Complete Disconnection: Third-degree AV Block

Third-degree atrioventricular (AV) block, also known as complete heart block, represents a complete failure of electrical conduction between the atria and the ventricles.¹⁸⁷ As a result, the atria and ventricles beat independently of each other, with the atria typically paced by the sinoatrial (SA) node and the ventricles paced by an escape rhythm originating from the AV junction or the ventricles themselves.¹⁸⁷

Analogy: “The upper and lower sections of the orchestra are playing entirely different songs at their own paces, with no coordination.”

ECG Characteristics

The ECG in third-degree AV block shows complete atrioventricular dissociation.¹⁸⁷ The P waves occur at a regular rate, reflecting the sinus rhythm of the atria, and the R-R intervals are also regular, reflecting the escape rhythm of the ventricles.¹⁸⁷ However, there is no relationship between the P waves and the QRS complexes; the PR interval will vary randomly because the atrial and ventricular activities are not synchronized.¹⁸⁷ The atrial rate (determined by the P-P interval) is usually faster than the ventricular rate (determined by the R-R interval).¹⁸⁷ The morphology of the QRS complexes depends on the location of the ventricular escape pacemaker. If the escape rhythm originates in the AV junction, the QRS complexes will typically be narrow (junctional escape rhythm, rate 40-60 bpm). If the escape rhythm originates in the ventricles, the QRS complexes will be wide (ventricular escape rhythm, rate 20-40 bpm).¹⁸⁷

Other Symptoms

Third-degree AV block can cause significant symptoms due to the lack of coordination between atrial and ventricular contractions and the often slow ventricular rate.¹⁸⁷ These symptoms may include fatigue, dizziness, lightheadedness, syncope (fainting), chest pain, and shortness of breath.¹⁸⁷ The slow ventricular rate can lead to bradycardia, which can be severe.¹⁸⁷ Patients with complete heart block may exhibit hemodynamic instability and are at risk of sudden cardiac death, especially if the ventricular escape rhythm is slow or unreliable.¹⁸⁷ On physical examination, jugular venous pressure may reveal cannon A waves due to the simultaneous contraction of the atria against closed AV valves.¹⁸⁷ Third-degree AV block can be congenital or acquired due to various cardiac conditions or medications.¹⁸⁷

Analogy

“Like two separate orchestras playing in the same room but completely out of sync, the atria and ventricles beat independently.”