4.2.9 Echocardiographic heart score
Electrocardiographic measurement of the QRS duration and conversion into a figure known as 'heart score' was a technique popularised by Steel and co-workers in Australia (see section 4.1.5). In view of the practical and theoretic limitations of the electrocardiographic measurement of heart score, there has been some interest in the use of echocardiography to measure the size of the LV and predict the racing potential of an individual horse. The practice is based on the assumption that those horses with the large hearts are more likely to win races than those with smaller hearts. This assumption is unproved and may be flawed. Although the general principle that the best horses have large hearts may be correct, there is no evidence to show that this is why they win races. Evidence in human athletes indicates that athletes have bigger hearts than sedentary individuals and that elite athletes have particularly large hearts. However, the differences are only great enough to be significant when populations are comrather than individuals. Furthermore, heart size changes in response to training so that those athletes that train harder develop larger hearts and this may be the reason that they win races, rather than any intrinsic differences in cardiac capacity.