Mechanical power normalisation methods to predict ICU mortality: a retrospective cohort study.

Abstract

Background: The optimal mechanical ventilation strategy to minimise ventilator-induced lung injury (VILI) remains uncertain. Mechanical power (MP) is a key VILI determinant, but whether and how MP should be normalised to individual patient characteristics is unclear. In this study, we aimed to evaluate whether the discriminatory accuracy of MP for ICU mortality in mechanically ventilated patients improves when normalised to physiologically relevant variables that reflect individual susceptibility to VILI. We also explored whether the relationship between MP, MP_ratio, and mortality is linear or exhibits a threshold effect.

Methods: In this retrospective observational study, we extracted granular electronic healthcare record data for mechanically ventilated adults in a single centre over a seven-year period. Primary exposures were MP with five normalisations: for dead space (expressed as corrected minute ventilation, ventilatory ratio, or end-tidal to arterial CO₂ ratio); aerated lung size (compliance), and normal idealised MP (MP_ratio). We used logistic regression to assess associations with ICU mortality. We calculated the Area Under the Receiver Operating Characteristic Curve (AUROC) to compare discriminative accuracy of individual models. Additionally, we evaluated the linearity or presence of a threshold for the relationships between MP, MP_ratio and ICU mortality.

Result: We included 3,578 patients in our analyses. We found MP normalised to compliance (AUROC 0.71, 95% confidence interval (CI) 0.69-0.73, p = 0.007 (DeLong's test)) and MP_ratio (AUROC 0.71, 95% CI 0.68-0.73, p = 0.0014) performed better than MP alone (AUROC 0.69, 95% CI 0.66-0.71) for predicting ICU mortality. Other methods of MP normalisation were no more discriminative than MP without normalisation. The relationship between MP and MP_ratio with ICU mortality showed a statistically significant but small departure from linearity.

Conclusions: Mechanical power normalised to compliance and MP_ratio had better discrimination for ICU mortality than MP, although the difference was modest and absolute predictive power remained limited.