Introduction
Atrioventricular (AV) blocks are common causes of bradyarrhythmia in dogs and cats, often identified during ECG or telemetry monitoring. While some AV blocks are benign and vagally mediated, others signal significant conduction system disease requiring pacemaker implantation.
This article provides a comprehensive, clinically relevant overview of types of AV block, ECG differentiation, underlying causes, and management strategies for small animal practitioners and veterinary cardiology enthusiasts.
What Is an Atrioventricular (AV) Block?
An AV block occurs when there’s delayed or failed conduction of electrical impulses from the atria to the ventricles through the AV node or His-Purkinje system. The severity of conduction disturbance defines the degree of block:
- First-degree AV block: delayed conduction
- Second-degree AV block: intermittent conduction failure
- Third-degree (complete) AV block: total conduction block
These conduction abnormalities can be transient, progressive, or permanent, making early recognition vital to avoid syncope, collapse, or sudden death.
Types of AV Block in Dogs and Cats
1. First-Degree AV Block
Definition:
A delay in AV nodal conduction without dropped beats.
ECG Characteristics:
- PR interval >0.13 sec in dogs, >0.09 sec in cats
- Every P wave followed by a QRS complex
- Constant PR interval prolongation
Common Causes:
- Increased vagal tone (e.g., athletic dogs, rest)
- Sedation or anesthesia
- Drugs (digoxin, beta-blockers, calcium channel blockers)
- Rarely, structural myocardial or nodal disease
Treatment & Prognosis:
- No treatment required if asymptomatic
- Address drug toxicity or systemic disease if present
- Excellent prognosis; typically physiologic and reversible
2. Second-Degree AV Block
Second-degree AV block occurs when some atrial impulses fail to conduct to the ventricles. It’s subdivided into two patterns with distinct prognostic implications.
Mobitz Type I (Wenckebach)
Definition:
Gradual PR prolongation culminating in a dropped QRS complex.
ECG Findings:
- Progressive PR lengthening until a P wave isn’t conducted
- Grouped beating pattern
- Often associated with high vagal tone
Common Causes:
- Physiologic vagal influence
- Sedation or sleep
- Occasionally seen in healthy young or athletic dogs
Diagnosis Tip:
Perform an atropine response test (0.04 mg/kg IV or IM).
- If conduction normalizes → functional/vagal
- If no response → pathologic
Treatment:
- None needed if physiologic
- Avoid exacerbating vagal stimuli
Prognosis:
Excellent; rarely progresses if not associated with structural heart disease.
Mobitz Type II
Definition:
Sudden non-conduction of P waves without PR interval prolongation.
ECG Findings:
- Constant PR intervals before dropped beats
- May appear in ratios (2:1, 3:1)
Clinical Significance:
Always pathologic, indicating disease in or below the His bundle.
Common Causes:
- Idiopathic fibrosis or sclerodegenerative conduction disease
- Myocarditis (infectious, immune-mediated)
- Neoplasia infiltrating the conduction system
- Ischemia, trauma, or digoxin toxicity
Treatment:
- Evaluate for systemic and cardiac disease (CBC, biochemistry, echo, infectious testing)
- Permanent pacemaker implantation recommended if persistent or symptomatic
- Withdraw negative dromotropic drugs
Prognosis:
Guarded without pacing; good post-pacemaker placement.
3. Third-Degree (Complete) AV Block
Definition:
No atrial impulses conduct to the ventricles; the atria and ventricles beat independently.
ECG Findings:
- P waves and QRS complexes unrelated
- Ventricular escape rhythm present
- Atrial rate > ventricular rate
Clinical Signs:
- Profound bradycardia
- Exercise intolerance, collapse, syncope
- Possible sudden cardiac death
Common Causes:
- Idiopathic fibrosis or sclerotic degeneration
- Myocarditis (infectious or immune-mediated)
- Infiltrative/neoplastic disease
- Endocardiosis involving the AV node region
- Postoperative or inflammatory injury
Treatment:
- Definitive therapy: permanent pacemaker implantation
- Temporary transvenous pacing for unstable patients
- Atropine, isoproterenol, or dopamine as short-term bridges only
- Evaluate for systemic or infectious causes (e.g., Bartonella spp., Trypanosoma cruzi)
Prognosis:
Excellent long-term survival with pacing; poor without it.
Diagnostic Approach to AV Block
A structured diagnostic work-up ensures accurate classification and management.
| Step | Key Actions | Purpose |
|---|---|---|
| 1. History & Physical Exam | Assess for bradycardia, collapse, drug history | Identify underlying causes |
| 2. ECG or Holter Monitoring | Confirm block type, pattern, and escape rhythm | Determine if persistent or intermittent |
| 3. Atropine Response Test | 0.04 mg/kg IM or IV | Differentiate functional vs. structural |
| 4. Echocardiography | Evaluate cardiac structure and function | Detect concurrent cardiomyopathy or endocardiosis |
| 5. Laboratory Tests | CBC, chem, electrolytes, digoxin levels, infectious panels | Rule out systemic or drug-induced disease |
Treatment Summary by AV Block Type
| AV Block Type | Common Causes | Treatment | Prognosis |
|---|---|---|---|
| First-Degree | Vagal tone, drugs | None; monitor | Excellent |
| 2° Type I (Wenckebach) | Vagal tone, sedation | None; atropine if symptomatic | Excellent |
| 2° Type II | Fibrosis, myocarditis | Permanent pacemaker | Guarded → Good (with pacemaker) |
| 3° (Complete) | Degenerative, inflammatory | Permanent pacemaker | Good (post-implant) |
Key Clinical Pearls
✅ Always determine whether AV block is functional (vagal) or pathologic; the atropine test remains invaluable.
✅ Persistent Mobitz Type II or complete AV block = pacemaker candidate.
✅ Avoid negative chronotropes in any patient with suspected conduction disease.
✅ Monitor progression; some second-degree blocks evolve into complete block over time.
