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From the *Department of Anesthesiology, Uniformed Services University of the Health Sciences, Bethesda, Maryland; Department of Anesthesiology, Walter Reed Army Medical Center, Washington, DC; Department of Anatomy, Physiology, and Genetics; and Graduate School of Nursing, Uniformed Services University of the Health Sciences, Bethesda, Maryland.
Address correspondence to Dale F. Szpisjak, MD, MPH, Department of Anesthesiology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Road, Bethesda, MD 20814. Address e-mail to dszpisjak{at}usuhs.mil.
BACKGROUND: Field anesthesia machines (FAM) have been developed for remote locations where reliable supplies of compressed medical gases or electricity may be absent. In place of electricity, pneumatically controlled ventilators use compressed gas to power timing circuitry and actuate valves. We sought to determine the total O2 consumption and ventilator gas consumption (drive gas [DG] plus pneumatic control [PC] gas) of a FAM's pneumatically controlled ventilator in mechanical models of high (HC) and low (LC) total thoracic compliance.
METHODS: The amount of total O2 consumed by the Magellan-2200 (Oceanic Medical Products, Atchison, KS) FAM with pneumatically controlled ventilator was calculated using the ideal gas law and the measured mass of O2 consumed from E cylinders. DG to the bellows canister assembly was measured with the Wright Respirometer Mk 8 (Ferraris Respiratory Europe, Hertford, UK). PC gas consumption was calculated by subtracting DG and fresh gas flow (FGF) from the total O2 consumed from the E cylinder. The delivered tidal volume (VT) was measured with a pneumotach (Hans Rudolph, KS City, MO). Three different VT were tested (500, 750, and 1000 mL) with two lung models (HC and LC) using the Vent Aid Training Test Lung (MI Instruments, Grand Rapids, MI). Respiratory variables included an I:E of 1:2, FGF of 1 L/min, and respiratory rate of 10 breaths/min.
RESULTS: Total O2 consumption was directly proportional to VT and inversely proportional to compliance. The smallest total O2 consumption rate (including FGF) was 9.3 ± 0.4 L/min in the HC-500 model and the largest was 15.9 ± 0.5 L/min in the LC-1000 model (P < 0.001). The mean PC circuitry consumption was 3.9 ± 0.24 L/min or 390 mL ± 24 mL/breath.
CONCLUSIONS: To prepare for loss of central DG supply, patient safety will be improved by estimating cylinder duration for low total thoracic compliance. Using data from the smaller compliance and greatest VT model (LC-1000), a full O2 E cylinder would be depleted in <42 min, whereas a full H cylinder would last approximately 433 min.
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