How large is the variability of relative blood volume during haemodialysis?

Carlo Basile, Rosa Giordano and Alessio Montanaro

Nephrology and Dialysis Unit, Hospital of Martina Franca, Italy

Sir,

We read with interest the paper ‘Variability of relative blood volume during haemodialysis’ by Krepel et al. [1]. They conclude that relative blood volume (RBV) monitoring is of limited value in the prevention of dialysis-related hypotension due to the large intra- and inter-individual variability [1].

We were struck, when analysing Tables 3 and 4 of the paper by Krepel et al. [1], by the data of patient 4, which are considerably different from those of the other nine patients of the study. We were confirmed in our opinion that patient 4 must be considered an ‘outlier’, from the statistical point of view, and by that way excluded from the statistical analysis, when examining the individual behaviours of RBV changes plotted against time and ultrafiltration volume (UV) (Figures 2a and 2b): in these graphs there is one single patient whose percent RBV increases consistently during ultrafiltration, at least in the first 60 min of dialysis: this is really a very bizarre behaviour! We hypothesize that this patient corresponds to patient 4 of Tables 3 and 4.

To consider patient 4 an ‘outlier’ is a crucial point in the interpretation of the results of the present work, due to the small number of patients (only 10) enrolled into the study.

Thus, if we reanalyse the data shown in Table 3 excluding patient 4, we can see that both intra- and inter-individual variability decrease considerably (Table 1Go): the mean intra-individual coefficient of variance (CV) by a percentage ranging from 18 (at 240 min) to 54% (at 120 min); the inter- individual CV by a percentage ranging from 15 (at 60 min) to 34% (at 180 min). As shown in Table 1Go, the largest differences are observed in mean intra-individual CV.


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Table 1. Mean RBV, inter-individual CV and mean intra-individual CV of the 10 patients enrolled into the study change considerably, when excluding patient 4

 
It is not excluded that a statistically significant correlation between RBV changes and UV may be found at all time points studied, and not only at 60 and 240 min, as found by the authors [1]. Furthermore, the statistical significance of the correlations between RBV and heart rate, blood pressure, and inferior caval vein measurement might change too. We could not calculate these correlations due to unavailability of the raw data.

In conclusion, both intra- and inter-individual variability do exist, but they are not as large as suggested by Krepel et al. [1].

References

  1. Krepel HP, Nette RW, Akçahüseyin E, Weimar W, Zietse R. Variability of relative blood volume during haemodialysis. Nephrol Dial Transplant2000; 15: 673–679[Abstract/Free Full Text]