Whether he's headed for a demanding cross-country course, an endurance ride or a hunter pace, your horse faces a challenge. Will he finish the ride with energy to spare? Or will he tire quickly or, worse, finish so drained by the effort that you have to scratch him from the event?
Your horse's heart will play a big role in determining the answers to those questions. It's an incredible athletic machine, and he can't reach his -potential unless it's well tuned. This article will explain why the heart is so important and how you can ensure it performs at its best.
The heart is the key to performance because, along with the blood vessels and the lungs, it delivers oxygen and fuel to working muscles. Like an internal combustion engine, your horse's muscles use the oxygen to burn the fuel--fatty acids and blood glucose--releasing energy for contraction. This aerobic (with oxygen) system is super-efficient, and it constantly adapts to meet the demands of the moment. When your horse's brain senses that the muscles need more oxygen, for example, it orders the heart and lungs to work harder. Respiration and heart rates increase to speed delivery of oxygen, and blood vessels dilate or contract to change patterns of blood flow.
A horse's heart rate can go from as low as 25 beats per minute (bpm) at rest to 10 times that at peak exertion--a far greater range than even the most elite human athletes can achieve. But if the muscles work hard enough, their needs outstrip the ability of the cardiovascular system to deliver. Then the horse switches to anaerobic (without oxygen) energy production, using muscle enzymes to burn glycogen and other fuels stored in muscle tissue. This method is much less efficient, and the horse can keep it up for only a few minutes before his muscles tire.
Cardiovascular fitness changes the equation. Like any muscle, the heart gets stronger with use, so hard work makes it stronger and more efficient. It can beat faster for longer periods of time, and its stroke volume--the amount of blood pushed out with each beat--increases. A fit horse also makes more oxygen-carrying red blood cells and clears old, spent red cells from his blood more quickly. New capillaries (tiny blood vessels) develop in the muscles and in the lungs, speeding the exchange of oxygen and carbon dioxide. He's able to do more before switching to the anaerobic system, and he recovers faster after exercise. Increased blood flow also delivers nutrients and has healthy effects throughout the body, from a shiny coat to healthy hooves.
Conditioning work increases your horse's aerobic capacity by strategically upping the demands on the oxygen delivery system. These changes take place slowly. To reach the level of fitness your horse needs for competition, you need to do the right amount of work, at the right level, over time. Too little, and he won't be up to the demands of the event; too much, and he'll be tired, stressed and possibly injured. How can you gauge if you're on target? Monitor his heart rate.
Know His Rates
Start by getting familiar with your horse's normal heart rates. You can use a stethoscope (inexpensive ones sell for less than $10) and a watch with a second hand. Listen along the heart-girth, behind the left elbow. (You may need to experiment a bit until you find a good listening spot.) Count beats for 15 seconds, and multiply by four to find the rate in bpm. Measure the rate in these situations:
At rest: Your horse's heart may laze along at around 25 or 30 bpm when he's just hanging out in his stall, but you're more likely to get a resting reading of 40 to 50 at first. That's because just by entering his stall and putting the stethoscope on his side, you'll cause the rate to go up. If you do this often, though, he won't be so interested. You'll find that you get lower readings.
The resting heart rate is a useful measure of health. Take resting readings at the same time of day for several days, and calculate the average. A marked departure from the average rate (an increase from 30 bpm to 50, say) can signal illness or some other type of stress.
After exercise: Check your horse's rate immediately after exercise and then at five-minute intervals to see how quickly he recovers. The readings you get will depend on how hard he worked, how fit he is and other factors (more on this later). Even after a cross-country gallop, though, you want to see the rate down around 100 bpm within five minutes and in the 50 to 60 bpm range within 30 minutes.
Checking his heart rate with a stethoscope allows you to hear the heart's rhythm, not just count the beats--and if you hear irregularity in the steady lub-dub pattern, you'll want your vet to check it out. But for conditioning, the stethoscope has disadvantages. You can't use it to monitor the heart rate during exercise or to find out where the rate peaks during work. The rate begins to slow as soon as your horse stops working, and it will have dropped significantly by the time you (or a helper) can get the instrument to his side.
To really know what's going on, use a heart-rate monitor. Monitors have steadily become more popular as they've dropped in price. You can find models designed for horses for less than $150, although you can pay much more if you want lots of bells and whistles.
Like an electrocardiogram, a monitor detects the electrical signals that prompt the heart to beat. Typical models have a small transmitter with positive and negative electrodes. The negative electrode goes under the girth on the near side, and the positive electrode goes under the saddle pad on the off side. A little gel helps the heart's electrical signal carry through the horse's coat to reach the electrodes, and the transmitter sends the data wirelessly to a wristwatch receiver that displays the rate.
Most heart monitors available today read accurately. But they're based on human equipment, and the horse's electrical signal is slightly different. Occasionally a monitor will miss beats in a particular horse and give a false reading. You can check the accuracy of your monitor by -using a stethoscope and comparing the machine reading to your own count.
Know His Trends
A monitor lets you see where the rate is as you work and train, and that's valuable information. As a general rule, heart rates below 150 bpm indicate that your horse is working aerobically--his muscles are using oxygen to burn fuel. Above 150 bpm, he begins to move past the anaerobic threshold. His muscles switch over to the less efficient anaerobic system. Watching his heart rate, then, tells you not only how hard his heart is working but also how his muscles are obtaining energy. That knowledge allows you to gradually build his aerobic capacity through conditioning. The idea is to stress his system enough to strengthen it but not enough to do harm.
Your cardiovascular conditioning program should be tailored to your individual horse--his starting level of fitness, his natural physical ability and the competition he's training for. Generally you start with at least a month of long, slow distance work at walk, trot and canter, keeping your horse's heart rate below the anaerobic threshold. That strengthens bones and muscles as well as building aerobic capacity. A typical week includes three conditioning days interspersed with two to three light rides and one day of rest. As your horse gets fitter, you can add more intense exercise, pushing his heart rate above 150 bpm in a short workout one day a week or in intervals--quick bursts of intense effort spaced out in a workout that's mostly at slower speeds.
Keep records of your readings so you can see how the rates change over time, always comparing your horse to himself, not to others. A common method for judging fitness is to clock your horse's speed at a heart rate of 200 bpm, which is high but submaximal. (Peak rates range as high as 250 bpm--but you do not want to work your horse to that rate.) To do the test accurately you need a measured track and a stopwatch, but you can develop your own yardstick if you compare his performance over the same ground and at the same pace on different days. Log his heart rates during work and the time it takes to recover. Remember that anytime the rate goes above 150 bpm, you want to see it return to normal (the 50-60 range) within 30 minutes. If your horse takes longer to recover, you've pushed too hard.
Along with heart rates, monitor his breathing--the rate and effort of his breaths, and the time it takes for his breathing to recover after work. Watch for variations, which could point to respiratory problems as well as poor conditioning.
You'll find that many factors will cause heart and respiration rates to vary from day to day. Chief among them are temperature and humidity, which force the heart and lungs to work harder. Unfamiliar surroundings can also send the rates up. Don't panic if you take your horse to an event and find that his heart rate shoots up to 150 before you've even warmed up. He's excited. Once he relaxes, it will drop.
Rather than daily changes, focus on trends. Over time, you should see him become more tolerant of work. He's able to do the same work at a lower heart rate--a canter at 400 meters a minute lifts the rate to 120 bpm instead of 130, say--or go faster or farther at a given heart rate. If the trend begins to go the wrong way and he becomes less tolerant of work, that suggests he's stressed. He may have an illness, or you may be pushing too hard and overtraining. It's time to back off and look for the reason.
Kim McGurrin, DVM, DVSc, Dip. ACVIM, is a veterinary internist, equine clinician and researcher at the Ontario Veterinary College, University of Guelph, Canada. In 2004 she and veterinarians Dan Kenney and Peter Physick-Sheard developed the electrical cardioversion procedure used to correct atrial fibrillation in horses. Since then she has trained specialists from other major veterinary hospitals--including the University of Pennsylvania's New Bolton Center, the University of California at Davis, the University of Florida, Cornell, Ohio State, and Washington State--to perform the procedure.
How the Heart Pumps Blood
By any measure, your horse is a big-hearted animal. An adult horse's heart is about the size of a basketball and weighs, on average, more than eight pounds. Like yours, it has four chambers, all with walls of solid, dense muscle.
The most powerful of the chambers is the left ventricle, the lower chamber on the left. When its strong walls contract, about a liter of blood is pumped out through the aorta, the main artery. The blood travels on through branching blood vessels to every part of the body, delivering oxygen and collecting carbon dioxide and other wastes.
Veins bring blood back to the right atrium, the upper chamber on the right, which contracts to force the blood into the lower chamber, the right ventricle. When the right ventricle contracts, it pushes the blood out of the heart through the pulmonary artery to the lungs, where the blood releases carbon dioxide and picks up oxygen. The blood travels back to the heart through the pulmonary vein, entering the left atrium. Contraction sends it on to the left ventricle to begin the round trip again.
A pattern of electrical signals, produced by a tiny structure known as the sinus node in the upper part of the right atrium, keeps the contractions in synch. One-way valves between the atria and the ventricles and between the ventricles and the arteries prevent blood from slipping back the way it came, keeping circulation heading in the right direction.
The "lub-dub" sound of the heartbeat is actually the sound of these valves shutting--a soft "lub" as the valves between the atria and ventricles close at the same time on both sides, and a more distinct "dub" as the valves between the ventricles and the aorta and pulmonary artery close simultaneously.
Horses don't get clogged arteries and suffer heart attacks as people do, but they do have heart problems. Fortunately, most of these are rare.
Atrial fibrillation is an irregular heartbeat (arrhythmia) that affects one to three percent of horses. In this condition the heart's upper chambers (atria) contract irregularly and don't keep time with the lower chambers (ventricles). Horses with atrial fibrillation (unlike people with this problem) generally don't have underlying heart disease. They can tolerate this condition well if they don't have to work very hard. But the irregular beat means that the heart pumps blood inefficiently. When the horse is asked for a big effort--as he may be in racing, eventing, endurance, jumpers or upper-level dressage--he tires quickly. Untreated, the problem can limit or end the horse's career.
Until recently the only treatment option was the drug quinidine. It's effective, but it can have serious side effects that include colic, laminitis and seizure--so it's given at a clinic, and the horse is monitored throughout the treatment. A -procedure adapted from human medicine, transvenous electrical cardioversion, is a new alternative. Two long intravenous catheters are threaded through the jugular vein to the heart; then a defibrillator delivers a mild shock through the catheters, jolting the rhythm back to normal. The procedure has a high success rate and no significant complications. But the horse has to be anesthetized, which poses some risks.
Other arrhythmias affect horses less often. Sometimes they can be very serious. Damage to the heart or some other cause disrupts the electrical signals that govern the heartbeat. The rhythm breaks down, and the heart can no longer pump blood. When an apparently healthy horse suddenly drops dead, a severe arrhythmia is often the cause.
Aortic rupture is another cause of sudden death. Aged breeding stallions sometimes go this way. The effort and excitement of breeding raises the stallion's blood pressure, which leads to rupture in the main artery leading from the heart. Pulmonary artery rupture also occurs. It has caused some racetrack deaths.
Leaky heart valves usually show up as poor performance. The vet checks with a stethoscope and detects a murmur. A leaky valve can't be treated, but it should be monitored. Many horses with heart murmurs do well, but that depends on the degree and the valve that's involved.
Over time a leaky valve can lead to other problems. The heart has to work that much harder to push the necessary blood out, and a lifetime of extra work causes the heart to enlarge. Eventually the horse can go into heart failure. This usually happens in an older horse, and it develops slowly. Medications may help the heart keep going a little longer. But the drugs have side effects, and the horse remains at risk with a limited quality of life.
This article originally appeared in the May 2008 issue of Practical Horseman magazine.