Continuous measurement of oxygen consumption during cardiopulmonary bypass: description of the method and in vivo observations

BACKGROUND: Systemic oxygen consumption is not routinely measured during cardiopulmonary bypass, despite its potential benefits. We aimed to develop a noninvasive method to continuously measure oxygen consumption using respiratory mass spectrometry during hypothermic cardiopulmonary bypass in pigs. METHODS: Nine pigs weighing 18.5 (1.6) kg underwent hypothermic (32 degrees C) cardiopulmonary bypass for 180 minutes with 120 minutes of aortic cross clamping. An AMIS 2000 mass spectrometer (Innovision A/S, Odense, Denmark) was adapted for the on-line measurement of oxygen consumption by sampling the inlet and outlet gases of the membrane oxygenator together with measurement of the "expired" gas volume. RESULTS: Active cooling for 60 minutes reduced the venous blood temperature by 2.9 (0.8) degrees C and VO(2) by 0.70 (0.33) mL/kg/min. The 40-minute active rewarming restored the venous blood temperature by 4.4 (0.4) degrees C and oxygen consumption increased by 1.36 (0.33) mL/kg/min. There was wide interanimal variability, however, particularly at higher venous blood temperatures. Immediately after the release of aortic cross clamp, there was a noticeably acute increase in oxygen consumption in all the pigs (0.64 [0.21] mL/kg/min). CONCLUSIONS: A simple and safe adaptation of mass spectrometry allows continuous measurement of oxygen consumption during hypothermic cardiopulmonary bypass. The wide interindividual variations observed in this pilot study underscore the need to more accurately describe changes in oxygen consumption and how they are affected by temperature, oxygen delivery, and other interventions during cardiopulmonary bypass. As such, the technique may have an important role in clinical research and management of oxygen transport in patients undergoing cardiac surgery.