The effects of different cardiovascular diseases on the athletic performance of horses are difficult to quantify. Valvular heart disease is the most common cardiac disorder seen in horses admitted to the Mid-Atlantic Equine Medical Center, however these conditions do not impact athletic performance as was once generally thought. Heart arrhythmias more commonly affect performance, however may not be detected without the aid of 24 hour heart monitoring or exercise ECGs (electrocardiogram). Echocardiography or ultrasonography of the heart allows an accurate estimate of the significance of valvular disease in horses. Many of these horses present with a history of poor performance or exercise intolerance.