It is important to consider why electrocardiography is being performed as this dictates what tests are required and how you interpret the findings. Electrocardiography is performed during the investigation of poor performance in athletes or during the investigation of an arrhythmia or cardiac murmur identified at routine examination. Important additional information includes any history of exercise intolerance, the horse's signalment and intended use.
Deciding which type of electrocardiogram
Resting electrocardiogram
This is easily achieved with basic equipment and is useful for investigating sustained arrhythmias present at rest. However, it may require follow up with an exercising electrocardiogram (ECG). An intermittent problem can be investigated using 24-hour Holter ambulatory ECG. A 24-hour recording generates a substantial amount of data to analyse.
Exercising electrocardiogram
This requires equipment that is able to record or transmit the ECG to a recording device. The exercise test should be representative of the horse's normal exercise level and should include the recovery period. It is used to assess whether an arrhythmia present at rest resolves during exercise or, conversely, whether an arrhythmia becomes apparent during exercise or recovery.
Acquiring the electrocardiogram
A good quality ECG trace makes interpretation much easier (Verheyen et al, 2010). The fundamental principles are ensuring that the ECG pads have good contact with the horse and limiting movement artefacts (Figure 1).
Tips for achieving a good quality ECG trace include:
- Only clip if the horse has a long haircoat.
- Apply pads to a dry coat.
- Adhesion of the pads can be optimised by:
- Pads that have been stored in a zip-lock bag/from a new packet.
- Extra glue (spray-on adhesive such as Opsite spray) around the edge of the pads.
- You can apply two sets of electrocardiogram pads (8 pads) at the beginning of the session in case any fall off, as it is very difficult to get new pads to stick to the horse once it is sweating.
- Covering the pads with self-adhesive foam (Polsterplast).
- Place the pads out of reach of the rider's leg and any other moving parts.
- Tuck the ECG leads in but make sure they will not put tension on the electrocardiogram pads.
Interpreting the electrocardiogram
It is important to ensure that there is a recording of sufficient length, intensity of exercise is appropriate and the ECG recording is of sufficient quality for interpretation. The ECG software may provide digital analysis, although visual assessment is also required. Further explanation of the use of digital analysis has been documented elsewhere (Mitchell, 2019).
Heart rate analysis
Resting heart rate should be evaluated to assess whether it is appropriate for the type of horse and environment. Heart rate varies slightly with bodyweight, which results in lower heart rates in large horses and higher heart rates in ponies: up to 44–48 beats per minute (bpm). Heart rate response to exercise and excitement should be interpreted in light of the horse's breed, fitness level and the exercise intensity undertaken (Table 1).
Table 1. Approximate heart rate ranges for different levels of athletic activity
| Activity | Heart Rate (bpm) |
|---|---|
| Rest | 30–40 |
| Walk | 60–80 |
| Trot | 80–120 |
| Canter | 120–150 |
| Gallop | 150–180>200 at maximum intensity |
Source: Allen et al, 2016
After high intensity exercise the heart rate would be expected to decrease below 100bpm within 5 minutes. Slow heart rate recov-ery may not be indicative of cardiac pathology, but it can indicate an underlying abnormality, such as equine asthma or orthopaedic conditions (Allen et al, 2016).
Heart rhythm analysis
The basis of rhythm analysis is assessment of the timing and morphology of complexes. Increased heart rate with exercise causes some expected changes in ECG appearance; P-R and Q-T intervals shorten without a big change in QRS complex duration (Box 1). The P wave will become buried in the previous T wave, and at high heart rates you will not be able to identify the P waves (Figure 2). However, there are other aspects of the ECG which should remain consistent regardless of exercise intensity, in particular R-R intervals should remain regular throughout and, although the T wave may change polarity, the QRS complex should keep the same polar-ity (although it may increase in amplitude).
Box 1.QRS complexesQRS complexes – The first downward deflection is the Q wave, upwards is the R wave and the next downward is the S wave. Horses generally have RS complexes when recorded on a base-apex lead configuration, with the largest deflection made by the S wave. However, for convention and understanding between species they are commonly referred to as QRS complexes and the interval between complexes is referred to as the R-R interval.
For areas of irregular rhythm, it can be helpful to apply questions to the ECG to try to identify the arrhythmia. These questions include:
- Is there a P wave for every QRS complex?
- Is there a QRS complex for every P wave?
- Is the P wave morphology normal?
- Is the QRS complex morphology normal throughout (do all the QRS complexes look the same)?
- Is an ectopic beat followed by a pause?
The answers to these questions can then be combined to define the irregularity as much as possible. Wherever possible, identify the origin of the arrhythmia as supraventricular or ventricular, and state whether the rhythm is sustained or paroxysmal.
The next step is to establish how frequent the abnormal beats are and in what situations they occur (at rest, exercise or recovery?).
Common abnormalities
Horses commonly have vagally-mediated dysrhythmias at rest, which disappear with exercise or excitement. These rhythms are sometimes described as physiological.
For second-degree atrio-ventricular block (2AVB) there is a P wave for every QRS complex but not a QRS complex for every P wave. P wave morphology remains the same throughout. This rhythm is considered physiological in most horses, if conducted beats are sufficient to maintain an appropriate cardiac output and the 2AVB disappears with exercise. High-grade/advanced 2AVB is a concern when two or more successive P waves are unconducted and persist with light exercise/stress; these horses are considered less safe to ride (Reef et al, 2014). Sinus arrhythmia occurs as a gradual increase and decrease in heart rate over the course of several beats. It is most commonly seen in recovery from exercise. The P-P and R-R interval will vary, but the P-QRS relationship should be constant and the appearance of QRS complexes will be normal. This is of no concern if the arrhythmia resolves over the recovery period.
Arrhythmias which most commonly cause concern in equine patients include supraventricular and ventricular arrhythmias and atrial fibrillation. These are described below.
Supraventricular premature complexes (SVPC) occur as a premature P wave, accompanied by a QRS complex. P wave amplitude may vary, the QRS complex can appear normal or may vary in morphology, and there may be variation in T wave morphology (Broux et al, 2016). This is typically followed by a non-compensatory pause (the interval of the premature R to the next R wave is similar but slightly longer than the normal R-R interval) (Figure 3). SVPCs can be infrequent and disappear with exercise, in which case they are considered clinically insignificant. They are considered clinically relevant when they occur more frequently. Although they are unlikely to be life-threatening, they can cause an excessively high heart rate at exercise or predispose large horses to develop atrial fibrillation.
Supraventricular tachycardia is defined as four or more SVPCs in a row (Figure 4). P waves and QRS complexes have normal morphology but occur at a higher rate than expected for the horse's level of excitement or pain present.
Ventricular premature complexes (VPCs) are identified when the QRS complex occurs early and is not associated with a P wave (Figure 4). Often the QRS complex has abnormal morphology, often described as ‘wide and bizarre’ (due to the slower ventricular depolarisation). However, in some cases the morphology can be closer to normal. They are commonly followed by a compensatory pause (from the R wave preceding the VPC to the R wave following the VPC is twice the normal R-R interval). Infrequent, isolated VPCs, particularly those occurring at the end of exercise/in recovery, are considered normal. However, due to the potential risk for deterioration to ventricular tachycardia and fibrillation leading to sudden collapse or death, careful assessment is required. Abnormalities causing concern include frequent VPCs, variable morphology of complexes, short coupling intervals and paroxysmal ventricular tachycardia, as well as concurrent structural abnormalities (which would be identified by electrocardiography).
Ventricular tachycardia is the occurrence of four or more VPCs in a row. This may be paroxysmal or sustained. This has the potential to lead to fatal ventricular fibrillation and warrants further investigation.
Atrial fibrillation is seen as an undulating baseline (F waves), with no P waves apparent (Figure 5). This leads to an irregular rhythm of QRS complexes. QRS morphology and duration appears normal.
Atrial fibrillation can be paroxysmal (lasting less than 3–5 days) or sustained. There may be no other clinical signs at rest and low intensity exercise, but during high-intensity exercise it can result in a very high heart rate and poor performance. A full cardiac examination should be performed to identify whether other cardiac abnormalities are present (for example, mitral regurgitation and left atrial enlargement). For horses performing at a lower-exertional level, atrial fibrillation is unlikely to limit performance. However, the safety to continue ridden exercise should be evaluated with exercising ECG and electrocardiography, as horses with atrial fibrillation may have an inappropriately high ventricular rate for the level of exercise or ventricular ectopy, which poses a risk for collapse or sudden death. For horses performing at high/maximal exercise intensity, atrial fibrillation is likely to be performance limiting and treatment is recommended. Treatment is also recommended in any horse showing high maximal heart rates during regular exercise (>220bpm) or abnormal QRS complexes at exercise. Prompt treatment is recommended as atrial fibrillation becomes more difficult to convert to sinus rhythm the longer it continues.
As well as identifying arrhythmias, it is important to be able to identify artefacts and differentiate them from arrhythmias. Arrhythmias are deflections on the ECG not caused by cardiac electrical activity, but mostly caused by movement of horse, electrodes or wires. In general, they are sharp, repeating deflections that are often narrower than the QRS complex. An artefact will not be followed by a T wave, whereas every QRS will be followed by a T wave.
Interpreting the findings
The ECG should be interpreted in light of the signalment of the horse, the context of examination and required outcome. For example, this may involve interpreting whether the current arrhythmia is contributing to poor performance and whether the horse is safe to continue with ridden exercise.
The type and size of horse is likely to impact on the interpretation of clinical findings This is particularly significant for atrial arrhythmias where, in large horses, there is an increased risk of developing atrial fibrillation. The horse's expected work load and use should be taken into consideration. For example, will it be ridden by a child or an adult? This information is important as it indicates the rider's understanding of the risk posed and whether informed consent is possible (Reef et al, 2014).
If the ECG was performed as part of investigation of a cardiac murmur then the findings need to be interpreted alongside the remainder of the cardiac evaluation, in particular eletrocardiographic findings. For example, SVPCs may be considered of greater significance in light of left atrial enlargement and the potential for progression to atrial fibrillation. Ventricular ectopy may be considered particularly grave when encountered in combination with aortic regurgitation and signs of overload of the left ventricle.
What next?
The American College of Veterinary Internal Medicine consensus statement on decision making for equine athletes with cardiac disease (Reef et al, 2014) is an excellent resource to help you to formulate a plan for a horse. Identification of exercise-associated dysrhythmia should prompt evaluation of the entire animal in order to determine the significance of the arrhythmia. In cases of poor performance, this may include orthopaedic examination and respiratory evaluation (upper respiratory dynamic endoscopy/evaluation of the lower airways for inflammation). Further cardiac evaluation would include exercising ECG +/- 24-hour recording if not already performed, as well as assessing plasma cardiac troponin I concentration, electrolyte concentrations and electrocardiography.
Equipment
The equipment used by this author was Televet 100, Kruuse UK.
KEY POINTS
- The aim of equine electrograph interpretation is to determine whether. an abnormality is present and is significant, causing either poor performance or risk of collapse or death.
- A good quality electrocardiogram recording is essential.
- Most commonly an exercising electrocardiogram is required and sometimes a 24-hour recording.
- Vagally-mediated arrhythmias are common at rest but should disappear at exercise.
- Atrial fibrillation or frequent ectopic beats at exercise warrant further cardiac evaluation.