Friday, November 7, 2008

ST depression: is it ischemia? No, hypokalemia.


This patient presented with alcohol intoxication and possible overdose. No ischemic symptoms.

Rate 71
PR 224
QRSD 90
QT 424
QTc 461

There is ST depression in V4-V6 (minimal). She is in alcohol intoxicated: high risk for hypokalemia.

Differential of ST depression is:

Secondary to abnormal QRS (LVH, LBBB, RBBB, etc.)
Primary ischemia
Hypokalemia
Digoxin
Normal variant.

This patient had a K of 2.8. Difficult case because she does not have prominent U waves (though they are there). I just finished a study of patients with proven hypokalemia compared to a control group. The QTc was the single best differentiator, with 450 ms the best cutoff. ST depression had good specificity but very poor sensitivity. Prominent U-waves were specific but not sensitive, and presence of any U-wave was sensitive but not specific. The best combination of factors was: Subjective diagnosis + QTc > 450 + Prominent U-waves, vs. none of the 3, with sensitivity of 86%, specificity of 100%, and accuracy of 92%. This held true when the subjective interpretation was done by residents who had had a short tutorial by me.

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