Abstract
I/E ratios and I/E ratio alarm function of the Bennett MA-1 ventilator were examined at various flowrates and respiratory frequencies. In the assist mode, the I/E ratio alarm is inoperable; however, by varying respiratory frequency while maintaining a constant flowrate and tidal volume, one can significantly alter the I/E ratio. For a given tidal volume, we found that the flowrate setting that yields a safe I/E ratio at one frequency may create hazardous I/E ratios at higher frequencies. In the control mode, the inspiratory pause created by the expiratory resistance control seemed to be read by the ventilator circuitry as part of expiration. Therefore, hazardous I/E ratios can exist without alarm function, and this control should not be used unless it is monitored carefully. The Bennett MA-1 ventilator, a well-designed, reliable, and flexible unit, is a powerful therapeutic tool when applied in a knowledgeable manner.
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