Cardiac rhythm - new technology brings new insights | Trainer Magazine | European Trainer Article Index

Oct 7

Oct 7 Cardiac rhythm - new technology brings new insights

CARDIAC RHYTHM New technology brings new insights When a horse runs badly, lameness or respiratory disease tend to immediately spring to mind, and indeed these are the most common causes, in that order. The heart comes in third, albeit quite a way behind these other body systems. If sudden death occurs in an equine athlete, a heart problem is usually the first thing that’s suspected. A new study, published in The Equine Veterinary Journal, provides significant insights on the cardiac rhythm abnormalities that can develop during and after racing in standardbreds. Thehorse’s heart is an astounding capable of around 1 litre of blood with every heart beat, and with the ability to accelerate from a resting heart rate of around 30 – 40 beats per minute, to around 230 bpm at maximal exercise. This allows the horse to pump over 250 litres of blood every minute and in doing so, it supplies the oxygen it needs to fuel its muscles when galloping. Elvis Presley sang “Rhythm is something you either have or don't have, but when you have it, you have it all over”. But when it comes to the equine heart, was he right? The equine ECG The heart is a pump that is powered by spontaneous electrical activity. The electrical activity is what determines the heart’s rhythm, making it either regular or irregular, slow or fast. The main tool that is used to document cardiac rhythm is the electrocardiograph – or ECG. The ECG is essentially a roadmap showing how the electrical activity which co- ordinates cardiac contraction spreads from its starting point in the pacemaker through the chambers of the heart. If the rhythm is disrupted, the ECG pinpoints exactly where within the heart chambers the disruption originates. Until fairly recently, routine recording an ECG required attaching electrodes to the horse’s skin, which in turn were attached to the recording unit by leads or wires. Although it has been possible to record ECGs during exercise for several decades, using ECG units with integral digital recorders or those could transmit to a distant monitor by radio, inevitably, this sort of recording equipment limited availability. ECGs have been recorded in standardbreds during race conditions in a study performed by Dr Physick-Sheard in Canada and some surprising results emerged. It was found that over 18% of these horses had irregularities of the cardiac rhythm, particularly as the heart slowed immediately after intense exercise. Rhythm irregularity was more common in horses which broke stride or suffered interference raising the possibility that these are linked to stressful circumstances. For human athletes, screening programs have been developed in an effort to identify pre-existing cardiovascular abnormalities that have the potential to cause sudden death. These programs have been developed based on prevalence and risk factor data for various cardiac arrhythmias. In the horse, guidelines for screening for arrhythmias prior to racing are essentially non-existent and the likely effects of various cardiac arrhythmias on athletic performance or risk of sudden death is largely based on expert opinion rather than evidence. New ECG technology Vets on the racetracks have rarely carried traditional bulky ECG equipment. As a result, when a cardiac arrhythmia was suspected based on listening to the heart after racing, it was rarely possibly to document this or determine the exact nature. The usual outcome being that the horse returned home with an unsatisfactory explanation of why it might have run badly and uncertainty about what the future might bring. But, with recent technological advances, this situation looks set to change quite rapidly. It is now possible to attach a device to a mobile phone which can very effectively record an ECG without the need for leads, clips or electrodes. The Pennyslvania Standardbred study Researchers based at the New Bolton Centre, University of Pennsylvania have recently published the results of a large-scale study in Equine Veterinary Journal. The study was supported by Pennsylvania Harness Horsemen’s Association at Pocono Downs and Chester Downs and the Meadows Standardbred Owners Association. Dr JoAnn Slack, co-ordinator of the research team, recorded ECGs using clip-less, hand-held devices before and after racing. The study was performed over a 12 week period and involved Standardbred horses entered to race at a five eighth-mile race track where all races are 1 mile long. All horses entered to race on data collection days were enrolled in the study. Pre-race ECGs were obtained prior to horses warming up and post- race ECGs were obtained as soon as the horse returned to the paddock area following the race. The ECGs were obtained simply by placing the recorder over the heart just behind the triceps muscle, usually on the left side but sometimes, if the horse was unco-operative, the right. The total study population comprised 1,816 horses; 453 trotters and 1,363 pacers. Some horses raced several times during the study and in total Dr Slack’s team recorded 4335 pre-race ECGs and 4322 post- race ECGs – quite a considerable effort, particularly as Dr Slack analysed every one personally. Premature depolarisations The study showed a wide range of rhythm abnormalities both before and after racing. By comparing several ECGs from the sub-group of horses that ran and were recorded several times, it was clear that these events did not occur consistently. Of most interest were the rhythm disturbances that are currently considered to be clinically important. The simplest form of rhythm disturbance is premature beats, or more correctly, depolarisations. These can originate in the ventricles, the main pumping chambers, or within the atria, two chambers that sit at the top of the heart and contribute to its filling. Individual ventricular depolarisations were common, found post-race in one in every 361 race starts. Individual depolarisations are fairly unlikely to affect the horse because they have minimal impact on the overall rhythm and cardiac output. Arrhythmias that disrupt the heart rhythm for several seconds or minutes are of more concern. Two main forms of this category were found in the Pennsylvania Standardbred Study: atrial fibrillation, occurring in 1 in 720 racestarts and ventricular tachycardia occurring in 1 in 2161 racestarts. Atrial Fibrillation With atrial fibrillation, the co-ordinated electrical activity within the atria (filling chambers) is completely lost. In its place, random wavelets of depolarisation spread around the atria continuously, causing them to wobble rather than contract. The ventricles continue to pump, but because they are no longer filled efficiently, the overall cardiac output is reduced. This results in poor delivery of blood and oxygen to exercising muscles and therefore poor performance. Atrial fibrillation has often been recognised in racehorses finishing badly, but this is the first study to systematically document its prevalence in such a large group of horses where every horse in the race was examined. Five of the six horses that had atrial fibrillation were amongst the group of 254 horses that did not finish, were distanced or finished greater than five seconds behind the winner, giving a prevalence of 2.0%. But, one horse with atrial fibrillation paced the best mile of his career and placed second in the race, likely developing the arrhythmia at the end of or immediately after the race. Ventricular tachycardia Two horses had ventricular tachycardia identified after racing, one of which required emergency treatment. Both horses had normal heart rhythms prior to racing. Neither of these two individuals showed any signs of poor performance and most likely developed the arrhythmias at the end of the race or immediately afterwards. One horse had normal post-race behaviour and one horse was mildly agitated and slow to cool down. Ventricular tachycardia is a highly concerning arrhythmia because it can lead to sudden death. It is also very easy to mistake this arrhythmia for just a fast heart rate or even atrial fibrillation if only a stethoscope is used and not an ECG. Having a handheld ECG available at the track can potentially facilitate a life-saving intervention. Risk factors Dr Slack examined various risk factors that might possibly be linked to cardiac arrhythmias. Increasing age was a significant risk factor for post-race arrhythmia and for supraventricular premature depolarisations. Furosemide (Lasix) was administered in 2,588 out of 4,335 race starts (60%). However, there was no association between post-race arrhythmia and furosemide administration. Similarly, sex, gait and ambient temperature did not appear to have any link. What can a trainer take from this study? Don’t forget that the heart can be a cause for poor performance. Horses that perform poorly should have their heart listened to with a stethoscope, ideally in that early post-race period. Any irregularities in the heart rhythm or heart rates higher than expected (even if the rhythm is regular) should prompt an ECG evaluation. What might the future bring? We are still a long way behind human sports medicine where screening for hidden cardiac risk is widespread. But this study has provided a large and very useful database on which to build. There is a need to document the prevalence and risk factors for cardiac arrhythmias in other forms of racing. The Pennsylvania team has shown how this can be done using simple and accessible technology. The ECG is a roadmap of how electrical impulses travel through the heart. The atria are the filling chambers. The normal heart beat starts in the SA node, it passes across the right (RA) and left (LA) atria, to the AV node. There is a short delay then the impulse travels down into the pumping chambers, the right (RV) and left (LV) ventricles, through a network of specialised fibres. A beat originating in the sinus (the SA node), has a specific pattern of a p formed of a double peak, followed by the qrs and t complex. An SVPD is an abnormal impulse that starts in the atria and the p wave has a different shape. A VPD is an abnormal impulse that starts in the ventricles and the qrs-t have a different shape. If the abnormal impulses are starting at several sites, the qrs will have several shapes. If there are multiple abnormal impulses in succession, this is called tachycardia.

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