A study was conducted by Dr. Frank Hu, Dr. W.C. Willett, Dr. G.A Colditz and their team undertook a study to see the correlation between snoring and cardiovascular disease (CVD) in women. This was done specifically, as most of the other related studies were focused on men.
CVD or cardiovascular disease is an umbrella term used to define conditions that affect the heart and related blood vessels system. It is associated with increased risk of blood clots and atherosclerosis (build-up of deposits in the arteries). There are various types of CVD that includes coronary heart disease, aortic diseases and stroke.
Details On The 8-Year Nurses’ Health Study
As per the results of this study, they published a journal paper – “Snoring and Risk of Cardiovascular Disease in Women” in the prestigious Journal of the American College of Cardiology. In many of the epidemiologic studies, snoring has been frequently associated with a high risk of ischemic heart disease, hypertension and stroke. Though the extract reason isn’t clear but it is usually associated with Obstructive Sleep Apnea. In an independent study done by Dr. JWJ Shepard, the results were published in the medical journal, Clinical Chest Medicine in 1992, “Hypertension, cardiac arrhythmias, myocardial infarction and stroke in relation to obstructive sleep apnea.” It was found that there was a connection between snoring and cardiac issues.
Other studies found a correlation between sleep apnea which leads to hypoxemia and increases the risk of atherosclerosis. Conversely, it was found that though people snore, but not all of them have sleep apnea.
As per Dr. Frank Hu and his team of eminent doctors, they investigated the association of snoring and CVD during a period of 8 years of follow-up of those women that has been a part of the Nurses Health Study. A similar cohort study was conducted in the past by another team of scientists that solely focused on men and was followed up for only 3 years.
Methods of study
In 1976, the Nurses Health Study was established with 121,700 registered nurses that volunteered to be a part of the program. They were aged between 30 to 55 years and were living in 11 states in USA. Each of them completed a questionnaire that gave details about their lifestyle and medical history. Every two years a follow up was done and new questionnaires were sent for updated information, health risks and identify diagnosed cases of cardiac issues and other ailments.
In 1986, the last questionnaire was sent out where the nurses were asked to give details about the total hours of sleep in 24 hours, and their sleeping positions. Based on the 8 years data, analysis was done.
Additionally, 71,779 women in the age group of 40 to 65 years were included in 1986 that did not have any diagnosed cancer or CVD. This year is considered to be the baseline for analysis. As per collected data, in 1972, all the nurses were married and it was reassessed in 1980.
The end point analysis was done starting from 1986 to June 1994. The study included the diagnosis of non-fatal myocardial infraction or coronary heart disease, non-fatal and fatal stroke. During this period, permission was requested to evaluate the medical records of women that were diagnosed with stroke and/or non-fatal MI in the follow up questionnaires. It was given to doctors that were not aware of the study’s self-reported risks. All of the diagnosis were evaluated as per the World Health Organization’s standards. Stroke was confirmed on the basis of the National Survey of Stroke criteria that required neurological defects, sudden onset that lasted for twenty-four hours or more. These included hemorrhagic stroke, ischemic stroke and those of unknown causes. All deaths were reported by the family and were verified. Fatal coronary heart disease (CHD) was confirmed by hospital records and autopsy reports.
The grouping was done based on never snoring, occasional snorers and regular snorers. This was aggregated and incidence rates were calculated. The method of person-time exposure was done by dividing the number of events by the personal time in each category.
The RR or relative risk was calculated as exposure divided by the reference category with age adjustments. Using pooled logistic regression for multivariate analysis, they included:
- Age in intervals of 5 years
- Body Mass Index (BMI)
- Smoking status – 1 to 14 cigarettes a day, 15-24 cigarettes a day and 25 cigarettes a day
- Alcohol consumption
- Physical activity levels
- Menopausal status – (postmenopausal without hormone replacement, premenopausal, postmenopausal with past/current hormone replacement)
- Family history of early MI,
- Parental history of hypercholesterolemia/diabetes
- Hours of sleep and sleeping position.
In primary analysis, there is no data on the history of hypertension although snoring is one of associated risks. The development of hypertension is an intermediary step towards development of CVD. This study took into account the rotating night shift as assessed in 1988 that was associated with the CVD in the Nurses Health Study, as per the journal paper- “Prospective study of shift work and risk of coronary heart disease in women” by Dr. I. Kawachi I, Dr. GA Colditz and Dr. MJ Stampfer.
Results of the study
In the 8 years of follow-up, the authors of the study were able to document 644 cases of cardiovascular disease and 398 cases of stroke. Looking at the sleep pattern baseline on a per day basis, the results showed:
- 5% were sleeping less than 5 hours
- 26% were sleeping 6 hours
- 41% got 7 hours of sleep
- 28% slept for 8 hours
- 5% got to sleep for 9 hours or more
Additional data that was computed showed that –
- 7% slept on their back
- 11% slept on their front
- 81% slept on their side
Of those nurses that participated in the study, 65% were occasional snoring, 25% of women never snored, while only 10% regular snoring.
Considering the marital status, and duration of shift hours did not affect the parameters. However, snoring was related to obesity and that did affect the influence of BMI on snoring. Additionally, there was a link between snoring and CVD when it came to effect of smoking and BMI, though sleeping positions were a non-issue.
After taking into account smoking, age, risk of heart diseases and BMI, occasional snorers have 20% chances of developing CVD while regular snorers had 33% chances of getting CVD. Considering cross-sectional analysis, smoking and BMI remained as key risks associated with snoring in all groups.
Thus, their findings were similar to other studies that focuses solely on men. Cardiovascular diseases and stroke are associated with snoring to a certain level.
In the Wisconsin Sleep Cohort Study, undertaken by Dr. T. Young, Dr. L. Finn and their team, a cross-section study of 580 adults, it was found that snoring is associated with hypertension, irrespective of gender, age and BMI.
Thus, this study by Dr. Frank Hu and his team, observed that there was significant risk of developing CVD among occasional snorers. It supports the theory that even those that do not develop sleep apnea are at risk of developing sleep disordered breathing continuum. Thus, snoring inadvertently affects health and leads to higher risk of CVD.