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.