ECG Simplified – Part 7

ECG Simplified – Part 7

Now we will move one to heart rhythm disorders originating from the upper part of the heart known as supraventricular arrhythmias. Supraventricular means above the ventricles. Supraventricular ectopic beats are premature beats originating above the ventricles. Supraventricular ectopics are the commonest form of supraventricular arrhythmia. They can occur in isolation or in sequences of couplets. If more than three are seen in a sequence it will constitute a short run of supraventricular tachycardia.
Supraventricular ectopics are common in the setting of atrial dilatation due to chronic obstructive lung disease or reduced pumping function of left ventricle. Supraventricular ectopic beats have a narrow QRS complex preceded by an abnormal P wave. They are also called supraventricular premature complexes as they occur before the next expected sinus beat. In this ECG shown, the abnormal P wave is superimposed on the preceding T wave. QRS complex of SVPC is almost like that of the normal sinus beat. Abnormal beat is followed by a pause.
Supraventricular tachycardia is a fast regular rhythm. Supraventricular tachycardia can be recognized on the ECG as a narrow QRS tachycardia with either absent P waves or P waves which are distinctly different from those in sinus rhythm. There are several types of supraventricular tachycardias, based on the mechanism. Commonest form in adults is called AV nodal re-entrant tachycardia or AVNRT. It is due to a circus movement of signals within the AV node.
Supraventricular tachycardia is often initiated by a supraventricular ectopic beat which sets off the circus movement. This ECG shows a supraventricular ectopic with a different type of P wave, marked by the red line, initiating a run of supraventricular tachycardia. This sequence was captured from the storage memory of the intensive care central monitor. SVT is the short form for supraventricular tachycardia.
Preceding sinus rhythm shows regular sequence of P waves followed by QRS complexes. During SVT, P waves are not seen because they are occurring along with the QRS complexes in AVNRT type of supraventricular tachycardia.
In this case, the supraventricular tachycardia terminated spontaneously. This was followed by a pause and then a ventricular premature complex and two beats originating from the AV junction, marked as J in the tracing. Following beat looks like a ventricular ectopic beat super imposed on a sinus P wave. Last beat in the sequence is a normal sinus beat. Such irregular sequences can occur after termination of a tachycardia.
This ECG shows a regular junctional rhythm originating from the AV junction. Tall peaked T waves marked by pink arrows, is another associated abnormality, not due to the junctional rhythm. Tall peaked T waves can occur when the blood level of potassium is high. No P waves are visible in this junctional rhythm as atrial activation coincides with ventricular activation. This is because signals pass simultaneously upwards and downwards in this junctional rhythm. Junctional rhythm occurs as a subsidiary rhythm when the sinus node is suppressed due to some reason or when there is complete sinoatrial block.
This ECG shows another variety of junctional rhythm in which P’ waves are seen after the QRS complexes. This is because the rhythm originates from the lower part of the AV junction. It takes more time to reach the atria than the ventricles. So, a retrograde P’ wave is seen after the ventricular activation, which is the QRS complex. Timing of the T waves, which is different from that of the retrograde P’ waves have been marked in lead V2.
In this ECG, P waves, marked by blue arrows, are inverted in leads II, III, and aVF. These leads are directed towards the lower part of the body. Hence inverted P waves in these leads mean that the atrial activation is proceeding from below upwards. Retrograde activation from below upwards indicates that activity originates from the lower part of atrium, known as low atrial rhythm. This rhythm also occurs when the sinus node is not functioning well, as a subsidiary rhythm.
This ECG shows multiple shapes of P waves, marked M, seen in short runs of tachycardia. This is characteristic of multifocal atrial tachycardia. Sinus P waves marked S are seen after the pauses. Multifocal atrial tachycardia is sometimes called as chaotic atrial rhythm because of the variability in atrial activity. Multifocal atrial tachycardia can occur in those with chronic obstructive lung disease.
Atrial fibrillation seen in this ECG, is the most common sustained arrhythmia, originating from the upper chambers of the heart. AF can be recognized in the ECG with absence of organized atrial activity and the presence of fibrillary waves. AF can be either fine AF or coarse AF depending on the amplitude of fibrillary waves. In this ECG it is fine atrial fibrillation because the fibrillary waves are hardly seen in most leads.
In atrial fibrillation, the atrial rate is very high, of the order of 450-600/min. But most of the signals are blocked in the AV node so that the ventricular rate does not go that high. Still, it may be around 120/min in most cases. It can be slowed down by using medications. In this ECG, the ventricular rate is controlled.
Atrial flutter is a regular rhythm originating from the upper chambers of the heart. The atrial rate may be 250 to 350/min or even more. Difference from atrial fibrillation is that organized atrial activity is present. But all the signals are not usually conducted down to the ventricles in atrial flutter. In the ECG illustrated only alternate flutter waves are conducted down. Hence the ventricular rate will be only half that of the atrial rate. If all the flutter waves are conducted down, as occurs rarely, the ventricular rate will be dangerously high.