Determinants of ventilator weaning outcome in a medical-surgical ICU

Introduction

The purpose of this study is to prospectively evaluate the determinants of weaning outcome in a selected sample of ventilator-dependent patients.

Methods

After fulfilling a set of inclusion/exclusion criteria, 46 patients treated in a single medical-surgical ICU were prospectively evaluated between October 2011 and January 2013. The study protocol followed the generic course of mechanical ventilatory support and its discontinuation. A period of CMV was followed by stepwise reduction in pressure support. The outcomes were recorded at the second and 24th hour after the beginning of the PSV (8 cmH₂O PS) spontaneous breathing trial (SBT). A short period of ZEEP breathing was introduced after the CMV as an additional respiratory stressor. A large number of parameters (including those derived via indirect calorimetry and esophageal balloon catheter) were analyzed. The following statistical methods were used: Student's t test or its nonparametric alternatives for intergroup comparisons, repeated-measurements ANOVA for intragroup comparisons, cross-tabulation and Fisher's exact test to compare categorical variables. Logistic regression analysis was conducted with regard to success/failure classification.

Results

Twenty-two (63.04% of all) patients successfully passed the 2-hour SBT. Of them, five failed the subsequent 24-hour SBT. Almost all of the latter (four out of five) were ventilated due to chronic respiratory or cardiac diseases. The following parameters were significantly different between the groups of patients who successfully and unsuccessfully completed 2-hour SBT: pre-inclusion body weight; PEEPi (end-expiratory occlusion method), peak and mean airway pressures during CMV; PaO₂, PaO₂/FiO₂ and oxygenation index during the SBT; respiratory rate, tidal volume, f/Vt, patient work of breathing (modified Campbell method), total body oxygen consumption, respiratory quotient and energy expenditure during ZEEP and the SBT. The logistic regression model devised to predict the outcome of the 2-hour SBT included the following parameters: type of artificial airway (tracheotomy vs. translaryngeal), oxygen consumption, f/Vt and energy expenditure during the SBT.

Conclusion

The presence of serious respiratory and/or cardiac comorbidities might require longer duration of the SBT. In our study, respiratory load-muscle capacity balance and metabolic activity appear to play a major role in determining the weaning outcome. A short period of ZEEP breathing is safe and might have prognostic utility regarding the outcome of mechanical ventilatory support discontinuation.