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Article #75

The authors studied a subgroup of 17 patients with obstructive sleep apnea who also had evidence of nocturnal bradyarrhytmia (slowing of heart rate) during ambulatory sleep studies. These derived from a total of 239 patients with sleep apnea, the great majority of which (222, or 93%) showed no evidence of such an arrhythmia. They compared these two groups on various measures with an aim to predicting what factors might predict the occurrence of bradyarrhytmias in a population of sleep apnea patients.     Those with bradyarrhytmia also received more intensive inpatient sleep studies on two successive nights, along with 48 hours of Holter monitoring (ambulatory monitoring of electrocardiogram). Medication affecting heart rate was stopped a week before.     Comparison of sleep apnea patients with and without bradyarrhytmia showed that those with arrhythmias were (1) more obese (average Body Mass Index of 39 versus 30); and (2) more severely affected by sleep apnea as measured by the Respiratory Distress Index (RDI=90 versus 24).     In most (15/17) of those patients with nocturnal bradyarrythmia, there was no sign of heart rhythm abnormalities while awake. Bradyarrhytmias occurred only during sleep. These varied considerably in frequency and severity. All subjects showed episodes of slowing, but these ranged from only a few occurrences in six of the 17 subjects, to hundreds of occurrences in four. Six subjects showed a more severe arrhythmia, second-degree heart block, and three showed one still more severe, third-degree block. The episodes of arrhythmia occured in association with episodes of apnea or hypopnea, most often (67%) during Rapid Eye Movement or REM sleep, least often (0.3%) during deep or slow-wave sleep. The arrhythmic episodes were associated with lower levels of blood oxygen than were apnea/hypopnea episodes without arrhythmias, but there was no clear-cut correspondence between the two, nor any threshold value of oxygen below which arrhythmias occurred.     The authors considered that their major findings were as follows: Patients with apnea plus bradyarrhytmias were more obese, and had higher Respiratory Distress Indices of apnea severity, than patients with apnea without arrhytmias. The bradyarrhythmias occurred mainly during REM sleep.     They noted that seven patients were referred for pacemakers because of the severity of their arrhytmias, though none were symptomatic (for example, none had fainting spells).     They suggested that the association of arrhytmias with REM sleep might result from excessive physiological response to changes in arterial blood pressure, based on oversensitivity of receptors for blood pressure. Note that in normal people, the heart rate will slow as blood pressure rises, as a way of reducing strain on the heart. The phenomenon of bradyarrhthmia in apnea patients probably also relates to to excessive activity of the vagal nerve, which normally carries messages to slow heart rate.

The occurrence of heart arrhythmias in association with episodes of sleep apnea/hypopnea is, of course, added cause for concern that obstructive sleep apnea be treated, lest dangerous consequences on the cardiovascular system ensue. As the authors point out from their own large group of sleep apnea patients, almost half had arterial hypertension, itself a risk factor for more dangerous cardiovascular consequences, such as heart attacks and strokes. The specter of heart arrhytmias as a more immediate result of sleep disordered breathing only adds to concern. However, we should bear in mind that only a small minority of sleep apnea patients show this complication. The concurrent association with obesity and severity of apnea makes it more likely that these people will get treated. On the other hand, we should note that their arrhythmias failed to show up on daytime monitoring of the electrocardiogram, even though this went on for extended periods of time.     Therefore, people at high risk for such a complication of sleep apnea cannot reassure themselves with a normal reading on a routine electrocardiogram, usually covering only a few minutes in the waking state. Nor can a relatively slight degree of blood oxygen desaturation associated with sleep disordered breathing offer reassurance that arrhythmias cannot occur. Only examination of heart rhythm throughout sleep, especially during apneas and hypopneas associated with REM sleep, can rule out the occurrence of this type of arrhythmia. The fact that seven patients were referred for pacemakers underscores the seriousness of the arrhythmias, even when apparently not causing symptoms.

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