A PROSPECTIVE VALIDATION STUDY OF STRESS INDEX IN PREDICTING MECHANICAL STRESS IN ARDS PATIENTS

INTRODUCTION. 30% of ARDS patients ventilated according to NIH protocol presents CT
scan evidence of hyperinflation. Functional analysis of pressure-time curve (Stress Index=SI)
has been shown to identify this condition in experimental settings. We tested the hypothesis
that the SI is an accurate predictor of mechanical stress due to overdistention in ARDS patients.
METHODS. The presence ofmechanical stress was identified by CT scan analysis. The threshold
value of SI that best discriminated between patients with and without overdistention was
determined by Receiver Operating Characteristic (ROC) curve; the accuracy of this threshold
value was prospectively tested in a second group of patients. Accuracy of plateau pressure
(Pplat) to identify CT hyperinflation was also assessed.
RESULTS. A SI threshold value of 1.08 was chosen by ROC analysis (fig 1); sensitivity and
specificity of the selected threshold was 0.80 and 0.67 respectively. Sensitivity and specificity
of Pplat equal or >30 cmH2O to identify hyperinflation were 0.40 and 0.90 respectively. With
a threshold value of 28 cmH2O sensitivity and specificity were 0.70 and 0.75.
CONCLUSION. SI analysis is an accurate predictor of mechanical stress in the clinical settings
for ARDS patients with a higher sensitivity than Pplat.