Respiratory flow measurements and anesthetic drugs: some in vitro observations

Abstract

Study Objective: To illustrate the influence of anesthetic gases on respiratory flow measurements and to correct this influence. Design: In vitro evaluation. Setting: Laboratory. Measurements and Main Results: An in vitro method using a 120-L Tissot water-seal spirometer was used along with a Bicore CP-100, designed for use in intensive care units, and a Datex Ultima, designed for use in the operating room. The flow transducer of one of the instruments being tested was placed in the gas inlet of the Tissot so that simultaneous measurements could be made. Timed flows (2 to 60 L/min) of various gases (O 2 and air) and gas mixtures (halothane-O 2, isoflurane-O 2, N 2O-O 2, and N 2O-O 2-isoflurane) were used and the measurements made by the Tissot and the test instrument compared. The Datex Ultima, which includes software corrections for anesthetics, was able to accurately measure gas flows (2 to 60 L/min) of air, 100% oxygen, and anesthetic gas mixtures to within ±10% of measurements made by the Tissot. The Bicore CP-100, intended for use with mechanically ventilated patients, accurately measured air and 100% oxygen flow rate to within ±8% of the measurements made by the Tissot, but there were large errors (up to 40%) when anesthetics were used. Conclusions: This study illustrates the effects of anesthetic gases on measurements of ventilatory flow and the need to ascertain whether corrections are needed to improve the accuracy of flow transducers.