The effect of a bellows leak in an Ohmeda 7810 ventilator on room contamination, inspired oxygen, airway pressure, and tidal volume

We investigated the effect of a small bellows leak (bellows full at end-expiration) on inspired oxygen fraction (FIO₂), exhaled tidal volume (VT), airway pressure, and room contamination in an oxygen-driven anesthesia ventilator (Ohmeda 7810, Madison, WI). CO₂ concentration at the ventilator exhalation valve, FIO₂, VT, and airway pressure were measured (n = 3) while ventilating a CO₂-producing test lung at 8 breaths/min and an inspiratory/expiratory ratio of 1:2, with and without a bellows leak (4-mm-long tear). Set VT was 400, 600, 800, and 1000 mL. Fresh gas flow (FGF) was 0.3 L/min O₂ and (a) 5.0 L/min air, (b) 2.0 L/min air, and (c) 0.2 L/min nitrogen. There was no clinical difference in FIO ₂, VT, PIP (peak inspiratory pressure) and PEEP (positive end-expiratory pressure), with and without a 4-mm bellows tear, at all FGFs and VT settings. CO₂ at the ventilator exhalation valve was always nonzero with a bellows leak, indicating that CO₂-laden circuit gas was contaminating the drive gas via the bellows leak. A 4-mm bellows tear in an Ohmeda 7810 ventilator allows anesthetic gases to contaminate ambient air but does not cause clinically significant changes in FIO₂, exhaled VT, PIP, or PEEP.