). PD BV and PD EFV, both expressed as a percentage of the starting value, correlated significantly (r = 0.63, p < 0.005) and UF-volume (differences between the groups were not significant) correlated to EFV decrease (r = 0.45, P < 0.005). In conclusion, the combination of both non-invasive methods elucidates the pathophysiology of UF-induced hypotension and provides a means of reducing dialysis morbidity. The influence of tissue hydration state on these variables has been shown.
