Interpreting ECGs in horses can feel intimidating at first, but the truth is there are a few main arrhythmias to remember. Understanding equine physiology is one of the easiest ways to correctly identify arrhythmias and score points on the NAVLE®. In this guide, you’ll learn to recognize a classic and common physiologic arrhythmia (second-degree atrioventricular (AV) block), differentiate it from the most common pathologic arrhythmia (atrial fibrillation), and build a structured approach to equine ECGs.
Don’t Panic Over Dropped Beats
Horses are athletes by design. Their massive size and impressive cardiovascular efficiency allow them to maintain low resting heart rates, normally between 28-44 bpm. This is the slowest heart rate among common domestic species.
Horses (especially fit ones) also frequently display physiologic arrhythmias at rest due to high vagal tone when relaxed. This leads to common identification of second-degree AV block, as well as sinus arrhythmia and first-degree AV block.
The ability to recognize physiologic arrhythmias allows you to know when auscultatory or ECG abnormalities are significant.
Focus on the Big Three Equine Cardiac Principles
If you understand these three fundamentals, equine ECGs are straightforward:
1. Bigger Animals have Slower Heart Rates
Massive stroke volume allows horses to circulate blood with a slower resting rate.
2. Athletic Horses Display High Vagal Tone
Relaxed, athletic horses often develop normal physiologic arrhythmias (e.g., sinus arrhythmia, second-degree AV block) that are normal at rest and disappear with light exercise. The most common example is second-degree AV block, where there is a P wave without an associated QRS complex. The rhythm is normal between dropped beats, and resting heart rate might be slightly low (22-28 bpm). This is a physiologic (normal) arrhythmia seen in healthy athletes and should not warrant worry or intervention.
3. Atrial Fibrillation is the Most Common Pathologic Arrhythmia
It is critical to distinguish second-degree AV block from atrial fibrillation (Afib), the most common pathologic arrhythmia causing poor performance in horses.
| Assessment | Atrial Fibrillation | Second-Degree AV Block |
|---|---|---|
| Auscultation | Irregularly irregular No S4 | Regularly irregular Hear S4 (atrial contraction) in dropped beat |
| ECG | No P waves (instead fibrillation-waves) Irregular R-R interval | Occasional P wave not followed by QRS (not >2 in a row) Regular R-R intervals (except at site of dropped beat) |
| Response to Exercise | Tachycardia, exercise intolerance Arrhythmia persists, +/- more complex arrhythmias develop | Sinus Rhythm |
| History | Exercise intolerance/poor performance (can be incidental in minimally athletic horses) | None; incidental finding |
How to Interpret Equine ECGs
Veterinary students benefit from using a systematic, repeatable ECG interpretation strategy, as outlined below:
Interpret the ECG, Recognizing Normal Sinus Rhythm
Start by identifying the P wave, QRS complex, and T wave. Ask yourself:
- Is there a P wave for every QRS complex?
- Is there a QRS complex for every P wave?
- Are the R-R intervals regular or irregular?
- Do the QRS complexes all look similar?
Identify Second-Degree AV Block
In second-degree AV block, the ECG strip shows mostly normal P-QRS-T complexes, with occasional “dropped” beats (i.e., AV block):
- Normal P wave
- No QRS, instead a pause
- The next P wave is when you would expect it (consistent P-P interval)
- Normal rhythm between dropped beats
Distinguish Normal from Abnormal
Second-degree AV block is considered physiologic when:
- The horse is calm or resting
- The dropped beat is infrequent (i.e., not >2 dropped beats in succession)
- Resting heart rate is normal (or often a bit low, 22-28)
- Exercise or sympathetic stimulation causes sinus rhythm
Look for Red Flags
Escalate concern if you observe:
- High-grade AV block (i.e., >2 blocked P-waves in a row)
- Persistent AV block during exercise
- Syncope, weakness, or collapse
- Atrial fibrillation
- Identification of additional/other arrhythmias (e.g., atrial or ventricular premature contractions, ventricular tachycardia)
These may signal structural or inflammatory heart disease that warrant a full cardiac workup.
Study Habits that Build ECG Confidence
ECG interpretation improves with repetition, not memorization. Try these habits:
- Review normal and abnormal ECGs side-by-side
- Practice identifying second-degree AV block and atrial fibrillation
- Create your own ECG flashcards labeled only with waveforms and key clues
Students who recognize the normal patterns more confidently identify arrhythmias, allowing them to succeed quickly.
Your NAVLE®-Friendly ECG Interpretation Framework
Use this framework for every equine ECG question:
1. Classic Case Pattern
- Second-degree AV block: healthy/athletic adult horse, low resting HR with regularly irregular rhythm, occasional dropped beat (P wave not followed by QRS)
- Afib: Adult horse presenting for poor performance (most often a racehorse), irregularly irregular heart rhythm, no P waves, and irregular R-R intervals
2. Test of Choice
Careful auscultation followed by light exercise (when safe) and repeat auscultation. Physiologic arrhythmias resolve with increased heart rate. Use ECG to confirm when needed.
3. First-Line Response
Second-degree AV block is a physiologic arrhythmia that does not require treatment. Treatment of Afib varies depending on the cause and horse’s intended use: use exercising ECG and echocardiography to determine the best recommendation (i.e., to cardiovert or not).
Master the Normal to Recognize the Abnormal
Second-degree AV block represents one of the most common physiologic arrhythmias in horses and is not a cardiac emergency. A calm, methodical approach—identify the P wave, confirm the missing QRS, and evaluate the horse’s demeanor and clinical signs—allows you to move through NAVLE® questions with confidence. Remember the main rule-out is Afib, which usually presents differently and has different ECG findings. Master the normal patterns first, and both the exam and clinical practice become much more manageable.
More from Zuku Learning:
- Master NAVLE® concepts
- Subscribe to the Question of the Day to receive a NAVLE® question delivered to your inbox daily
- Review the No Zebras: How to Triage your NAVLE® Study for Maximum Points blog
- Review the How to Build a NAVLE® Study Plan that Works blog
