Parasystole
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Mohammad Salih, MD.
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 parasystolic focus can exhibit exit block, during which it may fail to depolarize excitable myocardium.
Although 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 (described as modulated parasystole). 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. Therefore appropriate diagnosis of modulated parasystole relies on the construction of a phase response curve as theoretical evidence of modulation of the ectopic pacemaker cycle length (CL) by the electrotonic activity generated by the sinus discharges across the area of protection.
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
Differentiating [disease name] from other Diseases
- [Disease name] must be differentiated from other diseases that cause [clinical feature 1], [clinical feature 2], and [clinical feature 3], such as:
- [Differential dx1]
- [Differential dx2]
- [Differential dx3]
Epidemiology and Demographics
- The prevalence of [disease name] is approximately [number or range] per 100,000 individuals worldwide.
- In [year], the incidence of [disease name] was estimated to be [number or range] cases per 100,000 individuals in [location].
Age
- Patients of all age groups may develop [disease name].
- [Disease name] is more commonly observed among patients aged [age range] years old.
- [Disease name] is more commonly observed among [elderly patients/young patients/children].
Gender
- [Disease name] affects men and women equally.
- [Gender 1] are more commonly affected with [disease name] than [gender 2].
- The [gender 1] to [Gender 2] ratio is approximately [number > 1] to 1.
Race
- There is no racial predilection for [disease name].
- [Disease name] usually affects individuals of the [race 1] race.
- [Race 2] individuals are less likely to develop [disease name].
Risk Factors
- Common risk factors in the development of [disease name] are [risk factor 1], [risk factor 2], [risk factor 3], and [risk factor 4].
Natural History, Complications and Prognosis
- The majority of patients with [disease name] remain asymptomatic for [duration/years].
- Early clinical features include [manifestation 1], [manifestation 2], and [manifestation 3].
- If left untreated, [#%] of patients with [disease name] may progress to develop [manifestation 1], [manifestation 2], and [manifestation 3].
- Common complications of [disease name] include [complication 1], [complication 2], and [complication 3].
- Prognosis is generally [excellent/good/poor], and the [1/5/10year mortality/survival rate] of patients with [disease name] is approximately [#%].
Diagnosis
Diagnostic Criteria
- The diagnosis of [disease name] is made when at least [number] of the following [number] diagnostic criteria are met:
- [criterion 1]
- [criterion 2]
- [criterion 3]
- [criterion 4]
Symptoms
- [Disease name] is usually asymptomatic.
- Symptoms of [disease name] may include the following:
- [symptom 1]
- [symptom 2]
- [symptom 3]
- [symptom 4]
- [symptom 5]
- [symptom 6]
Physical Examination
- Patients with [disease name] usually appear [general appearance].
- Physical examination may be remarkable for:
- [finding 1]
- [finding 2]
- [finding 3]
- [finding 4]
- [finding 5]
- [finding 6]
Laboratory Findings
- There are no specific laboratory findings associated with [disease name].
- A [positive/negative] [test name] is diagnostic of [disease name].
- An [elevated/reduced] concentration of [serum/blood/urinary/CSF/other] [lab test] is diagnostic of [disease name].
- Other laboratory findings consistent with the diagnosis of [disease name] include [abnormal test 1], [abnormal test 2], and [abnormal test 3].
Imaging Findings
- There are no [imaging study] findings associated with [disease name].
- [Imaging study 1] is the imaging modality of choice for [disease name].
- On [imaging study 1], [disease name] is characterized by [finding 1], [finding 2], and [finding 3].
- [Imaging study 2] may demonstrate [finding 1], [finding 2], and [finding 3].
Other Diagnostic Studies
ECG signals
On an ECG, parasystole may be recognized by:
- spotting ectopic P or QRS waves which either:
- 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.
- [Disease name] may also be diagnosed using [diagnostic study name].
- Findings on [diagnostic study name] include [finding 1], [finding 2], and [finding 3].
Treatment
Medical Therapy
- There is no treatment for [disease name]; the mainstay of therapy is supportive care.
- The mainstay of therapy for [disease name] is [medical therapy 1] and [medical therapy 2].
- [Medical therapy 1] acts by [mechanism of action 1].
- Response to [medical therapy 1] can be monitored with [test/physical finding/imaging] every [frequency/duration].
Surgery
- Surgery is the mainstay of therapy for [disease name].
- [Surgical procedure] in conjunction with [chemotherapy/radiation] is the most common approach to the treatment of [disease name].
- [Surgical procedure] can only be performed for patients with [disease stage] [disease name].
Prevention
- There are no primary preventive measures available for [disease name].
- Effective measures for the primary prevention of [disease name] include [measure1], [measure2], and [measure3].
- Once diagnosed and successfully treated, patients with [disease name] are followed-up every [duration]. Follow-up testing includes [test 1], [test 2], and [test 3].