Understanding an ECG takes practice and I feel like you can only really understand its parts a few at a time. Then once you understand those parts, imagining what we see on the ECG as it translates to cardiac function takes another level of thinking. I don’t claim to actually understand all this stuff (I really don’t know much about 12-leads) and I already shared incorrect information in my first ECG workshop at work, but with practice, my grasp has improved. So…let’s talk a little about some overlooked topics.
Please say this with me…’refractory period’. Don’t you feel smart just saying that? Refractory can mean stubborn or unmanageable or resistant to an impulse. I’m sure if you bring up ‘refractory period’ at a dinner party, you will definitely look like a genius. So what is it? In an ECG there are two types of refractory periods: absolute and relative. During absolute refractory extra impulses are completely blocked meaning you won’t have QRS complexes on QRS complexes. This allows the heart the time it needs to generate an impulse, cause depolarization and repolarization of the cells, and return to normal in an organized and predictable fashion. However, during relative refractory an untimely impulse may sneak it’s way through and cause a QRS complex (usually a premature ventricular complex or PVC) on the end of a T wave. Why is this a problem? It throws the heart out of its normal, predictable pattern in a violent way and the heart typically responds to it with it’s best organized (yet dangerous) ventricular rhythm: v-tach or the more unorganized rhythms of torsades de pointes or v-fib.
Is there anything that can help us anticipate this R-on-T phenomenon? Why, yes! It’s called the QT interval. This measurement basically tells us how long it takes an electrical impulse to travel through the ventricles and return to normal. Normally this occurs in about 0.34-0.43 seconds. We start to get worried if the impulse is between 0.44-0.50 seconds and >0.5 we should freak out a little. If we see the QT interval taking longer and longer, our patients are approaching a higher risk level for an R-on-T phenomenon. As this time lengthens, the patient runs the risk that the next beat generated could fall during relative refractory and send them into an unpleasant situation (like v-tach or torsades and a code situation…no one wants that).
Something to note with the QT interval: as the heart rate increases, the QT interval appears to get shorter. This does not mean that patients with prolonging QT intervals are safer at higher heart rates. Actually we account for this changing interval with the corrected QT or QTc. This calculation depends on the QT interval and the R-to-R interval (QTc=QT/(square root of R-R)).
What can cause this impulse to lengthen? Often it is medications, but it can also be caused by electrolyte imbalances and heart rate changes (as we just discussed). When an impulse gets through and occurs during relative refractory, we still label it a PVC. Substances that cause these PVCs are typically stimulants (caffeine, alcohol, nicotine, digoxin, epi, aminophylline) but hypoxia, fever, exercise and emotional stress can cause these as well. Of course, sick and diseased hearts are yet another group at greater risk for PVCs. Medications that we commonly give in Progressive care that prolong the QT interval include:
There are many others (including antibiotics and heart medications) which can be found at Credible Meds but it requires an account. Micromedex may give some of this information and (at least on our floor) we have access to this for free.
Hopefully this very basic overview gives people an understanding of the QT interval and why it is important. Um…so I don’t know exactly what to do to measure the QTc on a patient with a-fib or a-flutter. Anyone know how to do that?