Sinus bradycardia

Sinus bradycardia is one of the main causes of bradycardia, or slow heartbeat.

The electrical circuit of the heart is connected to four chambers, two upper (atria) and two lower (ventricles).

The heart rhythm is determined by a natural pacemaker: the sinus node. It is located in the right atrium.

The sinus node generates electrical impulses that lead to the contraction of the heart muscle and thus to a heartbeat.
Starting from the sinus node, the electrical impulses pass through the atria, causing contraction and the pumping function of blood into the ventricles.
After that, the electrical impulses reach a group of cells called the atrioventricular nodes (AV nodes).
The AV node conducts the signal to some special cells called His bundles.
These cells conduct the signal downwards to a left leg that serves the left ventricle of the heart and a right thigh that serves the right ventricle of the heart.
When an electrical impulse passes through these thighs, the heart chambers contract and pump the blood.
The right ventricle carries deoxygenated blood to the lungs and the left ventricle sends the oxygen-rich blood throughout the body.
Bradycardia occurs when the electrical signals are slowed down or blocked.

In sinus bradycardia, the slowdown occurs because the sinus node reduces its electrical impulses that make the heart beat.

Children have more heartbeats at rest than adults, especially when they are young.
Thus, bradycardia can be defined in an infant at a frequency of less than 100 beats per minute.


Types of fascicular block and its importance

The left leg of the bundle of His divides into 2 bundles: anterior and posterior bundle.
When a line is blocked by one of the legs, this is called an anterior or posterior hemiblock.
The anterior hemiblock is more common and causes the left axial deviation on the ECG.
The posterior hemiblock causes the right axial deviation in the electrocardiogram.

Bifascicular block
This is the combination of a right femoral block and a posterior or anterior left hemiblock.
The ECG shows a right femoral block and a hemiblock on the left, which can be anterior or posterior.

Trifascicular block
This occurs when the bifascicular block is caused by a heart block of the first degree.

Symptoms of sinus bradycardia

If the sinus node generates a heart rate that is too slow for the needs of the body, then symptoms may occur.
Most often, sinus bradycardia is not considered dangerous unless it is symptomatic.

With a slow heartbeat, the brain and other organs cannot receive enough oxygen.
As a result, bradycardic symptoms may occur, such as:

  • Fainting (syncope)
  • Vertigo
  • Weakness
  • Fatigue
  • Shortness of breath
  • Chest pain
  • Memory problems and mental confusion

When is a slow heart rate normal?

A resting frequency of the heart of less than 60 beats per minute can be physiological in some people, especially healthy adults, adolescents and athletes who exercise
Often athletes also have early repolarization, which is the regression of the electrical excitation of the heart at rest, which occurs too soon after contraction.
In others, a frequency below 60 may be caused by vagal hypertension (since the atrioventricular node is innervated by the vagus nerve), by some medications, or pathological.
A heartbeat of 56, 57 or 58 bpm is not within the limits, but is borderline and investigations should be carried out, because it could also be too low.

Causes of bradycardia

Many conditions can lead to problems with the heart’s electrical system, including:

  • Tissue damage associated with aging
  • Heart disease or infarction
  • Congenital heart disease (congenital cardiomyopathy)
  • Infection of the heart (myocarditis)
  • Complication in cardiac surgery
  • Insufficient thyroid activity (hypothyroidism)
  • Imbalance of electrolytes, minerals and substances required for electrical conduction of pulses
  • Sleep apnea, the patient has repeated interruptions in breathing during sleep
  • Inflammatory diseases such as rheumatic fever or lupus
  • Medications, for example medicines for cardiac arrhythmias, arterial hypertension and psychosis

Problems of the sinus node

As a rule, bradycardia begins in the sinus node. A slow heart rate can occur because the sinus node:

  • transmits electrical impulses more slowly than normal;
  • pauses or cannot rhythmically maintain the impulses;
  • discharges an electrical impulse that blocks before triggering the contraction of the atria;
  • in some people, as a result of the problems of the sinus node, there is an alternation between accelerated heartbeat and slow heartbeat (tachycardia-bradycardia syndrome).

Atrioventricular block

Bradycardia can also occur because the electrical signals do not spread from the atria to the ventricles (heart block or AV block).
The interruption of the electrical signal can occur in the AV node, in the bundle of His, or elsewhere along the right and left legs that conduct the electrical signals to the ventricles.

Classification of the line block

AV block of the first degree. In the lightest form of the heart block, the electrical signals from the atria reach the ventricles, but the signal is slightly delayed.
First-degree heart block rarely causes symptoms and does not need to be treated if there are no other abnormalities in electrical conduction transmission.

AV block of second degree. In the second degree heart block, not all electrical signals reach the ventricles.
Some impulses are lost, the result is a slower and sometimes irregular rhythm.

Heart block of the third degree (complete). In the case of heart block of the third degree, no electrical impulse can pass from the atria to the ventricles of the heart. If this happens, the bundle of His and the other ventricular tissues take over the pacemaker function for the ventricles. These alternative tissues work slowly and sometimes send irregular electrical impulses.
In this case, it is called sinus arrhythmia.

Femoral block. The interruption of the electrical signal in parts of the right or left leg that occurs at the end of the electrical impulse path is called the leg block. The severity of the disease depends on whether both legs are affected, whether other types of a heart block are present, and what the degree of damage to the heart tissue is.

Risks and complications

The complications of untreated bradycardia depend on:

  • the slowness of the heartbeat,
  • the place where the problem of electrical conduction occurs,
  • the severity of tissue damage to the heart.

If bradycardia is so pronounced that it causes significant symptoms, the possible complications are:

  1. frequent fainting,
  2. lack of pumping ability of the heart (heart failure),
  3. sudden cardiac arrest or sudden death,
  4. High blood pressure (hypertension).

Bradycardia in pregnant women deprives the mother and fetus of the oxygen they need.
The mother may experience symptoms such as fainting, chest pain, weakness, fatigue and shortness of breath.
If this is not treated, it can lead to fetal bradycardia, complications during childbirth, death of the fetus and premature birth of the child.

Diagnosis of sinus bradycardia

The doctor prescribes a series of examinations:

  1. measurement of heart rate,
  2. determining the relationship between slow heart rate and symptoms,
  3. Identification of the diseases that can cause bradycardia.

Electrocardiogram (ECG)
The electrocardiogram is an important tool for evaluating bradycardia. An ECG uses small sensors (electrodes) attached to the chest and arms to register the electrical signals that pass through the heart.
The device records curves with the intensity of the flowing current.
The doctor may look for patterns in these signals to determine the type of bradycardia.
The doctor may order to wear a mobile ECG device at home as well to get more information about heart rate and to establish a link between slow heart rate and symptoms.

These devices include:
Holter-EKG. This portable ECG device is carried in a bag or on a shoulder strap.
It can record cardiac activity around the clock during the entire time it is worn and gives the doctor an idea about the heart rhythm.
The doctor will probably ask to keep a symptom diary during the 24 hours of the examination.
If one knows the symptoms and the corresponding time, one can understand the connection with the abnormality of the blows registered in the Holter ECG.

Event recorder. This mobile ECG device is used to record cardiac activity over a few weeks or months.
If the patient feels symptoms, he has to press a button and the ECG registers an ECG strip of a few minutes.
This allows the doctor to determine the heart rhythm when symptoms occur.

Other investigations
The doctor will order a blood test to find out if diseases cause bradycardia, for example, an infection, hypothyroidism (hypothyroidism) or electrolyte imbalance.
If sleep apnea is suspected, examinations can be carried out that record the sleep situation.

Therapy of sinus bradycardia

Treatment of diseases that cause
bradycardia If a disease causes bradycardia, for example hypothyroidism or sleep apnea, disease treatment can correct bradycardia.

The doctor may prescribe medications to increase HR, for example:

  • Atropine
  • Isoprenaline

Variations of medications in iatrogenic bradycardia

Some medications can cause bradycardia, for example, those used for other heart conditions.

The doctor checks the medications that are taken and may prescribe other medicines.
By changing medications or reducing the dose, the problem of low heart rate can often already be solved.
For example, oxymetazoline could cause an increase in arterial blood pressure, which can lead to reflex sinus bradycardia due to vagal activation.
If no alternative therapy is possible and the symptoms are severe, there are indications for a pacemaker.


A pacemaker is a battery-powered device, about the size of a small cell phone, that is implanted under the collarbone.
The wires of the device lead to the heart.
At the end of the wires are electrodes that are attached to the heart tissue.
The pacemaker controls heart rate and releases electrical impulses to maintain heart rhythm.
Today, there are devices without cables (wireless) that are used directly in the ventricle.
The doctor goes through the femoral artery with the circulation to the heart and attaches the battery there.
This procedure offers various advantages, for example, no general anesthesia is needed, there is no incision in the chest and the risk of infection is reduced.

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