Phonocardiography Monitoring in Medicine

Introduction

The Definition and need for Phonocardiography

A Phonocardiogram (PCG) is an audio-visual display of acoustic vibrations made in the form of cardiac sounds and murmurs by the heart. These are often obtained using an instrument called as Phonocardiograph. The study of such various types of medical recordings made during cardiac auscultation is often termed as Phonocardiography.Heart sound analysis is a detailed step by step procedure adopted during cardiac auscultation particularly aimed at careful diagnosis of cardiac diseases, long term monitoring of patient health, as a biometric tool for patient authentication and for teaching, education and research purpose.

Phonocardiograms reveal critical health related statistics that correspond to the proper functioning of the cardiac valves. Normal heart sounds have two important and fundamental components the S1 (lub) and S2 (dub) sounds. Presence of additional sounds generally indicate abnormality. A PCG with third sound could signal critical heart failure, whereas as presence of pathological murmurs might indicate the defective valves or orifice in the septal wall.

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Phonocardiogram acquisition instruments are used for long time monitoring of patients suffering from various cardiac disorders. Real time Continuous monitoring helps the doctors to study the disease on a case to case basis and take necessary and corrective action. These Phonocardiogram instruments are now equipped with both wired and/or wireless technologies that make cardiac auscultation easy and user friendly. The new systems support telemetry thus enabling the doctors to serve the patients at remote locations where access to medical facilities is quite difficult.

Overview of Heart Sound Signals

The PCG signals are non-linear and non-stationary sound vibrations picked from the chest and that are usually associated with the contraction and relaxation of the muscles in the cardiac chambers (atria and ventricles), the valve movements, and the blood ‚ow.

An acoustic stethoscope helps the doctor to identify four classes of sounds during cardiac auscultation:

1. The fundamental heart sounds (S1 and S2),
2. The low-pitched diastolic sounds (S3 and S4),
3. The murmurs (systolic and diastolic), and
4. The high pitched sounds (clicks and snaps).

Heart sounds show varying levels of intensity, frequency, duration, and quality [13]- [18]. PCG signals may consists of four heart sounds: the rst heart sound (S1), the second heart sound (S2), the third heart sound (S3) and the fourth heart sound (S4).

• First Heart Sound (S1): The rst heart sound (S1) is caused by the closure of the mitral and tricuspid valves at the start of ventricular systole. The mitral component (M1) occurs 20-30ms before the tricuspid component (T1) [15]. The S1 is a low-pitch sound with longer duration [14]. The intensity of the M1 sound is much higher than the T1 sound intensity due to the abrupt rise in left ventricular pressure. The rst sound is usually evaluated with its quality, intensity, and degree of splitting [13]- [18]. A decrease in the intensity of S1 is associated with myocardial depression, ventricular septal defect and acute aortic regurgitation. The splitting of S1 sound may increase to 60ms in patients with right bundle branch block (RBBB), or ventricular tachycardia or premature ventricular contraction (PVC) [14].
• Second Heart Sound (S2): The second heart sound (S2) is caused by the closure of the aortic and pulmonic valves that occurs at the end of the systole. The S2 is shorter and slightly higher pitched sound. The S2 sound has frequency components of 10-400 Hz and the duration of 50-150ms [9]. The S2 is composed of aortic closure sound (A2) and the pulmonic closure sound (P2). These two components usually last less than 50ms [9]. The delay between the closure of the aortic and pulmonic valves causes a split S2 sound. The clinical evaluation of the S2 includes determination of the presence and degree of respiratory splitting and the relative intensities of A2 and P2. The amplitude and frequencies of A2 sound is slightly higher than the P2 sound [3]. The interval between the two sounds (splitting) widens on inspiration and narrows on expiration.The time interval between the A2 and P2 components is an indicator of aortic blood pressure [8]. The pathologies related to split S2 are: pulmonic stenosis, RBBB, left bundle branch block (LBBB), atrial septal defect and right ventricular failure. In normal cases, the S1-S2 interval (systole) is shorter than the S2-S1 interval (diastole) [30]- [40].
• Third Heart Sound (S3): The third heart sound (S3) produced by the sudden deceleration of blood flow within the ventricle, that occurs after 100-150 ms of the S2 during diastolic period. The S3 includes significantly low amplitude-frequency as compared with the first and second heart sounds. The S3 has frequency components of approximately 30-90 Hz with sound duration of 70 ± 15 ms during diastole period [43], [44]. S3 often occurs in patients with impaired myocardial reserve [16]. The clinical studies show that the S3 can provide clinical information about hemodynamic and systolic dysfunction, and evaluation of patients with congestive heart failure [41]- [48]. The presence of S3 in PCG of adults is correlated with heart failure.Fourth Heart Sound (S4):
• The fourth heart sound (S4) is caused by the contraction of atria to force blood into distended ventricle. The S4 is a low-pitched sound that occurs before the first heart sound. The S4 sound arises from low-frequency vibrations with frequency of 20 to 30 Hz. The PCG of patients with diminished left ventricular compliance show presence of S4 component [13]- [18]. The clinical studies show that the diastolic heart sounds combining with electrocardiogram (ECG) may improve the non-invasive diagnosis of myocardial ischemia.

Heart Murmurs and Other Pathological Sounds

Murmurs are caused by turbulence in blood flow or vibration of tissues. In pathological cases, different murmurs may be caused by valvular dysfunctions. Murmurs may be systolic, diastolic or continuous throughout systole and diastole. Heart murmurs are frequently categorized by the timing (early, mid, late, or pan), intensity, duration, pitch (low, medium, or high), quality (blowing, harsh, rumbling or musical), and shape-configuration crescendo (increasing intensity), decrescendo (decreasing intensity), crescendo-decrescendo (increasing then immediate decreasing intensity) [6], [14- 18]. The pitch and intensity are related to the velocity of blood flow that produces the murmur. The timing of a murmur is crucial for accurate diagnosis.The systolic murmurs can be grouped into: The early systolic murmur of acute mitral