Sled Dogs Have a Lot of Heart, Literally

When a long-distance musher says his favorite sled dog has a big heart, he might be describing more than the dog's loving disposition.

A team of five veterinarians from Alaska and around the country---including Jeanne Olson, a veterinarian who owns and operates Raven Veterinary Services in North Pole---found that sled dogs may develop an enlarged heart, just as human athletes sometimes do when they push themselves training for and anticipating in endurance events.

The study, which the vets volunteered their time to complete, compared 48 dogs that ran in the 1992 Yukon Quest International Sled Dog Race with two other groups of dogs. The Yukon Quest dogs had trained 2,000 to 3,100 miles before the race. Another group, called "lightly trained" dogs, ran from 200 to 500 miles during long-distance training. A third, the mongrels, had body types similar to the sled dogs but hadn't been on any endurance training runs.

By listening to the dogs' heart beats and analyzing electrocardiograph readings (the sharp peaks and valleys of a heartbeat represented on an oscilloscope), the veterinarians were able to tell if the dogs had heart growth associated with long-distance running.

They discovered that a correlation between the best sled dogs and large hearts seems to exist. However, a big heart may not always be a big asset. An enlarged heart also may be associated with Sudden Death Syndrome, a condition that causes a dog to drop dead in its tracks for no obvious reason.

Susan Butcher lost a dog to SDS on the 1994 Iditarod, the only dog to die on the whole race. One dog in the 1994 Copper Basin 300 also succumbed to SDS.

Pathologists haven't been able to make a connection between enlarged hearts and SDS, however, and Olson hopes that studies, such as one she will participate in this spring funded by the American Veterinary Medical Association, will provide some answers.

How does a heart grow in the first place? Olson said dogs, like people, may undergo a physiological adaptation to stress put on their hearts during training or races. In other words, the heart grows larger in response to the need to work harder, just as a weight lifter's arm grows larger as a reaction to pumping iron.

A dog's heart is similar to a human heart in that it has four main chambers: the right and left atriums and the right and left ventricles. The right atrium, on the upper right side of the heart, receives oxygen-starved blood from veins and passes it through a valve to the larger right ventricle, which functions as a pump. The right ventricle pumps blood through the pulmonary artery to the lungs, where wastes such as carbon dioxide are removed and the blood is recharged with oxygen.

The oxygen-rich blood enters the top of the heart at the left atrium. Then it moves on to the left ventricle, which is the pump responsible for delivering the performance-sustaining blood through arteries to the sled dog's muscles. The tissues absorb oxygen-rich blood and expel wastes into oxygen-depleted blood headed back through veins to the heart.

The left ventricle of the heart, the high-pressure pump that delivers oxygen-rich blood to the body, is the part of the heart that grows most noticeably in dogs with an enlarged heart, a condition known as Athletic Heart Syndrome.

In the study, 35 percent of Yukon Quest dogs had enlarged left ventricles, compared to 22 percent of the lightly trained dogs and seven percent (one dog) of the mongrels.

Although an enlarged heart may someday be linked to SDS, Olson said heart growth is a normal, beneficial reaction to high-stress training, and it's possible that a larger heart is what separates the great dogs from the good ones.