Electrocardiogram heart repolarization

Figure 13.4 Electrocardiogram. The electrocardiogram (ECG) is a measure of the overall electrical activity of the heart. The P wave is caused by atrial depolarization, the QRS complex is caused by ventricular depolarization, and the T wave is caused by ventricular repolarization.

Tricyclic antidepressants have effects on heart rate (HR), blood pressure (BP), and three distinct measures of the electrocardiogram (EKG). The EKG parameters affected are (1) the PR interval, which represents depolarization of the aorta (2) the QRS duration, which represents intraventricular conduction time and (3) the QTc, which represents the depolarization and subsequent repolarization of the ventricles, corrected for cardiac rate. 

Special focus is given to the cardiovascular system. For the core battery of the cardiovascular system according to S7A blood pressure, heart rate and electrocardiogram should be assessed, but also in vivo, in vitro and/or ex vivo evaluations, including methods for repolarization and conductance abnormalities should be considered. 

Electrical activity in the heart can be picked up by electrodes placed on the skin and recorded as the familiar electrocardiogram (ECG). The ECG is a record of the sum of all action potentials in the heart as it contracts. Action potentials are generated by depolarization followed by repolarization of the cardiac muscle cell membrane. Depolarization is initiated by an influx of sodium ions into the cardiac muscle cells, followed by an influx of calcium ions. Repolarization is brought about by efflux of potassium ions. The phases of a cardiac action potential are shown in Eigure 4.3 where the depolarization is the change in resting membrane potential of cardiac muscle cells from —90 mV to 4-20 mV. This is due to influx of sodium ions followed by influx of calcium ions. Contraction of the myocardium follows depolarization. The refractory period is the time interval when a second contraction cannot occur and repolarization is the recovery of the resting potential due to efflux of potassium ions. After this the cycle repeats itself. 

Classically the electrocardiogram (ECG) was believed to be a graphic tracing of the electric current produced by the myoneural activity associated with heart muscle excitation. The normal ECG, it was believed, showed deflections resulting from atrial and ventricular activity. The first signal, P, is due to atrial excitation with the QRS deflections arising from ventricular activity. T waves are believed to be due to ventricular recovery (repolarization) while the U waves are seen in the normal ECG and are accentuated in hypokalaemia (low potassium levels in the blood). 

This kind of electrodes is used to measure many sources of electrical signals in the body, such as the heart electrical signal. In this example the myocardial depolarization and repolarization of heart cells can be measured through a system called electrocardiogram (ECG) with electrodes placed on the patient s skin (Thaler, 2007). 

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