This study compares the effect of online-haemodiafiltration (o-HDF, post-dilution mode) with conventional haemodialysis (HD) and 'temperature-controlled' HD (Temp-HD) on the haemodynamic stability of hypotension-prone patients. Seventeen patients with a history of frequent hypotensive episodes during dialysis sessions were studied, each patient serving as his or her own control. The first 25 HD treatments in comparison with 25 o-HDF sessions were evaluated using identical dialysate temperature. In the second part of the study, o-HDF (n = 25) was compared with Temp-HD (n = 25). In the latter method, the temperature of the dialysate was adjusted to result in identical energy transfer rates to those in the corresponding o-HDF. The number of hypotensive episodes, blood temperature and blood volume regulation were assessed. Symptomatic hypotension was much more frequent during HD (40%) than during o-HDF (4%) (P < 0.001). During o-HDF, an enhanced energy loss within the extracorporeal system occurred (o-HDF, 16.6 +/- 4.0 W; HD, 5.4 +/- 5.1 W; P < 0.0001), despite identical temperature settings for dialysate and substitution fluid. As a result, the blood returning to the patient was cooler during o-HDF than during HD (o-HDF 35 +/- 0.2 degrees C vs HD 36.5 +/- 0.3 degrees C; P < 0.0001). In o-HDF, even in the patients' circulation, the mean blood temperature was lower (o-HDF 36.7 +/- 0.2 degrees C vs HD 36.9 +/- 0.3 degrees C; P < 0.0001) and blood volume was significantly more reduced (o-HDF, 91.8 +/- 3.1%; HD, 94.0 +/- 3.2%; P < 0.05). Energy transfer rates and blood temperature did not differ significantly between o-HDF and Temp-HD. The rate of hypotensive episodes was low and not different between o-HDF (4%) and Temp-HD (4%). Neither was there any significant difference in blood volume reduction. O-HDF showed a significant reduction of hypotensive episodes compared with HD. Surprisingly, o-HDF resulted in cooling of the blood via enhanced thermal energy losses within the extracorporeal system, despite use of replacement fluid prepared from pre-warmed dialysate. The incidence of symptomatic hypotension was reduced to that of o-HDF by using cooler Temp-HD. Thus, unexpected blood cooling appears to be the main blood pressure-stabilizing factor in o-HDF.
