Paroxysmal AV block

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Akash Daswaney, M.B.B.S[2]

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Historical Perspective

• There is limited information about the historical perspective of paroxysmal AV block.
• In 1931, Adolph Sachs et al reported one of the first cases on paroxysmal AV block. The patient had presented with multiple spells of palpitations, hot flashes, dizziness, ringing in the ears, weakness and diaphoresis accompanied by convulsions. During an acute episode, he noticed an irregular ventricular rhythm, irregular complexes and given a time interval of 30 seconds, the ventricle would beat once in the first 15 second window and 4-5 times in the succeeding window. It was noticed that the duration of the block progressively increased until it was present all the time. Mitral valvulitis was put down as the cause of the attack and the fact that a response to atropine does not rule out an intrinsic conduction defect was emphasized.
• In 1972, Philippe Coumel et al hypothesized that the cause of bradycardia/pause dependent AV block was the spontaneous depolarization of specialized conducting fibers in the late stages of diastole. It was during this ‘zone of opportunity’ that they noticed this ‘AV dissociation’. In this case, they found that the block occurred to be proximal to the division of the His bundle.
• In 1997. Brignole et al first described EI AVB in a group of 15 syncope patients with an initial negative work up. They fortuitously stumbled upon ECG findings indicating a paroxysmal AV block and reconfirmed this by performing an adenosine triphosphate test in each patient.

Classification

• Paroxysmal AV Block may be classified according to the cause into three types :

1. Intrinsic AV Block (I-AVB)
2. Extrinsic Vagal AV Block (EV- AVB)
3. Extrinsic Idiopathic AV Block (EI- AVB)

Paroxysmal AV Block classification based on cause

Intrinsic AV Block (I-AVB): Due to innate structural/ conduction defect

Extrinsic Vagal AV Block (EV- AVB): Due to vagal surge/reflex

Extrinsic Idiopathic AV Block (EI- AVB) : Due to innately low adenosine plasma levels

Tachycardia Dependent AV Block (TD- AVB)

Bradcardia/Pause Dependent AV Block (BD- AVB/PD-AVB)

Intrinsic AV Block

• Intrinsic AV block (I-AVB) is an AV block secondary to an innate anatomical defect.
• It is hugely recognized on an ECG as an atrial premature beat (APB) or ventricular premature beat (VPB) before and after a variable period of complete AV block/asystole.
  • Sinus rate increase/ decrease prior to the VPB/APB or during the period of asystole further divides it into Tachycardia Dependent AV block (TD-AVB) and Pause/Bradycardia dependent AV block (PD- AVB).

• Normal cardiac myocytes are associated with a more negative resting membrane potential, an increased amplitude of action potential and a fast depolarizing sodium current.
• An exact opposite is seen in diseased myocytes responsible for TD- PAVB. An imbalance between inward depolarizing sodium and calcium currents and outward repolarizing potassium currents causes an increase in recovery time and leads to a phenomenon called ‘post-repolarization refractoriness’.
• Despite repolarization being complete, a stimulus would not be able to induce an action potential.

• A hypothetical line of thinking that could be attributed to both PD-AVB and TD-AVB is a ‘concealed conduction’ in the intra His Bundle which serves as a source of a delayed escape rhythm, thereby disrupting the refractoriness and recovery time of the surrounding myocytes. This predisposes the patient to fatal complications such as syncope, presyncope, sudden cardiac death and atrial fibrillation with a rapid ventricular repose rate.
• Certain studies hypothesize that ventricular or supraventricular impulses reach this ‘concealed conduction’ at a time when there is a local phase 4 block (when sodium channels are inactive.) This subsequent long pause is reflected by the increased H-H interval in EPS studies and confirms an intra His Bundle block (an entity commonly missed and mislabeled as an infra-His Bundle block or AV- block on electrophysiological studies)
• Much debate surrounds this as it has also been documented that TD- AV/ PD-AV blocks are not related to phase 3 or phase 4 conduction defects, as previously hypothesized. It is related to myocardial ischemia, Mobitz type II block, RBBB and Intra His bundle conduction defects, retrograde ventricular premature beats and anterograde atrial premature beats; all factors that are independent of local phase 4 blocks.

Extrinsic Vagal AV Block

• An extrinsic vagally mediated AV block (EV-AVB) may occur due to a vagal surge or a condition causing an increase in vagal tone such as during tilt table tesing, carotid sinus massage, coughing, micturition, defecation, swallowing, myocardial infarction, injection of dypramidole and cardiac transplant rejection.
• It causes SA and AV node slowing and is therefore reflected on the ECG as sinus rate slowing, increasing/irregular PP and PR intervals prior to a period of compete AV block. A heterogenous presentation in terms of Mobitz type I or II and complete heart block may also be noted. This is followed by a period of sinus acceleration.
• Electrophysiological studies indicate a normal H-H interval and therefore it can be assumed that it does not have any effect on conduction in the bundle of His and is not associated with any anatomic involvement, as seen in intrinsic AV Block.
• The pathophysiology of EV-AVB may even be related to the autonomic control of the sinus and AV nodes. A parasympathetic predominance over the SA node and sympathetic predominance over the AV node is exerted in a normal autonomic nervous system.
• A disruption in this regulation may cause parasympathetic bursts and therefore, an AV block.
• The effect of vagal stimulation depends on the method and intensity of stimulation and the resting sympathetic activity.
• Vasalva maneuver, carotid sinus massage, water face immersion, tilt table testing may or may not induce an EV- AVB and in some cases a reversal may be seen on atropine administration.

Extrinsic Idiopathic AV Block

• The pathogenesis of extrinsic idiopathic paroxysmal AV block (EI-AVB) can be correlated to adenosine plasma levels (APL) and increased affinity of adenosine A1 receptors.
• There is a recurrent history of unexplained syncope, absence of ECG and cardiac abnormalities and a good prognosis.
• Due to innately low APL values seen in these patients, there is an upregulation of A1 receptors, such that even during a mild transient surge in endogenous adenosine levels, AV block occurs.
• A1 receptors, which are present more in the AV node than the SA node, impose an antiadrenergic action by antagonizing β1 receptors, the sympathetic nervous system, hyperpolarizing the SA and AV nodes through potassium channels and lowering intracellular cAMP levels.
• Therefore, in such patients an injection of adenosine or adenosine triphosphate (ATP) may reproduce the attack and adenosine antagonists such as theophylline may be an efficacious treatment option.
• On an ECG, there is an absence of signs of vagal stimulation, atrial/ventricular premature beats and there may be a presence of narrow QRS complexes prior to the period of complete AV Block/ asystole
• Certain studies have also noticed genetic polymorphisms in A2A receptors in a population of people experiencing recurrent unexplained syncope.

Screening

Diagnosis

Initial Approach | History and Symptoms | Electrocardiogram, Holter Monitoring, External Loop Recorder | Implantable Loop Recorder | [[Paroxysmal AV block Electrophysiologic studies|Electrophysiologic studies] | Echocardiography and Exercise stress tests | Carotid Sinus Massage | Tilt Table testing | Laboratory Findings and Stimulation tests |

Initial Approach

• The pathway to conclusively diagnosing a patient with paroxysmal AV block is not straightforward.
• Since most patients present with a history of recurrent unexplained syncope and fortuitous timing would be required to document classical ECG findings during an acute episode, it would be best to treat it as a diagnosis of exclusion.
• An initial evaluation strategy of taking a detailed history, physical examination, risk stratification, ECG recording and BP measurement should help decide what investigations should be ordered (based on whether the syncope is cardiac related, reflex/neutrally mediated, secondary to cerebrovascular disease or due to orthostatic hypotension).

IMAGE TO BE INSERTED - INITIAL APPROACH

History and Symptoms

• History of syncope, presyncope, duration of each episode, number of episodes, activities during the syncopal episode, aggravating or relieving factors, history of past medical illnesses, prodrome/ recovery phase description in terms of signs, symptoms and duration are important points to be addressed whilst taking a history of a syncope patient.
• A study of 341 syncope patients showed that the time between the first and last syncopal episode being less than 4 years, syncope during effort or supine position, a history of palpitations, convulsions or blurring of vision were important predictors of a cardiac syncope.
• Similarly, duration of prodrome > 10 seconds history of pallor, nausea, diaphoresis, dizziness, presyncope, abdominal discomfort and time between first and last syncopal episode being more than 4 years were important predictors of a neutrally mediated syncope.
• Based on a detailed history, one can decide whether a cardiac syncope was secondary to a rhythm dysfunction, structural cause or ischemia related and would warrant a work up of an ECG, Holter monitoring, echocardiography, electrophysiologic study, or an exercise stress test.
• Similarly, neutrally mediated syncope maybe vasovagal, situational, secondary to increased carotid sinus sensitivity or non classical and orthostatic hypotension may be due to a primary or secondary autonomic failure, secondary to drugs or hypovolemia. This may be further explored by a carotid sinus massage, tilt table testing, adenosine plasma levels or an adenosine triphosphate stimulation test.

Electrocardiogram, Holter monitoring, External Loop Recorder

• According to the European Society of Cardiology, indications for ECG monitoring are as follows :
  1. Immediate in-hospital monitoring (in bed or by telemetry) is indicated in high risk patients
  2. Holter monitoring should be considered in patients who have frequent syncope or presyncope (more than or equal to 1 episode per week)
  3. External loop recorders should be considered, early after the index event, in patients who have an inter symptom interval of less than or equal to 4 weeks

Classic ECG Findings seen inthe different types of paroxysmal AV Block are as follows : IMAGES TO BE INSETED, 3 ECG IMAGES

• The SYNARR- Flash study was one of the first multicentric observational studies wherein 395 patients with a history of unexplained syncope were monitored with an external ECG device for 4 weeks.
  • Based on certain criteria, events were classified as conclusive, significant, suggestive and negative.
  • It was found that diagnostic events were seen more in patients in which ECG recordings were initiated within 15 days from the index syncope and those with a history of supraventricular arrhythmias and frequent events. If the patient remained undiagnosed following this 4 week interval, more invasive modalities like implantable loop recorders (ILR) may be considered.

• Brignole et al demonstrated that bundle branch block findings on an ECG does not necessarily correlate to a cardiac related/ bradyarrhythmic syncope.
  • Bundle branch block (in particular, a monofasicular block) in a patient with atypical presenting symptoms and advancing age should prompt a physician to think of paroxysmal AV block or a neutrally mediated mechanism being behind the syncope.

Treatment

Medical Therapy | Interventions | Surgery | Primary Prevention | Secondary Prevention | Cost-Effectiveness of Therapy | Future or Investigational Therapies

Case Studies

Case #1

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