ECG Simplified – Part 6

ECG Simplified – Part 6

Now we will move on to abnormalities in the conduction of electrical signals within the heart. Conduction abnormalities between the sinus node and the atrium are known as sinoatrial conduction abnormalities. Abnormalities in the conduction between the upper and lower chambers is known as atrioventricular conduction abnormalities. Abnormalities of conduction within the ventricles are called intraventricular conduction defects. Similarly, there could be inter atrial and intra-atrial conduction abnormalities as well. The former is between the two upper chambers while the latter is within the upper chamber.
Sinoatrial conduction abnormalities could be first degree, second degree or third degree SA blocks. But it is not possible to identify first degree and third degree SA blocks on the surface ECG as the sinus node activity is not visible. First degree SA block means a delay in conduction from the sinus node to the atrium, without any loss of signals.
In second degree SA block, there is intermittent conduction across the sinoatrial junction. This is manifested by intermittent absence of P waves in the surface ECG. Second degree SA block is the only type which can be diagnosed from the surface ECG.
Third degree SA block means that there is total absence of conduction from the sinus node to the atrium. In the surface ECG this will manifest indirectly as absence of P waves originating from the sinus node. P waves originating from another part of the atrium may be there, with a different shape and electrical axis. Total absence of sinus node activity and total absence of sinoatrial conduction cannot be differentiated from surface ECG.
Sinus arrest is another condition different from sinoatrial block. In sinus arrest, sinus node fails to produce signals. It could be intermittent or prolonged sinus arrest. Intermittent sinus arrest will appear like skipping of P wave, while prolonged sinus arrest will appear as a prolonged pause. Sometimes a slower pacemaker at the atrioventricular junction or from the ventricles may take over signal generation.
Sinus arrest is one of the manifestations of a disease known as sick sinus syndrome. Symptomatic sinus pauses are treated by implantation of a permanent pacemaker. Pacemaker is an electronic device, usually implanted under the skin and connected to the heart using electrode wires introduced through blood vessels known as leads.
Interatrial block has been well recognized in ECG only recently. In partial interatrial block, the P wave is wide and notched, without left atrial enlargement. In advanced interatrial block, the P wave is wide and biphasic in leads II, III and aVF. Inversion of terminal portion of P wave is because of delayed activation of the left atrium from below upwards due to the block in the conduction bundle in the upper part. Upward activation is shown as negative in leads from the lower part of the body, II, III, aVF as the activation moves away from those leads.
Atrioventricular block is divided into first degree, second degree and third degree or complete heart block. In first degree AV block, there is only prolongation of conduction without any P wave being non-conducted. It manifests in ECG as a prolonged PR interval.
In second degree AV block, there are intermittent non-conducted P waves. There are two types of second degree AV blocks. In type I, there is progressive prolongation of PR interval followed by a non-conducted P wave. In type II there is no progressive prolongation of PR interval, but there are intermittent non-conducted P waves. Type II is more serious.
In complete heart block, also known as complete AV block, none of the P waves are conducted to the ventricles. The ventricles are activated by a subsidiary pacemaker either from the AV node or the ventricles. P waves are seen at a higher rate than the QRS complexes, without any relationship between the two. If the subsidiary rhythm is originating from the AV node, the QRS complex will be narrow. When the subsidiary rhythm originates from the ventricle, the QRS is wide and the heart rate lower.

Reference on Interatrial Block

1. Baranchuk A, Torner P, de Luna AB. Bayés Syndrome: What Is It? Circulation. 2018 Jan 9;137(2):200-202. doi: 10.1161/CIRCULATIONAHA.117.032333. PMID: 29311351.