INTRODUCTION

Electrocardiography is used for the diagnosis of cardiac rhythm disturbances that may have been identified during a physical examination. However, some arrhythmias are not present at the time of electrocardiography, despite being obvious during physical examination, or may be suggested by a clinical history of intermittent reduction in cardiac output and not identified clinically. There are several reasons why the arrhythmia may not be detected at the time of recording a patient-side ECG and may be related to changes in autonomic tone such that they only occur during exercise or during rest. In these situations ambulatory electrocardiography can be very useful in order to document cardiac rate and rhythm at the times when clinical signs have been reported, such that a dysrhythmia may be reproduced. In addition, ambulatory electrocardiography can be useful in the critical care setting where cardiac rhythm disturbances may occur as a result of electrolyte and fluid status or even following drug administration such as quinidine sulfate for the treatment of atrial fibrillation (AF).

Traditionally ambulatory ECG devices were defined as being either continuous ambulatory monitors that recorded the ECG signal for subsequent analysis or radiotelemetric devices that transmitted the ECG signal to a terminal where they could be viewed in real time. These boundaries have now been blurred and devices are able to both store and transmit ECG data to be viewed in real-time and interrogated more thoroughly at a later stage. Despite this, the traditional definitions are still valid in terms of how the ECG signal will be used, although since they are all ambulatory monitors, the clinical uses will be defined as resting or exercising ambulatory electrocardiography.

Continuous ambulatory (Holter) electrocardiography traditionally refers to an ECG device that records the ECG signal for subsequent analysis. There is no requirement for the ECG to be monitored during the recording, with the possible exception of ensuring an adequate signal is present. The ECG signal can be recorded using a variety of technologies ranging from analogue audiotapes, mini-discs, or solid state memory devices. The duration of the recording can be variable, traditional tape-based analogue systems used to provide a 24-hour recording period while modern solid state devices can record for up to a week. The longer periods of recording are more likely to be representative of any intermittent dysrhythmia but require more time to process and since the automated algorithms used are designed for use in human patients they are not always accurate when analysing equine ECGs. This form of device is most appropriate for stable/paddock rest or low intensity/endurance exercise where it is not possible to follow the path of the horse or stay within range of a radio signal.

Radiotelemetric ECG devices transmit an ECG signal to a distant terminal where it can be visualized without immediate detailed analysis. This brings with it a requirement for trained observers to be present to identify abnormalities and react to these as they occur. Recently, internet and mobile phone ready devices have been established allowing remote monitoring by trained personnel. The obvious advantage of this system is that it allows the observer to intervene should any dysrhythmia occur, and therefore is ideal for exercise and in a critical care setting.

ELECTRODE PLACEMENT FOR AMBULATORY ELECTROCARDIOGRAPHY

In order to obtain a diagnostic ambulatory ECG it is usual to use a chest-lead system so that electrodes can be placed underneath a girth or surcingle that will ensure appropriate contact between the electrode and the skin, will reduce movement artifact and will provide a point for attachment of the device. In order to optimize the ECG for automated processing the lead placement is usually such that the positive electrode (right arm) is placed at the top of the thorax on the left-hand side at the level of the vertebral bodies while the negative electrode (left arm) is placed on the sternum. If multiple leads are being used the neutral (left leg) is placed on the right-hand side of the thoracic wall. The earth (right leg) electrode can be placed anywhere around the girth area. It is usual to clip the horse where the electrodes will be placed and to use self-sticking electrodes that contain contact gel. It may be necessary to apply surgical spirit to maintain electrical contact especially in hot climates. The girth should be applied tightly to prevent movement of the electrodes. This arrangement of electrodes should record an ECG with a negative QRS, positive P and T waves (Fig. 10.1).

Figure 10.1 Continuous ECG of a horse with sinus rhythm and occasional supraventricular premature depolarizations (SVPD) obtained via a 24-hour continuous Holter recorder. Notice both SVPD have normal appearing premature QRS complexes that are preceded by a P wave and normal P–R interval. The sinus rhythm returns to normal following SVPD. Isolated ectopy is not uncommon in resting ambulatory recordings at a frequency of around one per hour.

Modified base-apex ECG, 25 mm/second paper speed.

RESTING AMBULATORY ELECTROCARDIOGRAPHY

Resting ambulatory electrocardiography can be useful in order to establish true resting heart rate and rhythm as well as to document the frequency of any intermittent dysrhythmia. Resting ambulatory electrocardiography is indicated in animals that are presented for investigation of poor performance and/or collapse where the purpose of the recording is to identify intermittent dysrhythmias that may themselves be clinically significant or that suggest underlying myocardial disease. Resting electrocardiography is also indicated in the clinical care setting.

When interpreting resting ambulatory ECGs some dysrhythmias are often found that are of no clinical significance; the prevalence of second degree atrioventricular block (AVB)¹ is much higher during ambulatory recordings at rest than during patient-side ECG recording as a result of reduced sympathetic stimulation when the animal is returned to a more natural environment. This is a normal physiological dysrhythmia and should be of no significance if it resolves during light exercise. Occasional ventricular (VPD) and supraventricular premature depolarizations (SVPD) can also be seen on resting ambulatory recordings and provided that they occur less than 1 per hour and are not present at exercise are not considered abnormal (Figs. 10.1 and 10.2).

Figure 10.2 Continuous ECG of a horse with sinus rhythm and occasional ventricular premature depolarizations (VPD) obtained via a 24-hour continuous Holter recorder. Notice the ventricular premature depolarization with its QRS complex that is widened and bizarre in appearance when compared to the other normally conducted QRS complexes. The T wave of the ventricular premature depolarization is oriented in the opposite direction of the QRS complex. The next normal sinus beat occurs but is not conducted through to the ventricle, resulting in a compensatory pause.

Modified base-apex ECG, 25 mm/second paper speed.

AF most frequently presents as a persistent dysrhythmia, but can also present as a paroxysmal form where the dysrhythmia spontaneously resolves. Although resting ambulatory electrocardiography is unlikely to identify this, if not present on a patient-side ECG, horses may have frequent isolated SVPD between episodes. If these are present and the horse has a history of a dysrhythmia following exercise, often associated with a sudden loss of performance,² then this may support a tentative diagnosis of paroxysmal atrial fibrillation. If isolated SVPD are frequent without a history that supports a diagnosis of paroxysmal atrial fibrillation the horse should undergo a full cardiovascular assessment. Although SVPD are unlikely to destabilize and compromise cardiac function they may be evidence of underlying atrial myocardial disease. ( AF, SFP, VMD)

Frequent VPD (see Fig. 10.2) that are present on an ambulatory resting ECG may be suggestive of underlying myocardial disease or may occur in horses with aortic valve disease and should prompt a thorough investigation in an attempt to document underlying pathology and determine whether treatment is likely to be effective (see Chapter 13). In a study of 38 horses with ventricular dysrhythmias, 14 had frequent VPD and were managed by pasture rest and the administration of corticosteroids based on a presumptive diagnosis of myocarditis. There was an improvement of dysrhythmia frequency in nine horses, in five of which the dysrhythmia was completely abolished suggesting that these dysrhythmias represent underlying myocardial pathology (Bowen IM, Marr CM, unpublished observations). Frequent or complex VPD are reported to be associated with sudden cardiac death in humans³ and horses⁴ and it is believed that they persist or deteriorate in horses that are kept in exercise. Therefore, rest is an important aspect of management of these animals and glucocorticoids are sometimes recommended in the belief that the period of rest required would be shorter. Where horses have frequent isolated ventricular premature complexes their prevalence should be evaluated during exercise. Although isolated single VPD have little effect on cardiac output, they may become more frequent with exercise or result in ventricular tachycardia, which could result in collapse during exercise or even sudden death should ventricular tachycardia develop into ventricular fibrillation. ( AF, AR, RCT)

Ventricular tachycardia is most likely to be seen in the critical care setting with the use of ambulatory or telemetric electrocardiography (see Chapter 21). There is a particular association between horses with gastrointestinal tract disease and ventricular dysrhythmias, and electrocardiography should be considered when heart rates are higher than would be expected for the degree of pain and hypovolaemia.⁵ These dysrhythmias may represent changes in electrolyte, acid-base or fluid status or endotoxaemia and therefore an assessment of the underlying cause is important. Cardiac troponin I concentrations may be increased in horses undergoing exploratory laparotomy for the correction of strangulating small intestinal lesions. This suggests a degree of cardiac myocyte damage, which in one study was shown to be related to hypovolaemia rather than endotoxaemia.⁶ Therefore, volume expansion should be considered in these animals. Criteria for diagnosis and specific management of ventricular dysrhythmias are detailed in Chapter 13. The aim of diagnosis, management and monitoring of these animals is to identify those at risk from progression to ventricular fibrillation which is rarely amenable to intervention.

EXERCISING AMBULATORY ELECTROCARDIOGRAPHY

Exercising electrocardiography can be used to assess the significance of dysrhythmias detected at rest and to identify dysrhythmias that were otherwise undetected and that only occur during exercise. The advantage of ambulatory electrocardiography in this setting is that the animal can be examined in the same conditions that provoke clinical signs. Exercising ambulatory electrocardiography is indicated for investigation of horses that are presented for investigation of poor performance and collapse and should form part of the evaluation of a horse with aortic valve regurgitation. In addition to providing information about cardiac rhythm cardiac rate can be obtained and can be compared to speed of the horse at the time using either a high-speed treadmill or GPS-based velocity devices. Further details regarding exercise testing and interpretation of exercising electrocardiography can be found in Chapter 11.

HEART RATE VARIABILITY

Heart rate variability describes and quantifies the beat-to-beat variability in heart rate that occurs due to neurohormonal control of heart rate. Currently the technique is not in routine clinical use for the diagnosis or monitoring of cardiovascular disease in the horse; however many large-scale studies have shown it to be a useful method of predicting prognosis in various cardiac disease states in human beings. Therefore an understanding of these methods may prove useful in further improving the veterinarian’s ability to stratify risk in horses with cardiac disease.

Heart rate is not static, even in the resting horse, and changes in beat-to-beat intervals occur in the normal healthy animal. As such, the classical view of the heart possessing a metronome-like character only applies to the decentralized heart. In vivo the heart is under the influence of both the autonomic nervous system and the neuroendocrine system in order to maintain normal arterial blood pressure and result in cyclical changes in heart rate (R–R intervals). Techniques of assessing heart rate variability therefore quantify neurohormonal control and do so independently of heart rate providing an indicator of cardiovascular well being. The greater the heart rate variability, the healthier the heart. For example, both respiratory sinus arrhythmia and second degree atrioventricular block are considered normal physiological dysrhythmias^1,7 and are a result of autonomic control of the heart.

Determination of heart rate variability

Heart rate variability can be determined from a recorded ECG by extraction of normal R–R intervals (termed N–N intervals; normal–normal) either manually or automatically. These data can then be used to create a tachogram of R–R data (Fig. 10.3) and used for subsequent analysis. The duration of recording and environment in which the recording is made is important to consider. Most heart rate variability indices are obtained from continuous ambulatory recordings where the influence of external stimuli such as handling and transport can be excluded while providing large amounts of data that can be analysed. Accurate detection of normal intervals is essential and therefore the quality of ECG recording is vital in calculating accurate heart rate variability indices. The author’s recommendations for obtaining ECG recordings for heart rate variability analysis are shown in Table 10.1.

Figure 10.3 Tachogram of normal R–R (N–N) intervals from a horse showing cyclic fluctuations in heart rate.

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