The four heart sounds are referred to as S1, S2, S3 and S4. In dogs and cats the presence of a third (S3) or fourth (S4) heart sound is considered abnormal and usually indicates cardiac pathology. The relationship of the normal heart sounds to the cardiac cycle is shown diagrammatically in site.
The first (S1) heart sound is associated with the closure of the atrioventricular valves and is preceded by a rapid increase in intraventricular pressure. The SI sound therefore indicates the onset of systole (ventricular depolarization) and correlates with the onset of the QRS complex on the ECG. Splitting of the first heart sound occasionally occurs in large breeds and is best heard over the cardiac apex and mitral valve region of the heart. S1 splitting is due to asynchronous closure of the atrioventricular valves. In some cases, for example in large breeds of dog, it may be regarded as a normal finding; in others it may be associated with bundle branch block or ventricular premature contractions.
The second (S2) heart sound is associated with the closure of the pulmonary and aortic valves which occurs once the pressure in the ventricles drops below that in the aorta and pulmonary artery. The S2 sound signifies the onset of the diastolic phase of the cardiac cycle. Splitting of the second heart sound is, in most cases, pathological and may occur with pulmonary hypertension, pulmonic stenosis, bundle branch block and ventricular premature contractions. Occasionally S2 splitting can be heard in normal dogs during inspiration.
The third (S3) heart sound is caused by rapid ventricular filling which occurs early in diastole. During diastole the pressure in the ventricles approximately equals atmospheric pressure and is slightly less than the atrial pressures therefore blood flows passively through the atrioventricular valves. Rapid ventricular filling allows the ventricles to fill to approximately 70% of their diastolic volume before the sinoatrial node discharges and the atria contract. The S3 sound is not normally heard in dogs and cats and its presence indicates ventricular dilation. It is a low frequency sound best heard over the cardiac apex or mitral valve region. It occurs towards the end of the T wave on an ECG.
The fourth (S4) heart sound is not normally heard in dogs and cats. It immediately precedes S1 and like the S3 sound its presence is usually associated with atrial and / or ventricular dilation. The S4 sound represents blood flowing rapidly into the ventricles as a result of atrial contraction (which accounts for the remaining 30% of the diastolic volume). It is a low pitched sound best heard over the pulmonic and aortic valve regions.
Systolic clicks are audible between the first and second heart sounds and may or may not be associated with a murmur. Their clinical significance is uncertain. The presence of a systolic click may indicate an abnormality in the formation and therefore the closure of the atrioventricular valves. Many animals with systolic clicks develop a mitral insufficiency murmur within 6-12 months.