By Francesca Worsman BVM&S MRCVS
Horserace Betting and Levy Board Senior Clinical Training Scholar in Equine Internal Medicine, Royal (Dick) School of Veterinary Studies, University of Edinburgh
Horses, through selective breeding for athletic excellence, have well-developed hearts that rarely cause problems compared to those encountered by humans. On occasions however, things go wrong. Due to their well-developed physiology, horses are at risk of a fibrillating heart (atrial fibrillation), while circumstantial evidence suggests that more severe heart rhythm abnormalities, somewhat akin to those experienced by human athletes, may cause sudden death. Ultrasound examination of the heart, known as echocardiography is a readily available tool for examining the heart and significant advances in ultrasound technology are likely to provide exciting information about the detailed function of the equine heart. A huge benefit of ultrasound is that it is non-invasive and can be carried out on a standing unsedated horse, so normal heart function is maintained during the examination.
The equine heart, like all mammalian hearts, has four chambers. The right atrium, which receives oxygen-depleted blood from the organs, passes it into the right ventricle which then pumps it to the lungs. Blood picks up oxygen in the lungs and then returns it to the left atrium, which then passes it onto the left ventricle for pumping to the organs of the body, including the muscles. Oxygen is thus delivered to the tissues and then the cycle repeats, more than 50,000 times per day! The left and right atria work in unison during heart filling (diastole), and the left and right ventricles work in unison during evacuation of blood from the heart (systole). Murmurs, often detected by veterinary surgeons when listening to hearts, are either caused by normal forward blood flow through the heart or by backflow leakage across the valves within the heart (regurgitation). Many of these murmurs are not a cause for concern, although some regurgitant murmurs are more severe and can cause problems with heart function. Similarly most heart rhythm abnormalities are innocuous and do not affect performance while others are more serious. In some horses, due to suspicion by a veterinary surgeon of a more significant problem, extensive evaluation of the heart is required; echocardiography is one of the key tools for diagnostic evaluation of the heart to assess the impact of any problems on athletic performance.
Real-time three-dimensional echocardiography (3DE) is an exciting new tool that has recently become available in equine medicine and may shed light on important heart problems in horses, including those that cause poor performance. Compared to standard two dimensional echocardiography (2DE), which evaluates a single scan plane, 3DE involves the simultaneous collection of multiple scan planes from the heart to create a pyramid of scan data. 3DE is preferable to 2DE because in theory it does not rely on geometric assumptions on chamber shape to calculate chamber volumes. Also assessment of heart architecture and function (including that of valves) is likely to be more accurate as the technique provides unlimited viewing planes. Finally, for assessment of regurgitant flow, this technique should also be better because the image can be manipulated to better assess the leaky flow from those valves. In humans, for certain heart volume and muscle mass measurements, 3DE is more comparable than 2DE to cardiac magnetic resonance imaging (MRI), which is the acknowledged gold standard. Unfortunately, owing to the significantly larger size of horses, there is currently no equipment available for equine cardiac MRI. Therefore, 3DE could provide the next best option for more detailed equine cardiac evaluation.
Mitral valve regurgitation for example is commonly encountered in the equine athlete and, while often of no consequence, in more severe cases, it may lead to poor performance. Pathological consequences are due to backflow leakage causing secondary left atrial volume overload, and this will lead to an increased likelihood of atrial fibrillation. Potential advantages of 3DE in this context are more accurate assessment of the degree of volume overload, the regurgitant orifice (i.e., size of the hole!), and the valve structure and motion. Mitral valve regurgitation can be easily confirmed by 2DE, however it can be more difficult to ascertain the cause and severity. With 3DE, the software allows manipulation and therefore anatomic evaluation of the valves from many angles including ‘face-on‘ views. This results in more detailed evaluation of subtle abnormal valve motion and confirmation of suspected findings, which can’t be reliably detected by 2DE - eg. mitral valve thickening, mitral valve prolapse or ruptured chordae tendineae as the cause of mitral valve disease. Geometric assumptions on volume are avoided as much more structural data is obtained.
A few specialist centres worldwide currently offer 3DE imaging for horses including the Equine Hospital at the Royal (Dick) School of Veterinary Studies, University of Edinburgh. In equine medicine we are still at an early stage of using 3DE as it is not validated in horses, therefore it is mostly used in research at the moment as opposed to routine diagnosis. As part of my research at Edinburgh, sponsored by the Horserace Betting and Levy Board, I have been assessing the left atrial volume using 3DE from thoroughbreds in training. One of my aims was to determine the variability of equine left atrial volume measurement using a special 3DE software analysis package to see how much variation there was between successive 3DE measurements by the same person. Forty-four National Hunt thoroughbreds in training were scanned to obtain the 3DE views of this chamber. We then graded them to exclude images of reduced image quality so that we were only assessing good quality images of the left atrium. In total 24 horses were included—aged 4-9 yrs, weighing 411-534kg. I analysed the images retrospectively, after the horses were scanned. I didn’t include any horses with grade >3/6 heart murmurs. This was because we first need to validate 3DE with normal, healthy hearts. Random generated order measurements were obtained by a single person on four occasions. Real-time three-dimensional end-systolic (ESV) and end-diastolic (EDV) left atrial volumes were measured using 3DE software, and the results were then statistically analysed.