By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program
of the Cardiovascular Institute at Mount Sinai School of Medicine, New York.

For some years it has been suspected that interruptions of the normal breathing pattern during the night, manifested by snoring and sleep apnea (apnea means stopped breathing) are associated with high blood pressure. The unresolved question has been whether the sleep-disordered breathing, as it is technically called, is causing the hypertension, which is characterized by high blood pressure during the day and night. One of the problems has been that both sleep-disordered breathing and hypertension are related to obesity, so it could be that the obesity is the cause of both. However, recent studies have found a connection between sleep-disordered breathing and hypertension even when obesity is not present. Another problem is that the existing studies have all been cross-sectional, that is made at one point in time, so that it is not possible to say which condition comes first.

An analysis of the Wisconsin Cohort Study has now provided an answer. This is a survey of several hundred government employees in Wisconsin, 700 of whom have been studied twice over a 4-year period. On each occasion their blood pressure was measured by 24 hour ambulatory monitoring, and their sleep and breathing patterns were monitored on a second night. Another 184 subjects were studied a third time, 8 years after the first study.

The main finding was that people with even the mildest form of sleep-disordered breathing (which is measured as the number of episodes of interrupted breathing per hour of sleep)- of less than 5 episodes per hour, had a 42% greater chance of becoming hypertensive 4 years later than people with no interruptions. For those with more frequent interruptions (5 to 15 per hour) the risk of hypertension was doubled, and for those with more severe sleep-disordered breathing (more than 15 per hour) the risk was tripled. The analysis accounted for the effects of many of the other factors, which are thought to influence the development of hypertension, including obesity and age.

Doctor’s Comments

The importance of this study is that it is the first prospective study to show a relation between sleep-disordered breathing and the onset of hypertension that is independent of obesity. One of the interesting findings is that the effects of interrupted breathing during the night are graded- the more common the interruptions, the greater the risk of developing hypertension. Even very minor interruptions, occurring at a rate of once every 15 minutes and lasting only 30 seconds or so, can have an effect. These are not changes that are likely to be noticed either by the patient or by his or her sleeping partner. Full-blown sleep apnea, which results in major snoring and daytime sleepiness, may be manifest by one interruption every 2 minutes. How these minor changes can influence blood pressure throughout the whole 24 hours remains a mystery.

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