An echocardiographic and auscultation study of right heart responses to training in young national hunt thoroughbred horses

Cardiovascular and locomotory parameters during training in Thoroughbred racehorses: a multi-national study

Studies assessing concurrent speed, cardiovascular physiology and biomechanical parameters in racehorses during training have been rarely reported, mostly due to a lack of available comprehensive technology. Here, using a validated device (the ‘Equimetre’™) that records all parameters simultaneously, we report the effect of increased training speed and thus intensity, from slow canter to hard gallop, on racehorse heart rate, heart rate recovery and on biomechanical parameters, such as stride length and frequency. We also demonstrate the effects of varying track surface, horse age and sex. Cohorts comprised a total of 509 Thoroughbred racehorses in-training in either Australia or France, completing a total of 1,124 and 6,016 training sessions, respectively. Horses varied by age (2-9 years old) and sex (including geldings) and were trained by either a single trainer (Australia) or two trainers (France); both using multiple training centres that varied by track surface (sand, turf or fibre). Data were analysed using Analysis of Variation (ANOVA) or linear mixed-effect models as appropriate. Racehorses increased speed predominantly by an increment in stride length, then frequency, both of which varied according to track surface (shorter stride on sand; P<0.001). Cardiovascular parameters (peak, and recovery of, heart rate) increased with training intensity, but not linearly (Ptrend<0.001). Older horses recorded lower heart rate at 15 mins after training (P=0.002), often due to lower achieved peak heart rate at most training intensities (P<0.001 for the cohort in Australia). In conclusion, this immersive field study demonstrates the value of technology for recording physiological and biomechanical parameters of racehorses-in-training. The data could inform trainers, jockeys and owners on aspects of individual racehorse health, welfare and suitability for certain races.

Fundamentals of arrhythmogenic mechanisms and treatment strategies for equine atrial fibrillation

Atrial fibrillation (AF) is the most common pathological arrhythmia in horses. Although it is not usually a life-threatening condition on its own, it can cause poor performance and make the horse unsafe to ride. It is a complex multifactorial disease influenced by both genetic and environmental factors including exercise training, comorbidities or ageing. The interactions between all these factors in horses are still not completely understood and the pathophysiology of AF remains poorly defined. Exciting progress has been recently made in equine cardiac electrophysiology in terms of diagnosis and documentation methods such as cardiac mapping, implantable electrocardiogram (ECG) recording devices or computer-based ECG analysis that will hopefully improve our understanding of this disease. The available pharmaceutical and electrophysiological treatments have good efficacy and lead to a good prognosis for AF, but recurrence is a frequent issue that veterinarians have to face. This review aims to summarise our current understanding of equine cardiac electrophysiology and pathophysiology of equine AF while providing an overview of the mechanism of action for currently available treatments for equine AF.

Exercise-associated rhythm disturbances in poorly performing Thoroughbreds: risk factors and association with racing performance

BACKGROUND

Exercise-associated cardiac rhythm disturbances are common, but there is a lack of evidence-based criteria on which to distinguish clinically relevant rhythm disturbances from those that are not.

OBJECTIVES

To describe and characterise rhythm disturbances during clinical exercise testing; to explore potential risk factors for these rhythm disturbances and to determine whether they influenced future racing.

STUDY DESIGN

Retrospective cohort using a convenience sample.

METHODS

Medical records were reviewed from two clinical services to identify horses with poor performance and/or respiratory noise with both exercise endoscopy and electrocardiography results. Respiratory and ECG findings recorded by the attending clinicians were described, and for polymorphic ventricular rhythms (n = 12), a consensus team agreed the final rhythm characterisation. Several statistical models analysing risk factors were built and racing records were reviewed to compare horses with and without rhythm disturbance.

RESULTS

Of 245 racehorses, 87 (35.5%) had no ectopic/re-entrant rhythms, 110 (44.9%) had isolated premature depolarisations during sinus rhythm and 48 (19.6%) horses had complex tachydysrrythmias. Rhythm disturbances were detected during warm-up in 20 horses (8.2%); during gallop in 61 horses (24.9%) and during recovery in 124 horses (50.6%). Most complex rhythm events occurred during recovery, but there was one horse with a single couplet during gallop and another with a triplet during gallop. Fifteen horses (one with frequent isolated premature depolarisations and 14 complex rhythms) were considered by clinicians to be potentially contributing to poor performance. Treadmill exercise tests, the presence of exercise-associated upper respiratory tract obstructions and National Hunt racehorses were associated with rhythm disturbances. The proportion of horses racing again after diagnosis (82%) was similar in all groups and univariable analysis revealed no significant associations between subsequent racing and the presence of any ectopic/re-entrant rhythm, or the various sub-groups based on phase of exercise in which this was detected.

MAIN LIMITATIONS

Reliance on retrospective data collection from medical records with no control group. Exercise ECGs were collected using only 1 or 2 leads. Variables examined as risk factors could be considered to be inter-related and our sub-groups were small.

CONCLUSIONS

This study confirms a high prevalence of cardiac rhythm disturbances, including complex ectopic/re-entrant rhythms, in poorly performing racehorses. Detection of rhythm disturbances may vary with exercise test conditions and exercise-associated upper respiratory tract obstructions increase the risk of rhythm disturbances.

Equine Echocardiography

This article provides an overview on the principles of transthoracic echocardiography in horses. Indications for echocardiography, equipment, and technical considerations are discussed and a systematic approach for a complete echocardiographic examination in horses is described. Methods for assessment of chamber dimensions, allometric scaling of measurements, assessment of systolic and diastolic ventricular function, assessment of atrial function, hemodynamic assessment, and evaluation of valvular regurgitation are explained, focusing on traditional 2-dimensional (2D), motion-mode, and Doppler echocardiographic methods. Selected applications of newer echocardiographic methods, such as tissue Doppler imaging and 2D speckle tracking are also described.

Right ventricular function during pharmacological and exercise stress testing in horses

The disproportionate rise of pulmonary artery pressure compared to systemic blood pressure during exercise can lead to detrimental right ventricular remodelling in endurance athletes. Horses may act as an extreme model of these athletic cardiovascular adaptations, as they show a three fold increase in pulmonary pressures during exercise. Right ventricular function was examined in ten healthy horses using post-exercise and pharmacological stress echocardiography in a randomised cross-over design. Exercise testing was performed on a treadmill while pharmacological testing was performed using an atropine-dobutamine infusion. Heart rate, systemic blood pressure and cardiac output during echocardiography were similar post-exercise compared to maximal pharmacological stress. Systolic pulmonary artery pressure was significantly higher during the exercise test (121±15mmHg) and during immediate post-exercise echocardiography (93±10mmHg) compared to maximal pharmacological stress (69±12mmHg). Right ventricular diameters as well as the ratio of systolic right to left ventricular area were higher post-exercise. Right ventricular fractional area change was significantly decreased post-exercise (40.5±6.2%) compared to a significant increase during pharmacological stress echocardiography (72.6±7.3%). Serum cardiac troponin I concentration was significantly higher 2h after the pharmacological test compared to baseline values and post-exercise, although the highest value was found post-exercise in the horse with the highest systolic pulmonary artery pressure and lowest right ventricular fractional area change. Pharmacological stress testing is not recommended in further studies on right ventricular adaptations in athletic horses, as this does not reproduce the effects of exercise.

Exercise-Induced Cardiac Remodeling: Lessons from Humans, Horses, and Dogs

Physical activity is dependent upon the cardiovascular system adequately delivering blood to meet the metabolic and thermoregulatory demands of exercise. Animals who regularly exercise therefore require a well-adapted heart to support this delivery. The purpose of this review is to examine cardiac structure, and the potential for exercise-induced cardiac remodeling, in animals that regularly engage in strenuous activity. Specifically, we draw upon the literature that has studied the "athlete's heart" in humans, horses, and dogs, to enable the reader to compare and contrast cardiac remodeling in these three athletic species. The available literature provides compelling evidence for exercise-induced cardiac remodeling in all three species. However, more work is required to understand the influence of species/breed specific genetics and exercise-related hemodynamics, in order to fully understand the impact of exercise on cardiac structure.

Recommendations for management of equine athletes with cardiovascular abnormalities

Murmurs and arrhythmias are commonly detected in equine athletes. Assessing the relevance of these cardiovascular abnormalities in the performance horse can be challenging. Determining the impact of a cardiovascular disorder on performance, life expectancy, horse and rider or driver safety relative to the owner's future expectations is paramount. A comprehensive assessment of the cardiovascular abnormality detected is essential to determine its severity and achieve these aims. This consensus statement presents a general approach to the assessment of cardiovascular abnormalities, followed by a discussion of the common murmurs and arrhythmias. The description, diagnosis, evaluation, and prognosis are considered for each cardiovascular abnormality. The recommendations presented herein are based on available literature and a consensus of the panelists. While the majority of horses with cardiovascular abnormalities have a useful performance life, periodic reexaminations are indicated for those with clinically relevant cardiovascular disease. Horses with pulmonary hypertension, CHF, or complex ventricular arrhythmias should not be ridden or driven.

Associations between exercise-induced pulmonary haemorrhage, right ventricular dimensions and atrioventricular valve regurgitation in conditioned National Hunt racehorses

REASONS FOR PERFORMING STUDY

Exercise-induced pulmonary haemorrhage (EIPH) and tricuspid (TR) and mitral valve regurgitation (MR) are conditions with high prevalence in conditioned Thoroughbreds. Stress failure of pulmonary capillaries, leading to EIPH, is most likely when pulmonary vascular pressure is increased, and mitral regurgitation can result in pulmonary venous hypertension.

HYPOTHESIS

There might be an association between MR and EIPH and the right ventricle (RV) of horses known to suffer repeated episodes of EIPH, as their hearts would be subject to higher pulmonary vascular pressures and cardiac output during training and hence increased RV afterload and preload.

METHODS

An echocardiographic and auscultation study was conducted in 121 race-fit National Hunt Thoroughbreds. Cardiac auscultation and echocardiography were performed. A guided M-mode image of the RV just below the tricuspid valve was obtained from a right parasternal location and colour flow Doppler (CFD) used to interrogate the tricuspid valve and right atrium. The mitral valve was similarly examined from the left hemithorax. Severity of TR and MR by CFD was graded. Binary data on EIPH, based on whether the horse was perceived to have a clinically significant problem with EIPH, were determined retrospectively for each horse by the horses' primary care veterinary surgeon from medical and other records. Data were analysed using a standard logistic regression analysis approach.

RESULTS

EIPH was significantly and positively associated with the systolic and diastolic dimensions of the RV (P = 0.017 and 0.011 respectively) and this association was not sensitive to the effects of age or weight. There were no significant associations between EIPH and TR or MR by auscultation or CFD (TR: auscultation P = 0.1; CFD P = 0.2 and MR: auscultation P = 0.07; CFD P = 0.37).

CONCLUSIONS AND CLINICAL RELEVANCE

This study was limited by the method used to classify EIPH, but there was no association between EIPH and horse age, weight, TR or MR in this population of Thoroughbreds. Nevertheless, RV internal dimension was greater in horses obviously affected by EIPH, suggesting that factors resulting in EIPH may directly or indirectly affect RV remodelling in athletic horses.

Measurements of right ventricular internal dimensions and their relationships to severity of tricuspid valve regurgitation in National Hunt Thoroughbreds

REASONS FOR PERFORMING STUDY

The effect of tricuspid valve regurgitation (TR) in right ventricular (RV) chamber size has not been reported.

METHODS

An echocardiographic and auscultation study was conducted on 394 race-fit National Hunt Thoroughbreds (TBs) in 4 training yards. Auscultation was performed on each horse and audible murmurs of TR graded using a 1-6 scale. Echocardiography was also performed and standard, 2-dimensional (2D), short (SA) and long axis (LA) images of the RV obtained from a right parasternal location. An m-mode image was recorded from an SA view of the RV just below the level of the tricuspid valve. Colour flow Doppler (CFD) echocardiography was used to interrogate the tricuspid valve and right atrium. If retrograde flow was visible, representative recordings were also acquired. Severity of TR by CFD was graded using a subjective 1-9 scale. Measurements of RV chamber size in systole and diastole were made from archived 2D, LA, SA and m-mode images by a single observer unaware of the auscultation and CFD findings. To determine day-to-day repeatability of RV measurements from each imaging plane and of TR by CFD, 5 horses were examined on 5 consecutive days using an identical echocardiographic protocol. Images from 30 horses in the original dataset were also measured on a second occasion to determine the intra-observer repeatability.

RESULTS

There were significant positive correlations between the severity of TR by CFD and RV internal diameter from each image plane. In contrast, there were no significant associations between grade of TR by auscultation and any RV dimension. Data also showed that only RV measurements derived from M-mode images have acceptable intraobserver repeatability and similarly the M-mode measurements had the best overall day-to-day repeatability.

CONCLUSIONS AND CLINICAL RELEVANCE

Data showed that TR assessed by CFD was positively associated with the diastolic and systolic dimensions of the RV, but failed to demonstrate a positive association between the presence of a murmur of TR and RV chamber size. M-mode and 2D echocardiography provided moderately reliable and repeatable methods for obtaining measurements of RV internal dimensions. M-mode images provide the most reliable and repeatable measurements, particularly for less experienced operators.