Second-Degree AV Block in Horses: Normal Finding or Heart Problem?
- Second-degree AV block is a common resting rhythm change in healthy horses, especially fit horses with high vagal tone.
- In many horses, the skipped beat pattern disappears with excitement, trotting, or light exercise. That pattern is usually considered physiologic rather than dangerous.
- It becomes more concerning if it does not go away with exercise, happens in long runs of dropped beats, or is paired with poor performance, weakness, fainting, or collapse.
- Your vet usually confirms the rhythm with auscultation and an ECG, then decides whether the horse needs only monitoring or a fuller cardiac workup.
What Is Second-Degree AV Block in Horses?
Second-degree atrioventricular, or AV, block is a heart rhythm pattern where some electrical impulses from the atria do not reach the ventricles. On exam, that can sound like a pause or a "dropped beat." In horses, this finding is often linked to strong parasympathetic, also called vagal, tone at rest. Because athletic horses commonly have high vagal tone, a second-degree AV block can be a normal physiologic finding rather than a heart disease diagnosis.
What matters most is context. A resting horse with an otherwise normal exam may show an occasional dropped beat that disappears with excitement or light exercise. That pattern is often considered benign. In contrast, a horse with repeated dropped beats that continue during exercise, very slow heart rates, poor performance, weakness, or collapse needs prompt veterinary evaluation because the rhythm may be pathologic.
This is why the same ECG label can mean very different things in different horses. For one horse, it may be an incidental finding during a routine exam. For another, it may be a clue that the conduction system or heart muscle needs a closer look.
Symptoms of Second-Degree AV Block in Horses
- No obvious signs at rest
- Irregular rhythm or occasional pause heard during auscultation
- Low resting heart rate
- Poor performance or exercise intolerance
- Weakness, stumbling, or seeming "off" during work
- Episodes of collapse or fainting
Many horses with a physiologic second-degree AV block have no symptoms at all. The rhythm is often found during a routine exam, pre-purchase evaluation, or workup for an unrelated issue. A skipped beat pattern at rest is not automatically an emergency.
When to worry: call your vet sooner if the horse has poor performance, tires unusually fast, seems weak, has repeated long pauses, or the rhythm does not disappear with light exercise. See your vet immediately if your horse has collapse, near-collapse, or any episode that could put the horse or rider at risk.
What Causes Second-Degree AV Block in Horses?
The most common cause is high vagal tone in a relaxed horse. Horses naturally have strong parasympathetic influence on the heart, and that can slow conduction through the AV node enough to create an occasional blocked beat. This is especially common in fit, calm horses at rest and may disappear once the horse becomes alert or starts moving.
Less commonly, second-degree AV block reflects a true conduction problem. That is more likely if the rhythm is frequent, occurs in high-grade patterns with multiple blocked beats in a row, persists during exercise, or is associated with clinical signs. In those cases, your vet may look for underlying heart disease, inflammation, structural heart changes, electrolyte problems, medication effects, or disease affecting the conduction tissue.
Auscultation alone cannot always tell the difference between a normal resting rhythm change and a clinically important arrhythmia. That is why exercise response and ECG findings matter so much. The same pause that is harmless in one horse may deserve a deeper workup in another.
How Is Second-Degree AV Block in Horses Diagnosed?
Your vet starts with a history and physical exam, including careful auscultation of the heart at rest. They will want to know whether the horse has poor performance, weakness, collapse, or any change under saddle or in harness. Because atrial fibrillation and other arrhythmias can also sound irregular, an ECG is usually the key test to confirm what rhythm is present.
A resting ECG often shows the blocked P wave pattern clearly. Your vet may then repeat the exam during excitement, hand-walking, trotting, or another controlled exercise setting. If the second-degree AV block disappears with light exercise, that supports a physiologic resting rhythm. If it persists, becomes more complex, or the horse has concerning signs, your vet may recommend a more complete cardiac workup.
That workup can include echocardiography to look for structural heart disease, ambulatory ECG or Holter monitoring to measure how often the rhythm occurs over time, and bloodwork to check for contributing problems such as electrolyte abnormalities or systemic illness. In performance horses, the goal is not only to label the rhythm but also to understand whether it changes safety, workload, or prognosis.
Treatment Options for Second-Degree AV Block in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm call or clinic physical exam
- Auscultation at rest and after excitement or light exercise
- Basic resting ECG if available
- Short-term monitoring and activity guidance from your vet
Recommended Standard Treatment
- Complete veterinary exam and history review
- Resting ECG plus exercise-response assessment
- Targeted bloodwork such as CBC, chemistry, and electrolytes when indicated
- Clear recommendations on riding, training, and recheck timing
Advanced / Critical Care
- Referral to equine internal medicine or cardiology
- Echocardiogram
- Ambulatory ECG or Holter monitoring
- Exercise ECG or monitored performance evaluation
- Expanded diagnostics and case-specific treatment planning
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Second-Degree AV Block in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Does this rhythm look physiologic at rest, or do you think it could be pathologic?
- Did the AV block disappear with excitement or light exercise?
- Is an ECG enough for now, or should we add an echocardiogram or longer monitoring?
- Is it safe for my horse to be ridden, driven, or competed while we are evaluating this?
- Are there any signs that would mean I should stop work and call right away?
- Could medications, electrolyte changes, illness, or another heart condition be contributing?
- How often should we recheck this rhythm if my horse has no symptoms?
- What level of work is reasonable for my horse based on these findings?
How to Prevent Second-Degree AV Block in Horses
You usually cannot prevent the common physiologic form of second-degree AV block, because it is tied to normal resting vagal tone in many healthy horses. In that setting, the goal is not prevention but recognition. A calm horse with an occasional dropped beat at rest may be normal, especially if the rhythm disappears with light exercise.
What you can do is reduce the chance of missing a more important problem. Keep up with routine veterinary exams, mention any poor performance or unusual fatigue, and tell your vet if the horse has ever stumbled, seemed weak, or collapsed. Performance horses and horses with a newly detected irregular rhythm may benefit from a more structured cardiac evaluation before returning to harder work.
Good general health care also matters. Work with your vet to manage hydration, electrolyte balance, underlying illness, and medication use. These steps do not guarantee prevention, but they help support normal heart function and make it easier to spot when a rhythm change is no longer acting like a normal finding.