Multipatient anesthetic mass spectrometry: rapid analysis of data stored in long catheters

A centrally located mass spectrometer sequentially samples airway gases from ten anesthetized patients through 30 m long, 1.07 mm, ID, nylon catheters and three way solenoid valves. End-tidal and inspired concentrations of O2, N2, CO2, N2O, and halothane, enflurane, or isoflurane are displayed on a computer terminal screen in each OR with trend plots. While a gas sample from one room is being analyzed, all other catheters are slowly sampled in order to continuously store 20-s concentration profiles ready for analysis. The stored gas sample is analyzed at twice the rate it was sampled. The computer switches catheters after one breath has been validated from two comparable end-tidal PCO2 values. Large flow changes produced by switching from one catheter to the next require regulation of the pump pressure in the mass spectrometer. This method reduces the time required to sample each room to 6.96 s (4-10 rooms). Catheter transit slows the response to a step increase in concentration by about 0.13 s (from 10 per cent-90 per cent) and prolongs the transit time through the catheter for a volatile anesthetic by about 0.04 s more than N2. The monitoring facility is used in each room for an average of 5.5 h/day. Two years of experience suggest that it can facilitate detection of faulty technique and equipment, reduce cost of anesthetic agents by encouraging use of closed systems, increase patient safety, aid research and teaching, and diminish exposure of OR personnel to anesthetics. Inherent problems have resulted in an inoperative time of less than 2 per cent.