What could be the cause of an intermittent third heartbeat and how is it treated?

Intermittent Third Heartbeat: Causes and Treatment

An intermittent third heartbeat most commonly represents either premature ventricular contractions (PVCs), an audible third heart sound (S3) from complete AV block with preserved sinus rhythm, or less commonly, ventricular bigeminy or trigeminy. The specific cause determines treatment, ranging from observation alone for benign ectopy to permanent pacemaker implantation for high-grade AV block.

Differential Diagnosis

Premature Ventricular Contractions (PVCs)

• PVCs are the most common cause of an "extra" heartbeat and are often benign in patients without structural heart disease 1
• Benign ventricular arrhythmias occur in patients without known heart disease and do not require treatment 1
• PVCs become potentially malignant when they occur at rates >10 per hour, are repetitive in nature, or occur in the setting of severely depressed ventricular function 1
• In patients with acute coronary syndromes, PVCs and non-sustained ventricular tachycardia occur frequently and are rarely of hemodynamic relevance, requiring no specific treatment 2

Complete AV Block with Preserved Sinus Rhythm

• An intermittently audible S3 can be a physical sign of complete atrioventricular block in patients with preserved sinus rhythm 3
• This occurs when strong atrial contraction develops exactly at the time of rapid left ventricular filling, creating an audible third sound 3
• The sound is best heard from the left sternal border to the apex during held breath over 5-6 consecutive beats 3

Intermittent Second or Third-Degree AV Block

• Intermittent bradycardia may result from variable contributions of intrinsic cardiac disease and extrinsic mechanisms 2
• Bifascicular block with intermittent complete heart block and symptomatic bradycardia is a high-risk scenario requiring intervention 4, 5
• Bifascicular block with intermittent type II second-degree AV block, even without symptoms, is considered high-risk 4, 5

Diagnostic Approach

Initial Evaluation

• Obtain a 12-lead ECG to identify the rhythm abnormality and assess for conduction system disease 2
• Assess for symptoms including palpitations (pounding, racing, skipped beats), presyncope (dizziness, lightheadedness), or syncope 2
• Evaluate for underlying structural heart disease, coronary disease, heart failure, or cardiomyopathy 2, 1

Extended Monitoring

• Ambulatory ECG monitoring or implantable loop recorder is essential when intermittent bradycardia is suspected but not documented on standard ECG 2
• Correlation between symptoms and documented arrhythmia is essential when deciding on treatment 2
• In patients with syncope and bundle branch block, implantable loop recorder studies show that approximately half require pacing 2

Echocardiographic Assessment

• Doppler echocardiography can identify the mechanism of an intermittent S3 by measuring transmitral flow velocities 3
• Maximal summation of E and A wave velocities >100 cm/sec with strong atrial contraction (A wave >80 cm/sec) correlates with audible S3 3
• Assess for left ventricular dysfunction, cardiomegaly, and structural abnormalities 2

Electrophysiological Study Considerations

• In patients with syncope and bifascicular block, electrophysiological study showing HV interval ≥100 ms or intra/infra-Hisian block at pacing rates <150 bpm predicts development of high-grade AV block 2, 5
• However, HV interval prolongation alone does not reliably identify patients at high risk of sudden death from bradyarrhythmia 4
• Electrophysiological study may be helpful to evaluate inducible ventricular arrhythmias in patients with bifascicular block 2

Treatment Based on Etiology

Benign PVCs (No Structural Heart Disease)

• No treatment is required for benign ventricular arrhythmias in patients without structural heart disease 1
• Reassurance and avoidance of triggers (caffeine, alcohol, stress) is appropriate 6

PVCs with Structural Heart Disease

• Beta-blocker therapy should be considered in patients with coronary disease or heart failure 2
• Frequent and complex PVCs (>10/hour) after myocardial infarction have independent prognostic significance and warrant closer monitoring 1
• Radiofrequency catheter ablation should be considered for recurrent symptomatic PVCs refractory to medical therapy 2, 6

Complete AV Block or High-Grade AV Block

Class I Indications for Permanent Pacemaker (Must Implant):

• Third-degree or advanced second-degree AV block at any anatomic level with symptomatic bradycardia, including heart failure or ventricular arrhythmias presumed due to AV block 2
• Third-degree or advanced second-degree AV block with documented asystole ≥3.0 seconds or escape rate <40 bpm in awake, symptom-free patients 2
• Bifascicular block with intermittent complete heart block and symptomatic bradycardia 4, 5
• Bifascicular or trifascicular block with intermittent type II second-degree AV block, regardless of symptoms 5
• Second-degree AV block with associated symptomatic bradycardia regardless of type or site of block 2

Class IIa Indications for Permanent Pacemaker (Reasonable to Implant):

• Syncope with bifascicular block when other causes have been excluded, especially if syncope may have been due to transient third-degree AV block 2, 5
• Asymptomatic second-degree AV block at intra- or infra-His levels found at electrophysiological study 2
• Markedly prolonged HV interval (≥100 ms) found during electrophysiological study 2, 5

Pacemaker Mode Selection

• Dual-chamber pacing (DDD) is generally preferred over single-chamber ventricular pacing (VVI) to avoid pacemaker syndrome 2, 5
• In intermittent bradycardia, adequate rate hysteresis should be programmed to allow spontaneous sinus rate to emerge and restrict pacing to periods when reflex bradycardia occurs 2
• Manual adaptation of AV interval (up to 250 ms) or programming of AV hysteresis prevents unnecessary right ventricular pacing 2

Important Clinical Pitfalls

Do Not Pace Asymptomatic Bifascicular Block

• Asymptomatic bifascicular block without evidence of intermittent AV block is not an indication for pacemaker implantation 5
• Routine prophylactic pacemaker implantation does not reduce mortality in asymptomatic patients with bifascicular block 4
• The rate of progression from bifascicular block to complete heart block is generally slow 5

Reversible Causes Must Be Excluded

• Permanent pacemaker implantation is not indicated for AV block expected to resolve (drug toxicity, Lyme disease, transient vagal tone increases, hypoxia in sleep apnea) 2
• Among patients with compromising bradycardia, adverse drug effects account for 21%, acute MI for 14%, intoxication for 6%, and electrolyte disorders for 4% 2

Syncope in Bifascicular Block Requires Careful Evaluation

• Although syncope is common in patients with bifascicular block, it is not always due to AV block 2
• Less than half of patients with bundle branch block and syncope have a final diagnosis of cardiac syncope; a similar percentage have reflex syncope 2
• Documented syncope with transient or permanent complete heart block is associated with increased sudden death risk 4
• Pacing relieves neurological symptoms but does not reduce the occurrence of sudden death in patients with syncope and bifascicular block 4

Congestive Heart Failure Is a Key Prognostic Factor

• The presence of congestive heart failure is a significant predictor of both all-cause mortality and sudden cardiac death in patients with bifascicular block 4
• Death in patients with advanced cardiac disease and HV prolongation is often due to underlying heart disease rather than AV block itself 2