Exercising electrocardiographic methods

Electrocardiography (ECG) is the gold standard for definitively diagnosing rhythm disorders,^1,2 and permits monitoring of the horse’s heart rate and rhythm during exercise. Continuous monitoring of the ECG can be accomplished with telemetry, with electrodes positioned in a modified base-apex configuration and attached to a transmitter that sends signals to a receiver.^3,4 Electrodes can either be placed on the neck base and flank or positioned, under a surcingle, at the level of the sixth or seventh intercostal space (Fig. 11.1). The signal is then displayed on an oscilloscope, which can be digitized or printed to permanently record the ECG. These types of systems can be used in conjunction with various exercise tests that best mimic or exceed the usual level of exercise of the horse. This can include simply lungeing the horse, riding the horse on the flat or over jumps, or exercising it at high speed on a racetrack, training gallop or on a high-speed treadmill.

Figure 11.1 Options for electrode placement during exercising ECG. The inset illustrates a transverse section and shows the relationship of the electrodes around the heart. RA, right arm; LA, left arm; LL, left leg.

Although less desirable than telemetry, continuous 24-hour ambulatory ECG monitoring (see Chapter 10) can also be used to monitor heart rate and rhythm during exercise. The main disadvantage with Holter monitors is that they do not display the ECG in real time. However, in some situations, when the exercise area is beyond the range of a telemetry unit, they may be more appropriate to use. For example, if it is not practical to reproduce exercising conditions using a treadmill, arena or racetrack, such as may occur with dysrhythmias suspected during endurance exercise, a Holter monitor can be placed during a long distance ride. This allows the potential to re-create similar environmental conditions and stresses. Holter montitors can also be used to identify the presence of dysrhythmias in a 24-hour period that warrants assessment for dysrhythmias during exercise.

Heart rate

Monitoring the heart rate at different exercise intensities and as the horse recovers from exercise has been utilized to determine fitness and the presence of possible cardiac disease. A range of heart rates for various exercise intensities has been described, with an expected heart rate of 80–120 beats per minute (bpm) at the trot (nonracing), 120–150 bpm at the canter, 150–180 bpm at a hand gallop and >200 (200–240) bpm at maximum exercise intensity⁴ (Fig. 11.2). If the heart rate deviates from these ranges, cardiac disease or lack of fitness may be a cause; however, it is important to remember that noncardiac factors may play a role in determining heart rate, including pain, respiratory disease, dehydration, or environmental conditions such as high heat or humidity. These other causes must first be ruled out before attributing heart rate deviations to cardiac disease. These ranges are also useful to determine if the horse is capable of performing its expected workload. If a horse can attain the desired level of work before reaching its maximal heart rate, it should theoretically be able to maintain that intensity, barring other problems. If it reaches a maximal heart rate at low speeds or exertion, it is unlikely it will be able to perform more vigorously. ( AF, VSD)

Figure 11.2 Continuous base-apex ECG of a horse exercising at various speeds on a high-speed treadmill. This shows the normal heart rate response to exercise and the appearance of the ECG. The resting ECG shows a bifid P wave and a bidirectional T wave. As the heart rate and sympathetic tone increase, the T wave becomes positive and increases in amplitude. Also, as the heart rate increases maximally, the P waves become more difficult to distinguish from the preceding T wave. However, the R–R interval remains regular, and the configuration of the R waves remain similar.

Heart rate decrease following exercise has also been used as a guide to fitness or cardiovascular problems. Generally, after very high-intensity exercise the horse’s heart rate decreases to below 100 bpm within 5 minutes, and returns to normal within 45 minutes. The same caveats for interpretation of delayed return of heart rate to normal values are true with heart rate recovery. This can be influenced by many other factors including pain, respiratory disease, dehydration, heat and humidity.

Variations in ECG configuration

Although changes in the configuration of the ECG, in particular, the T wave have been used to predict performance or organic heart disease, caution must be used in the interpretation of some deviations. Some of the components may have a different appearance from that expected, depending on changes in autonomic nervous system tone, with the most notable changes occurring in the T wave. During strenuous exercise the ECG becomes more difficult to critically evaluate because of high heart rates and motion artifacts. There are several changes that occur in the ECG during exercise. The T wave will often change polarity during exercise, from biphasic or negative to positive, with an increase in amplitude, if a modified base-apex lead system is used. The T wave may remain positive for some time after exercise has ended and the heart rate decreases, before returning to a resting configuration. The amplitude of the QRS may increase and be variable during exercise, and depending on the positioning of the electrodes, there may be motion-related artifacts. The P–R and Q–T intervals will shorten as the heart rate increases, however there will be little change in the QRS duration. The P wave will often become difficult to discern with increasing heart rates, as it can become buried in the preceding T wave. However, the R–R interval should always remain absolutely regular, and the polarity of the QRS should remain the same.

Dysrhythmias associated with exercise

Monitoring for the presence of dysrhythmias is the most important benefit of performing an exercising ECG. Continuous monitoring of the heart rhythm during exercise is vital to the diagnosis of dysrhythmias that may impact performance, or potentially even result in collapse or death. ( AF) Paroxysmal atrial fibrillation (PAF) is an important cause of poor performance;^5–7 however, it can be difficult to document because horses often convert to sinus rhythm before it is possible to complete a detailed electrocardiographic examination.^6,8 Therefore, it may be difficult to confirm PAF in those horses in which an irregularly irregular rhythm is auscultated after fading or stopping in a race, unless an ECG is obtained at that time. However, the presence of frequent supraventricular premature depolarizations (SVPD) in a 24-hour period might support a tentative diagnosis of PAF in these situations. If numerous ventricular premature depolarizations (VPD) are observed in a 24-hour period, shortly after an exercise-associated dysrhythmia is auscultated, this may suggest that the dysrhythmia could have been ventricular in origin. ( AF)

Horses have a high prevalence of dysrhythmias at rest that are vagally mediated and disappear with exercise. They may also have occasional premature depolarizations at rest; however, if they are infrequent, isolated and disappear with exercise they are unlikely to be of consequence (see Figs. 10.1 and 10.2). Conversely, horses may have a normal sinus rhythm at rest; however, this does not preclude them from developing dysrhythmias during peak exercise. That may impact performance. Cardiac dysrhythmias may be present before, during and after exercise in horses with no underlying cardiac disease and their interpretation can be difficult. A recent study examined the occurrence of dysrhythmias during warm-up, exercise and in the immediate post-exercise period in clinically healthy horses performing up to expectations⁹. In this study, isolated SVPD and VPD prior to exercise were a relatively common occurrence in these clinically normal horses. The etiology of these premature beats was unknown, although may relate to psychological factors. As many as 33 SVPD and 37 VPD were reported during that time period⁹. (Table 11.1).

Table 11.1 Prevalence of cardiac arrhythmias as detected using ambulatory recordings around exercise in normal race horses (n = 99) and in racehorses presented for assessment of poor performance (n = 88)