High Frequency Oscillatory Ventilator and Respiratory Measurement System: 5,555,880

A high frequency oscillatory ventilator for infants and adults using feedback control to maintain either the desired tidal volume or pressure delivered to the subject. The inspiratory to expiratory time ratio of the ventilator is variable. The ventilator corrects the measured pressure for arbitrarily-sized endotracheal tubes and calculates the actual pressure or tidal volume delivered to the subject. The ventilator also separates the source of the tidal volume oscillations from the patient circuit with a flexible membrane or diaphragm, allowing transmission of oscillating tidal volumes while blocking mean airway pressures. The patient circuit is flexible, but fabricated from low-compliance material to minimize the loss of tidal volume. The ventilator uses feedback control of the exhaust flow to maintain mean airway pressure in the presence of an independently controlled bias flow. Further, a respiratory impedance measurement system is built into or connected to the patient circuit which monitors changes in lung mechanics while the patient is connected to the high frequency ventilator without changing mean airway pressure or removing the patient from the ventilator.