Depolarization-repolarization cycle, phases

The depolarization-repolarization cycle consists of the following phases. 

Six helpful appendices are included Quick guide to cardiac arrhythmias summarizes the details of 20 arrhythmias Cardiac drug overview covers commonly used cardiac drugs Best monitoring leads shows the most beneficial leads to monitor for the most challenging arrhythmias Depolarization-repolarization cycle explains the five phases of this cardiac cycle Action potential curves reviews the cellular changes that occur during the depolarization-repolarization cycle and Cardiac conduction system reviews how electrical impulses affect heart function. 

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. 

The localization of adducts to the epicardial border zone suggested the possibility that IsoK/LG adducts contribute to cardiac arrhythmias. Ventricular tachycardia/fibrillation following myocardial infarction is a major cause of sudden cardiac death. Arrhythmias in ischemic myocardium arise from sodium channel blockade. Sodium channels are hypothesized to cycle between three conformational states a deactivated closed state, an activated open state, and an inactivated closed state. Upon depolarization, the deactivated state converts to the activated state and sodium current flows for a brief time before the channel enters the inactive state. The channel only converts from the inactive state to the deactivated state when the membrane repolarizes during the falling phase of the action potential. Changes in the ability to convert from the inactive to the deactivated state are critical to the initiation and perpetuation of arrhythmias. 

---