ECG Review

Periodic Weakness?

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.

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Scenario: The ECG above was obtained from a 24-year-old man with episodes of marked periodic weakness. He was otherwise healthy. What does the ECG suggest as a contributing cause of his symptoms? What disorder might the patient have?

Interpretation: The lead II rhythm strip shows the rhythm to be markedly irregular. We suspect that the underlying mechanism of the rhythm is sinus, although the constantly changing P wave morphology (and varying PR interval) makes it difficult to know for sure. The 1st, 6th, and 7th QRS complexes appear to be junctional escape beats, since they are not preceded by any P wave in this lead II rhythm strip. Of interest, there is evidence of some type of atrial activity notching the T wave of each of these beats. Whether this reflects retrograde R-P conduction from the junctional escape beats or blocked premature atrial contractions is impossible to tell from the short rhythm strip shown above. Otherwise, the QRS complex is of normal duration. However, the QT interval looks to be prolonged despite the slow heart rate. There is no chamber enlargement given the age of the patient. Next to the rhythm, the most remarkable finding on this tracing is dramatic alteration in the ST segment and T waves. There is marked ST segment depression in most leads that terminates in an upright deflection we suspect represents fusion of T waves with enlarged U waves (producing "Q-U" rather than QT prolongation).

In view of the history, the above ECG picture of diffuse ST-T wave depression with QU prolongation strongly suggests hypokalemia and/or hypomagnesemia as at least a contributing cause of the patient's symptoms. Serum potassium was 1.7 mEq/L at the time the above tracing was obtained. The ECG normalized with correction of this electrolyte disorder. The patient was diagnosed with periodic paralysis. This is a rare group of usually hereditary muscle disease disorders, characterized by intermittent episodes of flaccid muscle weakness lasting hours to days. There are many different forms of periodic paralysis, but most are associated with abnormalities of serum potassium (either hypo- or hyperkalemia), as was the case for the patient in this example.