Inspiratory pressure/maximal inspiratory pressure ratio: a predictive index of weaning outcome

Abstract

Objective: To compare the accuracy of PI/PImax ratio and other commonly used indices in predicting weaning outcome.

Design: A prospective study.

Setting: Intensive care unit.

Patients: 31 stable intubated patients ready to undergo weaning trial.

Methods: A simple method was developed to measure the PI and PImax in intubated patients. The accuracy of PI/PImax ratio and other commonly used indices in predicting outcome were compared. All indices were measured prior to weaning trial using standardized methods.

Measurements and results: Minute ventilation of the successful patients (13.00 +/- 0.67 (SE) l/min) was not significantly different from the failure patients (10.64 +/- 1.26 l/min, p = 0.10). The PI and PImax for the successful patients (11.48 +/- 1.25 cmH2O and 47.77 +/- 4.48 cmH2O, respectively) and the unsuccessful patients (14.32 +/- 2.31 cmH2O and 40.16 +/- 4.55 cmH2O, respectively) were also not significantly different (p = 0.28 and 0.24, respectively). The PI/PImax ratio was lower for the weaning successes (0.26 +/- 0.03) than for the weaning failures (0.36 +/- 0.04, p < 0.05). The threshold value of 0.3 for PI/PImax provided the best separation between weaning success and failure patients. The combined usage of rapid shallow breathing index and PI/PImax ratio provided the highest accuracy with sensitivity of 0.81 and specificity of 0.93.

Conclusion: The PI/PImax ratio provided a good separation between the patients who were successfully weaned and those who failed. It provides additional discriminative power to f/VT.