A comparative study of inspiratory muscle strength, neuromuscular drive to breath and its ratio in weaning outcome

Background and objectives

The inspiratory muscle strength and the neuromuscular drive to breath, evaluated by the maximal inspiratory pressure (MIP) and airway occlusion pressure (P 0.1), respectively, are important factors in weaning. The aim of this study is to evaluate the MIP, the P 0.1 and its ratio (P 0.1/MIP) in weaning outcome.

Methods

Seventy consecutive patients of several etiologies in the weaning process that remained up to 24 hours in mechanical ventilation were evaluated (all with PaO₂ ≥ 60 mmHg with FiO₂≤ 0.4 and PEEP ≤ 8 cmH₂O). All patients were submitted to a 2-hour trial of spontaneous breathing. Those who sustained 2 hours of spontaneous breathing without return to mechanical ventilation in the following 24 hours were considered weaned, while those who could not sustain 2 hours of spontaneous breathing or returned to mechanical ventilation in the following 24 hours were considered not weaned. MIP < -25 cmH₂O, P 0.1 <4.2 cmH₂O and P 0.1/MIP < 0.14 cmH₂O were used to predict the success in weaning outcome. The predictive performance of each index was evaluated through the sensibility, specificity, positive predictive value, negative predictive value and diagnostic accuracy. The results were also evaluated by the area under the receiver operating characteristic (ROC) curves.

Results

MIP presented an area under the ROC curves smaller than those for P 0.1 (0.52 ± 0.08 vs 0.76 ± 0.06, respectively; P = 0.004) and also smaller than those for P 0.1/MIP (0.52 ± 0.08 vs 0.78 ± 0.06, respectively; P = 0.0006). P 0.1/MIP presented excellent predictive performance in weaned patients, with sensibility of 98.08, but with the area under the ROC curves only slightly larger than those for P 0.1 (0.78 ± 0.06 vs 0.76 ± 0.06, respectively; P = 0.69).

Conclusion

In our study, Pi max was the criterion with the worst predictive performance. P 0.1 was shown to be a very important criterion to evaluate the respiratory center output, although with limitations in evaluating weaning failure. Patients with P 0.1/Pi max ratio >0.14 are not always associated with weaning failure, but values <0.14 were highly associated with success in weaning outcome.