Sinus bradycardia (or simply bradycardia) refers to the reduction in heart rate below the reference range. In the adult, the heart at rest beats at an optimal frequency of about 70-80 beats per minute, but it is considered:

• Normal: resting heart rate between 60-99 beats per minute (bpm);
• Tachycardia – resting heart rate above 100 bpm;
• Bradycardia: the resting heart rate below 60 bpm.

In the neonatal period the heart rate is instead higher (normal intervals between 90-180 bpm are considered), decreasing with age (up to 10 years, normal intervals between 70-110 bpm are considered).

Based on the entity, bradycardia can be considered:

• Mild : heart rate 50-59;
• Moderate: heart rate 40-49;
• Severe: heart rate less than 40.

Bradycardia is an important clinical condition because:

• It can be secondary to various causes, but most of the time it is not associated with any underlying pathology (physiological bradycardia);
• It is usually not a dangerous condition, however it can sometimes be the cause of a noticeable reduction in the blood supply to peripheral or central organs (such as the brain), with the risk of injury;

• Most of the time it occurs completely asymptomatically, however at times it can be responsible for the onset of symptoms such as

• Dizziness _
• Syncope,
• Loss of consciousness;

• Most of the time it does not require any treatment, but in symptomatic or severe cases it is possible that it may be necessary to undertake a targeted therapy;

Sinus bradycardia is an easily diagnosed clinical condition, in fact, the patient himself can perceive his own frequency by palpating the number of beats per minute of the radial (wrist) or carotid (neck) artery; in the presence of bradycardia, it is advisable to consult a doctor, to rule out that there are triggering causes.

Review of physiology

In order for the heart to contract, an electrical impulse is needed that spreads uniformly in the heart and stimulates its contraction.

The impulse for cardiac contraction originates at the level of the sinoatrial node, located at the level of the right atrium.

From this structure the electrical impulse propagates to the level of the atrioventricular node, located between the atria and the ventricles.

Modified cardiac fibers originate from the atrioventricular node capable of transmitting the impulse directly to the ventricles.

The sinoatrial node has an intrinsic physiological discharge capacity between 60 and 99 bpm, therefore it acts as a cardiac pacemaker, dictating the rate of contraction of the heart.

The discharge rate of the sinoatrial node can be modified by several factors, the most important being the nervous regulation by the nervous system:

Sympathetic (adrenergic and noradrenergic tone): determines an increase in heart rate;

Parasympathetic (vagal tone): causes a reduction in heart rate.

CAUSES

Bradycardia is a very common condition, especially in young people and in athletes. A second peak of incidence is found in old age, in which bradycardia can be physiological, or secondary to a heart disease.

Bradycardia can be:

Physiological: it is the most frequent, an isolated medical condition, encountered occasionally, in the absence of an underlying cardiac or systemic pathology;

Pathological or secondary, when it derives from the presence of other cardiac or systemic pathologies.

Physiological bradycardia occurs:

In the young subject, especially in athletes who perform aerobic activities (especially runners, cyclists and swimmers), in which the heart rate can even reach 30-40 bpm without giving significant clinical manifestations. This is linked to an increase in vagal tone , which slows the heart rate.

In the elderly, in which a modest bradycardia can be considered physiological.

During sleep, vomiting , valsalva maneuver (increase intra-abdominal pressure, as when preparing to perform an effort): in this phase the sympathetic tone is reduced and the parasympathetic increases, therefore there is also a modest and physiological reduction of heart rate.

Pathological bradycardia , on the other hand, can be secondary to:

• Heart disease.
• Systemic diseases.
• Substance intake.
• Bradycardia secondary to heart disease

Heart conditions that can cause bradycardia can be:

Sino-atrial node disease: it is a cardiac pathology defined by the alteration of the heart impulse formation. This condition is very common in the elderly, as the cardiac conduction system gradually reduces its activity due to aging. The result is a reduction in the speed of cardiac stimulation which determines bradycardia, up to in some cases causing asystole (absence of the heart impulse), which can last even for a few seconds. This condition usually occurs asymptomatically and is occasionally found during a cardiological visit; sometimes it can be symptomatic, especially when asystole of more than 3 seconds is found, and is called sick sinus syndrome.

Atrioventricular block: it is a cardiac pathology determined by an alteration of the transmission of the cardiac impulse from the sinoatrial node (where the impulse arises normally), to the atrioventricular node. This can determine

A delay in the passage of impulses from the atria to the ventricles,

Failure to transmit one or more impulses from the atria to the ventricles,

Or a complete dissociation between the atrial heart rate (which is normal) and the ventricular heart rate (which is what is perceived by palpating the peripheral pulses, which will be reduced).

Myocardial infarction : heart attacks, especially those of the lower wall, can be the cause of a dysfunction of the cardiac conduction system, causing bradycardia.

Myocarditis, and endocarditis: these are pathologies characterized by inflammation of the heart muscle or its innermost lining. Inflammation can damage the conduction system, leading to bradycardia.

Bradycardia secondary to systemic diseases

Sinus bradycardia can be caused by systemic conditions, including:

• Hypothyroidism: reduced thyroid function can also reduce heart rate.
• Electrolyte disturbances: in particular, the increase in potassium can be the cause of a reduction in heart rate.
• Intracranial hypertension: the increase in pressure inside the cranial theca, secondary to many pathologies (for example brain tumors, and meningitis ) can consequently trigger a reduction in heart rate.
• Hypothermia: the noticeable drop in body temperature can be associated with a reduction in heart rate.
• Infectious diseases: fever usually causes an increase in heart rate, however, some infections such as typhoid fever and brucellosis can cause it to decrease.
• Bradycardia secondary to substances
• Bradycardia secondary to substance intake is mainly related to drugs such as
• Β-blockers,
• Calcium channel blockers
• And digoxin,
• used to treat heart disease or hypertension, which also works by reducing the heart rate.

Nocturnal bradycardia

The presence of nocturnal bradycardias during sleep can be suggestive of the presence of an apnea disorder, especially in the case of patients characterized by the presence of specific risk factors such as:

• Overweight or outright obesity,
• Male sex,
• Old age,
• Familiarity,
• High blood pressure.

On the other hand, it should be remembered that in young and/or sports subjects it is common to find even very low values, equal to 30-40 beats per minute , sometimes even below 30  (cases, in any case, to be checked with the doctor).

SYMPTOMS

Clinically, bradycardia can be:

Asymptomatic: it is the most frequent condition, especially in young and healthy subjects;

Symptomatic: sometimes the reduction in heart rate can be such as to cause a profound reduction in cardiac output (amount of blood pumped by the heart in one minute) and therefore a reduction in organ perfusion. Therefore bradycardia may present with symptoms such as:

• Dizziness _
• Vision disturbances (blurred vision or bright flashes),
• Chest pain,
• Confusion,
• Syncope,
• Numbness of the hands and feet,
• Sense of cold,
• Difficulty breathing,
• Fatigue during physical activity,
• Asthenia,
• Chest pain.

COMPLICATIONS

In cases of:

Severe bradycardia,

• Asystole, more than 3 seconds,
• In the elderly subject, with impaired basic heart function,
• there may be a noticeable reduction in the perfusion of the central nervous system, which can cause irreversible brain damage ( stroke ), more or less extensive, depending on the severity of the reduction in perfusion.
• In symptomatic bradycardias, complications secondary to syncope (eg head trauma ) may occur.
• Among the most serious consequences we remember instead:
• Frequent fainting,
• The inability of the heart to pump enough blood ( heart failure ),
• Sudden cardiac arrest or sudden death.

DIAGNOSIS

The diagnosis of sinus bradycardia is very simple.

The first level exams include:

• Anamnesis: it is important to collect information relating to the symptoms, or to any pathologies the patient suffers from.
• Physical examination: it will be sufficient to palpate the peripheral pulses (mainly at the radial or carotid level), to find the reduction in the frequency of the heartbeat.
• Ecg: the execution of the electrocardiogram is essential to identify any anomalies in the tracing, which may be indicative of the dysfunction of the cardiac conduction system. In subjects with physiological bradycardia, the ECG will show no noticeable alteration.
• Second level examinations are performed in some cases, as a diagnostic study:
• Holter-ECG: it is an exam that provides for the continuous recording of the electrocardiographic trace for 24 hours; this examination is necessary for patients who have occasional bradycardia phenomena, which cannot be identified in the short period in which the visit is held.
• Echocardiography: Cardiac ultrasound may be necessary when a cardiac disease is suspected that cannot be studied with a simple ECG (myocarditis, endocarditis).

Blood sampling: needed to detect electrolyte or thyroid hormone abnormalities, as a trigger for bradycardia.

When previous investigations do not show alterations, a diagnosis of physiological bradycardia can be made.

TREATMENT AND REMEDIES

Treatment varies according to the severity of the bradycardia and the underlying cause.

Lifestyle changes: in mild or non-symptomatic bradycardias, it is preferable not to start any therapy, especially in young and healthy subjects, instead of promoting the adoption of healthy lifestyles:

• Do not smoke,
• Do not drink excessive amounts of alcohol,
• A healthy and balanced diet,
• Regularly  engage in aerobic physical activity,
• Maintain an optimal body weight for age and height.

• Pace-maker: in symptomatic bradycardias, or in severe bradycardias, especially in elderly subjects with compromised heart function, it is preferred to treat the bradycardia with the implantation of a pacemaker. It is a device implanted subcutaneously and connected to the heart by means of a catheter, which works by stimulating the heart’s contraction at a programmed rate. With the pacemaker implant, the patient will be able to lead a normal life and will have to undergo periodic cardiological checks. You will also need to inform the medical staff that you have a pacemaker before undergoing tests such as magnetic resonance imaging.

In the presence of a basic condition, it must obviously be treated, for example through:

• Correction of electrolyte imbalances;
• Correction of thyroid function;
• Discontinuation or revision of the dosage of bradycardic drugs.