ECG Simplified – Part 10

ECG Simplified – Part 10

Coming back to our previous discussion on conduction disturbances of the heart, we have intraventricular conduction abnormalities or conduction defects in the ventricles. There could be blocks in the right bundle branch or left bundle branch, with specific patterns in the ECG. Right bundle branch block is known in short as RBBB, while left bundle branch block is known as LBBB.

ECG showing right bundle branch block with RSR’ pattern in V1 and slurred S wave in I, aVL, V5 and V6. QRS duration is 120 ms. Shallow T inversion is noted in V1,V2. R’ wave is due to delayed electrical activation of the right ventricle. When there is right bundle branch block, activation of the right ventricle occurs by conduction from the left ventricle through the heart muscle.

This is slower than the conduction through the bundle branches and causes delayed activation of the right ventricle, at a time when it is not opposed by electrical signals from the left ventricle. Slurred S wave in lead I also represents delayed activation of right ventricle. The negative wave is because lead I is oriented towards the left ventricle. Increased QRS duration is also due to the delay in activation. If the QRS duration is less than 120 ms, with the same RSR’ pattern, it is called incomplete right bundle branch block pattern or IRBBB.

Left bundle branch block or LBBB manifests with M shaped pattern in leads I, aVL, V5 and V6. A somewhat W like pattern may be seen in lead V1. But sometimes the notch in the W pattern may not be there and the pattern in V1 may be just a wide Q wave as seen in V1-V3 in this ECG. Widening of QRS is due to delayed activation of left ventricle from the right ventricle, by slow conduction through the heart muscle. Conduction velocity is much lower in heart muscle, compared to the bundle branches which are part of the specialized conduction system of the heart. The wide Q wave in left bundle branch block may be mistaken for the Q wave in old myocardial infarction.

Another ECG with LBBB pattern with much wider QRS complex. The notched R waves resembling M pattern in leads I, aVL, and V5 are more evident in this case, marked by blue arrow. Almost W pattern is seen in V1 and V3, marked by violet arrow. The wider QRS complex in this ECG indicates associated heart muscle damage. In this case it was due to a previous heart attack.

A pattern similar to LBBB is seen when the right ventricle is paced using a pacemaker, either temporary or permanent. This is because the right ventricle is activated first and left ventricle later, by conduction through the heart muscle, just as in left bundle branch block. Pacemaker signal known as pacing stimulus artifact is seen just before the QRS complex. It is a sharp vertical deflection, which can be sometimes ironed out by a lower low pass filter setting as discussed earlier. If the pacemaker artifact is not visible due to technical problem, the ECG will be mistaken for left bundle branch block. In this ECG also, pacing artifact is hardly visible in lead I.

Another interesting conduction system abnormality is the presence of an accessory atrioventricular conduction bundle. This accessory pathway bypasses the normal AV nodal delay and activates the ventricles earlier than expected. This is known as ventricular pre-excitation. Presence of accessory conduction pathway can lead to fast heart rhythms known as atrioventricular re-entrant tachycardia (AVRT) due to circus movement of signals between the normal and abnormal pathways.

Characteristic ECG finding in an accessory pathway is a delta wave due to early activation of the ventricles. Delta wave gets its name from the resemblance to the Greek alphabet Delta. Early activation of the ventricles, bypassing the AV nodal delay also causes a decrease in the PR interval. Accessory pathway with recurrent tachycardia is known as Wolff-Parkinson-White (WPW) syndrome.

Another ECG abnormality which can cause heart rhythm abnormality is Brugada syndrome. In Brugada syndrome, there is ST segment elevation in leads V1-V3, which may mimic a STEMI. There is also an R’ wave resembling right bundle branch block pattern. Brugada syndrome can lead on to life threatening heart rhythm abnormalities.

Long QT syndromes are a group of genetically mediated diseases which are prone for life threatening heart rhythm disorders. Like Brugada syndrome, it is due to defect in ion channels of the heart. Hence, they are also called cardiac channelopathies. The QT interval is prolonged in this group of disorders, which leads to heart rhythm abnormalities. Though there are several formulae for correction of QT interval for the heart rate, a simple rule of the thumb is that if QT interval is more than half of the RR interval, it is likely to be prolonged.

This is another simple ECG finding known as electrical alternans, in which the amplitude of QRS complexes alternates between beats. This occurs in a condition known as pericardial effusion, in which large quantity of fluid collects within the layers of the covering of the heart. In one cardiac cycle the heart comes closer to the ECG lead and records a higher amplitude while in the next beat it moves away, within the fluid surrounding the heart.

In this echocardiogram, an ultrasound image of the heart, fluid surrounding the heart has been marked as PE, in short for pericardial effusion. It can be seen that between the two frames, heart is swinging to either side. This changes the conduction of signals from the heart to surface ECG electrodes, producing electrical alternans. Due to the fluid covering the heart, the ECG signals are dampened, producing low voltage complexes in pericardial effusion, in addition to electrical alternans.

A simple birth defect of the heart is having the heart on the right side of the chest instead of the normal left side. This swings the electrical axis of the heart to the right. Lead I records negative waves instead of the normal positive complexes. In a normal ECG, the amplitude of R waves increases as we proceed from V1 to V5. There is a reverse progression in dextrocardia as the heart is on the opposite side. Amplitude of QRS complexes decreases from V1 to V6. Actual amplitudes can be recorded by keeping the chest leads in reverse order on the right ride of the chest, known as right chest leads. Right chest leads are designated as V3R, V4R, V5R and V6R.

This has been a very concise description of the concepts in electrocardiography in a simplified manner. The field of electrocardiography has grown a lot over the past century. Automated algorithms for analysis have come a long way due to the enhancements in information technology. One of the latest advances is ECG imaging (ECGi) which uses a large number of chest electrodes worn as a vest. ECGi can give various types of maps of the electrical activity of the heart like a similar mapping using multiple electrodes within the heart, but non-invasively. These images are integrated with computed tomography (CT) images of the heart to give valuable information.