Third-Degree AV Block in Donkeys: Fainting, Collapse and Pacemaker Options

Third-degree atrioventricular (AV) block, also called complete heart block, is a serious rhythm disorder where the electrical signal from the upper chambers of the heart does not reach the lower chambers. That means the atria and ventricles beat independently instead of working together. In a donkey, this can lead to a very slow ventricular escape rhythm and poor blood flow to the brain and body.

In donkeys, complete AV block has been linked with weakness, fainting episodes, and collapse. Review articles on donkey internal medicine note that third-degree AV block appears to be reported with some frequency in donkeys, particularly miniature donkeys, and case reports describe successful pacemaker treatment in affected animals.

This is different from the more common second-degree AV block often seen in resting horses, which can be a normal vagal rhythm change. Advanced AV block that causes syncope is not considered a normal finding. If your donkey is passing out, tiring easily, or has a very slow pulse, your vet should treat it as a true cardiac emergency until proven otherwise.

The exact cause is not always clear. In some donkeys, especially young miniature donkeys, the problem may be congenital, meaning present from birth. UC Davis described a miniature donkey foal with fainting episodes whose ECG confirmed third-degree AV block; routine lab work did not support infection or electrolyte imbalance, so a congenital cause was suspected.

In other cases, complete AV block may be associated with structural heart disease or changes affecting the conduction system. Published miniature donkey cases included concurrent valve abnormalities, patent ductus arteriosus, aortic root dilation, and episodes of ventricular tachycardia. In horses, advanced AV block has also been associated with inflammatory heart disease such as myocarditis, so your vet may look for evidence of myocardial injury or systemic illness.

Your vet will also consider reversible contributors such as electrolyte disturbances, toxin exposure, or medication effects, even though these are not the most common explanation for persistent complete AV block. The key point is that the rhythm problem itself is dangerous whether the underlying cause is congenital, degenerative, inflammatory, or mixed.

Diagnosis starts with a physical exam, heart rate assessment, and careful history. Your vet will want to know exactly what the collapse episodes look like, how long they last, whether they happen with exercise or stress, and whether your donkey has had any recent illness, toxin exposure, or medication changes.

An electrocardiogram, or ECG, is the key test to confirm the diagnosis. In complete AV block, the ECG shows AV dissociation, meaning the atria and ventricles are beating independently. In reported donkey cases, ECG documented third-degree AV block with very slow ventricular escape rhythms, and in some donkeys there were also intermittent abnormal conducted beats or ventricular tachycardia.

Most donkeys also need echocardiography to look for valve disease, congenital defects, chamber enlargement, or other structural problems. Bloodwork is commonly used to check electrolytes, inflammatory changes, and overall organ function. Depending on the case, your vet or referral hospital may also recommend cardiac troponin testing, ambulatory ECG monitoring, or referral to a large-animal internal medicine or cardiology service before discussing pacemaker candidacy.

There is no guaranteed way to prevent third-degree AV block, especially when it is congenital or related to changes within the heart’s conduction system. Still, early detection can reduce the risk of traumatic collapse and may improve the chance of timely referral. Ask your vet to evaluate any unexplained fainting, exercise intolerance, unusual quietness, or very slow heart rate instead of assuming it is normal behavior.

Routine wellness exams matter. Careful auscultation can pick up dropped beats, murmurs, or marked bradycardia that deserve follow-up ECG testing. If your donkey has had a collapse episode, keep a written log or video if it is safe to do so. That information can help your vet distinguish syncope from seizures, weakness, or musculoskeletal falls.

Good general health care also supports the heart. Work with your vet on parasite control, vaccination, dental care, nutrition, and prompt treatment of systemic illness. Avoid access to known toxins and discuss all medications or supplements before use. These steps may not prevent every case, but they can help your vet identify contributing problems sooner and guide safer care.