BACKGROUND: Although several investigators have assessed the effects of pressure support ventilation on tidal volume and breathing patterns, none have investigated the combination of breathing patterns and end-tidal carbon dioxide in ventilator-dependent patients. OBJECTIVES: To determine the differences in end-tidal carbon dioxide and breathing patterns at varying pressure support ventilation levels in ventilator-dependent patients. METHODS: Breathing patterns were measured with a plethysmograph and a ventilator. End-tidal carbon dioxide was measured by connecting the capnography sampler to the exhalation port of intubated patients. All equipment was connected to a five-channel recorder for data collection. The respiratory rate, tidal volume, minute ventilation, end-tidal carbon dioxide concentration, and chest and abdominal movement were recorded at 10-minute intervals at four pressure support ventilation levels (0, 10, 15, and 20 cm H2O). RESULTS: As pressure support ventilation increased, the respiratory rate, end-tidal carbon dioxide concentration, and asynchronous movement of chest and abdomen decreased. Tidal volume increased with higher pressure support ventilation levels. CONCLUSIONS: Pressure support ventilation prevents asynchronous chest and abdominal movement and lowers the level of end-tidal carbon dioxide. Pressure support ventilation offers clinicians a way to lower the elevated carbon dioxide level that often occurs in critically ill patients. Increasing tidal volume and reducing the work of breathing by using pressure support ventilation may reduce diaphragm fatigue in ventilator-dependent patients.