Cardiac activity is provided by the following functions: automatism, excitability, conductivity, and contractility. ECG monitors cardiac conductivity and represents efficiency of automatism, excitability, and conductivity.Myocardial contraction induces significant changes in blood flow rate and acoustic signals. Cardiac contractility can be monitored using the phonocardiogram (PCG). Alternating pressure in blood vessels varies blood filling value and, therefore, varies light transparency of blood vessels. Photoplethysmography (PPG) can be used as a method for monitoring myocardium contractility (variable vascular tonicity).
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