Additional inspiratory work in intubated patients breathing with continuous positive airway pressure systems

Abstract

Spontaneous breathing with continuous positive airway pressure (CPAP) increases arterial oxygenation in patients with adult respiratory distress syndrome (ARDS). Two types of CPAP systems are currently used, continuous flow and demand valve systems. Clinically, the latter has had advantage of being more economical with fresh gas but is often poorly tolerated by patients. Studies in normal volunteers who were sitting have shown the total work of breathing to be increased more with demand valve systems than with a high-flow CPAP circuit without valves. However, no data were available in supine patients whose tracheas were intubated. Under these conditions, both the changes in pleural pressure and the values of pleural pressure relative to atmospheric pressure are required to distinguish the part of work done by the patient and that done by the CPAP device. The use of esophageal pressure for this determination is questionable in supine subjects. However, the increase in external inspiratory work added by a CPAP system can be evaluated. This superimposed inspiratory work (SIW) can be determined from simultaneous recordings of the flow and pressure difference across the CPAP device. We measured SIW in supine intensive care patients spontaneously breathing with CPAP and compared three demand valve systems with a continuous flow circuit.