Phonocardiography

Phonocardiography (PCG) is a noninvasive method for obtaining recordings of cardiovascular sound, i.e. acoustic phenomena which are perceivable by auscultation at the cardiac region on the chest wall. Essentially, vibrations of the chest surface are observed which are generated in cardiac structures and transmitted to the chest wall. In physiological terms heart sounds and murmurs are mentioned: Heart sounds have a transient, musical character; Murmurs, except for the musical types, have a random, noisy character. Normal heart sounds are initiated by valve closing (the first and the second sound) or by filling phenomena (the third and the fourth sound); besides the normal ones pathological sounds exist e.g. generated at opening of degenerated valves. Heart murmurs of the random type are generally resulting from turbulence in blood flow; the latter can be caused by flow through a narrowed valve opening or a leaking valve. Besides of a small number of normal types most heart murmurs are pathological and many of these can be related to malfunctioning of cardiac valves. Phonocardiography and auscultation thus aim at the recognition of pathological sounds and murmurs and their relation to malfunctioning heart structures. Recording of heart sounds at the chest surface, as for any vibration recording, requires a vibration transducer (also called a heart vibration pickup, a heart sound microphone), a suitable amplifier with specific frequency characteristics and a storage, visualization and processing device. Contact transducers (accelerometers) and air-coupled transducers are used. The first type can be indicated as absolute, the second as relative as it measures a weighted difference in vibration between the air chamber surface and the surface at the edge of the rigid housing of the transducer. The sensing chest piece of the (electronic) stethoscope accords with the second type. As a general rule quantitative measurement requires a transducer which does not affect the vibration to be measured. This is hard to achieve in vibration measurement on soft tissue as is the case for the chest wall. Affecting the vibrating surface is e.g. a technique applied in heart auscultation: with the use of a stiff membrane and by applying firm pressure on the stethoscope chest piece higher frequencies are better observed. In order to obtain equivalent results as from auscultation (due to the qualities of the human hearing sense) high-pass frequency filter sets have to be applied on recorded vibrations in phonocardiography. A single signal representation does not provide the same information as heard with the stethoscope. On the other hand heart sound recording can exceed the hearing abilities and inaudible sounds can be recorded. In conventional phonocardiography signals are stored on paper with a suitable graphic recorder; Evidently, modern systems make use of information technology for storage, processing and visualization. As a result of different transducers, an important interference with the chest wall vibration and different signal conditioning, heart sound recording is not standardized, as compared e.g. with electrocardiography which has a clear millivolt scale on its ordinate axis: The method can be indicated semi-quantitative as results depend on the measuring apparatus used. Nevertheless knowledge of heart sounds and murmurs has been greatly increased with the technique. Signal analysis, more specifically time-frequency analysis, has proven to be very useful in the identification and classification of heart sound components and murmurs and their relation to cardiac structures and hemodynamic variables. Whereas auscultation still holds its position as a diagnostic tool for general physicians and cardiologists, phonocardiography in its classical form has lost interest mostly as a result of the availability of techniques as Doppler echocardiography and cardiac imaging methods, which give more direct information on cardiac structures and hemodynamic variables. The historical value of conventional phonocardiography has to be stressed. On the other hand, recording and processing of heart sounds continue to be subjects of scientific research and remain beneficial for training and for supporting of diagnosis of physicians. Electronic stethoscopes coupled to a laptop and connected to internet for automated or for remote diagnosis by a specialist may grow in importance in the next years.