Hypercapnia Test As a Predictor of Success in Spontaneous Breathing Trials and Extubation

BACKGROUND: The ventilatory capacity of the respiratory neuromuscular system can be studied with the hypercapnia test.

OBJECTIVE: To determine whether decreased response to the hypercapnia test is associated with failure to pass a spontaneous breathing trial (SBT) or extubation failure.

METHODS: We studied 103 intubated patients ready for SBT. We used a hypercapnia test in which we approximately doubled the dead space and thus caused re-inhalation of expired air. We calculated 3 ratios: the ratio of P_0.1 (airway occlusion pressure 0.1 s after the onset of inspiratory effort) during hypercapnia test to baseline P_0.1; the ratio of the change in minute volume [ΔV_E] to the change in P_aCO2 (we call this ratio the hypercapnic ventilatory response); and the ratio of the change in P_0.1 [ΔP_0.1] to the change P_aCO2 (we call this ratio the hypercapnic-respiratory-drive response).

RESULTS: Thirty-six patients failed the SBT, and 11 patients failed extubation. The mean values for the SBT/extubation-success group, the extubation-failure group, and the SBT-failure group, respectively, were: ratio of hypercapnia-test P_0.1 to baseline P_0.1: 4.3 ± 2.7, 3.7 ± 1.3, and 3.0 ± 1.8 (P = .03); hypercapnic ventilatory response: 0.60 ± 0.35 L/min/mm Hg, 0.50 ± 0.26 L/min/mm Hg, and 0.31 ± 0.21 L/min/mm Hg (P < .001); hypercapnic respiratory-drive response: 0.48 ± 0.24 cm H₂O/mm Hg, 0.42 ± 0.19 cm H₂O/mm Hg, and 0.27 ± 0.15 cm H₂O/mm Hg (P < .001). For predicting SBT/extubation success, the sensitivities and specificities, respectively, were: ratio of hypercapnia-test P_0.1 to baseline P_0.1 0.80 and 0.47; hypercapnic ventilatory response 0.86 and 0.53; hypercapnic respiratorydrive response 0.82 and 0.55.

CONCLUSIONS: The SBT/extubation-failure patients had less response to the hypercapnia test than did the SBT/extubation-success patients, and the hypercapnia test was not useful in predicting SBT or extubation success.