[abstract] EFFECT OF VARYING ATMOSPHERIC PRESSURE ON COMPRESSIBLE VOLUME OF MECHANICAL VENTILATORS.

Abstract

This study determined ventilator circuit compression factors (Cf) at one hypobaric and various hyperbaric pressures. Bottles with volumes of 1070 cc, 2495 cc, and 4320 cc were used as models of ventilator circuit volume (Vc)' Air was injected into each bottle with a 60-cc syringe, which pressurized the bottle to approximately 50 em H20, and its volume was noted. Pressure (p) was measured with an aneroid gauge (Marshalltown Gauge) attached to each bottle. Three measurements were made for each bot tIe at -15, 0, 15, 30, 45, 60, 120, and 165 feet of sea water. Compression factors were then calculated with the formula: Cf = vip. Data were averaged and plotted. Ventilator circuit factors and therefore compressional gas losses were directly related to Vc and were inversely proportional to atmospheric pressure (PB)' The ideal gas law was used to derive the equation Cf = Vc/PB, which may explain the effect of varying PB on Cf. Clinicians operating volume-cycled mechanical ventilators in hyperbaric chambers or on aircraft should be aware that Vc should be minimal. We recommend that clinicians measure delivered tidal volume after any significant alteration in PB' This will prevent the possibility of dangerous increases or decreases in tidal volume as compressional gas increases or decreases.