Left Bundle Branch Block (LBBB) in Dogs: 2025 Vet Guide 🩺🐾
In this article
Left Bundle Branch Block in Dogs: 2025 Vet Guide 🩺🐶
By Dr. Duncan Houston BVSc
Hi there—I'm Dr Duncan Houston, BVSc, veterinarian and founder of Ask A Vet. In this in-depth 2025 guide, we'll explore left bundle branch block (LBBB) in dogs, covering ECG recognition, causes, clinical importance, monitoring strategies, telehealth tools, and long-term care.
1. 🫀 What Is Left Bundle Branch Block?
In LBBB, electrical impulses traveling down the left bundle branch are delayed or blocked. This forces the ventricles to depolarise asynchronously, resulting in a prolonged and abnormal QRS complex on ECG—all hallmark signs of impaired conduction.
2. 🧮 ECG Features in Dogs
- Wide QRS complex longer than 0.08–0.12 s
- Monophasic R in lead I, QS or rS in V1
- A leftward shift in the electrical axis
- Discordant T waves opposite to the QRS complex
3. 🧬 Common Causes
LBBB typically signals underlying heart disease:
- Cardiomyopathies (dilated, hypertrophic)—fibrosis or myocardial injury
- Aortic stenosis or LV pressure overload
- Prior myocardial ischemia—rare but noted
- Degenerative fibrosis of conduction tissue, myocarditis, neoplasia
- Reversible causes: electrolyte imbalances, drug toxicity, trauma
4. ⚕️ Clinical Significance & Signs
Often, LBBB is found incidentally on an ECG in asymptomatic dogs. However, when symptoms occur, they usually reflect the underlying condition:
- Exercise intolerance or fatigue
- Coughing, breathing difficulty, or signs of heart failure
- Occasional syncope—rare, often due to severe conduction or HR issues
5. 🔬 Diagnostic Approach (2025 Edition)
- History & physical: assess murmurs, rhythm irregularities, exercise capacity
- ECG: confirm LBBB pattern, distinguish from other intraventricular blocks
- Echocardiography: evaluate LV function, chamber sizes, and valve quality
- Bloodwork: measure electrolytes, thyroid, renal markers, cardiac biomarkers (e.g., NT-proBNP)
- Holter monitoring: check for intermittent conduction anomalies or arrhythmias
- Imaging (CT/MRI): for detailed cardiac structure if a congenital disease is suspected
6. 🛠 Management & Monitoring
- Treat underlying disease: e.g., ACE inhibitors, beta blockers, diuretics for CHF or cardiomyopathy
- Routine follow-ups: echo & ECG every 6–12 months if cardiac abnormalities present
- Holter rechecks: if symptoms like fainting or worsening exercise intolerance emerge
- No pacemaker necessary unless it progresses to high-degree AV block or syncopal episodes occur
7. 📈 Prognosis
- Depends on the underlying heart disease
- In healthy dogs, incidental LBBB may remain stable
- With cardiomyopathy, LBBB indicates dyssynchrony and increased risk of CHF
8. 🌐 2025 Telehealth & Tech Support
- Wearable ECG patches: real-time home monitoring linked to vet systems
- AI ECG analysis: flag axis shifts or QRS abnormalities via the Ask A Vet platform
- Remote Holter reviews: specialists assess rhythm remotely
- Virtual consults: review ECGs, discuss test results via video
- App reminders: schedule diagnostics, medications, and follow-ups
9. 🏡 Home Care Tips
- Monitor exercise tolerance, respiratory rate, appetite, and energy
- Avoid strenuous activity unless cleared by a vet
- Use wearable monitors to catch changes early
- Keep up scheduled echo and ECG check-ins via Ask A Vet
- Report any syncopal events or cough episodes promptly
10. 🐕 Breed & Age Notes
- Large breeds like Boxers, Dobermans, and Great Danes—prone to DCM and conduction abnormalities
- Senior dogs may develop idiopathic conduction block due to fibrosis
- Previously healthy dogs may show intermittent, anesthesia-related LBBB
11. 💬 FAQs from Pet Parents
- Is LBBB dangerous?
- Not usually, but it's a signal to investigate underlying heart health.
- Can it be resolved?
- Transient blocks (related to drugs or stress) may improve, but persistent LBBB rarely reverses.
- Do I need a pacemaker?
- No—unless the dog has a high-grade AV block or syncopal episodes.
- Should I restrict exercise?
- Moderate activity is safe. Restrict only if symptoms like cough or collapse occur.
12. ❤️ How to Ask A Vet Support You
- Upload ECG and echo data for specialist interpretation
- Receive AI-based alerts on rhythm shifts or worsening QRS patterns
- Keep track of medications and test schedules within the app
- Schedule teleconsults to discuss test findings and care strategies
- Integrate with your local vet to coordinate diagnostics and interventions
Download our app for seamless cardiac care and ensure your dog's heart remains strong. 🐾
13. 🔚 Final Thoughts
Left bundle branch block in dogs represents delayed electrical conduction in the left ventricle and often signals deeper heart disease. Though incidental LBBB can be benign, persistent or symptomatic cases warrant thorough evaluation. Thanks to wearable ECGs, AI analysis, and telehealth, 2025 offers proactive remote monitoring and management, making it easier than ever to keep your dog's heart on track.
— Dr Duncan Houston, BVSc
