Parasystole

# Overview
Parasystole is a type of arrhythmia caused by the presence and function of a secondary pacemaker in the heart, which works in parallel with the SA node. Parasystolic pacemakers are protected from depolarization by the SA node by some form of entrance block. This block can be complete or incomplete.
Parasystolic pacemakers can exist in the atrium or the ventriculus. Atrial parasystolia are characterized by being a type of narrow QRS complexes Arrhythmia.

# Historical Perspective
- Parasystole was first reported by Schamroth in 1967.
- Parasystole as an entity was studied by Kaufmann and Bothberger, but their studies are now interpreted differently.

# Classification
- Parasystole may be classified according to location of their origin but in literature no such classification mentioned.
- Parasystole consists of stimultaneous activity of two (rarely more) independent impulse-forming centers, one of which is "protected" from the other, each competing to activate the atria or ventricles or both.
- The parasystolic pacemaker may be located anywhere in the heart but is commonly located in the ventricles, less commonly in the A-V node and rarely in the atria.

# Pathophysiology
- Parasystole is a result of interaction between two fixed rate pacemakers having different discharge rates.
- Parasystolic pacemakers can exist in either the atrium or the ventricle.
- The latent pacemaker is protected from being overdriven by the dominant rhythm (usually NSR) by intermittent or constant entrance block (impulses of sinus origin fail to depolarize the latent pacemaker secondary to block in the tissue surrounding the latent pacemaker focus).
- The depolarized level of membrane potential at which abnormal automaticity occurs can cause entrance block, leading to parasystole. This would be an example of an arrhythmia caused by a combination of an abnormality of impulse conduction and impulse initiation.
- Such block must be unidirectional, so that activity from the ectopic pacemaker can exit and produce depolarization whenever the surrounding myocardium is excitable.
- The protected pacemaker is said to be a parasystolic focus. In general, under these conditions, a protected focus of automaticity of this type fires at its own intrinsic frequency, and the intervals between the discharges of each pacemaker are multiples of its intrinsic discharge rate (sometimes described as fixed parasystole).
- Therefore on the surface electrocardiogram (ECG) the coupling intervals of the manifest ectopic beats wander through the basic cycle of the sinus rhythm.
- The traditional ECG criteria used to recognize the fixed form of parasystole include:
The presence of variable coupling intervals of the manifest ectopic beats.
Interectopic intervals that are simple multiples of a common denominat.
The presence of fusion beats.
- The presence of variable coupling intervals of the manifest ectopic beats.
- Interectopic intervals that are simple multiples of a common denominat.
- The presence of fusion beats.
- The parasystolic focus can exhibit exit block, during which it may fail to depolarize excitable myocardium.
- The parasystolic focus is protected, it may not be totally immune to the surrounding electrical activity.
- The effective electrical communication that permits the emergence of the ectopic discharges can also allow the rhythmic activity of the surrounding tissues to electrotonically influence the periodicity of the pacemaker discharge rate.
- Electrotonic influences arriving during the early stage of diastolic depolarization result in a delay in the firing of the parasystolic focus, whereas those arriving late accelerate the discharge of the parasystolic focus.
- As a consequence, the dominant pacemaker can entrain the partially protected parasystolic focus and force it to discharge at periods that may be faster or slower than its own intrinsic cycle and give rise to premature discharges whose patterns depend on the degree of modulation and the basic heart rate, occasionally mimic reentry, and occur at fixed coupling intervals.
- All these features of abnormal automaticity can be found in the Purkinje fibers that survive in regions of transmural MI and cause ventricular arrhythmias during the subacute phase..

# Clinical Features
- Parasystole may feel that heart is racing or beating too slowly.
- Heart may not pump effectively due to the fast or slow heartbeat.
- These include shortness of breath, weakness, dizziness, lightheadedness, fainting or near fainting, and chest pain or discomfort.

# Differentiating  from other Diseases
The differentiating features are largely based on both EKG findings and cardiovascular examination.
- Atrial fibrillation is irregularly irregular, while the other rhythms such as atrial flutter, sinus tachycardia, AV nodal reentry tachycardia and paroxysmal supraventricular tachycardia are all much more regular.
- An atrioventricular nodal reentry tachycardia will often break with either carotid sinus massage or AV nodal blocking agents.
- If the patient has Wolff-Parkinson-White syndrome there may be much more rapid conduction.  The presence of the delta wave on EKG is characteristic.

# Epidemiology and Demographics
- The incidence of parasystole is approximately 0.13 per cent of all electrocardiograms taken in a general hospital and is seen twice as frequently in males.
- In the majority of the cases studied with parasystole there was demonstrated some form of heart disease and 65 per cent of the patients were older than 60 years of age.
- The most commonly associated heart diseases were arteriosclerotic heart disease and/or hypertensive cardiovascular disease (60 per cent) and half of these subjects had congestive heart failure.

## Age
- Patients of all age groups may develop Parasystole.
- Parasystole is more commonly observed among patients older than 60 years old.
- Parasystole is more commonly observed among elderly patients.

## Gender
- Males are more commonly affected with Parasystole than females.
- The male to female ratio is approximately 2 to 1.

## Race
- There is no racial predilection for Parasystole.

# Risk Factors
- Parasystole is a kind of arrhythmia caused by the presence and function of a secondary pacemaker in the heart,so any heart rhythm problem or condition can be a risk factor.
- The most commonly associated heart diseases were arteriosclerotic heart disease and/or hypertensive cardiovascular disease (60 per cent) and half of these subjects had congestive heart failure.
- Various other acquired and congenital heart diseases were also associated with this arrhythmia but 14 per cent failed to show any evidence of heart disease.

# Natural History, Complications and Prognosis
- Early clinical features include palpitations, shortness of breath, chest pain.
- Although parasystole generally carries a benign prognosis, parasystolic beats may be capable of inducing ventricular tachycardia and ventricular fibrillation on rare occasions.
- The prognosis of individuals with arrhythmia is influenced by the presence, absence or status of the underlying heart disease.

# Diagnosis
- Diagnosis based on EKG.

## Symptoms
- Symptoms of may include the following:
- Palpitation
- Shortness of breath
- Chest pain
- Syncope

## Laboratory Findings
- There are no specific laboratory findings associated with parasystole.

## Imaging Findings
- There are no imaging study findings associated with parasystole.

# ECG signals
On an ECG, parasystole may be recognized by:
- spotting ectopic P or QRS waves which either:
-ccur at regular intervals
the time between them is always exact multiple of the smallest time between such two occurrences
- occur at regular intervals
- the time between them is always exact multiple of the smallest time between such two occurrences
- spotting "fusioned" P or QRS complexes (sometimes rare)
Another feature is the varying coupling interval between sinus and ectopic beats.

# Treatment

## Medical Therapy
- There is no treatment for parasystole, the mainstay of therapy is supportive care.

## Surgery
- Surgery is not an option for parasystole.

## Prevention
- There are no primary preventive measures available for parasystole.