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 (V_E) to the change in P_aCO₂ (ΔV_E/ΔP_aCO₂), the change in airway occlusion pressure at 0.1 second of inspiration (P_0.1) to change in P_aCO₂ (ΔP_0.1/ΔP_aCO₂), the ratio of the change in P_0.1 to baseline P_aCO₂ (ΔP_0.1/P_aCO₂) and the ratio of the change in V_E to baseline P_aCO₂ (ΔV_E/P_aCO₂).

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 ΔP_0.1/ΔP_aCO₂ (0.19±0.11 and 0.34±0.20 cm H₂O/mmHg respectively, P=0.006) and lower ΔV_E/ΔP_aCO₂ (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 P_aCO₂ 0.81 (95% confidence interval: 0.66 to 0.91), hypercapnic P_aCO₂ 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), ΔP_0.1/P_aCO₂ 0.76 (0.60 to 0.87) and for the ΔV_E/P_aCO₂ 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.