LIMITS OF HIGH FREQUENCY JET VENTILATION IN SMALL SUB JECTS

Abstract

To define limits of injector size/internal compressible volume (ICV) of the jet tubing in small subjects, we studied anesthetized paralyzed cats using the MK-800 jet ventilator & noncompliant tubing with ICV of 14 ml. FIO₂ of 1.0 & 30% duty cycle remained constant. In part I the injector size was varied from #12,14,16 to 18 (2.16-1.06mm)at rates of 150 & 300/min with normal and noncompliant lungs. The driving pressure was adjusted to keep a constant mean airway pressure (MAP) for each set of conditions. In part II the ICV was ↑ from baseline(B) to 1.5B, 2B and 3B through the same sequence as in part I, using #14 & #16 injectors. Tracheal pressure tracing was recorded. Despite constant MAP, PCO₂ rose with ↓ injector diameter though with normal lungs at 150/min PCO₂ was normal even with the smallest injector (#18). Increasing ICV up to 3B had no effect with #14 injector or with normal lungs. With #16 injector and stiff lungs, PCO₂ rose as ICV ↑.

When an unfavorable relationship exists between injector size, driving pressure & ICV excessive compression of gas in the patient circuit results in loss of pulsatility of gas flow & inadvertent PEEP leading to CO₂ retention.

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