A stable ICU patient had the following two EKGs obtained within minutes of each other.  Can you guess what is going on?

Answer:  The EKG in sinus rhythm reveals a LAD occlusion status post reperfusion in the cath lab.  There are some Q-waves and a wee bit of residual ST elevation.  T-wave inversion reflects reperfusion.

So what is going on with the paced EKG?  The patient had developed a Mobitz II block leading to insertion of a temporary transvenous pacer wire (before placement of the pacer, the heart rate was occasionally drifting into the 30s).  The pacer was set at a rate of 60 b/m, which turned out to be close to the patient's intrinsic rate.  So the patient was flipping between runs of intrinsically paced beats and runs of pacemaker-triggered beats.

The set of two EKGs points out how much information is covered up by the paced rhythm.  The paced rhythm is unable to convey the presence of Q-waves, some traces of persistent ST elevation, and reperfusion T-waves – all important bits of information which provide context regarding how this patient's infarct is evolving.

(It's important to note that paced EKGs can reveal ischemia in some situations – similar to LBBB EKGs, using the modified Sgarbossa criteria.  So don't assume that a paced EKG is worthless for ischemia.)

This suggests that in selected situations it might be reasonable to transiently reduce the pacemaker rate, with a goal of intentionally eliciting an EKG in sinus rhythm.