Hypercapnia test as a predictor of success in spontaneous breathing trials and extubation

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

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

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 [DeltaV(E)] to the change in P(aCO(2)) (we call this ratio the hypercapnic ventilatory response); and the ratio of the change in P(0.1) [DeltaP(0.1)] to the change P(aCO(2)) (we call this ratio the hypercapnic-respiratory-drive response).

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 H(2)O/mm Hg, 0.42 +/- 0.19 cm H(2)O/mm Hg, and 0.27 +/- 0.15 cm H(2)O/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 respiratory-drive response 0.82 and 0.55.

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.