What causes sudden cardiac arrest in sleep?

When you are asleep, you are not aware of what’s happening in your body and your surroundings. “It’s like you are under anesthesia,” says the Kolkata-based Dr MK Das, President of the Asian Heart Society.

So, what happens to the heart when you sleep? Usually, when you sleep, the heart rate slows down.

How does sleep affect your heart rate?

Dr. Kapil Kumawat, Senior Consultant Cardiology – Adult, Electrophysiology, Narayana Health, says, “There is a decrease in sympathetic outflow and increase in parasympathetic outflow.” Sympathetic system controls fight-or-flight response, while the parasympathetic system controls the rest and digestion process. The subconscious body activities are governed by these autonomic nervous systems and hormones. 

Dr Nileena N.K.M, Consultant, Nithra sleep clinic, Chennai, explains, “The cardiovascular system works differently in both rapid eye movement (REM) and non-rapid eye movement (NREM) sleep stages. In NREM sleep, the parasympathetic system is more active and reduces the heart rate and BP. But in REM sleep, there are bouts of sympathetic activity, causing an increase in heart rate. Sometimes, it might be the same as when you are awake, even though it is a deeper stage of sleep.”

Why does cardiac arrest happen in sleep?

Experts say people with underlying heart diseases are more at risk of sudden cardiac arrest in sleep. These include people who have suffered heart attack before, valvular heart disease (any valve of the heart is damaged or diseased), myocarditis with cardiac failure, certain dyselectrolytemia (sodium-potassium imbalance) and chronic kidney disease.

“Sudden electrical disturbances in the heart can lead to cardiac arrest in sleep. There are some genetic disorders of the heart in which persons, including young children, may die in their sleep,” says Dr Kumawat.

Dr Das, also the representative (Asia), assembly of international governors of the American College of Cardiology (ACC), adds that ventricular tachycardia and ventricular fibrillation are the main causes of sudden cardiac arrest in sleep. “There could be sudden blockage of coronary arteries, causing muscular dysfunction and different electrical activities in the form of ventricular tachycardia. While sleeping, you lack the defence mechanism to rise, cough or seek help. Hence, the risk of death is higher when asleep,” he says.

Obstructive sleep apnea (OSA) could also be a reason for sudden death in sleep. Ms Nilena says, “A person with severe OSA is at risk of high BP and when struggling for oxygen, the sympathetic system gets more active and puts your heart at more strain.”

Other risk factors, says Dr Das, include stress and severe viral infections like Covid-19 as it can lead to tear in the coronary arteries. “This can happen to both young and old people. Those with pre-existing comorbidities like diabetes, cancer, hypertension and myocarditis are at higher risk,” he adds.

Will a heart attack wake you?

Cardiologists say that one may not feel any pain or discomfort when they suffer a heart attack in sleep. Dr Das says, “Brain function is tuned to being asleep. You do not have any perception about what is happening around you.” Dr Kumawat adds that since the blood supply to the brain is cut off immediately due to cardiac arrest, one falls unconscious quickly and is unable to seek help.

Dr Nileena says a person may not feel a silent heart attack while sleeping. However, it may wake them up if it causes severe chest discomfort or pain.

Most cardiac events like these tend to occur in the early morning hours during REM sleep, say experts. Dr Nileena says, “The NREM and REM sleep cycles keep alternating. When you sleep, you first enter NREM sleep and then REM sleep in the first one and a half to two hours of sleep. Then, you slip into the NREM sleep again. As the night progresses, the duration of the REM cycle increases. So, it’s usually in the early morning hours that we tend to see such cases.”

Dr Das adds, “By 3 am, all hormones get surged as humans naturally shift to lighter sleep. The bad hormones responsible for arrhythmias are also released. Hence, the early morning hours from 3 am to 5 am are the most vital time.”

Best sleeping position for heart health

Experts say there is no specific sleep position which has been proven to benefit the heart but sleeping on your left side might help. Dr Das says, “Heart is usually on the left side while veins and arteries are on the right. So, if you lie on your left side, your veins and arteries will be free and your blood circulation to the heart may be more. If you sleep on your back (supine position), it can block the airway. If you sleep on your chest (prone sleep position), your chest may be compressed which also impairs your breathing.”

How to avoid cardiac arrest when asleep?

Dr Das says the most important thing is lifestyle changes. “There are 7 S’s to remember – avoid salt, sedentary lifestyle, smoking and ensure systolic BP control, systemic cholesterol control, sugar control, stress management.”

Dr Nileena adds that snoring should not be taken lightly. Many think that if you snore, you are sleeping well. But it is important to get your snoring checked and treated, especially if it is getting severe and causing breathing difficulties. “OSA can reduce oxygen levels and affect the heart. It feels like you are drowning in your sleep,” she says.

Takeaways

• People with underlying heart diseases are more at risk of sudden cardiac arrest in sleep.
• Ventricular tachycardia and ventricular fibrillation are the main causes.
• Obstructive sleep apnea can also lead to low oxygen, putting the heart at stress, during sleep.
• Most of these cardiac events tend to occur in the early morning hours during REM sleep.
• There is no specific sleep position which has been proven to benefit the heart but sleeping on the left side might help.
• Lifestyle modifications and treating obstructive sleep apnea can help reduce risk of cardiac arrest in sleep.