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The veterinary mystery of the California cluster
Equine researchers try to understand a spike in sudden deaths, possibly cardiac-related, among thoroughbreds.


Applying research to the problem

To try to better understand causes, a study was recently performed in standardbred racehorses in which heart rhythm was followed from harnessing until the end of the race during normal competition. "Disturbances in rhythm immediately after the race were identified that would be capable of causing a fatal outcome, but there were no deaths," says Physick-Sheard. "However, clear indications of strategies that might reduce risk of a fatal outcome were noted. Sudden cardiac death associated with racing occurs more commonly in the thoroughbred than in the standardbred, and it is possible that ventricular arrhythmias are taking place here also."

To resolve this question in racing thoroughbreds, researchers recently monitored heart rates and rhythms during normal scheduled racing at Woodbine Racetrack in Toronto, from saddling to unsaddling and into early recovery. "The objectives were to characterize the range of usual rate and rhythm variations and to provide guidance as strategies to minimize risk are developed," explains Physick-Sheard. He says the thoroughbred work was done after the standardbred work, and data are now being processed.

During the standardbred study, the research team monitored horses' heart rates and heart rhythms during normally scheduled standardbred racing.2 "We put the monitoring gear on when the horses came into the paddock, and it stayed on until the horses came off the track at the end," Physick-Sheard says. "We were able to monitor the entire exercise response. What we identified of particular significance was that about 18 percent of horses would have significant rhythm disturbances during recovery, fairly unusual ventricular rhythm disturbances, which were not previously identified in horses."

He goes on to say that although all of the horses in the study continued to race and none died, the arrhythmias did occur at the time when there appears to be a peak in post-exercise death, as the literature shows that most horses identified as dying of sudden cardiac death die in the period immediately after racing.

"When we saw these rhythm deficits, each of them had the potential to deteriorate into a malignant or fatal arrhythmia, though none of them did," Physick-Sheard says. They all recovered and went back to normal sinus rhythm. The principal finding was that horses subjected to racing exercise will tend to show disturbances during the recovery period.

As part of the study, the team also identified factors that predisposed horses to the probability of a horse experiencing cardiac rhythm disturbances. The factors included being a trotter, getting parked at the half-mile pole and breaking in the stretch. "Our analysis of those and associated data leads us to believe that some types of physical stress, predominantly stress that is associated with psychological stress, may be predisposing these horses," Physick-Sheard explains.

They also found that the arrhythmias frequently occurred in association with what is generally thought to be a normal variation in rhythm when the heart rate is decreasing. "The heart rates do not descend smoothly," says Physick-Sheard. "They tend to come down in a stepwise fashion, and sometimes that step is quite obvious, sometimes not. But there is always a tendency for the heart rate to come down in the middle ranges in a stepwise fashion. We identified that these episodes of sudden slowing appear to be a trigger for ventricular rhythm disturbances. That is another piece of information that we are also pursuing to indicate why sudden death may occur in all athletes during recovery from maximal effort. Some deaths occur in humans under similar circumstances."

Physick-Sheard says researchers don't know for certain whether these rhythm disturbances are just a usual variation from normal, or whether they actually represent pathology. "We are pursuing that issue with another study to try to find out whether there is pathology associated with these rhythm disturbances," he says. "It's not possible for us to be there when a horse dies because we don't want any horse to be dying, especially not while we're monitoring it. But that would be the only way to prove that these rhythm disturbances were in fact the cause of death. So we may never know for sure. The suspicion is that these disturbances on occasion deteriorate and result in death."

In human cardio stress testing, echocardiography is part of the normal routine. The difficulty is using this technology in horses. "Further studies to fully understand all the pieces to the puzzle would certainly necessitate echocardiographic studies," says Physick-Sheard. "For now, knowing that we have so much more information to gather, we are concentrating on real-world (racetrack) exercise. It's not possible to do ultrasound examinations on the racetrack at the present time."


Source: DVM360 MAGAZINE,
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