Appeared in: Journal of Hyperbaric Medicine, Vol 3, No 4, 1988
Publisher: Undersea and Hyperbaric Medical Society
Abstract: In our initial use of the Sechrist 500A hyperbaric ventilator operating within a Sechrist 2500B monoplace chamber, we observed that the patient's tidal volume (VT) decreased as chamber pressure (PCH) increased more than 10%, the maximum allowable decrement in VT from the ambient VT (500A operator's manual). Therefore we decided to quantitate this decrement in VT and to determine what variables were important for the ventilator to deliver an adequate VT. The 500A ventilator was set up within the chamber in the manner described by the 500A operator's manual. First, an adult Boehringer spirometer was calibrated over the range of monoplace PCH (0.85 to 2.9 ATA at our altitude). This was accomplished by collecting a volume of gas at various PCH, then measuring the volume of gas at atmospheric pressure and calculating the true VT. Comparing measured VT to true VT, the percent error discrepancy was: -1.5, +2.4, +2.8, +6.1, +6.0% at PCH = 0.85, 1.5, 2.0, 2.5, 2.9 ATA, respectively (the + values mean that the spirometer underestimates the actual VT by that %). Once the spirometer calibration was known, we varied the static complaince (C = 15 to 8.7 cc/cmH2O) of a test lung and ventilator control module inlet pressure (PIN) from 55 to 85 psig (the allowed range of PIN by Sechrist) and measured VT as the dependent variable. We found that VT is a function of C, PCH and PIN . Even with a normal V , VT decreased more than -10% when PCH exceeded 1.5 to 2.0 ATA. With a C = 15 cc/cmH2O, PCH = 2.5 ATA, and PIN = 55 psig, VT was reduced 38% from that at ambient pressure. We recommend monitoring VT continuously during mechanical ventilation of patients in a monoplace hyperbaric chamber with a hyperbaric-calibrated spirometer, making appropriate ventilator adjustments to maintain an adequate VT.