notes on ECG physiology

Ventricular depolarization consists of two sequential phases:

 1. The first phase of ventricular depolarization is stimulation of the ventricular septum from left to right. This produces a small (septal) r wave in the right chest leads and a small (septal) q wave in left chest leads.

 2. During the second major phase of ventricular depolarization, the stimulus spreads simultaneously outward through the right and left ventricles. Since the mass of the normal left ventricle overbalances the mass of the right ventricle, the spread of depolarization through the left ventricle predominates on the normal ECG. This spread of stimulus through the left ventricle produces a tall R wave in the left chest leads (such as V5 and V6) (in association with a small q wave initial r wave)

NB: Chest leads between these extreme positions show a transitional pattern with tall R waves and deep S waves.

 Standardization mark. The ECG needs to be standardized only before the first lead is recorded.
 PR interval. The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The normal PR interval varies from 0.12 to 0.2 second.

 QT interval. The QT interval is measured from the beginning of the QRS complex to the end of the T wave. It varies with the heart rate, becoming shorter as the heart rate increases

The heart rate can be calculated in ECG in two ways:

 Method 1: Count the number of large (0.2 sec) time boxes between two successive R waves and divide the constant, 300, by this number. If you want a more accurate measurement of the rate, divide the constant, 1500, by the number of small (0.04 sec) time boxes between two successive R waves.
 Method 2: Count the number of cardiac cycles that occur every 6 seconds and multiply this number by 10. A vertical mark appears on the top of the ECG paper every 3 seconds. A cardiac cycle is the interval between two successive R waves.

NB: A heart rate slower than 60 beats/min is called bradycardia; a heart rate faster than 100 beats/min is called tachycardia.

The ECG is a painless, simple test that takes 5 to 10 minutes.
• You lie on a bed or examination table.
• Several sticker sensors called electrodes are placed on the skin of your arms, legs, and chest. They can be cold at first.
• The electrodes are connected to wires called leads, which are connected to the ECG machine.
• The electrical activity of the heart is then recorded on a moving strip of paper in the ECG machine.