Long QT: Do not trust the computerized QT interval when the QT is long

A middle-aged male with h/o DM was found down.  He was hypoglycemic (mild, 45 mg/dl) and had pneumonia with hypoxia.  He had this ECG recorded:Sinus rhythm with slight right axis deviation and non-diagnostic T-wave inversionsHe received azithromycin and ceftriaxone for community acquired pneumonia.  Then he became very agitated in spite of correction of hypoglycemia, and was given a total of 15 mg of haloperidol.  His K returned at 2.8 mEq/L and ionized Calcium at 3.82 mEq/L (normal, 4.4 -5.2).  Magnesium was 1.5 mEq/L (normal, 1.3 - 2.0)A troponin returned slightly elevated, so another ECG was recorded:There is now sinus bradycardia with a bizarrely long QT interval.  The computer read the QT as 450 ms when it is really around 700 ms.This is a very dangerously long acquired long QT, and is due to a combination of azithromycin, haloperidol, hypokalemia, and hypocalcemia (note the long ST segments which are typical of hypocalcemia). He was given Magnesium, Calcium, and Potassium, monitored in the ICU, and the eventually the ECG corrected as the drugs metabolized.An echocardiogram showed no wall motion abnormality and the elevated troponin was not thought to be due to a Type 1 MI in this critically ill patient.CommentComputer algorithms for measure the QT interval are good for normal QT intervals but not for long QT intervals, and are particularly inaccurate for very long QT intervals.Acquired long QT, and how it causes torsade: It is usually due&n...
Source: Dr. Smith's ECG Blog - Category: Cardiology Authors: Source Type: blogs