ECG Review

Why is the QRS Changing?

By Ken Grauer, MD, Professor, Department of Community Health and Family Medicine, University of Florida Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book. Dr. Grauer reports no financial relationship to this field of study.

Figure. 12-lead ECG and lead II rhythm strip obtained from a mentally challenged 37-year-old prior to exercise testing.

Clinical Scenario: The ECG in the Figure was obtained from a mentally challenged 37-year-old man, who had been complaining of atypical chest pain. He was scheduled for an exercise treadmill test (ETT). Can you explain the "changing QRS" in the lead II rhythm strip of his pre-test baseline ECG? Will a stress test provide useful information?

Interpretation/Answer: As per the title of this ECG Review, the morphology of the QRS complex in the lead II rhythm strip at the bottom of the tracing is clearly changing! The QRS complex of beat #1 is predominantly positive (an Rs complex). It is biphasic (an RS complex) for beats #2 through 6, and then again becomes predominantly positive (Rs) for the rest of the tracing. Although there is slight variability in the P-P interval, consistent with sinus arrhythmia — the remarkable finding is the change in the PR interval. Slurring of the initial portion of the QRS complex of beats #7 through 11 results in PR interval shortening. The picture becomes clearer with inspection of simultaneously recorded leads V4 through V6. Thus, the patient has WPW (Wolff-Parkinson-White) syndrome, with intermittent conduction down the normal pathway (beats #2 through 6), alternating with conduction down the accessory pathway. Within the first 3-minute stage of exercise testing, the patient developed consistent conduction down the accessory pathway. This invalidated interpretation of any subsequent ST segment changes with exercise. The test was therefore stopped, and the patient referred for stress echocardiography to better assess the likelihood of coronary disease.