Congenital long QT (ECG)

The QT interval is measured (conventionally in lead II) from the onset of the QRS complex to the end of the T wave. As the QT interval varies with heart rate, it should be corrected (QTc) to a heart rate of 60 beats per minute by dividing the measured QT interval (QT) by the square root of the R-R interval. The upper limit of normal is 440 ms (0.44 s). In the illustration, the ECG machine has automatically measured the QT as 510 ms and calculated the QTc as 474 ms. (Illustration courtesy of Dr. Mike Blackburn, Consultant Pediatric and Adult Cardiologist, Leeds General Infirmary, Leeds, UK.)