Basics of Electrocardiogram
▪ An electrocardiogram (ECG) reflects the electrical activity of cardiac cells and records electrical activity at a speed of 25 mm/second. ▪ An electrocardiographic strip consists of horizontal lines representing seconds and vertical lines representing voltage. ▪ Each small square represents 0.04 second. ▪ Each large square represents 0.20 second. ▪ The P wave represents atrial depolarization. ▪ The PR interval represents the time it takes an impulse to travel from the atria through the atrioventricular node, bundle of His, and bundle branches to the Purkinje fibers. ▪ Normal PR interval duration ranges from 0.12 to 0.2 second. ▪ The PR interval is measured from the beginning of the P wave to the end of the PR segment. ▪ The QRS complex represents ventricular depolarization. ▪ Normal QRS complex duration ranges from 0.04 to 0.1 second. ▪ The Q wave appears as the first negative deflection in the QRS complex and reflects initial ventricular septal depolarization. ▪ The R wave is the first positive deflection in the QRS complex. ▪ The S wave appears as the second negative deflection in the QRS complex. ▪ The J point marks the end of the QRS complex and the beginning of the ST segment. ▪ The QRS duration is measured from the end of the PR segment to the J point. ▪ The ST segment represents early ventricular repolarization. ▪ The T wave represents ventricular repolarization and ventricular diastole. ▪ The U wave may follow the T wave. ▪ A prominent U wave may indicate an electrolyte abnormality, such as hypokalemia. ▪ The QT interval represents ventricular refractory time or the total time required for ventricular depolarization and repolarization. ▪ The QT interval is measured from the beginning of the QRS complex to the end of the T wave. ▪ The QT interval normally lasts 0.32 to 0.4 second but varies with the client’s heart rate, age, and gender.
