Bradycardia is a drop in heart rate below 60 beats per minute, which causes tissue hypoxia through poor blood circulation. Male individuals, athletes and performance athletes physiologically have a lower heart rate compared to the normal range, this feature is not accompanied by specific clinical manifestations. Bradycardia occurs as a result of a deficit in the generation or transmission of electrical impulses in the myocardium.
When should we go to the doctor?
Under resting conditions, the heart rate is between 70 and 90 beats per minute in healthy adults
During physical exertion, the pulse rate represents the difference between 220 and the age of the person concerned.
Physiological variations in the pulse rate, which depend on a person’s gender, allow the resting heart rate in men to be between 50 and 80 beats per minute, and in women between 60 and 100 beats per minute, due to the peculiarities of the parasympathetic nervous system (parasympathetic regulation causes the pulse rate to decrease, and sympathetic regulation causes the heart rate to increase).
The pump function of the heart propels blood from the ventricular to the vascular system of the body as follows:
- The right ventricle pumps non-oxygenated blood to the lung via the pulmonary artery
- The left ventricle pumps oxygenated blood to the aorta
Blood is taken up by the atria as follows:
- The right atrium receives non-oxygenated blood via the superior and inferior vena cava
- The left atrium receives oxygenated blood via the 4 pulmonary veins
At the cardiac level, blood circulates separately in the left and right heart separated by the interatrial and interventricular septum respectively, as follows: the right atrium takes the venous blood of the great circulation and distributes it to the right ventricle via the tricuspid valve to be pumped further to the lung. The left atrium takes oxygenated blood from the lung via the pulmonary veins and distributes it to the left ventricle via the mitral valve to be pumped to the aorta.
Causes of bradycardia
The main causes involved in the decrease in heart rate are:
- Abnormal generation of electrical impulses in the sinus node or their defective conduction along the electrical network of the heart
- Cardiac damage associated with the evolution of certain pathologies
- Acute myocardial infarction (cardiac tissue necrosis involving partial loss of pump function)
- Metabolic abnormalities leading to changes in serum potassium, magnesium, sodium and chloride concentrations
- Pulmonary thromboembolism
- Toxic/septic shock states
- Sleep apnea syndrome
- In some situations, bradycardia may be caused by the administration of drug therapies based on calcium blockers, digoxin or alpha-adrenergic agonists
Bradycardia risk factors
The main risk factors that favor the onset of bradycardia are:
- Older age
- Smoking
- Psycho-emotional stress
- Pathologies that interfere with the functionality of the heart’s electrical system: coronary artery disease, myocarditis, endocarditis, sarcoidosis, Lyme disease, hypothyroidism
- Excessive alcohol consumption and use of illicit substances
Bradycardia symptom
Mild or moderate bradycardia may evolve asymptomatically or with minimal manifestations limited to:
- Decreased exercise tolerance
- Increased fatigue or weakness
- Vertigo (dizziness)
- Headache
- Blurred vision
- Heavy sweating
- Palpitations (irregular heartbeat)
Severe bradycardia is frequently associated:
- Acute hypotension
- Acute alteration of mental status manifested by temporal-spatial disorientation, lethargy, psychomotor agitation, psychosis, attention deficit or hallucinations, along with angina pectoris (chest pain), cyanosis (blueness of the skin due to tissue hypoxia) in the extremities and loss of consciousness
When should we go to the doctor?
Chest pain, altered mental status, cyanosis or loss of consciousness, especially in the case of a person with a history of heart disease, require prompt referral to the emergency medical service or cardiology. If treatment is not promptly instituted, severe bradycardia may progress to heart failure and acute myocardial infarction.
Sinus bradycardia
Bradycardia can be classified according to the causes that favor its occurrence and their specific location as follows:
- Sinus bradycardia due to damage to the sinus node at which the electrical impulses of the heart are generated. Damage to this physiological pacemaker of the heart generates the appearance of a slow sinus rhythm, with a value of less than 60 beats per minute and can occur both in physiological conditions and secondary to other cardiac or metabolic pathologies.
- Bradycardia associated with aging occurs gradually under physiological conditions, without being generated by specific pathologies. Bradycardia in the elderly without specific manifestations is considered normal and does not require curative treatment.
- Sinus node disease is a condition that evolves with the occurrence of bradycardia or tachycardia secondary to a disease of the sinus node at which the electrical impulses of the heart are generated. Also known as sick sinus node syndrome, this condition can lead to benign sinus bradycardia (no potential for adverse outcome) to sinus arrest or bradycardia-tachycardia syndrome which manifests as paroxysmal atrial tachyarrhythmias in patients diagnosed with sinus bradycardia or sinoatrial block.
- Atrioventricular blocks are disorders of electrical impulse conduction that occur in the context of functional or organic cardiac abnormalities. They can be transient or permanent. Atrioventricular blocks can be of type I, II or III depending on the degree of interruption of impulse propagation from the atria to the ventricles and the severity of the patient’s manifestations.
Low heart rate in children
Physiological heart rate in children varies according to age, so in this population group bradycardia is defined as:
< 100 beats per minute in the newborn
< 90 beats per minute in infants aged 6 months to 1 year
< 80 beats per minute in children aged 2 to 3 years
< 70 beats per minute in children aged 4 to 5 years
< 60 beats per minute from the age of 10 years.
The main causes of bradycardia in children are:
- Congenital heart defects
- conduction disorders in the sinus node
- The administration of certain anxiolytic or beta-blocker therapies to treat certain conditions
- Thyroid pathologies involving hypothyroidism
- Hydroelectrolyte disturbances of various aetiologies favoring a decrease in serum potassium, sodium and calcium concentrations
- Central nervous system disorders that can cause faulty regulation of the heart rhythm through the action of the parasympathetic vegetative system
- Preterm infants are more prone to heart rhythm disorders such as bradycardia, apnea and desaturation which can reach up to 83% due to insufficient development of the cardiovascular and pulmonary system
Bradycardia treatment
The treatment of bradycardia varies according to the etiology (the cause of its occurrence) and the severity of the patient’s symptoms. Mild forms of bradycardia with minimal symptoms have a favorable evolution and do not require the institution of drug treatment.
The etiological treatment of bradycardia occurring in the context of thyroid diseases or hydroelectrolytic imbalances benefits from individualized treatment addressed to the underlying conditions.
Bradycardia of iatrogenic cause (medical cause) caused by certain drugs is improved by changing the doses of active substances or replacing them with other types of pharmaceutical preparations according to the indications of the attending physician.
Cardiac malformations that evolve with bradycardia require corrective surgical treatment of the minimally invasive or classical type, depending on the specifics of each case.
Cardiac pacemakers are reserved for patients with bradycardia due to a nerve impulse conduction deficit in the heart with an increased risk of adverse outcomes. The role of pacemakers is to continuously monitor the patient’s cardiac activity and to deliver low-intensity electric shocks in the presence of decreased heart rate.
Complications of bradycardia
Untreated, bradycardia can evolve unfavorably leading to cardiac and respiratory failure, acute myocardial infarction and injury during episodes of loss of consciousness.
In the long term, bradycardia affects the quality of life of the patient who is forced to limit his daily activities because of the feeling of fatigue and permanent weakness that occurs in the context of tissue hypoxia.
The main methods of preventing bradycardia are similar to those applied to avoid most cardiac pathologies and include:
- Maintaining an active lifestyle through regular physical activity
- Adopting a healthy diet
- Keeping body weight under control
- Drinking alcohol in moderate amounts and giving up smoking
References:
- Bradycardia, Mayo Clinic
- Cardiac asystole: A manifestation of neurally mediated hypotension-bradycardia, Science Direct
- Evaluation and management of bradycardia in neonates and children, European Journal of Pediatrics
- The Evaluation and Management of Bradycardia, J. Michael Mangrum, John P. DiMarco, The New England Journal of Medicine
