Background: Several studies have shown a slight discrepancy between ionic dialysance (D) and dialyser urea clearance (UK), even in the absence of access recirculation. As it has been suggested that this discrepancy could be due to the cardiopulmonary recirculation, we studied the relationship between these two parameters in a particular dialysis setting without cardiopulmonary recirculation.
Methods: Paired measurement of urea clearance and ionic dialysance were performed in five patients without arterio-venous access who were dialysed via an internal jugular vein twin catheter. Fifty paired measurements were used for statistical analysis. Vascular access recirculation was assessed by an ultrasound dilution technique. The measured value of ionic dialysance was corrected (D(0)) for the effect of vascular access recirculation and was compared with instant urea clearance calculated from the dialysate side.
Results: The difference between the paired measurements of D(0) and UK (n=50) was equal to 0.6+/-16.9 ml/min (NS). With a statistical power of 90% and taking into account this standard deviation, this study might have shown a difference of at least 10.9 ml/min. The correlation was highly significant (P<0.0001). The discrepancy of the two parameters varied with dialysis efficiency, with a decreasing D(0):UK ratio for the higher dialysis efficiency.
Conclusions: Compared with our previous results obtained in patients dialysed on arterio-venous access and performed with similar methods, the relationship between D(0) and UK is modified. This difference between D(0) and UK gets lower in patients dialysed on central catheters and this variance is in accordance with that expected when the influence of the cardiopulmonary recirculation on the measurement of ionic dialysance is taken into account. The limits of agreement (+/-2 SD) between D(0) and UK (+/-34 ml/min, Bland-Altman analysis) were higher than expected and raised questions about the accuracy of the measurement of each parameter via a central venous catheter.