The ECG Decoded: A Veterinarian’s Guide to the Heart’s Rhythm – Part 5: Rapid Rhythms from Above – Demystifying Supraventricular Tachycardias

by

Estimated reading time: 4.25 minutes

 

Welcome back to our series, The ECG Decoded: A Veterinarian’s Guide to the Heart’s Rhythm. After exploring the world of slow heart rhythms, we now turn to the opposite end of the spectrum: rhythms that are too fast. This installment focuses on tachycardias originating from above the ventricles, a group of arrhythmias that can be challenging to distinguish but are crucial to identify correctly for effective management.

 

The Supraventricular Tachycardias (SVTs): A Unified Approach

Supraventricular Tachycardias (SVTs) are defined as rapid, regular rhythms originating from the atria or the atrioventricular (AV) junction. While specific types exist, a practical clinical approach prioritizes recognizing their shared characteristics before diving into complex classifications. The key to identifying an SVT lies in the QRS complex: in the vast majority of cases, the QRS complex is narrow (< 70 ms in dogs), indicating that ventricular activation is proceeding via the normal His-Purkinje system.

The most common mechanisms behind SVTs are re-entry (an electrical impulse circling a small, abnormal circuit) and enhanced automaticity (a group of cells firing too quickly).

Let’s break down the most clinically relevant types you will encounter.

  1. Atrial Tachycardia (AT)
    This rhythm originates from a single, irritable focus in the atria outside the SA node.
  • ECG Hallmarks:
    • Rate: Typically 160-240 bpm in dogs.
    • P Waves: Often visible but have a different shape (morphology) from the normal sinus P wave, as they originate from a different part of the atrium. They may be buried in the preceding T wave, making them difficult to see.
    • Rhythm: Usually regular.
    • Context: Often associated with underlying structural heart disease (e.g., atrial enlargement).
  1. Atrioventricular Reentrant Tachycardia (AVRT)
    This is a classic re-entrant rhythm that utilizes an accessory pathway—an abnormal band of tissue connecting the atria and ventricles—as part of its circuit. This is the mechanism underlying Wolff-Parkinson-White (WPW) syndrome, which can be suspected on a normal ECG by a short PR interval and a slurred upstroke of the QRS complex (delta wave).
  2. Atrioventricular Nodal Reentrant Tachycardia (AVNRT)
    This is another common re-entrant rhythm where the circuit is contained entirely within the AV node itself. It is typically very fast and regular, and P waves are often not visible because atrial and ventricular depolarization occur simultaneously.

 

A Practical Tip: The “Regular, Narrow-Complex Tachycardia”
In an emergency or during a brief ECG recording, precisely distinguishing between AT, AVRT, and AVNRT can be difficult. The most important initial step is to recognize the pattern of a regular, narrow-complex tachycardia. This tells you the rhythm is likely an SVT and not the more dangerous Ventricular Tachycardia (VT), guiding immediate therapeutic decisions.

 

Junctional Rhythms: The AV Node Takes Over

The AV junction possesses innate pacemaker cells that can act as a backup if the SA node fails. A Junctional Rhythm occurs when this junctional focus accelerates and takes over as the primary pacemaker of the heart.

  • ECG Hallmarks:
    • Rate: A Junctional Escape Rhythm is typically slow (40-60 bpm in dogs), serving as a life-saving backup. An Accelerated Junctional Rhythm is faster (60-140 bpm) and competes with the sinus rhythm.
    • P Waves: The relationship of the P wave to the QRS is key. Because the impulse originates in the AV junction, it depolarizes the atria (retrogradely) and the ventricles simultaneously. This results in P waves that may be inverted in lead II and can occur immediately before, during (hidden), or just after the QRS complex.
    • QRS Complex: Narrow, unless a pre-existing bundle branch block is present.

Junctional rhythms are often a sign of underlying issues such as high vagal tone, drug toxicity (e.g., digoxin), or inflammatory heart disease.

 

Differentiating SVT from Sinus Tachycardia: A Critical Skill

A common diagnostic challenge is distinguishing a true SVT from a physiologic sinus tachycardia. This distinction is critical because treating an SVT with negative chronotropic drugs (like beta-blockers or calcium channel blockers) can be therapeutic, whereas using them inappropriately on a compensatory sinus tachycardia (e.g., from hypovolemia) can be disastrous.

Feature Sinus Tachycardia Supraventricular Tachycardia (SVT)
Onset/Offset Gradual (waxes and wanes) Often paroxysmal (abrupt start/stop)
P Waves Normal, upright morphology in lead II Abnormal (different shape), inverted, or absent
Heart Rate Usually < 180 bpm in dogs Often > 220 bpm in dogs
Response to Therapy Slows with treatment of underlying cause (e.g., fluids, pain relief) Requires specific antiarrhythmic therapy

 

At CardioBird, our AI is meticulously trained to analyze these subtle distinctions. It doesn’t just detect a fast heart rate; it measures QRS width, assesses P wave presence and morphology, and evaluates rhythm regularity to provide a specific diagnosis, such as “Atrial Tachycardia” or “Accelerated Junctional Rhythm.” This level of detail empowers you to move beyond simply identifying a tachycardia to understanding its origin, which is the foundation of selecting the right treatment.

In our next issue, we will tackle one of the most critical topics in emergency medicine: the identification and management of Ventricular Arrhythmias.

 

The CardioBird Team