Erroneous mass spectrometer data caused by a faulty patient sampling tube: case report and laboratory study.

We report an error due to faulty sampling of gas for mass spectrometry by side-stream analysis that occurred during a general anesthetic for a surgical procedure. Two defects in the patient sampling tube were present. First, a crack was discovered in the polyvinylchloride tubing at the connection to the patient circuit. Second, secretions had accumulated in the end of the tubing that caused a partial obstruction to gas sampling. This combination promoted the contamination of respiratory gases sampled from the anesthesia circuit with entrained room air. This entrainment, however, occurred only during exhalation while ventilation was being controlled with a descending (during exhalation) bellows. The particular sampling error was reproduced and characterized in a mock circuit to simulate the sampling tube defects. It was determined that both a leak and a partial obstruction were necessary conditions for the sampling error to exist.