Obesity Hypoventilation Syndrome: Early Detection of Nocturnal-Only Hypercapnia in an Obese Population

Study Objectives: Hypoventilation in obesity is now divided into five stages; stage 0 (pure obstructive sleep apnea; OSA), stages I/II (obesity-related sleep hypoventilation; ORSH) and stages III/IV (awake hypercapnia, obesity hypoventilation syndrome; OHS). Hypercapnia during the day may be preceded by hypoventilation during sleep. The goal of this study was to determine the prevalence and to identify simple clinical measures that predict stages I/II ORSH. The effect of supine positioning on selected clinical measures was also evaluated. Methods: Ninety-four patients with a body mass index > 40 kg/m(2) and a spirometric ratio > 0.7 were randomized to begin testing either in the supine or upright seated position on the day of their diagnostic sleep study. Arterialized capillary blood gases were measured in both positions. Oxygen saturation measured by pulse oximetry was also obtained while awake. Transcutaneous CO2 monitoring was performed during overnight polysomnography. Results: Stages I/II ORSH had a prevalence of 19% in an outpatient tertiary hospital setting compared with 61%, 17%, and 3% for stages 0, III/IV, and no sleep-disordered breathing respectively. Predictors for sleep hypoventilation in this group were an awake oxygen saturation of <= 93% (sensitivity 39%, specificity 98%, positive likelihood ratio of 22) and a partial pressure of carbon dioxide >= 45 mmHg (sensitivity 44%, specificity 98%, positive likelihood ratio of 24) measured in the supine position. Conclusions: ORSH has a similar prevalence to OHS. Awake oxygen saturation and partial pressure of carbon dioxide performed in the supine position may help predict obese patients with sleep hypoventilation without awake hypercapnia. Commentary: A commentary on this article appears in this issue on page 1455.