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5th practice Medical Informatics Biomedical Signal Processing TAMUS, Zoltán Ádám [email protected] The electrical manifestation of the contractile activity of the heart The ECG can be easily recorded with surface electrodes on the limbs or chest (Einthoven’s triangle) The waveshape is altered by cardiovascular diseases Einthoven’s triangle The ECG is an electrical signal (function of time) The SA node fires Slow-moving depolarization (contraction) of the atria The P wave in ECG • 0.1-0.2 mV • 60-80 ms Propagation delay at atriovenricular (AV) node Normally isoelectric segment PQ segment • 60-80 ms His bundle, the bundle branches, and the Purkinje system of specialized conduction fibers propagate the stimulus to the ventricles at high rate The wave of stimulus spreads rapidly from the apex of the heart upwards Rapid depolarization (contraction) of ventricles QRS wave • 1 mV • 80 ms The ventricular muscle cells possess a relatively long action potential 300-350 ms Result: isoelectric segment ST segment • 100-120 ms Repolarization (relaxation) of ventricles causes slow T wave • 0.1-0.3 mV • 120-160 ms Physiological interference • Respiration, squirms, coughs etc. • Mother’s ECG appearing with the ECG of the fetus Electromagnetic interference • TV and radiostations, CRTs, computers, fluorescent lamps Result: noise in the ECG signal Signal • Deterministic signal: the value at a given instance of time may be computed using a closed-form mathematical function of time or Predicted from a knowledge of a few past values of the signal • Nondeterministic signal (random signal): That does not meet the conditions of the deterministic signal The noise may be random, structured (deterministic) and physiological Random noise • ensemble average: 0 1 M x (t1 ) lim xk (t1 ) M M k 1 Reduction: Moving Average (MA) filter • general form: N y ( n) bk x ( n k ) k 0 • bk:filter coefficients Examples of MA filters • von Hann or Hanning filter: 1 y (n) x(n) 2 x(n 1) x(n 2) 4 • 8-point average: 1 7 y ( n) x ( n k ) 8 k 0 Rangaraj M. Rangayyan: Biomedical Signal Analysis, IEEE Press/Wiley, New York, NY, 2002. 1. Observation of noisy ECG signal 2. Filtering the noisy ECG signal by MA filter • von Hann or Hanning filter • 8-point MA filter sample files: http://zoltanadam.fw.hu software: MS Excel