Do chronic obstructive pulmonary disease (COPD) patients that snore have an increased risk of obstructive sleep apnea?

Obstructive sleep apnea (OSA) affects around 4% of middle-aged men (Young et al 1993). With this high prevalence of OSA and the rising worldwide increase in morbidity and mortality in chronic obstructive pulmonary disease (COPD) (WHO 2000), it is not surprising that research attentions have focused on the overlap between these two highly prevalent conditions with the aim of determining their relationship. A number of studies have investigated the association between OSA and COPD. In patients with COPD, studies have shown that as the depth of sleep increases so there is a reduction in minute ventilation with an increase in upper airway resistance (Ballard et al 1995) with up to 20% patients with severe COPD exhibiting co-existent OSA (Brander et al 1992). Although the Sleep Heart Health Study (Sanders et al 2003) found that OSA prevalence was not increased in mild COPD, undiagnosed airways obstruction can be present in ≥10 percent of patients with OSA (Lin and Huang 1992). These observations may simply reflect that the clinical relevance of the overlap between COPD and OSA only become apparent when one or both of these conditions are severe.

In this issue, Krieger and colleagues (2007) report on the respiratory disturbances that occur during sleep in COPD patients with mild airways obstruction. These researchers highlight that the differences in the literature pertaining to the incidence of nocturnal respiratory disturbance in COPD may be a consequence of inadequate techniques previously used to measure these events. Krieger and colleagues (2007) hypothesized that using more detailed measurements, such as measuring the arousal on the electroencephalogram in response to inspiratory flow limitation, or so called respiratory-effort related arousals (RERAs), in addition to standard respiratory measurements, COPD patients with daytime normoxia would have a greater severity of respiratory disturbance during sleep compared with non-COPD patients. However, in contrast to the original hypothesis, the authors showed that in a selected group of well-nourished (mean body mass index [BMI] 31 kg/m²) COPD patients with mild airways obstruction (mean forced expiratory volume in one second [FEV₁]/forced vital capacity [FVC] 63%), who were referred to a sleep centre to investigate snoring, the severity of respiratory disturbance and nocturnal hypoxemia were similar to patients without airways obstruction. In fact, the COPD patients overall had a tendency to have lower respiratory disturbances during sleep.

Are these data novel and useful? Although not wholly novel, these data identify a number of useful clinical messages. In the group of patients studied, RERA measurements adds little to the standard respiratory measurements of flow and thoraco-abdominal movement, the apnea-hypopnea index (AHI), and so for the majority of cases it is acceptable to use AHI, which is technically less cumbersome to perform. In addition, COPD patients with mild airways obstruction referred to sleep centers for further investigation of snoring have a similar severity of sleep-disordered breathing as compared with aged and BMI matched controls without COPD. These centres, therefore, can manage these COPD patients in a similar manner to other patients. Finally, the observation by Krieger and colleagues (2007) that the severity of airways obstruction, as evidence by FEV₁/FVC ratio, was inversely related to BMI, but AHI directly related to the severity of airways obstruction is an interesting result. Furthermore, these investigators showed that BMI is less predictive of the variance in AHI in COPD than those without COPD. These findings need some further discussion as body composition demographics in patients with COPD and OSA can be distinctly different. Indeed, it is established that there is a direct relationship between BMI and severity of OSA in patients without COPD such that a change in BMI is associated with change in AHI (Noseda et al 2006). In contrast, although body composition correlates with severe disease in COPD, there is an indirect relationship between body composition and disease severity such that a low BMI predicts poor outcome in COPD (Schols et al 2005). This adds to the complexity of the clinical problem and the results of the current study suggest that the correlations between severity of airways obstruction, BMI and severity of OSA do not necessarily extend to patients with COPD. In patients with COPD, one could predict that as airways obstruction increases so BMI would fall and thus AHI would decrease. However, this has still to be proven as the current study only investigated COPD patient with mild airways obstruction and preserved BMI. Despite AHI having 65% dependence on BMI, AHI had 85% dependence on airways obstruction, suggesting there could be greater relative importance of airways obstruction than body composition in COPD patients. More research comparing nocturnal respiratory disturbances in mild COPD and severe COPD is required, with attention directed on the effect of body composition, severity of OSA and severity of airways obstruction.