Top 3 Heart Block Patterns in Dogs: Second-Degree, Third-Degree, and Sick Sinus Syndrome (NAVLE Notes)
Top 3 Heart Block Patterns in Dogs: Second-Degree, Third-Degree, and Sick Sinus Syndrome (NAVLE Notes)
Heart block patterns in dogs can vary in severity, with second-degree, third-degree, and sick sinus syndrome (SSS) being the most commonly encountered types. They often lead to exercise intolerance and syncope due to bradyarrhythmia.
ECG showing second-degree heart block – note P-wave (arrow) with absent QRS complex
ECG showing third-degree heart block
ECG showing SSS with sinus arrest
Classic Case
- Second-Degree Heart Block:
- Signs: Clinical signs generally only present in high-grade blocks.
- Symptoms: Exercise intolerance, occasional syncope.
- Third-Degree Heart Block:
- Dogs: Syncope is common due to lack of ventricular response.
- Cats: Rarely show clinical signs.
- Sick Sinus Syndrome (SSS):
- Signs: Severe bradycardia, sinoatrial arrest, or brady-tachy syndrome.
- Breeds: Miniature Schnauzers, West Highland White Terriers, Cocker Spaniels.
- Symptoms: Exercise intolerance, syncope.
Diagnosis
- Second-Degree Heart Block:
- ECG/Holter:
- Occasional unconducted atrial beats (P waves not followed by QRS complex).
- Types:
- Mobitz Type 1 (Wenckebach): Successive P-R interval prolongation until QRS is absent.
- Mobitz Type 2: Fixed P-R interval with intermittent failure of conduction.
- High-Grade: Multiple P waves not conducted.
- Third-Degree Heart Block:
- ECG/Holter:
- Complete atrioventricular (AV) dissociation with no relationship between P waves and QRS complexes.
- Sick Sinus Syndrome (SSS):
- ECG/Holter:
- Sinus bradycardia, long pauses (sinoatrial arrest), and possible brady-tachy syndrome.
- Atropine Response Test: Positive response indicates physiologic bradycardia; no response indicates pathologic bradycardia.
- Echocardiography:
- Recommended to assess for any structural disease, although most cases lack a structural cause.
- Can reveal chamber enlargement or valvular insufficiency secondary to bradycardia.
Treatment
- Second-Degree Heart Block:
- Medications:
- Theophylline or Terbutaline: Sympathomimetics.
- Propantheline: Parasympatholytic.
- Pacemaker: Indicated in high-grade blocks.
- Third-Degree Heart Block:
- Pacemaker: Essential due to the risk of sudden death.
- Sick Sinus Syndrome (SSS):
- Bradyarrhythmias: Treated with sympathomimetics or parasympatholytics.
- Pacemaker: Needed if bradycardia causes clinical signs.
- Brady-Tachy Syndrome: Manage bradyarrhythmia first; medications for tachyarrhythmia can worsen bradycardia.
Key Points
- Treatment:
- SSS or Second-Degree Heart Block: Treatment is only necessary if bradycardia causes clinical symptoms.
- Third-Degree Heart Block: Requires treatment with a pacemaker due to the high risk of sudden death.
NAVLE-Style Practice Questions on Heart Block Patterns
Question 1
A 10-year-old West Highland White Terrier presents with episodic syncope and an ECG showing long, inappropriate pauses between P-QRS complexes. What is the most likely diagnosis?
- A) Second-degree heart block
- B) Third-degree heart block
- C) Sick sinus syndrome
- D) Atrial fibrillation
Correct Answer: C
Explanation: Sick sinus syndrome (SSS) commonly presents with sinoatrial arrest and long pauses on ECG. This breed is predisposed to SSS, which often causes syncope.
Question 2
An ECG in a dog shows multiple P waves not followed by QRS complexes, with a fixed P-R interval on conducted beats. What type of second-degree heart block is this?
- A) Mobitz Type 1
- B) Mobitz Type 2
- C) Third-degree heart block
- D) Sick sinus syndrome
Correct Answer: B
Explanation: Mobitz Type 2 second-degree heart block has a fixed P-R interval with occasional non-conducted P waves, indicating an interruption in AV conduction.
Question 3
A dog with third-degree heart block is brought in for recurrent syncope. What is the most appropriate treatment?
- A) Theophylline
- B) Terbutaline
- C) Pacemaker implantation
- D) ACE inhibitors
Correct Answer: C
Explanation: Third-degree heart block requires a pacemaker due to the risk of sudden death from complete AV dissociation.
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