ECG Simplified – Part 5

ECG Simplified – Part 5

Now we will see how the cardiac chamber enlargements are inferred from the ECG. Enlargement of the atria are reflected in the P wave which represents the atrial electrical activity. Normal P wave has a maximum amplitude of 2.5 mm and width of 2.5 mm on the ECG.
An increase in the amplitude of P wave is an indication of right atrial enlargement and increase in width of P wave an indication of left atrial enlargement. When both atria are enlarged, both width and amplitude of the P wave are increased.
Diagrammatic representation of left atrial enlargement with wide and notched P wave, known as P mitrale.
Diagrammatic representation of right atrial enlargement with tall peaked P waves known as P pulmonale.
Ventricular enlargement is indicated in the QRS complex, the activity of the ventricles. There may be secondary changes in ST segment and T waves. ECG changes in left ventricular hypertrophy, the technical name for thickening of left ventricle, is divided into left ventricular volume overload and pressure overload patterns. Similarly, right ventricular hypertrophy is also divided into pressure and volume overload patterns.
In the ECG, left ventricular volume overload is indicated by small initial q wave, tall R wave and upright tall T waves in V5, V6. Left ventricular volume overload can occur if there is a leak in the aortic or mitral valve or large defect in the wall between the two ventricles.
Tall R wave in V5, V6 will be there in left ventricular pressure overload as well. ST segment will show a down sloping depression and the T wave will be inverted. These changes are called strain pattern. This pattern can occur in high blood pressure and in obstruction to the aortic valve.
A simple measurement for left ventricular enlargement on ECG is Sokolow-Lyon criteria, which is often printed on automated reports. Sum of S wave in V1 and R wave in V5 or V6 more than 35 mm OR R wave in V5 or V6 more than 26 mm is considered as significant as per this criteria.
This is based on tall R waves in leads facing the left ventricle and deep S waves in leads facing the right ventricle.
Right ventricular volume overload manifests as rSR’ pattern in lead V1, which is also known as incomplete right bundle branch block pattern. It is typically seen when there is a defect in the wall between the two upper chambers of the heart.
Right ventricular pressure overload manifests with tall R wave, down sloping ST segment depression and T wave inversion in V1. The pattern is similar to that of left ventricular strain pattern but noted in leads facing the right ventricle. It occurs when the pressure in the blood vessels of the lungs are high or when there is obstruction to the pulmonary valve.
Mean electrical axis can be calculated for the P wave, QRS complex or the T wave. It can even be calculated for the segments like ST segment, though it is seldom done. Most often only the mean QRS axis is calculated and when electrical axis is mentioned, generally only mean QRS axis is meant. QRS axis can be altered in various disease states like conduction defects within the ventricle, heart attack, enlargement of heart chambers and change in position of the heart.
For calculation of mean axis of the QRS complex, the amplitude of the negative waves is subtracted from that of the positive waves while plotting on the graph using lead I as X-axis and aVF as Y-axis. The electrical axis of lead I is zero and that of aVF +90 degrees. Resultant is taken as the mean QRS vector and the deviation from lead I the mean QRS axis measurement.