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Introduction to Heart Sound:
There are two classical sounds of heart in humans, known as the first and the second sounds (Fig. 7.56). They can be easily detected with a Stethoscope. Two other sounds have also been described – the third and the fourth – which, though difficult to detect clinically, are constantly found in graphic records.
The first and the second sounds are close to each other. After the second sound there is a longer pause. The sequence is like this; first sound → second sound → pause; first sound → second sound → pause. Thus the sounds go on.
Biophysical principle underlying the occurrence of sound in the heart or in the circulation:
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In general blood flows, under most circumstances, through blood vessels in a streamline or in a lamina. Accordingly streamline flow is silence and must not be noisy. But under certain condition when this laminar flow or streamline flow is changed into turbulent flow then a noise may be produced.
According to Sir Osborne Reynolds (1890), turbulent flow is noisy and this turbulence depends upon the critical velocity. It is the velocity at which the turbulence occurs. Reynolds has shown that the critical velocity depends upon the viscosity of blood, density of blood and also on the radius of the blood vessels, by the relation Vc = (K ƞ/ρϒ), where Vc = critical velocity, K = Reynolds number which is constant about 1,000, ƞ = viscosity of blood, ρ = density of blood and ϒ= radius of the tube (Fig. 7.55).
Cause of Heart Sound:
The cause of heart sound is due to- (a) vibration of the leaves of the valves during closure, and (b) occurrence of turbulence during rapid rash of blood from the atrium to the ventricle.
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During gradual closure of valves, the vibration that occurs, are transmitted from the valvular area towards the apices of the ventricles in case of first heart sound and along the arteries (aortic and pulmonary) in case of second heart sound. So the first and second heart sounds are not necessarily due to cause of turbulence but for the cause of closure of the tricuspid and mitral valves in case of first heart sound; and aortic and pulmonary valves for the second heart sound.
The third sound occurs during opening of atrioventricular valve and where the rushing of blood through the narrow opening, produces turbulence. The fourth heart sound is due to rapid inrush of blood from the atria to the ventricles due to forceful contraction of the atria.
However in the heart and in blood vessels abnormal sounds which are generally heard and presumably due to abnormally high velocity of blood flow through the valves and blood vessels. These abnormal sounds (murmurs) are mostly associated with the occurrence of turbulence. In mitral stenosis or in coarctation of aorta, murmurs are always associated with the occurrence of turbulence.
Because during such occurrence, the blood flow reaches its critical velocity during flowing of blood through the narrowed lumen of the valves and blood vessels. Besides this, during heavy exercise innocent systolic murmurs are heard in normal individual due to rapid rush of blood from the ventricles to the arteries at mid-systole. Experimental shortening of the lumen of blood vessels may produce sound during systole.
This procedure is generally used clinically in the measurement of blood pressure indirectly through Sphygmomanometer. The sound that is heard during gradual release of pressure from the cuff is due to production of turbulence for just rushing of blood through narrowed vessels. The sound disappears as soon as the normal blood flow is resumed through the fully released vessels.
Methods of Studying Heart Sound:
i. Clinical Stethoscope is the commonest instrument, used for detecting and demonstrating heart sounds.
ii. For more accurate work, a microphone is applied to the precordium and is suitably connected to an Oscillograph. It is also connected with a mirror arrangement which reflects a beam of light on a moving photographic plate. Thus the sounds can by graphically recorded (Fig. 7.56).
The first heart sound is manifested by a prominent set of vibrations. It occurs during the ventricular systole and is noted just before the onset of the c wave of the jugular pulse.
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The second heart sound is manifested by a set of vibrations not as prominent as the previous one and occurs during the ventricular diastole and coincides with the notch on the ascending limb of the v wave of the jugular pulse.
The third heart sound is manifested by a small set of vibrations and coincides with the end of the descending limb of the v wave of the jugular pulse.
The fourth heart sound is manifested by another set of vibrations which occurs during the atrial systole and coincides with a wave of the jugular pulse.
The sounds are briefly described below:
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1. First Sound:
It occurs at the onset of ventricular systole.
Nature:
Dull and prolonged, like the word L-U-B-B (Fig. 7.57).
Duration:
0.1—0.17 sec. (average, half the ejection period).
Clinical Identification:
It can be identified by the following features:
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i. Its nature.
iii. It is best heard over the left fifth intercostal space about 1.27 cm (half and inch) inside the midclavicular line. It coincides with the apex beat and with the commencement of the carotid pulse.
iii. It comes just after the pause and just before the radial pulse.
iv. It coincides with the spike of the R wave of the Electrocardiogram.
v. It precedes the onset of the c wave of the jugular pulse.
Cause:
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First sound is caused due to:
i. Sudden closure of the A.V. valves and the vibrations set up in the valve leaflets due to increase in the intraventricular pressure.
ii. Ejection of blood from the ventricles and the vibrations transmitted to the walls of the aorta and pulmonary artery.
iii. Contraction of the thick ventricular muscles (muscular element). It is doubtful whether the second factor takes any appreciable part.
In graphic records often two distinct groups of vibrations are seen. The first group corresponds to the isometric contraction period and the second group to the maximum ejection period. The latter suggests that aortic vibrations, due to entry of blood, take part in causing the prolonged character of the first sound. The frequency of vibration varies from 25 to 45 per second.
Significance:
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i. First sound indicates the onset of clinical systole of the ventricles.
ii. The duration and the intensity of the first sound indicate the condition of the myocardium. In a strong healthy heart these features are prominent and become more so in a hypertrophied heart. If the myocardium is weak, the first sound will be short and low pitched.
iii. A clear first sound indicates that the A.V. valves are properly closing, i.e., there is no incompetence.
2. Second Sound:
Nature:
Short and sharp like the word DUP, the pitch being higher and duration shorter than the first sound (Fig. 7.57).
Duration:
0.1-0.14 Sec.
Causes:
It occurs at the onset of diastole and is caused by the sudden enclosure of the semilunar valves in the aorta and pulmonary artery and just after the T wave of the E.C.G. At the end of systole, ventricular pressure falls below the pressure of the aorta and the pulmonary arteries causing a much pressure difference in between the aortic or pulmonary arteries and the ventricles.
This increased pressure in the aorta or the pulmonary artery tends to close up the valves through a backflow. The two valves-aortic and pulmonary, do not close simultaneously but the pulmonary pressure precedes the aortic one. During inspiration but not in expiration, the sound is thus split into an aortic component and later a pulmonary component. Its intensity depends on the blood pressure. It consists of three or four chief vibrations with a frequency of about 50 per second.
Clinical Identification:
Clinically it can be identified by:
i. Its nature.
ii. Its relation with first sound and the pause.
iii. Its occurrence just after the apex beat and the carotid pulse.
iv. Getting the sound.
v. Its occurrence at the end of the T wave of the Electrocardiogram.
vi. Its coincidence with the notch on the ascending limb of the v wave of the jugular pulse.
Significance:
i. It indicates the end of systole and beginning of diastole.
ii. Its pitch is directly proportional to the blood pressure.
iii. A clear second sound indicates that the semilunar valves are closing properly, i.e., there is no regurgitation.
iv. The interval between first and second sounds is taken as the clinical systole and that between the second and first as the diastolic period of heart. The diastolic period, generally known as the pause, is a little longer than the systolic period. When the heart rate rises, the long pause shortens and the sounds appear to be equidistant.
3. Third Sound:
Nature:
It takes place just after the second sound and coincides with the opening of the A.V. valves, i.e., with the commencement of the ventricular filling.
Duration:
About 0.04 sec.
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Causes:
It is caused by the sudden rush of atrial blood into the ventricles when the A.V. valves open. Although it is stated that it can be detected in 60% of normal subjects yet, in actual practice, it is difficult to detect clinically.
Clinical Identification It can be identified by:
i. Its relation with the second sound.
ii. Its coincidence with the end of the descending limb of the v wave of the jugular pulse.
Significance:
It indicates the beginning of the ventricular filling.
4. Fourth Sound:
It is also called the atrial sound.
Cause:
It is caused by the contraction of the atria and the consequent rush of blood into the ventricles. It is difficult to detect clinically but is found in the graphic records. It coincides with the rise of a wave of the venous pulse.
Significance:
It occurs just before the first sound and indicates the end of ventricular filling.
It should be noted that each of the above sounds is really produced at two places—for instance, the first sound is produced at both the A.V. valves, the second sound at both the semilunar valves (aortic and pulmonary) and so on. Yet only one first sound, one second sound, etc., is heard.
This is due to the fact that the two sets of valves move exactly at the same time. If any discrepancy occurs in the pressure and time relations between the different chambers of heart, instead of one sound, two sounds may be heard at each time. Thus reduplication of the sounds occurs in cardiac diseases.
