ECG Review: The P Holds the Key

By Ken Grauer, MD

Figure. 12-lead ECG obtained from a 79-year-old woman with "skipped beats."

Clinical Scenario: The ECG in the Figure was interpreted as showing PVCs (premature ventricular contractions) and chamber enlargement. Do you agree? Can you identify at least four findings "keyed by the P" in this ECG?

Interpretation/Answer: The underlying rhythm on this tracing is sinus, as indicated by the presence of an upright P wave preceding each QRS complex in lead II. There are two widened and abnormal appearing beats seen in simultaneously recorded in leads V1, V2, V3 (labeled X and Z). Sandwiched between these two beats is a normal, sinus conducted impulse (beat y). The two abnormal appearing beats are not PVCs. Instead they are PACs (premature atrial contractions) conducted with RBBB (right bundle branch block) aberration. Premature P waves precede beats X and Z, and hold the key for this definite diagnosis.

P wave morphology in this tracing suggests other findings. Distinct notching and slight prolongation of the P wave in lead II, as well as the deep negative component to the P wave of the normal sinus conducted beat in lead V1 (beat y) is strongly suggestive of LAE (left atrial enlargement). The prominent point to the small amplitude initial positive deflection of this P wave preceding beat y also suggests the possibility of clearly also suggests the possibility of RAE (right atrial enlargement), although this much more subtle finding is clearly a less reliable indicator of RAE than would be the finding of tall peaked P waves in the inferior leads (which is not seen here).

The other noteworthy finding on this tracing is shallow symmetric T wave inversion in leads V4 through V6, which may indicate ischemia. Note that T wave inversion is not seen in the normally conducted beats (under y) in leads V1, V2, and V3. Here again, the "P holds the key" because the PAC preceding beats X and Z tells us that these QRS complexes are aberrantly conducted, which means that the deep T wave inversion following these beats can not be interpreted as indicative of ischemia. Thus, there is evidence of ischemic T wave inversion, but only in the lateral precordial leads (V4-V6), and not in leads V1, V2, and V3.

Dr. Grauer, Professor, Assistant Director, Family Practice Residency Program, University of Florida, is Associate Editor of Internal Medicine Alert.