ECG or Electrocardiogram, is recording of electrical activity in the heart.
The elctrocardiography is based on two basic principles.
1. When the electromagnetic force flows towards the positve electrode of a lead, the ecg would record an upward or positive deflection.
2. When the electomegnetic force flows away from the positve electrode of a lead or towards the negative electrode the ecg would record a downward or negative deflecton.
The electrocardiografic deflections are termed as P, QRS, T, and U.
Diagramatic representation of basic electrocardiografic deflections
The P wave represents atrial activation, the QRS complex represents ventricular activation, T wave represents ventricular recovery. The interpretation of U wave is uncertain. The ST segment, T wave and U wave together represent the ventricular recovery. The other significant portions of the ecg deflection are PR interval, QRS duration, ST segment.
A routine electrocardiogram consists of 6 Limb leads and 6 Chest leads. The Limb Leads are marked as I, II, III, aVR,aVL, aVF and the Chest Leads are V1, V2, V3, V4, V5, V6. The limb leads are placed on the four limbs and the chest leads are put on the mid chest and the left half of the chest.
Normal pattern of electrocardiografic deflections in various leads