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Oxygen Consumption Rate in a Field Anesthesia Machine
Charles L. Lamb, M.D., Kenneth D. Klions, M.D., Dale F. Szpisjak, M.D.
Anesthesia Department, Fleet Hospital 08, Bremerton, Washington.
Introduction: Patients often require anesthesia outside the traditional hospital-based operating room. Field anesthesia machines have been developed for remote locations. If a central supply of oxygen becomes unavailable, use of oxygen cylinders will be necessary. Gas-powered mechanical ventilators may rapidly deplete the oxygen cylinder(1), especially in patients with decreased pulmonary compliance. The goal of this study was determine oxygen consumption rates with a contemporary field anesthesia machine in models of high and low pulmonary compliance.

Methods: Oxygen consumption rates were tested using D-cylinders and the Narkomed® M field anesthesia machine, with a semi-closed circle breathing system. The filling pressure of the D-cylinders was 1700 psi. Three different tidal volumes were tested (500, 750, and 1000 ml) with both low and high compliance lung models. Four D-cylinders were tested in each group. A new disposable anesthesia circuit was used for each group (Sim Portex, Fort Meyers, FL). Each high compliance test used a new three-liter breathing bag. Each low compliance test used the same Siemens® Test Lung 190 (Siemens AG, Munich, Germany). Respiratory rate was 10 breaths/minute , I:E ratio was 1:2, and fresh gas flow was 1L/min. After verifying ventilator settings, the pipeline supply was stopped, the D-cylinder opened, and the time until the low supply pressure alarm sounded was recorded. For calculations, a full D-cylinder's pressure and volume were assumed to be 1900 psi and 400 L, respectively. The low oxygen supply pressure alarm threshold was assumed to be 35 psi. The volume exhausted from the D-cylinders was 351 L. Between-group comparisons of groups with the same tidal volume were analyzed using the independent samples t-test. Among-group comparisons with the same lung model were analyzed using repeated-measures analysis of variance with the Student-Newman-Keuls test for multiple comparisons. P < 0.05 accepted as significant. Data reported as mean (± SD).

Results: The time until the low oxygen supply pressure alarm sounded was longer for the high compliance lung model than for the low compliance lung model for each tidal volume tested (table 1). This time varied inversely with tidal volume for both lung models. The high compliance lung model oxygen consumption rate was slower than the low compliance lung model oxygen consumption rate for each tidal volume tested (table 2). The oxygen consumption rates varied directly with tidal volume.

Ref: 1. Anesth Analg 2002;95:148-50.

Anesthesiology 2004; 101: A566
Anesthesia Machine Tank Duration (min)
Tidal Vol. (ml)High Comp.Low Comp.P
50073.6 (1.0)*64.8 (1.2)*< 0.001
75068.0 (0.9)*61.5 (1.2)*< 0.001
100063.4 (0.6)*56.8 (0.4)*< 0.001
*P < 0.05 compared to same compliance group
Tank Consumption (L/min)
Tidal Vol. (ml)High Comp.Low Comp.P
5004.8 (0.07)*5.4 (0.10)*<0.001
7505.2 (0.07)*5.7 (0.12)*<0.001
10005.5 (0.05)*6.2 (0.05)*<0.001
* P < 0.05 compared to same compliance groups