Hypercapnia Test and Weaning outcome from Mechanical Ventilation in COPD Patients

Abstract

We evaluated the hypercapnia response test as a weaning outcome predictor from mechanical ventilation in patients with chronic obstructive pulmonary disease (COPD). We studied 44 COPD intubated patients considered ready for a weaning trial. The hypercapnia test was based on the modified method of re-inhalation of expired air. Through the hypercapnic test we calculated the ratio of the change in minute volume (VE) to the change in PaCO2 (ΔVE/ΔPaCO2), the change in airway occlusion pressure at 0.1 second of inspiration (P0.1) to change in PaCO2 (ΔP0.1/ΔPaCO2), the ratio of the change in P0.1 to baseline PaCO2 (ΔP0.1/PaCO2) and the ratio of the change in VE to baseline PaCO2 (ΔVE/PaCO2). Nineteen patients were successfully weaned and 25 patients failed. No differences in baseline clinical characteristics were found between the two groups. Weaning failure COPD patients had lower ΔP0.1/ΔPaCO2 (0.19±0.11 and 0.34±0.20 cm H2O/mmHg respectively, P=0.006) and lower ΔVE/ΔPaCO2 (0.21±0.15 and 0.40±0.22 l/min/mmHg respectively, P=0.002) than successfully weaned patients. The area under the receiver operating characteristic curve to discriminate weaning outcome was for the baseline PaCO2 0.81 (95% confidence interval: 0.66 to 0.91), hypercapnic PaCO2 0.76 (0.61 to 0.88), hypercapnic drive response 0.74 (0.59 to 0.86), hypercapnic ventilatory response 0.76 (0.60 to 0.87), ΔP0.1/PaCO2 0.76 (0.60 to 0.87) and for the ΔVE/PaCO2 0.81 (0.67 to 0.91). COPD patients with weaning failure have a significantly more blunted response to the hypercapnia response test than weaning success patients. This test could be useful to predict weaning failure patients if the combined values of the hypercapnic drive and hypercapnic ventilatory response were below the threshold values.