4.2 Arrhythmias & Conduction Blocks

Arrhythmias & Conduction Blocks

Quick Answer: Arrhythmias are classified by origin (sinus, atrial, or ventricular) and by rate and regularity. Atrial fibrillation is irregularly irregular with no P waves, while ventricular tachycardia is a wide-complex run of three or more PVCs at a rate over 100 bpm. AV blocks are graded first-degree, Mobitz I, Mobitz II, and third-degree (complete) based on how the PR interval and conduction ratio behave, with Mobitz II and complete block carrying the highest clinical risk.

Building on the sinus rhythm criteria from Section 4.1, this section works through the arrhythmias and conduction abnormalities a CEP must recognize during resting and exercise ECG monitoring, since several are indications to stop a test or activate emergency response.

Sinus Node Arrhythmias

• Sinus bradycardia: Normal P-QRS-T morphology and conduction, but rate under 60 bpm. Common in trained athletes at rest; concerning if symptomatic (dizziness, syncope) or if it develops abnormally during exercise.
• Sinus tachycardia: Normal morphology and conduction, rate over 100 bpm. A normal response to exercise, but at rest can signal anxiety, fever, dehydration, anemia, or cardiac compromise.

Atrial Arrhythmias

New-onset AF or a rapid, uncontrolled ventricular response during exercise testing warrants prompt evaluation and is a relative indication to stop the test.

Ventricular Arrhythmias

• Premature ventricular contraction (PVC): An early, wide (over 0.12 sec), bizarre-looking QRS with no preceding P wave, usually followed by a compensatory pause. Single, occasional PVCs are common and often benign; frequent, multifocal, paired (couplets), or runs of PVCs are more clinically significant and are relative indications to stop a graded exercise test.
• Ventricular tachycardia (VT): Three or more consecutive PVCs in a row, wide QRS, rate typically over 100 bpm. Sustained VT is hemodynamically unstable and is an absolute indication to terminate exercise testing immediately and initiate emergency response.
• Ventricular fibrillation (VF): A chaotic, disorganized rhythm with no identifiable P waves, QRS complexes, or T waves. There is no effective cardiac output — VF is a cardiac arrest rhythm requiring immediate defibrillation and CPR per emergency protocol (Chapter 11).

AV (Heart) Blocks

AV blocks describe a delay or interruption in conduction between the atria and ventricles, graded in four tiers:

1. First-degree AV block: Every P wave conducts to a QRS, but the PR interval is prolonged beyond 0.20 seconds and stays constant. Usually benign and often an incidental finding.
2. Second-degree, Mobitz Type I (Wenckebach): The PR interval progressively lengthens with each beat until one P wave fails to conduct (a "dropped" QRS), then the cycle repeats. Usually a benign block at the level of the AV node.
3. Second-degree, Mobitz Type II: The PR interval stays constant, but P waves are intermittently and unpredictably non-conducted (dropped QRS) without any preceding lengthening pattern. This block is typically located below the AV node (infranodal) and carries a higher risk of progressing to complete heart block — it is considered more clinically dangerous than Mobitz I.
4. Third-degree (complete) AV block: No atrial impulses reach the ventricles. P waves and QRS complexes occur independently of each other (AV dissociation), each at their own regular rate, with no fixed relationship between them. This is a medical emergency requiring prompt evaluation and often pacing.

Clinical Significance During Exercise Testing

New-onset Mobitz Type II block, third-degree AV block, or a new bundle branch block that cannot be confidently distinguished from ventricular tachycardia appearing during a graded exercise test are all relative indications to terminate the test. Because Mobitz II and complete heart block can progress abruptly to a slow ventricular escape rhythm or asystole, any patient found to have these blocks at rest should be evaluated and cleared by the supervising physician before undergoing exercise testing.

Bundle Branch Blocks

A bundle branch block delays depolarization through the right or left ventricle, widening the QRS to 0.12 seconds or greater:

• Right bundle branch block (RBBB): Classic "rabbit ears" (rSR' pattern) in V1, with a wide, slurred S wave in lead I and V6.
• Left bundle branch block (LBBB): Broad, monophasic R wave in leads I and V6, with absent Q waves in those leads. LBBB also distorts the ST segment and T wave in a way that obscures the ability to diagnose ischemia from standard ST-segment criteria, which is an important limitation to recognize before exercise testing a patient with known LBBB.