Biophysics of Chest Vibrations

The purpose of this mini-review is to outline diversity of chest wall vibrations, specify biomechanical peculiarities and point out a few problems crucial for development of diagnostic and therapeutic applications of chest wall vibrations. The review considers two types of chest wall vibrations: spontaneous, induced by breathing, and forced (or artificial), induced by external vibration forces. Spontaneous vibrations emerge in airways and lung tissues due to vortexes, flatter and stress relaxation in pulmonary parenchyma. The vibrations propagate in lung to the chest wall and could be registered by a stethoscope at the chest wall surface as respiratory sounds. Another type of vibrations emerges due to heart contraction and become apparent at the chest wall surface as heart sounds. Respiratory and heart sounds are used for diagnostics of respiratory and heart diseases. Computerized respiratory sounds analysis (CRSA) is a new technique emerged last years and based on respiratory acoustics and biomedical engineering. Forced vibrations are the lung and chest wall vibrations induced by external vibrations effecting airways orifice and/or chest wall. Diagnostic and therapeutic forced vibrations differ in both frequency and amplitude. Diagnostic vibrations imposed at the airway orifice include forced oscillatory technique, estimation of airway cross-section by analysis of acoustic pulse response measurements. Stress and deformation oscillations used for diagnostic techniques usually are small to avoid any mechanical nonlinearity. Diagnostic vibrations imposed at the chest surface include a special kind of forced oscillatory technique with pressure oscillation around the entire chest and a technique of percussions imposed locally to the areas of interest on the chest wall. Elastic waves propagate in airways, pulmonary parenchyma and chest wall during vibrations. Peculiarities of elastic waves propagation in these structures are discussed. A high sound or low ultrasound window presents new emerging perspective area for biomedical engineering aimed to develop a technique for lung sound/ultrasound imaging. There are a few kinds of therapeutic forced vibrations aimed to enhance airway sputum removal if the production of sputum is too large due to decease.