Today's Expert is Dr. KEN GRAUER, MD is Professor Emeritus (Dept. Community Health/Family Medicine, College of Medicine, University of Florida in Gainesville).
Dr. Grauer has been a leading family physician educator for over 30 years. During that time he has published (as principal author) more than 10 books and numerous study aids on the topics of ECG interpretation, cardiac arrhythmias, and ACLS (including an ongoing Educational ECG Blog) .
ANSWER:
 The QT interval is the period that extends from the beginning of ventricular depolarization - until the end of ventricular repolarization. Along with the PR interval and QRS duration - it is one of the 3 key intervals that we measure. Although severe hypercalcemia (usually only with serum levels exceeding 12 mg/dL) may produce QT interval shortening - this is rare (as well as being very difficult to recognize! ). So, for practical purposes - we really only need to concern ourselves with determining whether the QT interval is normal or long.
That said - accurate QT interval can be complex involving tables based on age, sex, and heart rate ... Fortunately - this isn't needed in most cases. Instead - a very handy "eyeball method" may be used to rapidly assess the QT. For practical purposes, the QT is prolonged - IF it clearly measures more than half the R-R interval. All you need do is select a lead where you can clearly see the end of the T wave. Measure the QT in the lead where the QT interval appears to be longest. If you don't have a pair of calipers handy - mark the QT you measure on a piece of paper, and see if the R-R interval is more than twice this amount. If it isn't - then the QT is probably prolonged. The principal exception to this "eyeball method" is when the heart rate is rapid (i.e.r., over 90-100/minute) - in which case it becomes more difficult to measure the QT and determine its clinical significance (CLICK HERE FOR ADDITIONAL RESOURCES.)
This of course brings up the question, "Why care if the QT is long?" We answer this question in the form of a LIST. Other than myocardial infarction/ischemia and bundle branch blocks (which will usually be obvious on the tracing) - there are 3 principal causes of QT prolongation where the only abnormality on the ECG may be the long QT. These 3 entities are conveniently remembered by the saying, "Drugs/Lytes/CNS". Thus, antiarrhythmic drugs/tricyclic antidepressants/phenothiazines - low potassium, magnesium, or calcium - AND - almost any CNS catastrophe (stroke, seizure, coma, intracerebral or brainstem bleeding) may cause QT prolongation.
 
SUMMARY: We assess the 3 intervals (PR/QRS/QT) early in the process of ECG interpretation, usually right after determining the rhythm. Assuming the heart rate is not overly rapid (under 100/minute) - We look to see if the QT measures more than half the R-R interval. If it doesn't - then the QT is normal. If it does - then the QT is probably long. Correlate clinically by looking for, "Drugs/Lytes/CNS" as a possible cause for the long QT.
 
Ken Grauer, MD