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      2.3.13 Heart Sounds and Murmurs

Discrete bursts of auditory vibrations that differ in intensity (volume), frequency, quality, and duration are known as heart sounds, the sounds for a normal heartbeat being first heart sound (called lub) and second heart sound (called dub). The first heart sound consists of multiple high-frequency elements where only the first two among them can be heard normally. These two normally audible sound waves occur simultaneously when the mitral and tricuspid valves close. The first heart sound indicates the beginning of ventricular systole and the end of mechanical diastole. The second heart sound is described as a discrete burst of vibration that occurred due to the immediate closure of the aortic valve and pulmonic valve. The second heart sound consists of two components – the aortic component and the pulmonic component – with the aortic valve and pulmonic valve being the sources of the second heart sound. The second heart sound is produced at the end of ventricular contraction and it indicates the beginning of ventricular diastole and the end of mechanical systole.

      Apart from the normal first and second heart sounds, unusual third and fourth heart sounds are found in healthy as well as unhealthy individuals. The third heart sound, or ventricular gallop, is an additional unusual sound produced after the first and second heart sound. It is generated when a large amount of blood strikes the left ventricle and happens when the mitral valves open after the second heart sound, allowing passive blood flow to the left ventricle. The fourth heart sound, or atrial gallop, is a low-pitched sound that occurs before the first heart sound as the ventricle fills late in its diastole due to atrial contraction. The fourth heart sound results from vibrations produced within the ventricle, and it is also known as the atrial sound because its development requires an effective atrial contraction.

Generally, the cardiologist examines the heart sound by cardiac auscultation technique using a stethoscope. The unusual pattern of heart sound can indicate a serious underlying heart condition. The first and second heart sound are common, while the third and fourth are unusual in healthy individuals. However, while the third heart sounds can be audible in healthy individuals below 40 years of age [14] and pregnant women [15], in people younger than 40 they are considered abnormal and a possible early indication of heart diseases including heart failure. The fourth heart sound is very rare in healthy individuals, but common in patients with heart diseases.

      Heart murmurs are the type of abnormal sound generated in the heart as a result of the turbulent flow of blood. These murmurs are generated either by blood flow through an abnormal valve or by abnormalities in the internal chamber of the heart. In most cases, the murmurs are not serious and do not require quick medication or a hospital visit. However, in a few cases, the abnormal murmurs are due to heart valve problems present at birth, or caused by cardiac shunts or septal defects.

      2.4 Evolution of the Devices for Conventional Clinical Cardiac Examination

Multiple devices have been used by physicians and paramedical experts to diagnose the existence of any heart defects or disease. Further, they continuously witness the evolution of these devices each year, in terms of size, accuracy, and specificity. This section discusses the very common devices used by the physicians and paramedical experts, and their brief history.

      2.4.1 Stethoscope

The stethoscope is one of the common devices used by the physician for auscultation examination. Stethoscopes were first invented by a French physician, René Laënnec, in 1816. The prototype consisted of a rolled paper cone and a wooden tube, the purpose being to hear the heart’s sounds. In 1819, it was improved by replacing the wooden tube with a rubber tube. In the following years, many improvements were made to the design, with the introduction of the diaphragm, bell, and binaural version. In 1956, several further improvements were implemented, resulting in the current stethoscope now used by doctors [16]. The evolution of the rubber-based stethoscope is given in Figure 2.5.

Figure 2.5 Evolution of rubber-based stethoscopes.

      The evolution of the stethoscope is not limited simply to design and appearance, but involves functionality, accuracy, and outcome. The first stethoscope only allowed doctors to hear the pumping of the heart through one ear, whereas the binaural stethoscope design, allowed its users to hear and compare sounds of each bell. Then came the stethoscope with its classic design of two sides and one bell, which helped physicians use it for the respiratory and cardiovascular systems. After that, the diaphragm was introduced, helping to distinguish low-pitched and high-pitched sounds for accurate diagnosis.

After several years, the development of digital stethoscopes began. A digital stethoscope (seee Figure 2.6) has the ability to convert acoustic sounds into electronic signals, which can be transmitted to a computer for further processing. The digital stethoscope has three different modules, namely data acquisition, preprocessing, and signal processing. The main benefit of these modules is to enhance the quality of the sound acquired [17]. Other recent improvements include embedded noise reduction and cancelation features that give better accuracy.

      Figure 2.6 A digital stethoscope.

      2.4.2 Electrocardiogram

The electrical activity of skeletal muscles was first noticed by Dr. Luigi Galvan, a physician in Italy, who pointed out that this activity could be recorded using electrical currents. In 1842, another physicist, Dr. Carlo Matteucci, found the electrical activity or responses for heartbeats in a frog. Several years later, a physiologist from London, Augustus Desiré Waller, showed the first human electrocardiogram, which used a capillary electrometer and electrodes placed on the chest and back. From this experiment, many demonstrations grew, including triphasic electric cardiac activity using improved capillary electrometers, followed by an improvement in refining electrometers by using five measured deflections in the electrocardiograph [18].

      The electrocardiogram machine initially had three electrodes forming a triangle. Two were later removed due to the poor yielding. The machine contained cylinders of an electrolyte solution and patients placed their arms and one leg inside these cylinders [18]. Examiners continued to use this machine until a new design was introduced in 1928 which was much lighter than the previous design and had five electrodes. Further improvements consisting of 12 electrodes were then made, giving more accurate measurements.

      Today, ECG systems are available in much smaller and portable forms, such as smartwatches, ECG patches, etc., which allow users to place their fingers on them to generate the ECG results within seconds.

      2.4.3 Echocardiogram (Ultrasound Devices)

Echocardiogram devices were first introduced in 1950, when the first M-mode echocardiography was developed by two physicists in order to study mitral stenosis and its diagnosis. Their work inspired many cardiologists, as they carried out their work to develop Doppler, two-dimensional (2D), contrast, and transesophageal echocardiograms. The echocardiogram was inspired by stethoscopes, and actually enables users to observe what occurs underneath the human skin [19]. The general setup of the echocardiography method is given in Figure 2.7.

      Figure 2.7 The echocardiography method. The patient lies in the left lateral decubitus position, and the ultrasound probe generates sound waves, receives an echo from body tissues, and sends the results to the ultrasound processing unit/computer to create a sonogram image.

      The echocardiogram

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