Cardiac arrest and characteristics of heart sounds
THE CONTEXT
Cardiac arrest is a sudden, complete loss of cardiac output requiring prompt treatment. The target passage also describes normal and abnormal heart sounds.
Cardiac arrest: mechanisms and recognition
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Cardiac arrest may result from asystole, ventricular tachycardia, ventricular fibrillation or loss of mechanical cardiac contraction, termed pulseless electrical activity.
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Clinical diagnosis is based on unconsciousness and absence of a pulse. Breathing may take some time to stop completely after arrest.
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Death is virtually inevitable unless effective treatment is given promptly.
Sudden cardiac death
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Sudden cardiac death is usually caused by a catastrophic arrhythmia.
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It accounts for 25%–30% of deaths from cardiovascular disease, with an estimated 70 000 to 90000 deaths annually in the UK. Many of these deaths are potentially preventable.
First and second heart sounds
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The first heart sound (S1) occurs at the onset of systole, is usually single or narrowly split, and results from mitral and tricuspid valve closure.
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S1 may be loud in hyperdynamic circulation associated with anaemia, pregnancy or thyrotoxicosis, and in mitral stenosis. It may be soft in heart failure or mitral regurgitation.
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The second heart sound (S2) occurs at the end of systole and results from aortic and pulmonary valve closure. It splits on inspiration and is single on expiration.
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Fixed wide splitting of S2 occurs with an atrial septal defect.
Third and fourth heart sounds
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The third heart sound (S3) occurs early in diastole, just after S2. It is low pitched, often heard as a gallop, and arises from the ventricular wall because of abrupt cessation of rapid filling.
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S3 can be physiological in young people and pregnancy, or pathological in heart failure and mitral regurgitation.
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The fourth heart sound (S4) occurs at the end of diastole, just before S1. It is low pitched and has a ventricular origin related to a stiff ventricle and augmented atrial contraction during atrial filling.
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S4 is absent in atrial fibrillation and is a feature of severe left ventricular hypertrophy.