Factors influencing successful discontinuance of mechanical ventilation after open heart surgery: a clinical study of 41 patients.

Circulatory, respiratory, and metabolic variables were measured with a mobile clinical bedside unit in 41 patients during the first 48 hours after open heart surgery. Calculations were carried out off-line by a computer program. The variables were measured during controlled mechanical ventilation and compared with those obtained during spontaneous breathing and after resumption of mechanical ventilation; attempts at spontaneous breathing were categorized as successful or unsuccessful. The variables were compared before, during, and after the successful and the unsuccessful attempts at spontaneous breathing. In the series as a whole, the onset of spontaneous breathing was characterized by increases in cardiac output, radial arterial and pulmonary arterial pressures, and mixed venous oxygen tension (PVO2) and content (CVO2), as well as diminished arterial and mixed venous oxygen content differences (avDO2); no significant changes in oxygen consumption (VO2) were seen. Unsuccessful attempts disturbed the patient's physiological equilibrium by reducing oxygen delivery and not increasing VO2, while increasing ventilatory work. In general, resumption of controlled ventilation restored the physiological variables to their control conditions. Successful attempts at spontaneous breathing did not greatly affect the physiological variables. The indication for resumption of controlled ventilation after periods of spontaneous breathing is the combination of increased ventilatory work with diminished circulatory and respiratory functions.