Ventilatory responses to hypercapnia and hypoxia in relatives of patients with the obesity hypoventilation syndrome

Background-It is unclear why some morbidly obese individuals have waking al veolar hypoventilation while others with similar obesity do not. Some evide nce suggests that patients with the obesity hypoventilation syndrome (OHS) may have a measurable premorbid impairment of ventilatory chemoresponsivene ss. Such an impairment of ventilatory chemoresponsiveness in OHS, however, may be an acquired and reversible consequence of severe obstructive sleep a pnoea (OSA). We hypothesised that, in patients with OHS who do not have coi ncident severe OSA, there may be a familial impairment in ventilatory respo nses to hypoxia and hypercapnia. Methods-Sixteen first degree relatives of seven patients with OHS without s evere OSA (mean (SD) age 40 (16) years, body mass index (BMI) 30 (6) kg/m(2 )) and 16 subjects matched for age and BMI without OHS or OSA were studied. Selection criteria included normal arterial blood gas tensions and lung fu nction tests and absence of sleep apnoea on overnight polysomnography. Vent ilatory responses to isocapnic hypoxia and to hyperoxic hypercapnia were co mpared between the two groups. Results-The slope of the ventilatory response to hypercapnia was similar in the relatives (mean 2.33 l/min/mm Hg) and in the control subjects (2.12 l/ min/mm Hg), mean difference 0.2 l/min/mmHg, 95% confidence interval (CI) fo r the difference -0.5 to 0.9 l/min/mm Hg, p=0.5. The hypoxic ventilatory re sponse was also similar between the two groups (slope factor A: 379.1 l/min mm Hg for relatives and 373.4 l/min mm Hg for controls; mean difference 5. 7 l/min mm Hg; 95% CI-282 to 293 l/min mm Hg, p=0.7; slope of the Linear re gression line of the fall in oxygen saturation and increase in minute venti lation: 2.01 l/min/% desaturation in relatives, 1.15 l/min/% desaturation i n controls; mean difference 0.5 l/min/% desaturation; 95% CI -1.7 to 0.7 l/ min/% desaturation, p=0.8). Conclusion-There is no evidence of impaired ventilatory chemoresponsiveness in first degree relatives of patients with OHS compared with age and BMI m atched control subjects.