Health: Quantifying Normal Variation in EKG’s

One of the factors the Federal Drug Administration (FDA) looks for when evaluating a drug is the potential for that drug to induce a cardiac arrhythmia. When a drug is administered to a patient, some of them can interfere with the normal electrical signals of the heart. In a normal heartbeat, the heart polarizes and then depolarizes, which makes the heart muscle contract then return to normal. The EKG (also known as an ECG) monitors the polarization-depolarization signal. Drugs are screened to detect whether there is any adverse effect of the depolarization of the heart. The screeners look at the termination of what is called the T-wave. The T-wave is the final visible wave in the ECG, which follows the “active phase” for what is called the QRS-complex. If the depolarization fails to complete, or if there is an over-extension of this process, the heart carries a risk of fibrillation or of potentially lethal arrhythmias.

The methodology that is currently used dates back to the 1920s method of measuring the duration of the QT interval. A strip of one to ten minutes of the EKG is looked at and a series of three “typical beats” is identified and a single measurement is made of the middle beat. The QT interval prolongation is used as a biomarker. If there is a prolongation found in the reading, then concern is raised that the drug may cause an arrhythmia. Currently, the physician is the one who decides where the end of the T wave occurs. There is not a standard measurement for this. The control subject’s heart rate also varies greatly, even when the person is in a fairly controlled environment.

The National Institute of Statistical Sciences is developing a new methodology for this measurement.