To the Editor:
In the recent issue of European Respiratory Journal, Kiely and McNicolas have sophisticatedly demonstrated that the increased risk of future cardiovascular diseases is independent of obstructive sleep apnoea (OSA) in patients with obstructive sleep apnoea syndrome (OSAS) 1. Although many investigators have reported that there is a significant link between OSA and the cardiovascular events, they insisted that the patients with OSAS are at high risk of future cardiovascular diseases from factors, i.e. diabetes mellitus, hypertension, hypothyroidism, hyperlipidemia, other than OSAS. In addition, these independent risk factors of cardiovascular diseases are not associated with OSAS severity, i.e. increasing apnoea/hypopnoea index (AHI) level 1. The results are very interesting, but could not be entirely accepted. It has been reported that OSA is associated with hypertension, independent of the confounding factors of age and obesity and that nondipping of 24 h blood pressure profile is related to apnoea severity 2. Indeed the nocturnal high blood pressure in patients with OSAS is easily controlled by the nasal continuous positive airway pressure, but hardly by ordinary hypertensive therapy 3. Interestingly, Weichler et al. have reported that the treatment of hypertension may reduce the frequency of sleep disordered breathing 4. Furthermore, two recent studies have suggested there is a significant causal relationship between hypertension and sleep-disordered breathing. Peppard et al. reported a dose-response association between sleep-disordered breathing at baseline and the presence of hypertension 4 yrs later, that was independent of known confounding factors 5. They also suggested that persons with minimal sleep-disordered breathing (less than AHI 5) had a higher odds ratio of hypertension than those with no episodes of sleep-disordered breathing. Bixler et al. have also reported that sleep-disordered breathing is independently associated with hypertension when potential confounders including age, BMI, sex, menopause, alcohol use and smoking were controlled for in the logistic regression analysis 6. Although the mechanism of hypertension and cardiovascular events in OSAS patients are not fully elucidated, it has been suggested that abnormalities in cardiovascular variability is implicated in the subsequent development of overt cardiovascular disease in patients with OSA 7. It has recently been reported that blood pressure measured in young adult men is positively associated with increased cardiovascular mortality in later life 8.
Considered together, it is reasonable to speculate that OSA per se contributes to raised blood pressure in adulthood, and is implicated in the pathogenesis of cardiovascular events in later life.
- Received October 27, 2000.
- Accepted October 27, 2000.
- © ERS Journals Ltd