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Heart rate is considered normal when it is 60 to 100 beats per minute at rest. Bradycardia is considered to exist when the resting heart rate is less than 60 per minute. It can be due to several reasons as listed below:
It is a situation when the electrical impulse does not pass down from the atria (upper chamber of the heart) to the ventricle (lower chamber of the heart). In this case, the lower chamber activity is completely dependent on its own escape beat which is usually very low, approximately 30 to 40 per minute. This is commonly due to age-related degeneration of the AV node through which the impulse passes from the atria to the ventricle. This usually happens in the elderly age group (beyond the age of 70). Rarely it can happen at a younger age also. Congenital complete heart block is a situation when the above said situation persists from birth. Other conditions which can give rise to complete heart block are ischemic heart disease, myocarditis, post valve replacement surgery due to inadvertent injury to AV node, drug-induced etc.
In this situation impulse from the upper chamber passes down to the lower chamber erratically. At times it passes down normally and at times gets blocked giving rise to a low ventricular rate. This also happens commonly due to age-related degeneration of the AV node.
In this situation, the sinus node, a region in the right upper chamber from where the impulse originates, is weak giving rise to slower impulse generation at a rate of less than 60 per minute. This is known as sick sinus syndrome when the slow impulse generation is not due to any secondary factor. The secondary factors which can reduce the sinus rate are drug-induced sinus bradycardia, hypothyroidism, electrolyte imbalance, myocarditis, increased intracranial pressure due to any reason etc. In young athletes, due to predominant vagal nerve activity, the heart rate tends to remain low (between 40 to 60 per minute). This is a normal physiological situation and in the absence of any symptoms does not warrant any treatment.
There are various symptoms due to a low heart rate.
Syncope: Transient loss of consciousness leading to falling is known as syncope. Low heart rate can cause this.
Giddiness, lightheadedness, transient blackout: These are various symptoms patients can present because of low heart rate.
Easy fatigue or tiredness: Due to low heart rate the patient sometimes complains of easily getting tired even if the workload is minimal. Often the patient complains of not being able to walk a short distance and needs to take rest often.
If the slow heart rate is due to any secondary cause, then the correction of that problem can restore normal heart rate. Sometimes some medicines like atropine are administered intravenously to increase heart rate acutely if sinus bradycardia is noted as an emergency measure. If it is age-related degeneration or irreversible damage of the sinus or AV node then pacemaker implantation becomes necessary. A permanent pacemaker is a small device, which is implanted below the skin and fat tissue beneath the collar bone. One or two leads are passed through a vein into the right upper and lower chamber and fixed to the inner layer of the heart muscle. The leads are connected to the pacemaker. Pacemakers can sense the heart's electrical activity. As long as it senses the heart's electrical impulse it remains in the standby mode and if it does not sense then generates an electrical impulse artificially to stimulate heart muscle thereby maintaining normal heart rate.
(Inputs by Dr Santosh Kumar Dora, Senior Cardiologist, Asian Heart Institute, Mumbai)
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