Cardiology II: Heart Sounds, Cardiac Output And Arterial Blood Pressure
Localization of the Different Heart Sounds
In the first Episode, we discussed the general physiology of the heart and its functions. Now we are getting to specific functions, of which one is the Heart Sounds.
Heart Sounds are produced by deceleration of blood impinging on elastic structures in the heart, giving rise to vibration. More rapid deceleration gives rise to louder sounds and vice versa. The pitch of the sound (frequency) is determined by the relative contribution of the mass of blood and the elastic properties of the structures which are responsible for producing the sounds.
First heart sound (S1) occurs at the commencement of systole and coincides with the closure of the A-V Valves. Second heart sound (S2) marks the onset of diastole and indicates closure of the semi-lunar valves. The third heart sound (S3) occurs in the earlier part of diastole and it coincides with rapid filling. The fourth heart sound (S4) is produced by atrial contraction and this is a late diastolic sound. When S3 and S4 becomes pathologically accentuated giving rise to a characteristic cadence resembling the canter of a horse, it is termed "gallop rhythm". The terms "Proto-diastolic" and "presystolic gallop" were used to denote S3 or S4 respectively.
When both these are audible and the diastolic interval is shortened due to rapid heart rate, these may merge (Summate) and such a phenomenon is called 'summation gallop'
The Cardiac Cycle
This is the volume of blood pumped by the left ventricle in one minute. Normal average is 5-6 litres in healthy adults. It is also expressed as the cardiac index, when related to the surface area of the individual. Normal cardiac index is 2.8 to 4.2 (mean is 3.4) litres/min/m2. Cardiac output is controlled by several factors such as heart rate, effective fillings pressure of the ventricles, compliance of the chambers, contractile force of the ventricles and the blood pressure.
Generally, the increase in cardiac output is achieved by increasing the heart rate and stroke volume. Exercise and emotion normally increase the cardiac output, whereas extreme tachycardia, extreme bradycardia, atrial fibrillation, myocardial dysfunction and anatomical or functional obstruction to the outflow of blood reduce it.
Heart receives its oxygen supply and nutrition from the coronary arterial blood flow which is 72-85 ml/100g of cardiac muscle/min in the resting phase. The heart utilizes 8-10ml of oxygen/100g every minute. The oxygen demand during systole is thrice that during diastole.
Arterial blood pressure
The arterial blood pressure is influenced by several factors such as the cardiac output, peripheral resistance in the arterial system, blood volume in the arterial system, viscosity of blood and elasticity of the arteries. The control of blood pressure is achieved mainly by alteration in the cardiac output and peripheral resistance. Normally, arterial blood pressure fluctuates widely depending on exercise and emotional stress, time of the day and posture. The systolic blood pressure falls by 15-30 mmHg during sleep. In the erect posture diastolic blood pressure is greater than during inactivity. Average systolic blood pressure readings are:
- Neonates: 40mmHg,
- infants aged 2 weeks-70 mmHg
- Children up to 12 years-105 mmHg and
- above 17 years- 120 mmHg.
In adults, for a considerable period of life (up to the sixth or seventh decade) the blood pressure remains more or less steady, but with old age, due to reduction in elasticity of the arterial wall, the systolic pressure may tend to rise.
© 2013 Funom Theophilus Makama