Bradycardia classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:

Overview

Bradycardia is a decrease in the heart rate due to abnormalities in the atria, AV node or ventricles.

Classification

Classification of Bradycardia According to The Origin of Impulse

Bradyarrhythmia

The origin of the impulse: Atria

The origin of the impulse: AV junction

The origin of the impulse: Ventricles

Sinus node dysfunction: Respiratory sinus arrhythmia Sinus bradycardia Sinoatrial block Sinus arrest Sick sinus syndrome Normal variant: Respiratory sinus arrhythmia

Abnormality in the atria: Low atrial focus bradycardia Atrial bigeminy

AV node dysfunction: First degree AV block Second degree AV block Complete or third-degree AV block

Junctional escape rhythm Junctional bigeminy

Isorhythmic A-V dissociation Slow VT (idioventricular rhythm) Ventricular escape rhythm Escape capture bigeminy

Atria

Respiratory Sinus Arrhythmia

• Respiratory sinus arrhythmia, is usually found in young and healthy adults.
• The heart rate increases during inhalation and decreases during exhalation.
• This is thought to be caused by changes in the vagal tone during respiration.
• If exhalation is associated with a drop in the heart rate below 60 bpm with each breath, this type of bradycardia is usually deemed benign and is considered a sign of good autonomic tone.

Sinus Bradycardia

• Sinus bradycardia is a sinus rhythm of less than 60 bpm. It is a common condition found in both healthy individuals and those who are considered well-conditioned athletes.
• Studies have found 50-85% of conditioned athletes have benign sinus bradycardia, as compared to 23% of the general population studied.
• The heart muscle of athletes has become conditioned to have a higher stroke volume, so it requires fewer contractions to circulate the same volume of blood.

Sick Sinus Syndrome

• Sick sinus syndrome covers conditions that include:
  • Severe sinus bradycardia, sinoatrial block, sinus arrest, and bradycardia-tachycardia syndrome (atrial fibrillation, flutter, and paroxysmal supraventricular tachycardia).

AV Junction

• An atrioventricular nodal bradycardia or junctional escape rhythm is usually caused by the absence of the electrical impulse from the sinus node.
• This usually appears on an EKG with a normal QRS complex accompanied with an inverted P wave either before, during, or after the QRS complex.
• An AV junctional escape is a delayed heartbeat originating from an ectopic focus somewhere in the AV junction. It occurs when the rate of depolarization of the SA node falls below the rate of the AV node.
• Dysrhythmia also may occur when the electrical impulses from the SA node fail to reach the AV node because of SA or AV block.
• This is a protective mechanism for the heart, to compensate for an SA node that is no longer handling the pacemaking activity, and is one of a series of backup sites that can take over pacemaker function when the SA node fails to do so.
• This would present with a longer PR interval. A junctional escape complex is a normal response that may result from excessive vagal tone on the SA node. Pathological causes include sinus bradycardia, sinus arrest, sinus exit block, or AV block.

Ventricles

• Ventricular bradycardia, also known as ventricular escape rhythm or idioventricular rhythm, is a heart rate of less than 50 bpm.
• This is a safety mechanism when there is a lack of electrical impulse or stimuli from the atrium. Impulses originating from or below the His bundle, also known as ventricular, will produce a wide QRS complex with heart rates between 20 and 40 bpm.
• Those above the His bundle, also known as junctional, will typically range between 40 and 60 bpm with a narrow QRS complex.
• In a third degree heart block, approximately 61% take place at the bundle branch-Purkinje system, 21% at the AV node, and 15% at the His bundle.
• AV block may be ruled out with an EKG indicating "a 1:1 relationship between P waves and QRS complexes. Ventricular bradycardias occurs with sinus bradycardia, sinus arrest, and AV block."
• Treatment often consists of the administration of atropine and cardiac pacing.

Infantile Bradycardia

• For infants, bradycardia is defined as a heart rate of less than 100 bpm (normal is around 120-160).
• Premature babies are more likely than full-term babies to have apnea and bradycardia spells; their cause is not clearly understood.
• Some researchers think the spells are related to centers inside the brain, that regulate breathing, which may not be fully developed.
• Touching the baby gently or rocking the incubator slightly will almost always get the baby to start breathing again, which increases the heart rate.
• Medications (theophylline or caffeine) can be used to treat these spells in babies if necessary.
• NICU standard practice is to electronically monitor the heart and lungs for this reason.