Cardiology Teaching Package
A Beginners Guide to Normal Heart Function, Sinus Rhythm & Common Cardiac Arrhythmias
ST Elevated Myocardial Infarction
With a STEMI, the artery is completely occluded and tissue beyond the clot starts to die within 30-45 minutes from onset of pain. Because the body cannot break down the fibrin itself, a thrombolytic, or Fibrinolytic, drug is introduced as soon as possible. Most of the tissue beyond the clot will be dead within 6 hours so the sooner we can give the fibrinolytic drug, the more heart muscle can be saved.
Again, depolarisation takes place as normal but when the stimulus reaches the dead tissue it can no longer get through as dead tissue cannot conduct electricity at all. The ECG machine picks up this change as ST Elevation. (see earlier examples)
TIME IS MUSCLE!
As an analogy, think of depolarisation of the heart as your route home. Each night you go home exactly the same way and the shape of your route on the map is the same. If you encounter an accident and detour on the way home, you still get there but by a more circuitous route. Add this to the map and you have a slightly changed shape but with the same start and finish line.
On ECG, if the rhythm is still sinus, you will still see the p wave, QRS and t wave, but the ST segment will be above or below the iso-electric line giving you a slightly changed shape as depolarisation in that area has to detour.
Below is a cross-section of a ventricle showing the development of necrosis.
Image: Wavefront Phenomenon of Ischaemic Cell Death
