Noninvasive measurement of intrapulmonary shunting.

ABSTRACT

OBJECTIVE: To investigate the accuracy and precision of a noninvasive approach to measurement of pulmonary shunt fraction using simultaneous application of 2 fundamental respiratory mixing equations the direct Fick equation for oxygen and the shunt equation of Berggren. This can be performed without mixed venous blood sampling and requires measurement of oxygen uptake and pulmonary blood flow. DESIGN: Comparison with invasive shunt fraction measured using mixed venous blood sampling and with estimated shunt fraction using an assumed arteriovenous O(2) content difference. SETTING: Major teaching hospital. PARTICIPANTS: Nine patients undergoing anesthesia for cardiac surgery. INTERVENTIONS: Pulmonary blood flow was measured using an indirect Fick technique (nitrous oxide throughflow) and by bolus thermodilution for comparison. MEASUREMENTS AND MAIN RESULTS: The mean shunt fraction measured by the invasive method was 0.145 (range 0.057-0.263). When pulmonary blood flow was measured using an indirect Fick technique (nitrous oxide throughflow), the absolute mean bias for noninvasive shunt fraction was -0.005 with a standard deviation of 0.012. Correlation was excellent (r(2) = 0.95, p < 0.001). Agreement was less precise when pulmonary blood flow was measured using thermodilution (mean bias + 0.001 with a standard deviation of 0.018). CONCLUSIONS: The noninvasive method is an accurate substitute for invasive shunt fraction measurement with mixed venous blood sampling.