Significance of clinical abnormalities of the heart in soundness

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.

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

REASONS FOR PERFORMING STUDY

There are few data available to determine the effect of training on cardiac valve function.

OBJECTIVES

To investigate the effect of commercial race training on right ventricular (RV) and tricuspid valve function in an untrained group of National Hunt Thoroughbreds (TB).

MATERIAL AND METHODS

Cardiac auscultation, guided M-mode echocardiography of the RV, and colour flow Doppler (CFD) tricuspid valve and right atrium were performed in 90 TB horses (age 2-7 years) 1998-2003. Forty horses were examined at least once and 48 horses were examined on at least 2 occasions. Examinations were then classified as: i) before commencement of race training, ii) after cantering exercise had been sustained for a period of 8-12 weeks and iii) at full race fitness. Tricuspid valve regurgitation (TR) murmurs were graded on a 1-6 scale and CFD echocardiography TR signals were graded on a 1-9 scale. Right ventricular internal diameter (RVID) in diastole and systole (RVIDd and RVIDs) was measured by guided M-mode. Associations between continuous RVID and TR measures and explanatory covariates of weight, age, heart rate, yard and stage of training were examined using general linear mixed models with horse-level random effects.

RESULTS

On average, RVIDd and RVIDs increased by 0.08 and 0.1 cm, respectively, per year increase in age (P = 0.1 and 0.02) and by 0.3 and 0.4 cm, respectively between pretraining and race fitness (P = 0.07 and 0.005). Tricuspid regurgitation score by colour flow Doppler increased by 0.6/year with age (P<0.0001) and by 1.8 between pretraining and race fitness (P<0.0001). No significant associations were found between any outcomes and weight, heart rate and training yard. Due to the high level of colinearity between age and training, multivariable models including both terms were not interpretable.

CONCLUSIONS AND CLINICAL RELEVANCE

Athletic training of horses exerts independent effects on both severity and prevalence of tricuspid valve incompetence. This effect should therefore be taken into account when examinations are performed. Dimensions of RV increase with age and training in TB horses in a manner that appears to be similar to that of the LV.

Prevalence and clinical importance of heart murmurs in racehorses

OBJECTIVE

To determine the prevalence of various types of heart murmurs in Thoroughbred racehorses and assess their association with performance by echocardiography and review of the horses' race records for the preceding 2 years.

DESIGN

Clinical and retrospective study.

ANIMALS

846 Thoroughbred racehorses.

PROCEDURE

Cardiac auscultations were performed by 3 individuals; for 30 horses, Doppler echocardiographic examinations were also performed. Statistical analyses of race records for 753 horses were performed to assess association of heart murmurs with performance.

RESULTS

Heart murmurs were detected by cardiac auscultation in 686 of 846 (81.1%) horses. Systolic murmurs over the heart base were most common; 365 (43.1%) horses had systolic murmurs that were loudest over the pulmonary valve area, and 232 (27.4%) horses had systolic murmurs that were loudest over the aortic valve area. Systolic murmurs over the tricuspid valve area were detected in 241 (28.5%) horses, whereas systolic murmurs over the mitral valve area were detected in only 32 (3.8%) horses. Diastolic murmurs were much less common than systolic murmurs. Review of race records did not reveal a significant association between murmurs and performance.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that heart murmurs are a common finding in racehorses; most of these heart murmurs do not appear to be clinically important.

Effect of age and training on murmurs of atrioventricular valvular regurgitation in young thoroughbreds

Cardiac auscultation was carried out on 111 Thoroughbred horses age 2-5 years to test the hypothesis that athletic training might influence the development of atrioventricular (AV) valve regurgitation in young Thoroughbreds. Murmurs of valvular regurgitation were identified and graded on a 1-6 scale. There were 2 sources of auscultation data: 1) 55 2-year-old horses that were examined by auscultation before training commenced and 9 months later when at race fitness; 2) 56 horses age 2-5 years that were examined on one occasion only (25 2-year-olds, 23 3-year-olds, five 4-year-olds and five 5-year olds). All horses in the second data set were in full training and racing regularly at the time of the examination. To conclude the study, 35 horses were selected randomly from both groups of horses and examined with colour-flow Doppler echocardiography. The aim of the final part of the study was to check specificity and sensitivity of auscultation for detection of AV valve murmurs and therefore validate the auscultation findings. Prior to training, the prevalence in 2-year-old racehorses of murmurs of mitral regurgitation and tricuspid regurgitation was 7.3% (4/55) and 12.7% (7/55), respectively. After training, the prevalence proportions increased to 21.8% (12/55) and 25.5% (14/55). After training, one horse developed a murmur characteristic of aortic regurgitation. The differences in murmur prevalence were statistically significant for mitral and tricuspid regurgitation (paired t test results: mitral regurgitation, P = 0.019; tricuspid regurgitation, P = 0.007), as were the differences in mean murmur grade (P = 0.018 and P = 0.0006, respectively). There were no significant effects of age on the prevalence of valvular regurgitation in 56 horses examined at race fitness. Auscultation was a specific (specificity 100%) and reasonably sensitive method for detection of murmurs of mitral and tricuspid regurgitation (mitral regurgitation: positive predictive value 100%, negative predictive value 84%, tricuspid regurgitation: positive predictive value 100%, negative predictive value 65%). These data suggest that the prevalence and grade of murmurs of mitral and tricuspid valvular regurgitation increase in 2-year-old Thoroughbreds after 9 months of athletic training. Whereas the effects of age and growth on the prevalence of murmurs cannot be ruled out from these data, this study suggests that there is an influence of athletic training on the development of atrioventricular valvular regurgitation in flat-racing Thoroughbreds.

Heart murmurs in horses: determining their significance with echocardiography

Physiological flow murmurs occur frequently in horses and may be difficult to distinguish from murmurs associated with underlying cardiac disease. The significance of heart murmurs auscultated in horses is often difficult to determine if the horse is not exhibiting any clinical signs or if the signs, such as poor performance, are nonspecific. A complete echocardiographic examination (M-mode, 2-dimensional (2-D) and Doppler) provides an objective assessment of the severity of the horse's underlying cardiac disease. Valvular regurgitation and ventricular septal defects (VSDs) occur frequently and may impair performance, result in the horse's premature demise or have no apparent effect on the horse's life expectancy or performance capabilities. The echocardiographic findings that are used to formulate a prognosis for longevity and performance in horses with valvular regurgitation include the abnormalities detected on the valve leaflets, degree of cardiac chamber enlargement, severity of the resultant volume overload, size of the regurgitant jet, and relative relationship of jet size to chamber size. The echocardiographic findings that are used to formulate a prognosis for horses with VSDs are the number, size and location of the defect(s), degree of left ventricular volume overload, maximal velocity and direction of shunt flow through the defect and the presence and severity of concurrent valvulus regurgitation. Knowledge of the natural progression of the common types of cardiovascular disease in horses, coupled with the echocardiographic findings, clinical history and owner's or trainer's expectations can help the veterinarian form an accurate prognosis for life and performance in horses with heart murmurs.

Colour flow Doppler echocardiography in horses with cardiac murmurs

Thirty-two horses with cardiac murmurs typical of tricuspid, mitral and aortic regurgitation were studied using colour flow Doppler echocardiography. The dimensions and duration of any regurgitant signals, recorded at the cardiac valve suspected as being the site of origin of the murmur, were measured. Results were compared with flow signals described at the valves of normal horses (Blissitt and Bonagura 1995). Horses with murmurs suggestive of tricuspid (n = 8) and aortic (n = 8) regurgitation showed larger regurgitant signals at the tricuspid and aortic valves respectively, than has been reported in normal horses. Horses with murmurs suggestive of mitral regurgitation (n = 8) had a regurgitant signal of longer duration than occurs in normal horses, but the jets were not larger. Comparison of two groups of horses with low and high grade murmurs of tricuspid regurgitation, showed that horses with louder murmurs had significantly longer jets of larger area than those with low grade murmurs. This study demonstrates that colour flow Doppler echocardiography is a sensitive technique for the detection of valvular regurgitation in horses. However, quantification of jet size and duration are important for distinguishing physiological backflow which occurs in normal horses (Blissitt and Bonagura 1995) from valvular regurgitation associated with cardiac murmurs.

Colour flow Doppler echocardiography in normal horses

Colour flow Doppler echocardiography is a technique that is used with two-dimensional (2-D) echocardiography to study blood flow patterns in the heart and blood vessels. This method was used to define normal flow patterns and to evaluate valvular function in 40 clinically normal Thoroughbred and Thoroughbred cross horses. Flow patterns from 10 standardised echocardiographic images were described in relation to anatomic landmarks and timing during the cardiac cycle. Consistent intracardiac flow patterns were identified in the normal horses. High velocity flow signals or regurgitant jets were recorded at the tricuspid (77.5%), mitral (67.5%), aortic (47.5%) and pulmonary valves (40%) in clinically normal horses. Most of these signals were transient, and many were associated with valve closure. This study demonstrates that colour flow Doppler echocardiography is a sensitive technique for the detection of intracardiac flow in horses. It will provide a basis by which to compare studies in horses suspected of having valvular heart disease.

A survey of cardiac auscultatory findings in horses

Auscultation was used to examine 545 horses for cardiac murmurs and some arrhythmias. The most significant finding was the high prevalence of right-sided holosystolic or pansystolic plateau-type murmurs compatible with a diagnosis of tricuspid regurgitation in National Hunt racing Thoroughbreds of all ages (16.4%). This condition was found in 4.7% of flat-racing Thoroughbreds, 3.7% of non-racing Thoroughbreds and Thoroughbred crosses, and in none of the ponies examined. High prevalences of left-sided early systolic murmurs (53%), left-sided early diastolic murmurs (23%), right-sided early diastolic murmurs (22%) and second degree atrioventricular block (23%) were found in racehorses of both types.