Horses—sport horse breeds in particular—have large hearts built for demanding physical exertion and stamina. However, cardiac arrhythmias are still extremely common. And although horses will typically present for a cardiovascular workup on the basis of “poor performance,” the cause and treatment can vary widely depending on the underlying etiology and type of arrhythmia.
While some arrhythmias are not cause for alarm, in some cases vets need to perform and evaluate an electrocardiogram (ECG) in order to confirm their diagnosis and determine the necessary treatment. The Piavet Platform displays a full equine ECG from the sensor readings of a palm-sized device (but more on that later…).
EQUINE CARDIAC ARRHYTHMIAS
By its most basic definition, a cardiac arrhythmia is an irregular heartbeat. More specifically, they are disturbances in heart rate, rhythm, or conduction, resulting in abnormal heart sounds. There are several types of equine cardiac arrhythmias, which are typically classified according to their origin (atrium or ventricles) and their rate (bradycardia or tachycardia). They may also be classified as pathological or physiological, meaning they result from disease or from normal functions of the body, respectively.
Cardiac rhythms are considered either normal or abnormal, although arrhythmias are extremely common in horses. As equine cardiovascular expert Dr. Peter Physick-Sheard explains, having a steady rhythm is abnormal. “What we’ve learned is that variations in heart rhythm don’t necessarily mean disease, and disease is not necessarily associated with variations in heart rhythm. The normal variation is huge. It’s absolutely normal for all horses to show some rhythm disturbances day to day. In fact, an absolutely steady, regular rhythm is abnormal.”
He emphasizes that it’s important to know what a horse’s normal heart rhythm looks like to help diagnose and treat problems when they’re not healthy. This is why regular, long-term ECG monitoring is the best way to get a complete picture.
Supraventricular arrhythmias originate in the upper chambers of the heart (in the atria), in the atrioventricular node, or in proximal junctional tissue.
Sometimes, supraventricular premature depolarizations (SVPDs) will accompany these arrhythmias. SVPDs are an abnormal event resulting from the existence of an ectopic pacemaker either in the atrium or in the AV bundle. While these depolarizations may be transmitted to the ventricle, those that are not transmitted are often associated with second degree atrioventricular block.
Supraventricular tachycardia (SVT) occurs when a horse has more than four SVPD in a row, and occurs because of a sudden release of vagal tone at the atrioventricular (AV) node. There are two types: sinus tachycardia and atrial tachycardia. Sinus tachycardia is mostly normal and is associated with increased sympathetic tone (excitement or pain), while atrial tachycardia can underline atrial myocardial disease or even systemic disease.
High-grade (advanced) second degree atrioventricular block
The term high-grade or advanced is added to second degree AV block when the ECG shows two or more non-conducted P waves in a row. This means the periods of AV block are prolonged and don’t disappear with exercise. Horses with this diagnosis can be unsafe to ride.
Third degree atrioventricular block
Third degree atrioventricular block is a pathological and rare arrhythmia characterized by a lack of transmission through the AV node between the atria and ventricles. Horses will experience severe exercise intolerance and fainting spells.
Atrial fibrillation (AF) is the most common atrial arrhythmia in horses. Defined by irregular or non-functional contracting of the atria, AF is associated with poor performance and exercise intolerance in horses. Most horses with AF will not have signs of cardiac disease, but it can be connected to an enlargement of the atrium or congestive heart failure. Symptoms can include poor performance in equine athletes, respiratory alterations, myopathies, and colic.
Ventricular arrhythmias—those originating in the lower chambers (ventricles) of the heart—are less common than supraventricular arrhythmias but are the most concerning type. Within the ventricular myocardium, “extra” depolarizations that result in abnormal QRS-T complexes (not preceded by P waves) can occur with these types of arrhythmias.
Cardiac problems like bacterial and viral infections, damage to the heart muscle, accidental poisoning, dehydration or electrolyte imbalance can all lead to ventricular arrhythmias. These inflammations can lead to necrosis or fibrosis of the myocardium, toxemia or even hypoxia.
Ventricular premature depolarizations (VPDs) are the result of abnormal electrical activity that originates in the ventricles. Infrequent VPDs—those that occur less than once per hour—are considered normal when found at rest and/or following exercise. However, when they’re frequent, polymorphic (i.e. different shapes, indicating origin in more than one site in the ventricles), or occur during exercise, they’re considered abnormal.
Ventricular tachycardia, when sustained, is a severe and life-threatening dysrhythmia. It usually comes secondary to severe underlying cardiac disease or electrolyte and metabolic imbalances.
Ventricular fibrillation is most often fatal and occurs when there are no organized ventricular depolarizations or contractions. It’s associated with increased myocardial irritability and is usually preceded by ventricular tachycardia.
VITAL SIGN MONITORING WITH THE PIAVET SYSTEM
The Piavet Measuring Device—one of four components that make up the Piavet System—uses innovative sensor technology to capture vital signs such as heart rate, body temperature and respiration rate. The Piavet Platform also displays a full ECG with R-R interval measurements as well.
In addition to these parameters, activity is also measured. And when identifying arrhythmias, this can be important. For instance, the prevalence of a second degree atrioventricular block can be more readily identified during an ECG monitoring at rest. And comparing the prevalence of these at-rest readings during light exercise will provide insight for clinical diagnosis.
Benefits of a remote, long-term ECG
A continuous ECG recording allows vets to better diagnose equine cardiac arrhythmias that would otherwise be difficult to detect. The mere presence of a veterinarian can cause white coat hypertension in horses, potentially altering the heart rhythm during a standard ECG.
To avoid this, the Piavet System enables remote ECG monitoring of cardiac arrhythmias. The Piavet Measuring Device is a palm-sized, multi-sensor electrode that fits securely into a surcingle-like belt that the horse can wear unsupervised. (Yes, it’s roll proof). It’s designed to be remotely accessible, meaning vets don’t have to be physically present with the patient, allowing the vet to capture the horse’s true heart rate and rhythm.
In general, severe arrhythmias usually appear after intense exercise or anesthesia. Continuous monitoring following these events is very important for vets to get a complete picture.
Equine cardiology is a complex topic, but vets who care for sport horses are no stranger to the associated conditions. At Piavita, we are proud to bring vets a remote vital sign monitoring system that displays a full equine ECG from the sensor readings of our Piavet Measuring Device. Click here for more information about our technology, or contact us at any time to learn more.