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 PaCO2), and (2) hypercapnic ventilatory response (the ratio of the change in minute volume to the change in PaCO2). 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.
Raurich et al. (Mon,) studied this question.