Hypercapnic Respiratory Failure in Obesity-Hypoventilation Syndrome: CO₂ Response and Acetazolamide Treatment Effects

OBJECTIVE: In obesity-hypoventilation-syndrome patients mechanically ventilated for hypercapnic respiratory failure we investigated the relationship between CO₂ response, body mass index, and plasma bicarbonate concentration, and the effect of acetazolamide on bicarbonate concentration and CO₂ response.

METHODS: CO₂ response tests and arterial blood gas analysis were performed in 25 patients ready for a spontaneous breathing test, and repeated in a subgroup of 8 patients after acetazolamide treatment. CO₂ response test was measured as (1) hypercapnic drive response (the ratio of the change in airway occlusion pressure 0.1 s after the start of inspiratory flow to the change in P_aCO2), and (2) hypercapnic ventilatory response (the ratio of the change in minute volume to the change in P_aCO2).

RESULTS: We did not find a significant relationship between CO₂ response and body mass index. Patients with higher bicarbonate concentration had a more blunted CO₂ response. Grouping the patients according to the first, second, and third tertiles of the bicarbonate concentration, the hypercapnic drive response was 0.32 ± 0.17 cm H₂O/mm Hg, 0.22 ± 0.15 cm H₂O/mm Hg, and 0.10 ± 0.06 cm H₂O/mm Hg, respectively (P = .01), and hypercapnic ventilatory response was 0.46 ± 0.23 L/min/mm Hg, 0.48 ± 0.36 L/min/mm Hg, and 0.22 ± 0.16 L/min/mm Hg, respectively (P = .04). After acetazolamide treatment, bicarbonate concentration was reduced by 8.4 ± 3.0 mmol/L (P = .01), and CO₂ response was shifted to the left, with an increase in hypercapnic drive response, by 0.14 ± 0.16 cm H₂O/mm Hg (P = .02), and hypercapnic ventilatory response, by 0.11 ± 0.22 L/min/mm Hg (P = .33).

CONCLUSIONS: Patients with obesity-hypoventilation syndrome and higher bicarbonate concentrations had a more blunted CO₂ response. Body mass index was not related to CO₂ response. Acetazolamide decreased bicarbonate concentration and increased CO₂ response.