In this excellent article from the March 2007 Journal of the Emergency Medical Services, Tim Phalen discusses the importance of performing serial 12 lead ECGs.
Here are some of the highlights.
â€œAcute myocardial infarctions (AMIs) arenâ€™t like broken bones and, therefore, ECGs arenâ€™t static like X-rays. If an EMS crew were treating a hip fracture and could somehow perform the X-ray on scene, what would it show? A broken hip, of course. And if the X-ray wasnâ€™t performed until the patient arrived at the emergency department (ED), would the broken hip still be visible?
When dealing with a fracture, whether the X-ray is obtained immediately, in 10 minutes or in 10 hours, the interpretation and diagnosis usually wonâ€™t change. But whatâ€™s true for X-rays isnâ€™t necessarily true for ECGs. In fact, an ECG can significantly change in a very sort period of time â€” as can the corresponding interpretation.â€
â€œ[I]t can be difficult to determine if the presence of LBBB on the ECG of a suspected AMI patient is preexisting or is a new onset. If the LBBB is infarct-induced, it has a high mortality rate â€” up to 60%. Therefore, the patients who may need reperfusion the most are the least likely to receive it. However, dynamic changes on serial ECGs shed light on the situation. A hallmark of infarct is change over time. If a patient has had an LBBB for the past 15 years, itâ€™s not likely to change much during the next 15 minutes. But when changes occur in a short period of time, suspect AMI.â€
Those are some excellent points.
Itâ€™s also helpful to understand the expected appearance of baseline abnormalities. For example, the rule of â€œappropriate T-wave discordanceâ€ states that with bundle branch blocks and paced rhythms, the T-wave should be deflected opposite the terminal deflection of the QRS complex. So LBBB is an abnormal finding, but discordant T-waves (and ST-segments) within the context of LBBB are a normal finding (to a point).
On the other hand, while baseline abnormalities like LBBB or paced rhythm may cause a discordant shift of the ST-segment and T-wave, the ST-segment should not be moving! A moving ST-segment suggests dynamic changes in supply v. demand characteristics.
In other words, ischemia.