THE FIRST HEART SOUND. This is caused by the closure of the mitral and tricuspid valves and is best heard at the cardiac apex. The sound is usually single but may be slightly split. If split, this ‘double’ sound at the beginning of systole must be distinguished from the combination of the first heart sound with a fourth heart sound or with an ejection click. The first heart sound is loud when the patient is thin and when the circulation is hyperdynamic, e.g. due to anaemia or thyrotoxicosis. The sound is also loud if the valve is still open when ventricular systole begins, e.g. in mitral stenosis.
A soft first heart sound occurs in patients with obesity, emphysema or pericardial effusion. It is also present when the valve leaflets are immobile, e.g. in severe calcific mitral stenosis, or when the leaflets are partly closed when systole begins, which occurs when the PR interval is long. A soft first heart sound also occurs when the valve does not close properly, as in mitral regurgitation. Heart failure and cardiogenic shock are also associated with a soft first heart sound.
The intensity of the first heart sound is variable when the relationship between atrial and ventricular systole is not constant, e.g. during ventricular tachycardia or complete heart block: when the PR interval is short the sound is loud, and when the PR interval is long the sound is soft. THE SECOND HEART SOUND. This is caused by the closure of the aortic and pulmonary valves. Unless excessively loud, the pulmonary component of the second sound is only heard in the pulmonary area. Left heart emptying is usually finished just before right heart emptying; therefore the pulmonary component of the second sound closely follows the aortic component. Inspiration results in increased venous return to the right heart, which further delays right heart emptying. The pulmonary sound is therefore delayed further on inspiration and the second heart sound becomes audibly split. Splitting of the second heart sound on inspiration is known as normal or physiological splitting and is most commonly heard in children or young adults.
Reversed splitting of the second heart sound (when the aortic component follows the pulmonary component) occurs on expiration. It is due to a fixed delay in left heart emptying caused by aortic stenosis, left bundle branch block or left ventricular failure. Thus, when right heart emptying is delayed during inspiration, the two sounds move together, and when the right heart empties more quickly during expiration, the sounds move apart. The fixed delay in the emptying of the right ventricle produced, for example, by right hundle branch block or pulmonary stenosis will result in wide splitting of thesecond heart sound. With an atrial septal defect there is usually some degree of right bundle branch block, and because of shunting of blood from the left to the right atrium the right-sided cardiac output is high and ventricular emptying is further delayed. The second heart sound is therefore widely split. Because communication at atrial level prevents differential changes of the venous return during inspiration and expiration, the wide splitting of the second heart sound is not varied by respiration. This is called fixed splitting. The aortic second sound is louder in systemic hypertension and when a hyperdynamic circulation is present. It is soft in aortic stenosis because the valve is relatively immobile, and it is soft in cardiac failure because of low blood flow. Similarly, the pulmonary component of the second heart sound is loud in pulmonary hypertension and soft in pulmonary stenosis.
ADDITIONAL HEART SOUNDS. Third and fourth heart sounds are diastolic in timing, representing ventricular filling, and are heard as soft ‘thudding’ noises immediately before the first sound (fourth sound) or after the second sound (third sound). The presence of a third or fourth sound produces a triple rhythm that, when associated with sinus tachycardia, sounds like a galloping horse-a gallop rhythm. The cadence of a gallop rhythm due to a third heart sound has been likened to ‘Kentucky’, whilst that due to a fourth heart sound resembles ‘Tennessee’. When both third and fourth heart sounds occur there is usually a marked sinus tachycardia, which results in a short diastolic period so that third and fourth sounds occur simultaneously. This is known as a summation gallop.
THE THIRD SOUND is due to rapid ventricula filling as soon as the mitral and tricuspid valves open. It is a normal finding in children and young adults when it is heard at the apex, especially in the left lateral position. In those over 40 years it represents heart failure or volume overload, e.g. due to mitral regurgitation. It is therefore sometimes referred to as a sound of ‘distress’. A right ventricular third sound is heard best at the left sternal edge, and a left ventricular third sound is heard at the apex.
THE FOURTH SOUND is caused by the surge of ventricular filling that accompanies atrial systole. It therefore occurs in late diastole. It may be a normal finding in an elderly subject, but in younger patients it usually indicates increased ventricular stiffness associated with hypertension, aortic stenosis or acute myocardial infarction. It is called the sound of cardiac ‘stress’, and can sometimes be felt.
ABNORMAL HEART VALVES may cause an audible signal when opening. An ejection click sound occurs immediately following the first heart sound. It is produced by the sudden opening of a deformed but mobile aortic or pulmonary valve. It is most commonly heard in association with a bicuspid aortic valve when it is easily heard throughout the respiratory cycle. A stenotic pulmonary valve also produces an ejection click, but this is best heard on expiration. A dilated aorta or pulmonary artery may also give rise to an ejection click.
A STENOTIC MITRAL OR TRICUSPID VALVE may produce a high-frequency opening snap that occurs just after the second heart sound. It can be distinguished from a split second sound or a third sound by the site at which it is best heard, its higher frequency and its lack of respiratory variation.
A MID-SYSTOLIC CLICK (OR CLICKS) is due to sudden prolapse of the mitral valve into the left atrium during ventricular systole. It occurs when the mitral valve is congenitally deformed or has undergone myxomatous degeneration, as in the mitral valve prolapse syndrome. These auscultatory features are inconsistent and wax and wane with time.
TUMOUR PLOPS are low-frequency sounds produced by the sudden checking of the travel of an atrial tumour when it reaches the valve. Such sounds occur after an opening snap, but before a third sound would be expected. The pericardial knock is another sound that occurs in this position in the cardiac cycle. Like a third heart sound, it is low frequency. It is heard in constrictive pericarditis and is due to rather early, sudden and marked halting of ventricular filling due to constriction.