CardioBird

Estimated reading time: 4.37 minutes

 

Welcome back to our series, The ECG Decoded: A Veterinarian’s Guide to the Heart’s Rhythm. Having explored how arrhythmias originate and how the ECG reflects structural changes, we now turn our focus to rhythms that are too slow. This installment provides a clear framework for understanding and identifying bradyarrhythmias and conduction disturbances—conditions where the heart’s electrical system fails to generate or properly conduct impulses, resulting in a dangerously low heart rate.

 

The Two Main Culprits: Failure to Fire vs. Failure to Conduct

Bradyarrhythmias fundamentally arise from two problems: a failure of impulse generation or a failure of impulse conduction. This distinction is crucial for accurate diagnosis and treatment.

1. Disorders of Impulse Generation
   These occur when the heart’s natural pacemakers don’t fire as they should.

• • Sinus Bradycardia: This is a simple, slow sinus rhythm where the SA node fires at a rate below the normal species-specific range. It’s common in athletic dogs at rest and can be a normal finding in some patients. However, it becomes clinically significant when it causes weakness or collapse, often indicating sick sinus syndrome (SSS) or high vagal tone.
  • Sinus Arrest/Sinoatrial (SA) Block: Here, the SA node temporarily fails to generate an impulse (arrest) or the impulse is blocked before it can exit the node (SA block). On the ECG, this appears as a pause with no P wave and no subsequent QRS complex. The length of the pause is not a multiple of the underlying R-R interval. Prolonged pauses (>3-4 seconds in dogs) are a hallmark of Sick Sinus Syndrome, a disorder of the SA node that can also include periods of tachycardia (“tachy-brady syndrome”).

2. Disorders of Impulse Conduction: Heart Block
   These occur when an impulse is generated but is delayed or blocked entirely as it travels through the conduction system.

• • Atrioventricular (AV) Block:
    • First-Degree AV Block: This is a consistent delay in conduction through the AV node. Every P wave is followed by a QRS complex, but the PR interval is prolonged. While often benign, it can be a sign of heightened vagal tone or early degenerative disease.
    • Second-Degree AV Block: This is an intermittent failure of conduction. Some P waves are followed by QRS complexes, and others are not. Mobitz Type I (Wenckebach) is characterized by a progressively lengthening PR interval until a QRS complex is dropped. It is often vagally mediated. Mobitz Type II involves a sudden, unexpected non-conducted P wave without a change in the PR interval. This type is more serious and often indicates structural disease below the AV node, frequently progressing to third-degree block.
    • Third-Degree (Complete) AV Block: This is a complete failure of conduction. No impulses from the atria reach the ventricles. The atria and ventricles beat independently—the P waves “march through” the rhythm at their own rate, while a slow escape rhythm (typically 20-40 bpm in dogs) originating from the ventricles controls the heart. This is a life-threatening arrhythmia that almost always requires pacemaker implantation.
  • Bundle Branch Block: This is a conduction disturbance within the ventricles, where one of the bundle branches fails to conduct. The impulse must travel through the muscle to activate the affected ventricle, resulting in a widened QRS complex. The key is that the rhythm is regular, and a P wave precedes every QRS. A bundle branch block itself does not cause bradycardia but is an important indicator of underlying conduction system disease.

 

Clinical Signs: When is a Slow Heart Rate a Problem?

The clinical significance of bradyarrhythmia depends entirely on its hemodynamic impact. Ask yourself: Is the heart rate sufficient to maintain cardiac output?

• Compensated: An animal may be asymptomatic with a slow rate if it can maintain adequate blood pressure and perfusion to vital organs.
• Decompensated: Clinical signs arise from poor perfusion and include lethargy, weakness, exercise intolerance, syncope (fainting), and even sudden cardiac death.

The onset of clinical signs is your primary indicator that bradyarrhythmia requires intervention.

 

A Practical Approach to the ECG

When you identify a slow heart rate, follow this systematic approach:

1. Identify the P Waves: Are they present and regular?
2. Assess the Relationship between P waves and QRS complexes: Is there a consistent P:QRS ratio (1:1)? Are P waves being blocked?
3. Measure Intervals: Is the PR interval constant, progressively lengthening, or absent?
4. Examine the QRS Complex: Is it narrow (suggesting a high escape focus) or wide (suggesting a ventricular escape focus)?

This logical process will lead you directly to the correct diagnosis, whether it’s sinus bradycardia, a type of AV block, or sinus arrest.

 

At CardioBird, we understand that distinguishing between these nuanced rhythms can be challenging in a busy practice. Our AI-ECG analysis is specifically designed to detect these pauses, measure critical intervals, and differentiate between the various types of bradyarrhythmias with high accuracy. We provide you with a clear, evidence-based interpretation, helping you determine the urgency of the situation and make confident decisions for your patients.

In our next issue, we will shift gears to the opposite end of the spectrum as we explore the rapid and often complex world of supraventricular tachycardias.

 

The CardioBird Team