The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

ACCURATE TIDAL VOLUME DELIVERY DURING MECHANICAL VENTILATION WITH HELIUM/OXYGEN MIXTURES.

Max Kirmse, MD, Dean Hess, PhD, RRT, Hideaki Imanaka, MD, Robert M. Kacmarek, PhD, RRT. Respiratory Care and Department of Anesthesia, Harvard Medical School, Massachusetts General Hospital, Boston.

Helium-oxygen mixtures (He/O_{2}) may be useful in the management of severe airway obstruction and asthma. The use of He/O_{2} mixtures during mechanical ventilation may alter the function of a ventilator. We developed a mathematical correction for tidal volume at different He/O_{2} mixtures. Methods: Air, oxygen and heliox (He 80%/ O_{2} 20%) were attached to the low pressure inlet of a Servo 900C. A total flow of 15 L/min of air or five different He/O_{2} mixtures (80/20, 70/30, 60/40, 50/50, 0/100) were used. The ventilator was set on volume control ventilation, rate 15/min, inspiratory time fraction 33% or 50% without plateau, and minute ventilation adjusted to display V_{T} (V_{disp}) between 300 and 800 mL in steps of 100 mL. A single-compartment lung model (Model 1600, Michigan Instruments) was used to obtain the actual delivered V_{T} (V_{act}). Prior to the experiment, a pressure-volume curve of the system was established, allowing us to calculate V_{act} as measured by the end-inspiratory pressure in the model. Before using He/O_{2} mixtures, the accuracy of the ventilator pneumotachometer was evaluated with air, by comparing V_{disp} vs. V_{act}. The error was < 3% in all settings. Three breaths at each setting were obtained and used to perform a multiple regression analysis, predicting the V_{act} according to V_{disp} and FiO_{2}. Results: The relationship to calculate the delivered V_{act},according to V_{disp} and the FiO_{2} was: V_{act}=0.94 x V_{disp} x FiO_{2}^{-0.15} (R^{2}=0.998). The bias ± precision for this correction was 0.8 ± 23.4 mL (see below). Calculating separate regression equations for each inspiratory time did not improve the results. The largest error, 40mL, was seen with FiO_{2}=1.0. Conclusion: This correction allows prediction of the delivered volume according to the displayed value for any He/O_{2} mixture when the Servo 900C is used in volume control mode.

(See original for figure)

Reference: OF-96-101

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