Continuous positive airway pressure (CPAP) and intermittent mandatory ventilation (IMV) have been applied to patients on mechanical ventilators by the use of continuous flow systems and demand valves. In this study we proposed to determine the effectiveness of continuous flow and demand valve systems in maintaining airway pressure (Paw). Using a lung model to simulate a spontaneously breathing patient, we determined the Paw for four levels of CPAP and three inspiratory times. Continuous flow systems were more effective in maintaining airway pressure than were demand valve systems. Patients who cannot tolerate increases in inspiratory or expiratory effort may not tolerate demand valves as well as continuous flow systems.
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