Tidal Volume Correction of Mechanical Ventilation Device for Use in Hyperbaric Therapy Settings

Background: The delivery of tidal volume (VT) through mechanical ventilation is impacted by variations in atmospheric pressure. According to Boyle’s law (P1V1 = P2V2), the pressure and volume of gas exhibit an inverse correlation when temperature is kept consistent. The performance of mechanical ventilators is also compromised by these physical changes. Commercially available mechanical ventilators are designed to function during use at or near sea-level atmospheric pressure and will typically exhibit some variation when exposed to pressure changes in a hyperbaric chamber. This study aimed to evaluate the Zoll 731 Series Z-Vent and determine a method to correct for the tidal volume changes seen during alterations of atmospheric pressure. Methods: A commercially available mechanical ventilation system (Z Vent- 731 Series, Zoll Medical, Chelmsford, MA) was evaluated for use in a clinical hyperbaric therapy setting. Validation of flow characteristics (delivered VT, and breathing frequency) was measured using a pneumotachometer (PNT-4813, Hans Rudolph, Shawnee, KS), connected to an analog-to-digital converter, which was calibrated with a 1-L syringe at each hyperbaric depth. VT settings (300, 400, and 500 mL) and frequency (12, 18, and 24 breaths/min) were measured at 1.0, 2.0, 2.4 (Figure 1), 2.8, and 3.0 ATA (atmospheres absolute). A PEEP of 5 cm H2O and an inspiratory time of 1 s were maintained at all VT settings. A modification formula was then derived from simple linear regression to correct for the desired VT output at all barometric pressures (Figure 2) and retested to evaluate the desired VT delivery after correction. Results: Across all ATA conditions, the ventilator showed a strong linear relationship between output and measured VT (all, R > 0.9, P < 0.01). The frequency was unaffected by changes in barometric pressure. Variations in output VT ranged from 1.0 mL (2.0 ATA) to 4.8 mL (3.0 ATA). At ATA >1.0 the ventilator output volume underestimated the measured volume (Figure 1). After applying a correction factor unique to each ATA condition, the mean difference between the desired and measured VT was ± 2.1 mL. Conclusions: In hyperbaric conditions between 2.0 – 3.0 ATA the 731 Series Z Vent VT delivery output was underestimated compared to the expected value as indicated by the VT setting on the machine. This difference can be effectively minimized through a correction factor derived from simple linear regression, tested at various levels of hyperbaria.