Objective: To examine whether an appropriately designed combination of an index of ventilatory endurance and the frequency divided by tidal volume ratio (f/VT) provides prognostic information for weaning outcome not offered by any index alone.
Design and setting: Prospective study in a multidisciplinary intensive care unit, university hospital.
Patients: 124 consecutive mechanically ventilated patients.
Interventions: We designed an index of ventilatory endurance (load/force balance) calculated as the mean inspiratory airway pressure (PI) during controlled mechanical ventilation/maximum inspiratory pressure (MIP) [PI equals the triplicate of mean airway pressure (Paw) displayed by the ventilator] and tested its capacity in predicting weaning outcome at 48 h along with f/VT and many other indices in 75 consecutive mechanically ventilated patients ready to wean. A stepwise discriminant function analysis was used to test the performance of appropriately designed index combination. Threshold values of indices and their combination were prospectively validated in another group of 45 consecutive patients.
Results: Stepwise discriminant analysis showed that PI/MIP and f/VT were the only indices that remained in the model with the function D=7.628xmean Paw/MIP+0.0158xf/VT-2.374. The cutoff point of D=0.5 had 94% sensitivity, 67% specificity, and 87% correct classifications. Prospective validation demonstrated similar results. The simplified discriminant function D=15xmean Paw/MIP+0.003xf/VT-5 and the cut-off point of D=1.0 had 89% sensitivity, 67% specificity, and 85% correct classifications.
Conclusions: The combination of mean Paw/MIP and f/VT in a simplified discriminant function is useful in predicting weaning outcome.