From the Guidelines
A 3rd degree AV block (complete heart block) presents on ECG as a complete dissociation between atrial and ventricular activity, with the P waves and QRS complexes having no relationship to each other, and the atria and ventricles beating independently at their own rates. The ECG findings in a third-degree atrioventricular (AV) block can be characterized by the following features:
- Complete dissociation between atrial and ventricular activity
- P waves and QRS complexes have no relationship to each other
- Atria and ventricles beating independently at their own rates
- Atrial rate is typically faster than the ventricular rate
- Ventricular rate usually between 20-40 beats per minute if originating from a junctional escape rhythm, or even slower (15-30 beats per minute) if from a ventricular escape rhythm
- QRS complexes may be narrow if the escape rhythm originates from the AV junction or wide if it comes from the ventricles
- PR interval varies continuously throughout the ECG because the P waves and QRS complexes are completely independent, as noted in 1.
This arrhythmia occurs due to complete failure of electrical impulse conduction from the atria to the ventricles, often resulting from damage to the His-Purkinje system, as discussed in 1. Patients with 3rd degree AV block frequently require immediate intervention with temporary pacing followed by permanent pacemaker implantation, as this condition can cause symptoms ranging from fatigue and dizziness to syncope and even sudden cardiac death due to the profound bradycardia. The decision to implant a pacemaker in a patient with abnormal AV conduction depends on the presence of symptoms related to bradycardia or ventricular arrhythmias and their prognostic implications, as stated in 1.
From the Research
ECG Findings in Third-Degree AV Block
The electrocardiogram (ECG) findings in a third-degree atrioventricular (AV) block are characterized by:
- Complete dissociation between the P-wave and QRS complex, as seen in a case of takotsubo cardiomyopathy with a third-degree AV block 2
- Atrioventricular dissociation, where the atria and ventricles function independently, resulting in a lack of coordination between P-waves and QRS complexes 3
- The presence of an escape rhythm, which can originate from the AV node or the ventricles, resulting in a narrow or wide QRS complex, respectively 4
- The ventricular rate is dependent on the location of the block and the functioning secondary pacemaker within the conduction system, with rates typically ranging from 20-60 bpm 4
Types of Escape Rhythms
The ECG findings can also vary depending on the type of escape rhythm present:
- For escape rhythms originating at the AV node or high in the His-Purkinje system, the QRS complex is typically narrow, with ventricular rates in the 40-60 bpm range 4
- For blocks with ventricular escape rhythms, the QRS complex appears wide, with rates of 20-40 bpm 4
Clinical Presentation
The clinical presentation of third-degree AV block can vary, with some patients being asymptomatic 5, while others may present with symptoms such as syncope 6, bradycardia, or hypotension 4