IWI: An integrative weaning index of discontinuation from mechanical ventilation
"Indexes predicting weaning outcome are frequently inaccurate," said Sérgio N Nemer RRT PhD of the Intensive Care Unit, Hospital de Clínicas de Niterói, Rio de Janeiro, explaining why he and a team of colleagues have developed a new integrative weaning index that aims to improve the accuracy of the traditional indexes. Their online provisional abstract published by Critical Care has received a notably high viewing level, which demonstrates the interest in and need for such an index.
During the research, the team evaluated 331 patients who had been mechanically ventilated for over 24 hours. ‘Initially, the threshold values of each index that best discriminate between a successful and unsuccessful weaning outcome were determined in 115 patients. In the second phase, the predictive performance of these values was tested prospectively in the other 216 patients. Frequency/tidal volume ratio (f/Vt ratio), tidal volume (Vt), tracheal airway occlusion pressure 0.1 s (P 0.1), the product of P 0.1 and f/Vt (P 0.1 x f/Vt), respiratory rate (f), static compliance of the respiratory system (Cst,rs), ratio of arterial oxygen tension to fraction of inspired oxygen (PaO2/FiO2 ratio) and the new integrative weaning index IWI (Cst,rs x arterial oxygen saturation / f / Vt ratio) were evaluated in all patients.
‘The readiness for weaning and the decision to return to mechanical ventilation was made by the physician in charge, based on the signs of poor tolerance. The receiver operating characteristic (ROC) curves were calculated in order to evaluate the predictive performance of each index. The Bayes' theorem was used to assess the probability of each test of predicting weaning.’
In the prospective-validation set, the researchers observed successful weaning in 183 patients (84.7%) and weaning failure in 33 (15.27%). ‘IWI presented the highest accuracy, with the area under the ROC curves larger than that under the curves for the f/Vt ratio (0.96 x 0.85 respectively; P = 0.003), and also larger than that under the curves for the other indexes. IWI presented a higher probability of successful weaning when the test was positive (0.99) and a lower probability when the test was negative (0.14). Measurement of Cst,rs during the weaning process was considered one of the study limitations.’
The team concluded that IWI was ‘the best predictive performance index of weaning outcome and can be used in the intensive care unit setting.’
Trial Registration: controlled-trials.com ISRCTN92117906
* Source: Critical Care – September 2009
18.11.2009
