Equivalent Circuit Analysis of High-Frequency Ventilators Including a New High-Impedance Flow-Interrupting Ventilator

The small tidal volumes (VT) delivered to the lungs by highfrequency ventilators can be very sensitive to changes in the patient's respiratory mechanics. Analysis of a Thevenin equivalent circuit, consisting of a ventilator internal oscillatory pressure source in series with a ventilator internal impedance and a patient's respiratory impedance, reveals the need of a high-internal-impedance ventilator to minimize this VT sensitivity problem. We present a general methodology to estimate the internal impedance of any type of ventilator. The internal impedance, at a given frequency and flow setting, is calculated from the slope of the relationship between the generated peak-to-peak pressure and the VT delivered into a calibrated rigid tank through a varying constriction. We tested a typical high-frequency jet (HFJ) ventilator and a new high-impedance flow-interrupting (HIFI) ventilator consisting of a flow source, a rotary valve, a high-impedance expiratory tube, and a servocontrolled mean proximal airway pressure (MPAP) regulator. We found that the VT delivered by the HIFI ventilator was independent of MPAP and decreased by 12 percent after a fivefold increase in the constriction-tank system impedance. In contrast, the VT delivered by the HFJ ventilator decreased by 80 percent after a similar change in load. We therefore conclude that the VT delivered by the HIFI ventilator should be significantly less sensitive to changes in patient's respiratory impedance than the VT delivered by an HFJ ventilator.