Electrocardiogram (ECG or EKG) is a tool for recording electrical activity of the heart for the purpose of assessing electrical and mechanical functioning.
In a normally functioning heart, electrical impulse is generated at the sinuatrial node, from where it travels to the atrioventricular node, then to the Bundle of His, bundle branches and then to the Purkinje fibers. As the impulse is conducted, the electrical activity is recorded on the ECG as a pattern.
P wave depicts atrial depolarization
QRS complex shows ventricular depolarization
T wave displays ventricular repolarization
PR interval depicts atrial depolarization and the delay in atrioventricular junction (AV node/Bundle of His). This delay allows time for the atria to contract before the ventricles contract
Pacemakers of the Heart
SA Node is the dominant pacemaker with an intrinsic rate of 60 – 100 beats/minute.
AV Node is the back-up pacemaker with an intrinsic rate of 40 – 60 beats/minute.
Ventricular cells are the back-up pacemaker with an intrinsic rate of 20 – 45 beats/minute.
In an ECG paper one small box represents 0.04 seconds horizontally while one large box depicts 0.20 seconds. Vertically, one large box represents a voltage of 0.5 mV.