Measuring exhaled volume with continuous positive airway pressure and intermittent mandatory ventilation. Techniques and rationale.

When patients breathe spontaneously through a ventilator circuit, a fall in airway pressure during the inspiratory cycle may increase inspiratory effort. A system of delivery which incorporates a distensible reservoir bag and delivers a constant flow of gas that is two or three times the patient's minute volume will prevent a significant drop in inspiratory airway pressure. Unfortunately, the constant flow of gas mixes with the patient's exhaled gas and makes continuous monitoring of exhaled volumes difficult. Two modifications of circuits are described which allow accurate continuous measurement of volume. One of these circuits allows analysis of the concentrations of expired gases. When spontaneous ventilation occurs, tidal volume and minute ventilation demonstrate an intact connection between the ventilator and the patient, continuously indicate the patient's ability to sustain independent ventilation, and give early warning of a change in respiratory status.