Nephrol Dial Transplant (2004) 19: 1010-1011
Nephrol Dial Transplant Vol. 19 No. 4 © ERA-EDTA 2004; all rights reserved
Characteristics of hypotension-prone haemodialysis patients: is there a critical relative blood volume?
Sir,We read with interest the article of Barth et al. [1], in which they tested the hypothesis that haemodialysis patients have an individual critical relative blood volume (RBV-crit) below which dialysis-hypotension will occur [1]. Identification of such a RBV-crit could contribute to the prevention of dialysis-hypotension by keeping the actual blood volume on the safe side of the RBV-crit. Based on the observation that the standard deviation (SD) of RBV-crit was <5% in three-quarters of their total study population (n = 60) the authors concluded that an individual RBV-crit exists for nearly all patients. We do not support this conclusion of the authors. The SD of the RBV-crit was >4% in 47.2% of their patients. If we assume a patient with a mean RBV-crit of 90% and a SD of 4% this implies by definition (normal distribution) that one-third of the RBV-crit values of this patient will be outside the 86–94% range. With this variability, we think it will be difficult—if not impossible—to use a RBV-crit for the prevention of dialysis-hypotension.
Other groups, not cited by Barth et al., have reported a poor intra-individual relationship between blood volume and blood pressure during haemodialysis [2,3]. Several factors may explain why most studies report considerable intra-individual variation of the RBV-crit. First, haemodynamic stability is not only determined by the course of blood volume but also by the response of the compensatory mechanisms to hypovolaemia. The response of these compensatory mechanisms (a decrease in venous capacity, an increase in vascular resistance and an increase in cardiac contractility and rate) is affected by several patient and treatment factors that may vary between dialysis sessions. For instance, differences in the ambient and dialysate temperature, food intake and the timing of the intake of anti-hypertensive medication as well as postural changes may all influence the compensatory responses to hypovolaemia in hypotension-prone patients and will thus contribute to the variability of RBV-crit. The second factor relates to the influence of the hydration status on blood volume. Most studies, including that of Bart et al, measured relative instead of absolute blood volumes. Since blood volume increases as the extra-cellular volume increases [4,5] the pre-dialysis absolute blood volume will vary according to the hydration status of the patient. The more over-hydrated the patient is, the larger the blood volume compartment is and the more the relative blood volume can decrease to end up at the same absolute blood volume level. Thus, variations in the hydration status at the start of the dialysis sessions will contribute to the variability of the RBV-crit. Barth et al., suggest that this might also be the case in their patient population since they state that ‘patients with a high mean ultrafiltration volume (relative to body weight) tended to tolerate a lower RBV-crit than patients with less interdialytic weight gains’. It is not clear from their data whether the same applies to the intra-individual RBV-crit variation.
With regard to the study of Barth et al., there are two additional factors that may explain some of the variability in RBV-crit. The authors used a rather liberal definition of dialysis-hypotension that also included patients who developed headache or cramps in combination with a medical intervention (ultrafiltration stop or change in body position) but without a documented decrease of blood pressure. The pathophysiology of headache and cramps, however, is multi-factorial and may not be related to hypovolaemia. Some patients with headache or cramps may not have had hypovolaemia and excluding these episodes may result in a decrease of the RBV-crit variability in some patients. Finally, Barth et al., used both first and following episodes of dialysis-hypotension to calculate the mean and SD of RBV-crit. In our experience, the first episode of dialysis-hypotension usually occurs at a lower relative blood volume than the following episodes during that same dialysis session. Since it was the authors’ ultimate goal to define an individual RBV-crit in order to avoid dialysis-hypotension we think it would have been more appropriate if they had included only the first episodes of dialysis-hypotension in their analysis. In conclusion, we still believe that because of the intra-individual variability, which was even shown in many of the patients in the study of Barth et al., the concept of the RBV-crit to prevent dialysis-hypotension will only have limited applicability.
Conflict of interest statement. None declared.
1Dialysis Centre Groningen 2Department of Internal Medicine Division of Nephrology University Hospital Groningen The Netherlands Email: c.f.m.franssen{at}int.azg.nl
References
- Barth C, Boer W, Garzoni D et al. Characteristics of hypotension-prone haemodialysis patients: is there a critical relative blood volume? Nephrol Dial Transplant 2003; 18: 1353–1360
[Abstract/Free Full Text] - Krepel HP, Nette RW, Akçahüseyin E, Weimar W, Zietse R. Variability of relative blood volume during haemodialysis. Nephrol Dial Transplant 2000; 15: 673–679
[Abstract/Free Full Text] - Oliver MJ, Edwards LJ, Churchill DN. Impact of sodium and ultrafiltration profiling on hemodialysis-related symptoms. J Am Soc Nephrol 2001; 12: 151–156
[Abstract/Free Full Text] - Guyton AC, Hall JE. Textbook of Medical Physiology. W.B. Saunders Company, Philadelphia, 2000; 332
- Movilli E, Cassamali S, Maffei C et al. Interdialytic variations of blood volume and total body water in uremic patients treated by dialysis (abstract). Nephrol Dial Transplant 2003; 18 [Suppl 4]: 177–178
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