Department of Internal Medicine University Hospital Maastricht Maastricht The Netherlands
Sir,
We read with interest the article by Altieri et al. [1] comparing haemodynamic effects of pre-dilution on-line haemofiltration and haemodialysis in a cross-over study. The main finding of a reduction in intra-session hypotensive episodes is in line with earlier data regarding post-dilution haemofiltration [2]. Nevertheless, we do not entirely agree with the interpretation of the results.
From the observation that the episodes of symptomatic hypotension progressively decreased and pre-dialytic mean arterial pressure increased, the authors conclude that it is inaccurate to attribute the difference in haemodynamic response during haemodialysis and haemofiltration treatment to a unique simple phenomenon such as temperature difference or sodium retention. Nevertheless, interpretation of the pre-dialytic blood pressure appears to be hazardous in this respect, as the authors suggest in their discussion (without mentioning in the results section) that the use of antihypertensive drugs was less during the episodes with haemofiltration treatment.
What thus remains is the interpretation of the number of hypotensive episodes. Available data strongly suggest that the difference in haemodynamic response between haemodialysis and haemo(dia)filtration is due to differences in blood volume preservation [3] or response of the resistance or capacitance vessels [4,5], the latter probably being of greater importance [5,6]. It has actually been shown in earlier studies, in which the thermal effects and haemodynamic response during different treatment modalities were assessed in detail, that all differences in vascular or blood pressure response between haemodialysis on one hand, and isolated ultrafiltration and haemo(dia)filtration on the other, were abolished when treatments were matched for extracorporeal energy balance [57]. This also held true for the difference in vascular response between haemodialysis and pre-dilution on-line haemofiltration, as shown in an earlier study by our group [5]. In this respect, it is important to note that pre-dilution on-line haemofiltration with an infusate temperature of 37°C will have a far more pronounced cooling effect than haemodialysis with a dialysis temperature of 37°C [5,8]. Indeed, in our earlier study, the substitution fluid had to be warmed to 39°C in order to achieve the same thermal energy balance as haemodialysis treatment with a dialysate temperature of 37.5°C [5].
In summary, we propose that before differences in haemodynamic response between haemodialysis and haemofiltration are to be attributed to a yet unknown factor, as suggested by Altieri et al. [1], treatments should be compared when matched for extracorporeal energy transfer.
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Divisione di Nefrologia Ospedale San Michele Cagliari Italy
Sir,
I thank Kooman and colleagues for their interesting comments on our recently published article [1].
Kooman disagrees with our interpretation of the results of the above study, which showed a progressive reduction of episodes of hypotension during the haemofiltration treatment period. Kooman's principal comment is that the only relevant, well-documented factor which causes better cardiovascular reactivity, is the cooling effect of haemofiltration treatment on the patient's blood, which increases the response of resistance and capacitance vessels; warming the substitution fluid negates the above effect.
I would like to stress that the temperature effect on blood pressure stability during haemodialysis and haemofiltration, first described by Maggiore et al. [2] and confirmed in many subsequent studies, including the elegant studies performed at the Maastricht University, is an acute intra-dialytic effect, demonstrated only in acute prospective short-term studies.
The above effect is not in contrast with the long-lasting effect described in our prospective long-term studies performed in stable patients [1,3]. In the present study [1] 23 patients were sequentially treated in three different study periods lasting 6 months each, namely: predilution on-line haemofiltration (HF1); high flux haemodialysis (HD); and predilution on-line haemofiltration (HF2).
All three phases of treatment were conducted with the same Kt/V, the same treatment time, the same membrane and fluid composition. The study showed the following relevant clinical differences in the same group of patients during HF and HD phases, which were unlikely to be attributable to a sole energy balance change, since they were not acute but progressive and persistent changes:
A complete understanding of the above findings is probably not possible with present knowledge. However, the following hypothesis is in agreement with the above results: haemofiltration treatment progressively improves blood pressure control through its favourable effect on cardiovascular compliance, leading to better control of both intra-treatment hypotension and inter-treatment hypertension. As a consequence, during HF, less fluid removal is required intra-treatment and less anti-hypertensive therapy inter-treatment to keep patients normotensive. A more physiological dry weight and a reduction of anti-hypertensive medication, together with a cooling effect, are the most likely factors determining better cardiovascular stability and less frequent symptoms during HF treatment.
The present hypothesis is not in contrast to the acute effect on vascular stability of a different thermal balance. The better haemodynamic profile observed in our patients during HF treatment, was caused by changes due to convective treatment which differs markedly from diffusive treatment: different solute and water removal [4]; higher beta-2-microglobulin removal [5]; adequate nutritional stimulus together with a sufficient generation of PCR at low Kt/V [3]; and different ionic balance [6]. The different thermal balance described during HF is very important but not the only clinically favourable effect of HF.
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