Post-Race Collapse - Prevention & Management
By Dr. David Marlin
Fortunately, incidents of post-race collapse are relatively rare following racing, however if they do occur, it’s important to know what steps can be taken. Common causes of post-race collapse include cardiac arrhythmias, neurologic events, internal bleeding due to large blood vessel rupture, airway obstruction and overheating. All of these are a serious cause for concern and likely to require veterinary support. However, overheating is likely to be one of the most common reasons for post-race collapse, but it is often not recognised as such and can lead to horses not receiving prompt treatment that may ensure a swift and uneventful recovery with no long-lasting injury.
During races, horses get hot because for every unit of energy they use which makes the muscles contract, four times as much energy is produced as heat. The harder and longer the horse works, the more heat it produces. Although horses lose heat by sweating (around 85%) and through breathing (around 15%) during a race, around 90% or more of the heat produced is stored in the muscles. Even so, on a hot day, horses may come in at the end of a race with body temperatures 1-2°C higher than they would for the same race in cool conditions.
It would not be unusual for horses to finish races with rectal temperatures of 40-41°C. But taking rectal temperature can also mislead us as the temperature inside the working muscles may be much higher; and it can take five minutes for the rectal temperature to reach a peak after a horse pulls up, increasing by another 1-2°C. It’s as we get to rectal temperatures of 42°C that the risk of collapse due to hyperthermia (high body temperature) becomes significant. Let’s look at why high body temperature can lead to collapse.
Firstly, very high body temperature leads to direct and damaging effects on the brain, the nervous system as a whole and the heart, which may lead to collapse. These effects are related to how high the temperature is and how long the horse stays at that elevated body temperature. For example, if a horse was not cooled off following a race, then it may take 5-10 minutes for the onset of collapse. However, post-race collapse on pulling-up and/or returning to the winners enclosure or stables is not uncommon, and this has a different underlying cause.
During the race, the horse actually reduces blood flow to the skin and chooses instead to send as much as possible to the muscles. This is very different to the situation in people where a significant amount of blood is always sent to the skin to help cooling (thermoregulation). The consequence of blood being directed to the muscles is that the muscle temperatures increase rapidly even over a few minutes of a race. When the horse starts to pull-up, this is reversed and blood is suddenly redirected to the skin. This is most pronounced when the horse comes to a stop. The effect is similar to fainting in people; the flow of blood to the surface causes a fall in blood pressure and effectively the horse faints.
Clearly, collapse of horses is undesirable and has the potential to cause further injury, so it is important to recognise the risks for post-race collapse with respect to overheating and what to do if the situation arises. One of the common misconceptions of post-race collapse is that this is due to “lack of oxygen”. Whilst this could be true in some cases, this is likely to occur in a very small number of horses and only in those with airway obstruction. From studies on treadmills, for example, we know that within a few seconds of starting to slow down, the low oxygen levels in the blood are immediately reversed and even become higher than they were before exercise. People will often cite the ‘blowing’ of horses after a race as an attempt by the horse ‘to get more oxygen in’, however, it’s clear from a number of studies that blowing/breathing after exercise is directly related to body temperature and not oxygen levels. Rapidly reducing body temperature by aggressive cooling results in a more rapid cessation of blowing.
When should heat stress and overheating be suspected? A horse that is hot to touch, blowing very hard and also ataxic (wobbly) when pulled up should be suspected as suffering from overheating, and cooling should be started immediately. If possible avoid turning in tight circles but keep walking as this helps increase blood pressure. Even in cases where overheating is not the main problem, cooling is extremely unlikely to have any negative effects. As mentioned previously, overheating is frequently not considered as a possible cause for post-race ataxia/collapse and may therefore not be recorded as such.
Some time ago Professor Tim Parkin and I examined data from the British Horseracing Authority over three seasons of diagnosed cases of post-race heat stress. Over a three-year period, 108 cases had been recorded by on-course veterinary surgeons post-race. A number of factors significantly increased the risk of a horse suffering overheating. Perhaps not surprisingly, races run in the spring or summer were eight times more likely to include at least one horse with heat stress as races run in the autumn or winter. National Hunt races were almost three times more likely to have a horse with heat stress due to the longer duration of the races compared with the flat. Also, for any type of race, there was an increasing risk for every five furlongs (1,000m). Races run in the afternoon were also three times more likely to have a heat stress case than a race run in the evening. Finally, faster races also increased the risk of horses suffering overheating.
This should all be expected: long races and/or faster races in the afternoon on warm days in spring and summer carry an increased risk of overheating/heat stress and collapse. As not racing in such conditions is not likely to be an option, it’s essential that racecourses and trainers are aware of the signs and risks of overheating and the risk of post-race collapse and take appropriate and prompt action if necessary.
Aggressive cooling is now used extensively in professional endurance racing and eventing, as well as in all equestrian disciplines at major events such as FEI World Championships and Olympic Games. The principle is simple. Applying cold water (0-5°C), either from a hose or from a large container of ice in water, rapidly cools the blood in the skin which in turn more slowly cools the muscles. In horses that are very hot and at risk of heat stroke/collapse, there is no requirement or benefit to scraping water. The key to minimising risk is continuous application of water over as much of the body surface as possible until the breathing starts to recover (i.e., until the blowing reduces). This is the best and most practical indicator of the effectiveness of the cooling. It’s also essential to continue aggressive cooling for 5-10 minutes to bring a horse’s temperature down 1-2°C. As mentioned previously, the cessation of blowing is the best indicator of effective cooling.
Applying cold towels, fans, shade, ice packs on large blood vessels, ice in the rectum, spraying with alcohol are all minimally effective in comparison with continuous application of large volumes of cold water all over the body. In contrast to widely held misconceptions, this approach to cooling does not cause the horse to heat up due to constriction of blood vessels in the skin nor does it cause muscle or kidney damage. The key to preventing collapse and or permanent injury due to heat stroke is rapid instigation of cooling. Literally, seconds count. Delaying cooling by thirty seconds may result in a collapsed horse.
Even in cases where the cause of ataxia/collapse is not primarily due to overheating, starting cooling until veterinary help arrives will not make the situation worse.
Compared with their jockeys, horses are actually able to tolerate much higher body temperatures. A jockey with a temperature of 41°C would be comatose and at risk of serious injury or even death, whilst a horse at 41°C would still be running. However, it is possible for both jockeys and horses to acclimatise to heat. Acclimatisation is the process whereby the body becomes more tolerant of heat as a result of regular daily exercise in the heat. Of course racehorses are most commonly trained early in the morning in the cooler part of the day, yet the majority of races are held in the warmer times of the day, so it’s conceivable that most racehorses are not heat acclimatised. It may also be of interest that heat acclimatisation also improves performance.
In summary, overheating of horses during races is more likely in longer, faster races at warmer times of the year. Horses that are hotter than normal are at an increased risk of heat-related collapse, often when returning to the paddock and standing. Horses that are very hot to touch, blowing hard, wobbly and possibly ‘excited’ are likely to be at risk for collapse. Starting cooling aggressively immediately can lead to rapid recovery and prevent collapse and the risk of more serious injury.
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