I was interested in the letter by Claudio A. Rabec (October 2002),1–
who mentions some important points. As Dr. Rabec suggests, the obesity-hypoventilation syndrome (OHS) needs a stricter definition. It should include only obese subjects with chronic daytime hypercapnia, when other causes of hypercapnia such as severe COPD are excluded. Awareness of OHS is poor among clinicians, with many patients treated for asthma or COPD—often with the inappropriate use of steroids and bronchodilators.2
I differ with the author in his suggestion that individuals with obstructive sleep apnea be excluded from the diagnosis of OHS. This would effectively eliminate many, if not most, obese patients with unexplained chronic hypercapnia. Sleep apnea syndrome may well be, in part, secondary to the chronic hypoventilation central to the OHS. Many subjects with obstructive sleep apnea are not hypercapnic; however, most obese subjects with daytime hypercapnia will prove to have obstructive sleep apnea. For instance, in an ongoing study at our institution, defining OHS as a body mass index > 30 and a daytime Paco2 of > 45 mm Hg in the absence of COPD, all 18 patients had obstructive sleep apnea. As the author points out, hypoventilation in obese subjects is complex. Only a few obese subjects have chronic hypoventilation, and the body mass index does not correlate well with this. Not all obese subjects have sleep apnea, and even fewer have daytime hypercapnia when causes such as COPD are excluded. Obese patients, who have other causes for hypercapnia such as severe COPD excluded, should have a diagnosis of OHS regardless of whether they have obstructive sleep apnea.