We have always been taught that both left bundle branch block and right ventricular pacing obscure the ECG diagnosis of acute myocardial ischaemia and infarction. Although correct in many instances, the ECG may provide valuable clues which should not be ignored.
This 12-lead resting ECG was during severe retrosternal chest pain in a heavy smoker with a VVI pacemaker.
Two hours later, the pain had settled
There was only minor coronary artery disease. Despite ventricular pacing, the ECG helped confirm Prinzmetal or variant angina due to coronary artery spasm.
This patient with a dual chamber pacemaker also had severe chest pain:
Despite ventricular pacing, the features of an acute septal infarct can be seen (red highlight). Ventricular fusion is very likely.
Even when the ventricular pacing ECG diagnosis of acute septal infarction is non-confirmatory, repeat ECGs can be very helpful.
Unipolar VVI pacing.
On day 1 marked T wave inversion (red highlight), which resolved by day 4.
Where possible, slowing the pacing rate to allow the native rhythm to emerge may be helpful.
Unipolar VVI pacemaker in a patient with severe retrosternal chest pain.
Left: Unipolar ventricular pacing.
Right: The pacing rate was slowed until the underlying native rhythm emerged. Sinus rhythm, bifascicular block, septal ST elevation, T wave inversion and developing Q waves (red highlight) consistent with an acute myocardial infarction.
These changes in the native rhythm must be differentiated from pacemaker T wave memory, which is a non-pathological phenomenon usually seen as persistent T wave inversion in the infero-lateral leads after pacing inhibition and AV conduction restored. In the above ECG, other features of infarction, such as Q waves and ST elevation were present.
T wave inversion has also been reported after transient left bundle branch block, ventricular tachycardia or pre-excitation and is also referred to as cardiac or T wave memory and the changes may last weeks. Obviously, the T wave inversion can cause confusion with diagnosis of myocardial ischaemia, particularly if the precipitating cause of the T wave memory such as a transient run of ventricular tachycardia, remains undiagnosed.
Normal pacemaker function can be damaged by myocardial ischaemia or infarction. Both pacing and sensing, particularly in the ventricle can be seriously compromised, leading to exit block and asynchronous pacing. This in turn may result in “R on T” and lethal ventricular arrhythmias.
VVI pacing. Acute myocardial infarction with loss of sensing and intermittent exit block. Normal ventricular pacing (red highlight). Ventricular exit block (yellow highlight). Native escape sinus rhythm (blue highlight). There is R on T (red arrow) resulting in Torsade de Pointes and asynchronous ventricular pacing continues (blue arrows).
VVI pacing. Acute myocardial infarction with intermittent failure of sensing and exit block (red arrows) resulting in Torsade de Pointes (red highlight) and asynchronous pacing (blue arrows).
In 49+ years as a practicing cardiologist, Dr Harry Mond has published 260+ published manuscripts & books. A co-founder of CardioScan, he remains Medical Director and oversees 500K+ heart studies each year.
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