ECG Review

How Many PACs?

By Ken Grauer, MD

Figure. Rhythm strip showing PACs. How many PACs are there?

Clinical Scenario: The rhythm strip shows premature atrial contractions (PACs). How many PACs are seen on this tracing?

Interpretation: This is a tricky tracing to interpret. The starting point (and perhaps the most difficult part of the interpretation) is to determine what an unaffected sinus beat looks like. We propose that the beat marked X represents the only such unaffected beat on the tracing. Note that this beat manifests a rounded (coved) ST segment with a shallow, symmetrically inverted T wave. All other T waves on this tracing are distorted by premature P waves (PACs). Many of these PACs are blocked—some subtly (producing slight peaking in the T wave of the 4th, 6th, 9th, and 11th beats)—with other blocked PACs being much more obvious (note peaked PACs in the T waves of the very first beat on this tracing, as well as for the 8th beat). In all, we count a total of 11 PACs—but expect that others might not quite count the same. The answer is, therefore, that there are a lot of PACs on this tracing, with slight variation in their time of occurrence accounting for the continual change in ST-T wave morphology (due to superposition of these PACs on the normal T wave, which is negative in this lead).

(Dr. Grauer, Professor and Assistant Director, Family Practice Residency Program, University of Florida, ACLS Affiliate Faculty for Florida, Gainesville, is on the editorial board of Emergency Medicine Alert.)