Predicting the need for prolonged ventilatory support in adult cardiac patients.

Forty-nine cardiac surgical patients had ventilatory function tests and measurements of maximum inspiratory (MIP) and maximum expiratory (MEP) pressures preoperatively. The differences between the values of various function tests were compared for patients requiring less than 24 hours of ventilator support and those requiring more than 24 hours. There was a significant difference in the mean values for the two groups in vital capacity (VC) first-second forced expired volume (FEV) midexpiratory flow between 50 and 75 (MMEF 50--75) and 75--85 percent of expired volume (MMEF 75--85), and MEP. The standard deviations of each of the variables were so large that the clinical usefulness was limited. However, when discriminant analysis was used for more than one variable, the combination of MMEF 75--85 and MEP predicted success or failure to wean in 24 hours correctly in 90 percent of instances. On the basis of these simple tests, patients predicted to succeed should be weaned from ventilator support on recovery from anesthesia. Those predicted to fail should be placed on intermittent mandatory ventilation (IMV) and should be weaned following a planned, logical process.