Lub Dub, Lub Dub, Click. Wait..... What?: Extra heart sounds. What do they mean?
In dogs and cats, we typically hear 2 heart sounds associated with every heart beat: lub, and dub. These heart sounds represent the times of closure of the AV values (mitral and tricuspid) and the semilunar valves (aortic and pulmonic) respectively. However, on occasion an extra, or third heart sound can be appreciated. The three most common extra heart sounds are gallops, mid systolic clicks and a split S2.
Gallop sounds occur during diastole. The sound is created as blood rushes into a non-compliant, stiff left ventricular chamber and is most commonly heard in cats with hypertrophic cardiomyopathy (HCM) or dogs with dilated cardiomyopathy (DCM). Gallop sounds can be heart rate dependent, being more audible at higher heart rates. Note that the correct term is a gallop sound, not a gallop rhythm. The distinction being that arrhythmias can be detected on an ECG recording, and extra heart sounds can not.
Mid systolic clicks and split S2 sounds occur during systole - the contraction phase of the heart. Mid systolic clicks represent early chronic degenerative valve disease with valve prolapse. They are rarely heard as these dogs typically also have a heart murmur which masks the extra heart sound. But during the very early phase of CDVD, a murmur is not always present, allowing the mid systolic click to be heard.
The crisp S2, or dub, sound of the heart is related to simultaneous closure of the pulmonic and aortic valves. If the valves do not close at the same time, the dub, or S2 sound becomes split into 2 sounds. The most common cause of this phenomenon is pulmonary hypertension (PH). PH causes resistance of blood flow through the main pulmonary artery and results in delayed closure of the pulmonic valve - leading to a split S2 sound.
How can you tell if the extra heart sound is a gallop, mid systolic click, or a split S2?
If you can determine if the extra heart sound is occurring during diastole or systole, that can be very helpful. But, at higher heart rates in particular, this can be very challenging. You can use breed to help guide you. Extra heart sounds in cats are typically gallop sounds associated with hypertrophic cardiomyopathy. Extra heart sounds in large to giant breed dogs are typically also gallop sounds, but associated with dilated cardiomyopathy.
Extra heart sounds in small breed dogs with no clinical signs typically represent mid systolic clicks associated with early chronic degenerative valve disease. And, extra heart sounds in small breed dogs with respiratory distress (particularly Westies) typically represent split S2 sounds associated with pulmonary hypertension.
What diagnostic tests are appropriate for dogs and cats with extra heart sounds?
Thoracic radiographs, a 6 lead ECG and an echocardiogram should be performed on all patients with detectable extra heart sounds so a definitive diagnosis can be obtained and appropriate monitoring and/or intervention can be recommended.
So embrace those extra heart sounds. They can be very telling.