The ECG was recorded from an 82 year old woman admitted with an acute myocardial infarction. Monitoring on the coronary care unit had shown the development of dropped beats.
ECG. The ECG shows sinus rhythm. Dropped beats are recorded, so second degree AV block is present. The PR interval shows an increase with successive beats until the dropped beat occurs: Mobitz type I second degree AV block (or Wenckebach block) is present.
Morphology: The QRS complexes are abnormally wide and there is a broad slurred secondary R wave in VI which has an rSR’ appearance.
There are late slurred S waves in leads that face the left ventricle: I, aVL, V5 and V6. This S wave is recorded from the last part of the ventricular myocardium to be depolarized- the free wall of the right ventricle. Because the spread of depolarization is directed away from leads such as these a negative deflection is recorded. However this phase of ventricular depolarization occurs towards leads VI and V2 which record a broad slurred positive R’ wave. Right bundle branch block (RBBB) is present.
Because the initial depolarization of the left ventricle in RBBB follows the normal route (ie with the interventricular septum activated in the normal direction), the presence of morphological changes has the same significance as in normal conduction. There are pathological Q waves in leads II, III, aVF and V4 – V6. There are raised ST segments in I, II, III, aVF and V2 – V6. The changes are therefore indicative of recent inferolateral myocardial infarction. The pattern of R wave progression in the precordial leads in RBBB is different to that seen during normal ventricular activation and changes in R wave progression that may reflect infarction are more difficult to interpret.
Left axis deviation is also present in this ECC and in the presence of RBBB signifies block in the anterior fascicle of the remaining left bundle, ventricular depolarization occurring via the posterior fascicle of the left bundle. When first or second degree AV block occurs in the presence of RBBB associated with left anterior hemiblock, that block is usually situated in the remaining fascicle; this is an unstable situation and complete (third degree) AV block may develop suddenly.
This patient received a temporary pacemaker before the development of more serious block occurred and she required pacing for three days before sinus rhythm returned.