Response time studies of a new, portable mass spectrometer.

Objective: Mass spectrometers are frequently used by anesthesiologists perioperatively to monitor patients' respiratory function and levels of inhaled anesthetics. Due to size, complexity and expense, they are typically used in a time-sharing manner which degrades their performance. We assessed the accuracy of the Random Access Mass Spectrometer (RAMS), Marquette Electronics) which is small enough to be dedicated to a single patient.

Methods: We compared the 10-90% rise times for O2, CO2, N2O and isoflurane for the RAMS with different catheter configurations to those of a MedSpect mass spectrometer (Allegheny International Medical Technology) operating under ideal conditions. For CO2 the lag of the RAMS relative to the MedSpect was also measured. Next, perioperative conditions were stimulated by ventilating anesthetized dogs with a variety of inhalatory gases and ventilatory parameters, and the interchangeability of the two devices was assessed.

Results: When fitted with a catheter with minimal dead space the MedSpect had rise times of 0.11-0.12 sec while the RAMS had rise times of 0.07-0.12 sec and a delay of 0.19 sec compared to the MedSpect. The rise times and delay of the RAMS increased when using a larger catheter and water trap. Although there were statistically significant differences in some values for inhaled and end-tidal gases under simulated perioperative conditions, particularly at the higher frequencies, these differences were small and for most purposes not clinically significant.

Conclusions: Our results demonstrate that the RAMS configured for clinical conditions performs nearly as well as the MedSpect under ideal conditions. The small differences between the two, confined almost entirely to their end-tidal CO2 values, could be due to differences in instrument calibration, by the larger sampling catheter commonly used in clinical settings, or by a combination of both factors. Therefore the RAMS is sufficiently accurate for clinical use and would alleviate problems associated with time-shared mass spectrometers.