Effective refractory period

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Image of a myocardial action potential. Effective refractory period in green.

In electrocardiography, during a cardiac cycle, once an action potential is initiated, there is a period of time that a new action potential cannot be initiated. This is termed the effective refractory period (ERP) of the tissue. This period is approximately equal to the absolute refractory period (ARP), it occurs because the fast sodium channels remain closed until the cell fully repolarizes. During this period, depolarization on adjacent cardiac muscles does not produce a new depolarization in the current cell as it has to refract back to phase 4 of the action potential before a new action potential can activate it. ERP acts as a protective mechanism and keeps the heart rate in check and prevents arrhythmias, and it helps coordinates muscle contraction. Anti-arrhythmic agents used for arrhythmias usually prolong the ERP. For the treatment of atrial fibrillation, it is a problem that the prolongation of the ERP by these agents also affects the ventricles, which can induce other types of arrhythmias.

Physiology of the cardiovascular system

Heart

Cardiac output	Cardiac cycle Cardiac output Heart rate Stroke volume Stroke volume End-diastolic volume End-systolic volume Afterload Preload Frank–Starling law Cardiac function curve Venous return curve Wiggers diagram Pressure volume diagram
Ultrasound	Fractional shortening = (End-diastolic dimension End-systolic dimension) / End-diastolic dimension Aortic valve area calculation Ejection fraction Cardiac index Left atrial volume
Heart rate	Cardiac pacemaker Chronotropic (Heart rate) Dromotropic (Conduction velocity) Inotropic (Contractility) Bathmotropic (Excitability) Lusitropic (Relaxation)
Conduction	Conduction system Cardiac electrophysiology Action potential cardiac atrial ventricular Effective refractory period Pacemaker potential Electrocardiography P wave PR interval QRS complex QT interval ST segment T wave U wave Hexaxial reference system
Chamber pressure	Central venous Right atrial ventricular pulmonary artery wedge Left atrial ventricular Aortic
Other	Ventricular remodeling

Vascular system/
hemodynamics

Blood flow	Compliance Vascular resistance Pulse Perfusion
Blood pressure	Pulse pressure Systolic Diastolic Mean arterial pressure Jugular venous pressure Portal venous pressure Critical closing pressure
Regulation of BP	Baroreflex Kinin–kallikrein system Renin–angiotensin system Vasoconstrictors Vasodilators Autoregulation Myogenic mechanism Tubuloglomerular feedback Cerebral autoregulation Paraganglia Aortic body Carotid body Glomus cell