Snoring and heart attack risk

A study has found that “heavy snorers are six times more likely to suffer a heart attack while asleep”, the Daily Express reported. It said the risk of heart disease could be increased by blood pressure, and nerve and hormonal changes caused by snoring, and that people who suffer from obstructive sleep apnoea are most at risk. The newspaper quoted the British Snoring and Sleep Association which said that anyone who snores should get treated immediately.

The study investigated the time of day that heart attack patients experienced their first symptoms of pain, and if they had signs of having obstructive sleep apnoea. It did not look at whether snoring leads to heart attacks. This research alone does not provide conclusive evidence. However, the researchers also refer to associated research, and a growing body of evidence that suggests that obstructive sleep apnoea can cause acute coronary syndromes, such as heart attacks. People should bear in mind that one of the symptoms of obstructive sleep apnoea is heavy snoring, but for accurate diagnosis a full set of sleep studies are required.

Where did the story come from?

Dr Fatima H. Sert Kuniyoshi and colleagues from the Division of Cardiovascular Diseases at the, Mayo Clinic and Foundation in Minnesota and from the Federal University of Espirito Santo, Vitoria, Brazil carried out the research. The study was supported by several grants, including grants from the Respironics Sleep and Respiratory Research Foundation, and National Institutes of Health.

The study was published in the peer-reviewed medical journal: the Journal of the American College of Cardiology.

What kind of scientific study was this?

In this case-control study, the researchers compared people who had their first heart attack symptoms during the sleeping hours (midnight to 6.00am) with a group who had their symptoms during the day (6.00am to midnight). They were interested in comparing the frequency of obstructive sleep apnoea in these two groups.

The patients were recruited to the study when they had been admitted to the researcher’s hospital with myocardial infarction (heart attack). A diagnosis of heart attack was confirmed by standard indicators (a rise in cardiac enzymes and in a marker of heart muscle damage called troponin T). The time at which the heart attack began was given by the patient. The researchers excluded those patients who did not give this information or who were uncertain. They also excluded those who had atypical chest pain, and those who had been previously treated for obstructive sleep apnoea. They said that although consecutive patients were eligible, recruitment was based on these exclusion criteria, on the availability of research personnel, and on the patient’s consent to participate.

All the participants underwent comprehensive polysomnography, a test for diagnosing obstructive sleep apnoea, about two to three weeks after their heart attack. This requires an overnight stay in a sleep lab where the number of times a person pauses in their breathing is continuously monitored, along with the oxygen levels in their blood. The researchers recorded an apnoea-hypopnea index (AHI) for all the participants. This score is an index of severity that combines pauses in breathing with reduced depth of breathing, and gives an indication of the disruptions and desaturations (a low level of oxygen in the blood). Those who scored five or more events per hour on this index were defined as having obstructive sleep apnoea.

What were the results of the study?

Ninety-two patients (71 men) with an average age of 61 years and a body mass index of 30 kg/m2 were selected and, using a threshold of AHI of five events per hour, obstructive sleep apnoea was diagnosed in 70% of them. People with obstructive sleep apnoea were older and more likely to have diabetes, heart failure and high cholesterol.

Heart attacks occurred between midnight and 6.00am in 32% of patients with obstructive sleep apnoea, and 7% of patients without it. Patients who had heart attacks between midnight and 6.00am were six times as likely to have obstructive sleep apnoea as those who had heart attacks during the other 18 hours of the day (95% confidence interval: 1.3 to 27.3). Of the patients who had a heart attack between midnight and 6.00am, 91% had obstructive sleep apnoea.

What interpretations did the researchers draw from these results?

The researchers point out that the novel finding of this study is that patients with obstructive sleep apnoea have an increased risk of heart attack between midnight and 6.00am compared with patients without the condition. They say that the “data suggest that obstructive sleep apnoea may be a trigger for heart attack, with a striking reversal in the expected diurnal timing of heart attack onset.” Meaning that as most heart attacks usually start in the daylight hours this is unexpected.

What does the NHS Knowledge Service make of this study?

The strengths and weaknesses of this study are described by the researchers:

• The study’s main limitation is in the selection process that was used to select the patients that took part. It was not randomised, and more people in the study had obstructive sleep apnoea (70%), than would be expected in the general population. This suggests that some sort of selection bias occurred, reducing the reliability of the results.
• Despite the researchers claim that the two groups were well-balanced, they showed a strong tendency towards other selection biases. For example, all five of the participants with heart failure occurred in those with obstructive sleep apnoea, and they also tended to be older, diabetic, and to have higher blood pressure, cholesterol and weight. It is possible that the severity of the underlying coronary heart disease can play a part in determining when pain begins.
• This study was conducted in patients who survived heart attacks and the researchers comment that their findings may not necessarily apply to people who die from cardiac disease.
• The confidence interval described in the results is wide and reduces confidence in the finding that people admitted to hospital with heart attacks between midnight and 6.00am are six times as likely to have obstructive sleep apnoea as those admitted at other times of the day.

Overall this study confirms and further defines the relationship between obstructive sleep apnoea and heart attack. However, because of the small numbers of patients and the way that they were selected for the study, it is not possible to be completely confident that the strength of the association approaches a six-fold increase in risk.

Two suggestions by the researchers deserve attention: that people with the onset of MI during sleep hours should be evaluated for obstructive sleep apnoea, and that the interventions known to treat obstructive sleep apnoea should be further researched to test whether they are effective in preventing heart attacks and sudden cardiac death.