The EGG was recorded from a 46 year old woman who was aware of irregular heart beats and “thumps” in the chest.
ECG. Rhythm: The basic rhythm is sinus wiith P waves preceding narrow complex beats conducted normally. The rhythm is interrupted by abnormal broad complex beats that occur earlier than the next anticipated sinus beat, ie they are premature beats. These abnormal complexes arise from an ectopic focus, and the course of depolarization is abnormal spreading slowly through ventricular myocardium instead of through the specialized conducting tissues. The consequence is that the QRS complex is wide and slurred. Secondary changes in the T waves are seen after these beats; the T wave is inverted when the QRS complex is dominantly upright and upright when the QRS complex is dominantly negative. These are similar to the changes seen in bundle branch block and are a consequence of abnormal depolarization.
The R – R interval between two consecutive sinus beats is exactly half the distance between the two sinus beats lying either side of a premature beat. This occurs because sinus node discharge continues uninterrupted, but an impulse arriving at the AV node or ventricle following the premature beat finds it refractory and is not conducted. Non conducted P waves can be seen distorting the ST segments of the ectopic beat in several places on the trace – particularly in the rhythm strip. The next sinus beat occurs at the normal time and causes ventricular depolarization in the usual fashion. Under these circumstances the pause following the premature beat – the compensatory pause – is complete and compensates exactly for the prematurity of the extrasystole.
Morphology: The sinus beats show normal QRS morphology.
Comment Ventricular premature beats are present. Ventricular ectopic beats may arise from a focus anywhere in the ventricular myocardium. When the ectopic beat occurs earlier than the next anticipated sinus beat it is known as a premature beat and when it arises later, as a ventricular escape beat. Both premature and escape ventricular beats arise from an ectopic focus and can be described as ‘ectopics’ or ‘extrasystoles’- and both terms are often used synonymously in clinical practice. The use of this imprecise terminology is to be discouraged.
In this case there is a complete compensatory pause following each premature beat but this is not seen in all cases; if a ventricular ectopic beat can penetrate the AV node retrogradely and depolarize the atria and sinus node prematurely the next ‘normal’ sinus beat will also occur early and the compensatory pause will be incomplete.
Ventricular premature beats are frequently seen in apparently healthy people and are of no prognostic significance under these circumstances although the patient may complain bitterly about the palpitation that may be caused. They may occur in valvular heart disease but are also a common accompaniment of serious myocardial disease e.g. cardiomyopathy or ischaemia.