The Use of Calculated Relative Inspiratory Effort as a Predictor of Outcome in Mechanical Ventilation Weaning Trials

Abstract

Evaluation of patients to determine when they are likely to be able to wean successfully from mechanical ventilation commonly takes into account half a dozen or more variables, such as spontaneous tidal volume (V_T), minute ventilation ( ${\dot{V}}_E$ ), respiratory frequency (f), negative inspiratory force (NIF), static lung compliance, resistance to gas flow, and flow itself. However, many patients fail weaning trials even though they appear to meet criteria for sustaining spontaneous ventilation; perhaps this is because clinicians cannot integrate all the variables. We therefore sought a simplified method for predicting weanability, hypothesizing that total drop in intrapleural pressure (ΔP_t) divided by NIF could quantify the fraction of a patient's NIF that is necessary for adequate spontaneous ventilation. Method: We studied 50 mechanically ventilated medical and postsurgical patients who satisfied traditional weaning criteria—measuring ${\dot{V}}_E$ and f, deriving average V_T, measuring NIF, estimating spontaneous inspiratory flowrate, computing elastance, calculating corrected V_T or reading it from the ventilator, and estimating resistance. By formula we calculated the AP, necessary for a spontaneous breath. The patient was then given a weaning trial with a T-piece and appropriate supplemental oxygen. A patient who remained off the ventilator 24 hours without ventilatory deterioration was considered weaned. Results: Of the 50 subjects, 27 had relative inspiratory efforts (ΔP_t/NIF) < 0.40, and 23 had values > 0.40. All 27 with a relative inspiratory effort < 0.40 weaned successfully. Of the 23 with values > 0.40, two weaned, and 21 failed the weaning trial. The two who weaned had values of 0.42 and 0.43, respectively. Conclusions: We conclude that in mechanically ventilated patients who satisfy traditional criteria for weaning, calculation of relative inspiratory effort permits the clinician to predict success or failure in a weaning trial. In our study, the critical value was about 0.40. The use of this tool has the potential to reduce the incidence of premature extubation and the morbidity associated with the exercise of fatigued muscles.