Intense running can increase its heart rate to a range of 300 to 360 beats per minute. The Greyhound has a much larger heart than most other dogs. It can contract and refill with blood five times per second during a race, and at rest is able to pump more blood per beat per kilogram than other dogs. The Greyhound's red blood cells are also capable of carrying more oxygen per millilitre of blood. His cardiovascular system certainly complements his body frame as well as his respiratory capacity, which is built for running, but, like all body systems, there is a limit to cardiovascular output and performance.
Cardiac arrest can strike any breed, and occurs in adolescent and adult dogs.
Adolescent dogs may be predisposed to sudden cardiac failure because of hidden genetic disorders such as patent ductus, pulmonic stenosis or septal defects between chambers. Adult dogs with an undetected genetic disorder may also be affected, or may have developed a myocardial focus of irritability from a previous infection, which would predispose the heart to electrical misfiring. Chronic high blood pressure may also initiate cardiac arrest. Dogs racing, trialing or competing in the show ring can suddenly drop to the ground as a result of cardiac arrest. Immediate treatment in the field must be exercised in order to save the life of the victim.