Changes over time in echocardiographic measurements in young Standardbred racehorses undergoing training and racing and association with racing performance

The ACE gene and human performance: 12 years on

Some 12 years ago, a polymorphism of the angiotensin I-converting enzyme (ACE) gene became the first genetic element shown to impact substantially on human physical performance. The renin-angiotensin system (RAS) exists not just as an endocrine regulator, but also within local tissue and cells, where it serves a variety of functions. Functional genetic polymorphic variants have been identified for most components of RAS, of which the best known and studied is a polymorphism of the ACE gene. The ACE insertion/deletion (I/D) polymorphism has been associated with improvements in performance and exercise duration in a variety of populations. The I allele has been consistently demonstrated to be associated with endurance-orientated events, notably, in triathlons. Meanwhile, the D allele is associated with strength- and power-orientated performance, and has been found in significant excess among elite swimmers. Exceptions to these associations do exist, and are discussed. In theory, associations with ACE genotype may be due to functional variants in nearby loci, and/or related genetic polymorphism such as the angiotensin receptor, growth hormone and bradykinin genes. Studies of growth hormone gene variants have not shown significant associations with performance in studies involving both triathletes and military recruits. The angiotensin type-1 receptor has two functional polymorphisms that have not been shown to be associated with performance, although studies of hypoxic ascent have yielded conflicting results. ACE genotype influences bradykinin levels, and a common gene variant in the bradykinin 2 receptor exists. The high kinin activity haplotye has been associated with increased endurance performance at an Olympic level, and similar results of metabolic efficiency have been demonstrated in triathletes. Whilst the ACE genotype is associated with overall performance ability, at a single organ level, the ACE genotype and related polymorphism have significant associations. In cardiac muscle, ACE genotype has associations with left ventricular mass changes in response to stimulus, in both the health and diseased states. The D allele is associated with an exaggerated response to training, and the I allele with the lowest cardiac growth response. In light of the I-allele association with endurance performance, it seems likely that other regulatory mechanisms exist. Similarly in skeletal muscle, the D allele is associated with greater strength gains in response to training, in both healthy individuals and chronic disease states. As in overall performance, those genetic polymorphisms related to the ACE genotype, such as the bradykinin 2 gene, also influence skeletal muscle strength. Finally, the ACE genotype may influence metabolic efficiency, and elite mountaineers have demonstrated an excess of I alleles and I/I genotype frequency in comparison to controls. Interestingly, this was not seen in amateur climbers. Corroboratory evidence exists among high-altitude settlements in both South America and India, where the I allele exists in greater frequency in those who migrated from the lowlands. Unfortunately, if the ACE genotype does influence metabolic efficiency, associations with peak maximal oxygen consumption have yet to be rigorously demonstrated. The ACE genotype is an important but single factor in the determinant of sporting phenotype. Much of the mechanisms underlying this remain unexplored despite 12 years of research.

Subclinical diseases underlying poor performance in endurance horses: diagnostic methods and predictive tests

Thirty-eight endurance horses underwent clinical and ancillary examinations, including haematological and biochemical evaluation, standardised exercise tests both on a treadmill and in the field, Doppler echocardiography, impulse oscillometry, video endoscopy and collection of respiratory fluids. All of the examined poorly performing horses were affected by subclinical diseases, and most of them had multiple concomitant disorders. On the contrary, the well-performing horses were free of any subclinical disease. The most frequently diagnosed diseases were respiratory disorders, followed by musculoskeletal and cardiac problems. Poor performers exhibited lower speeds at blood lactate concentration of 4 mmol/l (VLA4) and at heart rates of 160 (V160) and 200 bpm (V200) on the treadmill and in the field, as well as slower recovery of heart rate.

Effect of light exercise on valvular regurgitation in Standardbred trotters

REASONS FOR PERFORMING STUDY

Although valvular regurgitation is diagnosed frequently by colour Doppler echocardiography in racehorses at rest, there is currently no information about the effect of exercise on valvular regurgitation.

OBJECTIVES

To investigate the effect of light exercise on valvular regurgitation in a population of trotters.

METHODS

Standardbred trotters (n = 73) were examined by resting auscultation and echocardiography, of which 50 fullfilled the criteria of the study. Of these, 4 had reduced performance according to the trainers and were not included in the statistical analyses. Horses that showed valvular regurgitations diagnosed by colour Doppler echocardiography at rest, were light exercised and the regurgitant valves were then re-examined by colour Doppler echocardiography.

RESULTS

Of the 46 horses, 41 (89%) had one or more valvular regurgitations at rest with tricuspid regurgitation present in 83% of horses, mitral regurgitation 46%, pulmonary regurgitation 37% and aortic regurgitation 57%. After exercise, one or more valvular regurgitation persisted for 23 of the 41 horses (56%). The regurgitation decreased significantly for mitral, pulmonary and aortic valves; the mitral regurgitation persisted for 21% of the horses, pulmonary regurgitation persisted for 6% and aortic regurgitation persisted after exercise for 23 % with regurgitation diagnosed at rest. Half of those with tricuspid regurgitation at rest still had tricuspid regurgitation after exercise.

CONCLUSIONS

The results of this study showed that the majority of small valvular regurgitations disappear after light exercise. However, the severity of tricuspid regurgitation increased in 4 horses. Furthermore, these horses were not performing satisfactorily according to the trainers.

POTENTIAL RELEVANCE

Studies in man have shown that severity after exercise suggests pathological regurgitation. The present study indicates that echocardiographic examination of valvular regurgitation after exercise might be a relevant examination in horses. However, more studies are needed to evaluate the effect of strenuous exercise on valvular regurgitation in horses.

Clinical and echocardiographic features of mild mitral valve regurgitation in 108 horses

The clinical and echocardiographic characteristics of 108 horses with echocardiographically confirmed mild mitral valve regurgitation (MR) were investigated along with its clinical progression. Follow-up consisted of a re-examination of 28 horses and questionnaires were used to obtain information on a further 43 cases. Thirty-seven horses with mild MR were lost to follow-up. Horses with mild MR were re-examined between 2 and 9 years (3.8+/-1.8 years) following first presentation, with mild MR still present and a small, but statistically significant (P=0.049) increase of left ventricular diameter in end-diastole. These results suggested that mild MR has a good mid-term prognosis in sport and pleasure horses.

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.

ECHOCARDIOGRAPHIC REFERENCE VALUES IN WHIPPETS

The aim of the study was to establish reference echocardiographic values for whippets, to compare these values with previously published reference values for the general dog population, and to determine whether there is an influence of gender and breeding lines on echocardiographic measurements. Echocardiographic parameters from 105 apparently healthy whippets without cardiac symptoms were used to establish reference values for the breed and to compare these values with two previously reported reference ranges. The coefficients of the allometric equation Y= aM(b), useful to reconstruct normal M-mode and two-dimensional average values for whippets of varying weights, were calculated, as well as the lower and upper limits of the 95% prediction interval. First, we found that whippets have a significantly larger left ventricular diameter, increased left ventricular wall, and interventricular septum thickness than expected, in diastole as well as in systole. Fractional shortening was significantly lower than the reference value. Second, comparing males and females, taking body weight differences into account, females had a significantly larger left ventricular diameter in diastole and systole. Minor differences were found between racing and show pedigree dogs. In conclusion, the results of this study confirm that breed-specific reference values are needed in echocardiography. In whippets, the values found in this study can be used as references in order to avoid overinterpretation of cardiac dilation, hypertrophy, and/or decreased contractility in these dogs.

Use of color Doppler echocardiography to assess the development of valvular regurgitation in Standardbred trotters

OBJECTIVE

To evaluate the incidence and development of valvular regurgitation and its effect on racing performance in Standardbred trotters from 2 to 3.5 years of age.

DESIGN

Longitudinal observational study.

ANIMALS

103 horses.

PROCEDURE

Horses were examined via echocardiographic imaging 4 times at 6-month intervals. Two-dimensional and M-mode echocardiographic examinations were performed in each horse, and color Doppler imaging was used to detect and evaluate regurgitant blood flow at each valve.

RESULTS

For all cardiac valves, the frequency of valvular regurgitation increased significantly during the 18-month study period. Regurgitation was mild in all valves in which it was detected, and no progression in severity was observed at any valve. Tricuspid regurgitation was more prevalent in horses that raced regularly and was diagnosed more frequently in males and horses with larger left ventricular mass and left ventricular internal diameter during diastole. No relationship between racing performance and valvular regurgitation was observed.

CONCLUSIONS AND CLINICAL RELEVANCE

The incidence of valvular regurgitation increased during the study period, and it was common at 3.5 years of age. It was not possible to determine whether the increase was a result of maturity or training. The study revealed that there is a high prevalence of mild valvular regurgitation at this age, but apparently, the regurgitation had no effect on racing performance in this age group. Whether regurgitation will progress or influence future performance is not known.