Accuracy of a central mass spectrometer system at high respiratory frequencies

We have shown that in a laboratory setting the long lengths of sampling catheters necessary to access a central mass spectrometer restrict the high-frequency response of the system. Reported here are the results from a clinically operating Perkin-Elmer Advantage system. The sampling catheter was 50 meters long and the sampling flow was 240 ml/min. Rapid changes in carbon dioxide concentration were created by an electronically operated solenoid valve switching between 6.94% CO2 in 50% O2 balance N2 and 100% O2. The frequency of this simulated brething was varied between 10 and 100 breaths/min with the ratio of inspiration to expiration fixed at 1:2. Data were taken from the terminal in the operating room. Errors greater than 5% of the true value occurred at 35 breaths/min for the indicated inspired concentration and 73 breaths/min for the indicated expired concentration. For critical situations in which respiratory frequencies exceed 40 breaths/min, a centrally located mass spectrometer may not be adequate for measuring CO2 because of errors introduced by the long sampling catheters.