From a purely human perspective, the answer is no. Horses do not suffer from the coronary artery diseases plaguing millions of people. Arteriosclerosis, or clogging of the arteries that feed and nourish the heart, is a condition not typically associated with the equine species. Horses also are rarely victimized by high blood pressure. Nevertheless, horses do suffer from other diseases of the heart.
Common heart problems
One of the most common congenital heart problems (problems the horse is born with) are ventricular septal defects, or VSD's. A VSD is a defect, or hole, in the part of the heart that separates the main left and right chambers (ventricles). It develops as the heart forms during early pregnancy, and does not increase with size over time. When these defects are small (2cm), most horses are able to perform successfully as athletes, but larger defects do tend to cause performance problems. If the hole is so large that the heart cannot adequately pump blood to the body, the horse develops heart failure, usually at a relatively young age. A VSD can be diagnosed often by listening to the horse's heart with a stethoscope, because blood coursing through the defect produces an abnormal sound, or murmur. To determine the size of the hole requires that the heart be visualized with an ultrasound machine powerful enough to image a horse heart. With that, one can measure the size of the defect and try to predict if and when heart disease may develop.
Defects of the heart valves
Another common cause for heart murmurs in the horse are defects of the heart valves. Just as with people, horses have four valves in their hearts. Valves promote one-way blood flow through the four chambers of the heart, and abnormal development or injury to these valves can produce alterations in blood flow. Sometimes, the only indication of a valve defect is a murmur heard when listening to the heart. Many valve lesions cause no problem to the horse, but they do need to be monitored regularly, because some valve diseases do lead to progressive problems over time. Sudden and severe injuries to valves, though, can lead to rapid signs of heart failure and even death. Fortunately, these occur only very rarely.
Benign heart murmurs
Some murmurs are not serious at all, and are termed 'physiologic flow murmurs', often detected during routine physical examinations. These exist primarily due to the tremendous size and very slow heart rate (24-48 beats per minute) that horses maintain. Because the volume of blood being moved through the heart each time it beats is large, and the timing between each beat is long, veterinarians are often able to hear alterations in blood flow patterns. Physiologic flow murmurs are typically not associated with heart disease, but it is important that they be evaluated carefully. Some murmurs are only evident under certain conditions. It is not uncommon for horses in pain (i.e. colic, severe lameness) to develop detectable murmurs. These are transient and classically disappear when the animal is again healthy. Listening to the heart of an exercising horse can also identify murmurs only evident when the heart is working harder than it does at rest. It will also identify abnormalities in the heart beat rhythm.
Abnormal heart beat rhythm
Arrhythmia's, or abnormalities of the heart beat rhythm, are another type of abnormal heart condition. Some arrhythmia's are considered to be perfectly normal in the resting horse, and disappear when the horse exercises or gets excited. Others are heard only when the horse is exercising, particularly as the heart rate slows from that achieved during a canter or gallop down to a walk. Some arrhythmia's, such as atrial fibrillation, indicate problems within the heart, and should be evaluated with an EKG and ultrasound examination.
Annual examination important
Therefore, while horses don't typically have heart attacks, they do suffer from other forms of heart disease. It is important to know that your horse has a healthy heart, as both their safety and yours may be at stake. Thorough annual examinations by your veterinarian are an important part of detecting and preventing serious heart disease. When abnormalities of the heart are identified, further evaluation and diagnostic testing by your veterinarian or a veterinary specialist is warranted.