Hello, welcome everybody, this is doctor Bolad, board certified physician in cardiology, interventional
cardiology and internal medicine, certified by the American Board of Internal Medicine.
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Today I will talk to you about slow heart rate. The normal heart beats at a rate of
sixty to one hundred beats per minute. The pulse is generated by an electrical stimulus
that originates in the sinoatrial node, known as SA node for short. This node is located
in the right atrium, which is the right upper chamber of the heart. The right and left atria
are then activated and contract, and the electrical impulse travels down to the atrioventricular
node, known as the AV node for short. The AV node is located in the lower part of the
right atrium. In the AV node, the pulse is delayed for a few milliseconds, and then the
pulse travels down through the right and left bundles conduction system into the right and
left ventricles, leading the ventricles to contract and eject blood causing the pulse
to be generated, as shown in this demonstration. In the vast majority of patients, mild slowing
of the heart beating does not cause symptoms. When the heart slows down further, symptoms
related to the slow heart rate can occur. These include lightheadedness, presyncope
or syncope with loss of consciousness, worsening of angina in patients who suffer from coronary
artery disease, heart failure symptoms in patients who suffer from heart failure, cognitive
slowing, and exercise intolerance. Symptoms may be subtle, with many patients noting only
fatigue, which is frequently ascribed to aging rather than bradycardia. There is no specific
heart rate below which all patients develop symptoms, as cardiac output will vary depending
upon the ability to increase the blood volume output from the heart, related to underlying
conditioning and other diseases. Slowing of the heart can occur when normal
conduction of the heart is maintained, and this is referred to as Sinus Bradycardia.
It can also occur when there is blockage in conduction of electrical activity from the
upper part to the lower part of the heart, and this is referred to as heart block.
Sinus bradycardia can occur in healthy individuals in the following physiological conditions.
- In normal healthy children and adults, particularly during sleep when rates may transiently drop
as low as 30 beats per minute, and pauses of up to two seconds are not uncommon.
- Well-conditioned athletes, particularly in endurance athletic activities,
- Some elderly individuals, although this may represent an early manifestation of disease
in pulse generation, medically referred to as sinus node dysfunction.
Sinus bradycardia can also occur in the following non-physiological conditions:
- Sinus node dysfunction, also referred to as sick sinus syndrome, characterized by dysfunction
of the sino-atrial node due to disease of the node and surrounding atrial tissue.
- Medications – A number of medications can depress the sino-atrial node and slow
the heart rate. Examples of these medications include beta blockers, calcium channel blockers,
digitalis, antiarrhythmic medications and others.
- Heart Attack – Sinus bradycardia occurs in 15 to 25 percent of patients with acute
myocardial infarction, particularly those involving blockage in the right coronary artery.
- Obstructive sleep apnea – Individuals with obstructive sleep apnea frequently have
slow heart rate that may be severe during apneic episodes, sometimes leading to a pulse
rate as low or less than 30 beats per minute. - Exaggerated vagal nerve activity – as
occurs in vomiting, coughing,straining and carotid sinus stimulation.
- Brain conditions such as increased intracranial pressure , stroke, or trauma to the cervical
or thoracic spine. - Infectious causes – such as Lyme disease,
Chagas disease, legionella and others - Other causes – such as occurs in under
active thyroid function referred to as hypothyroidism, also occurs in anorexia nervosa, hypothermia,
and severe prolonged hypoxia. Bradycardia can also occur in the presence
of normal sinus node function due to block in conduction of electrical activity from
the Atrioventricular Node, leading to reduction of conducted impulses from the upper part
of the heart to the lower part of the heart. These heart blocks can be due to:
- Unknown causes due to progressive disease in the atrioventricular node and the conduction
system to the ventricles. - Ischemic heart disease – Blockage of the
coronary arteries account for about 40 percent of cases of AV block
- Cardiomyopathies such hypertrophic cardiomyopathy and infiltrative diseases such as amyloidosis
and sarcoidosis. - Myocarditis in conditions that lead to inflammation
of the muscle of the heart. - Patients with Congenital heart disease can
develop varying degrees of heart block. - There is also a familial form of heart block,
which is characterized by a progression in the degree of AV block in association with
a variable apparent site of block, and it is genetically transmitted.
- Miscellaneous causes, such as under active thyroid function, potassium and electrolyte
imbalance, neuromuscular disease and rheumatological conditions.
The treatment of slow heart rate depends on the cause of the bradycardia and whether or
not it is causing symptoms. If symptoms occur, and the bradycardia can not be reversed, then
patients would need an implantable permanent pacemaker.
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This is Doctor Bolad, helping you with your heart health, thanks for watching, and talk
to you soon.