Individuals with obstructive sleep apnea (OSA) have recurrent partial or complete upper airway obstruction during sleep. The effects of OSA extend beyond sleep, however, and physicians are increasingly recognizing OSA as an independent risk factor for hypertension and cardiovascular disease. Specifically, approximately half of patients with OSA have hypertension that corresponds with the severity of OSA.
Accumulating scientific evidence also suggests that having OSA predisposes patients to cardiovascular disease, although researchers have not been able to describe a precise mechanism for the association. Experimental, epidemiological and clinical trial studies have identified serum levels of uric acid as a potential cardiovascular risk factor. Moreover, studies have shown that serum uric acid levels are elevated in patients with OSA.
Asiye Kanbay, MD, an associate professor in the department of pulmonary medicine at Istanbul Medeniyet University School of Medicine in Turkey, and colleagues evaluated the connection between uric acid and OSA. They published the results of their cohort study in the European Journal of Internal Medicine, June 2014.1
For the study, Kanbay et al recruited patients who had undergone a formal sleep study for the diagnosis of OSA between January 2007 and December 2012. A total of 364 patients with mild, moderate or severe OSA, as well as 72 controls, were included in their analysis. Patients were classified on the basis of the apnea-hypopnea index (AHI) score, with AHI cutoffs being: <5 OSA events per hour for controls, 5 to 14.9 events for mild OSA, 15 to 29.9 events for moderate OSA and ≥30 events per hour for severe OSA.
Dr. Kanbay and colleagues questioned patients about cardiovascular comorbidity, including prior history and any established diagnosis of coronary artery disease, cerebrovascular accident, congestive heart failure due to coronary artery disease and arrhythmias. They also gathered information about any diagnosis of diabetes and/or hypertension, use of antihypertensive medications and body mass index. A routine blood chemistry panel, including serum levels of creatinine, alanine aminotransferase and uric acid, was also performed on all patients.
The investigators found that patients with severe OSA had higher levels of uric acid than patients in other groups. A regression analysis showed that uric acid levels were positively associated with AHI score. Hypertension rose and its prevalence increased as the severity of OSA increased.
The authors report that patients with severe OSA were more likely to have cardiovascular events. This association was not true for the other categories of sleep apnea. Researchers also found that individuals with any level of OSA were more likely to have elevated uric acid levels if they had experienced a cardiovascular event. The authors concluded that the relationship between hyperuricemia and sleep apnea is quite strong in this patient population and persists even after controlling for established risk factors of cardiovascular disease.
Dr. Kanbay and colleagues emphasize that the data are correlative and that it is not yet possible to conclude that uric acid is the cause of the increased cardiovascular risk. The data do suggest, however, that lowering uric acid may be one approach to decreasing the number of cardiovascular events in patients with OSA. (posted 7/18/14)
Lara C. Pullen, PhD, is a medical writer based in the Chicago area.