Basic principles of the ECG Voltage and timing intervals 1mV is represented by a deflection of 10 mm Each small square represents 40 milliseconds (ms) 5 small squares, thus representing 200 ms. Heart rate estimation HR = 300/ Number of large boxes between two RR waves If the number of large boxes: 5 - the HR is 60 beats per minute. 3 - the HR is 100 per minute. 2 - the HR is 150 per minute. Normal heart rate = between 60 to 100 beats per minute Electrical axis The overall direction of travel of the electrical depolarization wave through the heart is known as the electrical axis. Limb leads record ECG in the coronal plane, hence used to determine the electrical axis. The limb leads are I, II, III, AVR, AVL and AVF. The cardiac electrical axis is normally downward and to the left. The electrical axis is expressed in degrees and is normally in the range from -30 to + 90 degrees. Normal ECG P wave Depolarization of atria during sinus rhythm generates P wave PR interval Sh...