Instrumentation for Measuring Continuous Oxygen Consumption of Surgical Patients

A system has been developed for continuously monitoring the oxygen consumption (V02) of surgical patients. A replenishment technique is used whereby the oxygen removed by the patient from a closed rebreathing circuit is replaced. This is accomplished using a feedback-controlled pump to add oxygen at a rate necessary to maintain a constant inspired oxygen percent. A second feedback loop adds nitrous oxide to the circuit at a rate equal to the patient's nitrous oxide uptake rate, thus maintaining a constant circuit volume. These two feedback loops constitute a system to monitor oxygen and nitrous oxide uptake during nitrous oxide anesthesia. The instrument responds to a step change in V02 in 4 min. System accuracy has been shown to be ±6% in an in vivo comparison, the major error resulting from oxygen sensor or electronics dnift, leaks in the system, or changes in residual volume. The record of a surgical patient's VO2 while under balanced anesthesia shows a practical application of the system.