Continuous measurements of oxygen saturation during haemodialysis

A new technique for recording and analysing continuous measurements of oxygen saturation (SpO2) by pulse oximeter during haemodialysis was used to compare changes in SpO2 in eight patients during two 4 h periods of dialysis using a cuprophane membrane, once using an acetate dialysate, and once using bicarbonate. The computer-derived patterns of SpO2 show whether hypoxaemia was caused mainly by extrapulmonary abnormalities (ventilatory control) or intrapulmonary abnormalities (V/Q distribution). The patterns of oxygen saturation were analysed for (i) stability, (ii) the lower median 20th centile of SpO2, and (iii) time below a SpO2 of 90%. Not all patients had reduced oxygenation during acetate dialysis. Three of eight patients had a stable pattern with acetate dialysis and six of eight were stable with bicarbonate. Five of eight patients had a lower SpO2 with acetate but one patient had a lower SpO2 with bicarbonate. Four patients had prolonged, clinically significant periods of oxygen desaturation with SpO2 less than 90%; two of these had particularly prolonged periods during acetate (62 min and 12 min), but one patient showed a longer period during bicarbonate than acetate dialysis (7 min). In two patients the SpO2 declined to less than 84%. The patterns of SpO2 suggested that the decrease in oxygen saturation was due more to extrapulmonary abnormalities causing an instability in ventilatory control rather than to venous admixture. It is recommended that pulse oximetry is used to identify patients at risk of hypoxaemia, to monitor these patients during haemodialysis, and to administer oxygen to those whose SpO2 falls below 90%, particularly if they have anaemia or cardiovascular disease.