QT Interval

QT intervals can be tricky, representing one of the greatest challenges in electrocardiography,  but it's important to identify lengthening QT intervals as they could lead to potentially lethal ventricular arrhythmias.

The American Heart Association defines the QT interval as "the time for electrical activation and inactivation of the ventricles" and is measured from the onset of the QRS complex, ventricular depolarization, to the end of the T wave, ventricular repolarization.

When measuring the QT interval we want to use the longest QT on the 12 lead.  It is best to select complexes not exhibiting U waves (repolarization of the Purkinje fibers) but if you must, there are ways to measure that too.  It's worth noting that bradycardia increases the amplitude of U waves, making them appear in 90% of cases with heart rates less than 65! 

QT interval pathology is determined at a heart rate of 60 beats per minute.  Obviously not all heart rates are found at that pace and we must therefore calculate what's called the "corrected QT interval" (QTc).  There are several formulas for finding the QTc, most of which involve symbols that look like more Egyptian hieroglyphs to me than math, online calculators, and even apps for your phone if you're not mathematically inclined.  Thankfully for me, our company uses Lifepack15s and it's clearly displayed near the PR interval.  

QTc is defined as "prolonged" if it is >440ms in men and >460ms in women and is associated with risk of torsades de pointes when greater than 500ms.

There are many causes of long Qtc such as hypokalemia, hypomagnesemia, hypocalcemia, hypothermia, myocardial ischemia, post-cardiac arrest, congenital long QT syndrome, and even medications like Digoxin.


TL;DR

• The QT interval is the depolarizing and repolarizing of the ventricles.
• A long QT interval leads to potentially lethal ventricular arrhythmias.
• Pathology is determined by the "corrected QT interval" (QTc) and is listed just under the PR interval on our Lifepacks. 
• QTc >500ms is associated with risk of torsades de pointes.
• U waves appear in 90% of cases of heart rates less than 65. 
• Many causes such as hypokalemia, hypomagnesemia, hypocalcemia, hypothermia, myocardial ischemia, post-cardiac arrest, congenital long QT syndrome, and even medications like Digoxin.

References:
Lifeinthefastlane.com
American Heart Association
Journal of American College of Cardiology