Instrumentation for estimating pulmonary function in patients on positive end-expiratory pressure

For patients requiring PEEP during ventilation, e.g., patients with respiratory distress syndrome, performing traditional tests of lung function is a complicated problem. The pulmonary mechanics of these patients can change rapidly with potential for a negative impact on oxygenation. In an effort to address these problems, we designed a system with a three-way valve that permits the patient to be switched from the ventilator to a data collection system for the assessment of changes in the disease state. The short periods required for data collection (20 to 30 sec) do not interfere with patient care and are well within the limits of safety. A unique feature of the valved system is that it serves to maintain positive airway pressure during data collection. Tests show that, within the frequency range of interest, the mechanics of the measurement system are not altered by the DC pressures applied to the speaker and the valve itself does not contribute significantly to the serial impedance of the system.