The lack of effect of variable blood withdrawal rates on the measurement of mixed venous oxygen saturation

In a study of 12 critically ill patients, values for mixed venous oxygen saturation (Sv̄O₂) were measured from samples of pulmonary arterial blood drawn at five rates of withdrawal (1, 3, 10, 20 and 30 ml/min). Samples were drawn from properly placed pulmonary arterial catheters with the balloon deflated. Physiologic stability was documented at the beginning and end of the sampling period in all but one patient. Results in each patient demonstrated a close grouping of values for Sv̄O₂ obtained at the five rates of withdrawal (largest standard deviation, 1.9 percent). Evaluations by paired t-test comparing all rates of withdrawal with the slowest rate (1 ml/min) showed no significant differences. The rate of withdrawal of the sample is not associated with significant errors in Sv̄O₂ when samples are drawn from a properly positioned pulmonary arterial catheter.