The present study reports changes in echocardiographic dimensions of the heart in young Arabian type horses at the end of their growth period and at the beginning of their endurance training for competitions. All external variables included in the analyses influenced to some extent several of the measured dimensions. Morphometric dimensions, age and breed showed most consistently an influence on LA measurements, whereas BWT, breed and Km-career had most consistently an influence on LV measurements.
All the horses in the study population had at least one purebred Arabian parent. Our study population was chosen to be representative of the horses participating in endurance competitions. This choice was in accordance with literature data indicating that purebred or mixed-breed Arabians are largely overrepresented in endurance competitions, although some other breeds do participate [29, 30]. In view of the inclusion criteria, the division into several breed groups may seem arbitrary. However, there were significant differences between these breed groups. Purebred Arabians were the smallest and lightest - giving them advantage over other breeds in endurance competitions [29, 30]. Body dimension and BWT were highest in Anglo-Arabians that in turn led also to larger LVmass. Nevertheless, in the multivariate regression models, belonging to breeds other than purebred Arabians was associated with smaller LA measurements. This indicated that the smallest horses in our population did not necessarily have the smallest hearts and hence that other factors must be considered.
BWT and body dimensions were higher in Group 5y than in Group 4y, which was presumably due to on-going growth. In contrast, the smaller body dimensions in Group 6y (especially WH and BL, which are less influenced by body condition) might be due to the selection of smaller, lighter, more compact horses for more demanding competitions. Indeed, high BWT in endurance horses has been associated with failure to finish competitions, particularly due to lameness [29, 30]. Furthermore, horses in Group 6y had covered a greater distance in competition (Km-career) and thus were certainly subjected to harder training, which may have resulted in a lower BWT. Training reduces fat content and increases muscle mass. Therefore, it would have been interesting to measure lean body weight. BWT and especially fat-free mass were associated with better indices of performances in Standarbreds, [31, 32] and fat-free mass was also associated with a higher VO2max. However, too low BWT or too low percentage of body fat is detrimental in human athletes [33, 34]. The optimal percentage of body fat has not yet been established for horses, although body fat was around 8 % in well-performing athletic horses [29, 32, 35]. As is the case for human athletes, the optimal percentage of body fat doubtless varies from one discipline to another [34, 36].
All heart dimensions increased with age. The difference between 4 and 5 years of age was greater than the difference between 5 and 6 years of age. In univariate and multivariate regression analyses, for most heart dimensions a weak relationship was observed with BWT or TC (especially for LA measurements). The higher values for the R2 in regression models of LA measurements and of the dimensions of the great vessels, and their relationship with body dimensions (rather than BWT) might indicate that these dimensions are more strongly related to the horse’s morphotype. Al-Haidir et al. also have reported that some echocardiographic measurements are somewhat more strongly associated with TC than they are with BWT . However, due to the present study’s inclusion criteria, the range of body dimensions and weights was narrower than in the work by Al-Haidir et al. . Concomitant increases in all dimensions are seen with growth, and growth has to be taken into consideration when studying young individuals. Different breeds have differing growth rates. Thoroughbreds and Standardbreds are fast-growing breeds and reached 83 % of the adult BWT by the age of 2 years, whereas the growth rate is slower in other breeds and by the age of 2 years Warmbloods, Anglo-Arabians and Lusitanos reached 80 %, 75 % and 77 % of their adult BWT, respectively [20, 21]. Arabians have been classified as slow-growing horses . Accordingly, they are still growing between the ages of 4 and 6 years. This contrasts with Standardbred horses, which are considered to reach maturity between the ages of 3.5 and 5 years . Although growth certainly affects the LV dimensions, the latter are also greatly modulated by other factors. In particular, training is a strong stimulus for cardiac hypertrophy, and we detected a weak but significant association between Km-career and some of the LV measurements. This very weak association might be due to the poorly elaborated performance parameters (Km-career, d-career) and also due to the generally low level of endurance training seen at this age . A training-related increase in LV dimensions has been observed in Thoroughbreds , Standardbreds [5, 6] and elite Arabians . The absence of a change in RWT indicated that the heart’s adaptation to exercise in our population is more similar to that seen in Standardbreds [5, 6] than to that seen in Thoroughbreds,  or elite Arabians . Further, this finding might also illustrate differences in training practices among different breeds. We did not find any association between LA dimensions and Km-carrer. This is in contrast to studies in human athletes that showed also greater LA dimensions in presence of an Athlete’s heart [40, 41]. However, the endurance training in our study population was probably not intensive enough to induce changes in LA dimensions.
The results of our regression analyses agree with another study reporting small values for the R2 for LV measurements in young (2-year-old) Standardbreds . However, values for the R2 increased with age in this previous study, particularly for LVmass. This comparison also suggests that our population’s training was not yet hard enough to induce marked changes in heart dimensions.
In accordance with previous studies [42–44], there were no age-related differences in LA and LV functional indexes.
Females had the largest TC. Given that LA measurements were related to TC in our regression models, we were not surprised to find that females had also the largest LA measurements. On the contrary, intact males had the smallest LA dimensions. In contrast to other studies [5, 13, 44], gender did not appear to influence LV dimensions in our study population. One explanation could be that the intensity of the aerobic training in endurance horses at this age is not high enough to bring out the full potential for hypertrophy in intact males. Another explanation might be related to the custom of French endurance trainers/owners to breed females at the age of 5 years. Gestation may have induced changes in heart dimensions (as seen in humans and dogs), although these changes are usually reversible [45, 46].
The present study had a number of limitations. Due to field conditions and the limited time available per horse, not all echocardiographic views of heart chambers could be taken, and some views were of poor quality. Similarly, it was not possible to perform Doppler examinations and thus screen for possible regurgitations. Due to these technical issues there were several missing data. Nevertheless our analyses were based on a large body of data. Given that the measured heart chamber dimensions can be influenced by the transducer position and the views obtained [47, 48], not all of our measurements can be validly compared with the literature values. Our comparison with previous studies is therefore limited to the consideration of factors that influence heart dimensions. Second, the use of only longitudinal views meant that our LV measures could probably be underestimated . Furthermore, the measured HRs were slightly over resting values, and the shorter filling time at these HRs might have affected ours results, especially for diastolic measurements. Nevertheless, the HRs measured in our studies were only slightly higher than those reported in the literature for endurance horses [7, 48]. Lastly, the fact that the recordings and measurements were made by several operators probably decreased the precision of our measurements [25, 49].
Differentiating the effects of growth from the effects of training would have required the use of a control group. In principle, a control group should comprise untrained, non-competing horses of the same age. Unfortunately, this type of population was not available in our research setting. Furthermore, follow-up measurements of our study population 3-4 years later or measurements from a control group of mature, elite endurance horses would have provided information on the effects of more intensive aerobic training. In accordance with results from Sleeper et al.  reporting greater LVID, LVmass and SV in elite Arabians (mean age 21.1, age range 7-17) than in non-elite Arabians, data from our research group show that in a smaller group of endurance horses (n = 11; nine purebred Arabians and two others; four females, six geldings and one intact male; mean age ten, range 8-13 years) participating in races of least 120 km, the main differences relative to younger horses were seen for LVID, LVIA, LVIL and LVIV, with greater differences appearing in peak systolic measurements. Furthermore, in the older horses, LVmass was higher and LV FAC and LV FS were lower. The greater LV dimensions and LVmass agree with literature data on various breeds [4, 6, 7] and are certainly related to a greater aerobic training volume at this age. These considerations strengthen our hypothesis whereby the training load in younger horses was not intense enough to induce marked cardiac hypertrophy. Our findings with regard to LV FAC and LV FS are more difficult to explain, since the Athlete’s heart does not feature any changes in LV function at rest [1, 41]. The results might be best explained by the fact that older horses were more used to being handled and therefore showed a narrower range of heart rates than younger horses. Indeed, the HRs for adult endurance horses reported in the literature were slightly lower than in the present study [7, 48].
Furthermore, the performance parameters used in the present study (Km-career, d-career) were very rough. In France, purebred Arabians are not trained intensively between the ages of 4 and 6 years and training is not as standardised as it is in Thoroughbreds or Standardbreds . Furthermore, some horses were bred during this period, which decreased the training period. Therefore, it was not possible to analyse objective parameters, such as gains, speed at kilometre, or numbers of start [4, 5, 13, 14]. The most accessible parameters were Km-career and d-career, since this information was freely available in the FEE database. We hypothesised that a horse having participated in a greater number of competitions or longer competitions would have been trained harder. Even though we observed a weak relationship between Km-career and some LV measurements, better performance indicators are clearly needed. Long-term information on the endurance horses’ competitive performance was not yet available at the time of the present study and will be addressed in the future.
Lastly, we had no possibility to record and therefore to compare echocardiographic measurements taken before and after exercise. This comparison would have provided valuable information on cardiac adaptation to effort in competition [50, 51] but was outside the scope of the present study. Hence this topic merits further research.