What Is Refraction Rate of Heart?

There are two different refractory rates associated with the heart's electrical activity. One is known as the absolute refractory period, and the other is known as the relative refractory period.

The Absolute Refractory Period Defined

• The absolute refractory period (ARP) begins after the QRS complex has depolarized. The QRS is the waveform that generally contains the wave of greatest amplitude, in this case the R-wave. The ARP represents a duration in the cardiac depolarization-repolarization cycle where the cardiac muscle has spent its supply of action potential, thus it is an absolute that the heart cannot fire again during this period. The absolute refractory period ends in the middle of the following T-wave crest. That is to say, if the T-wave is split in half, the first half is considered part of the ARP.

The Relative Refractory Period Defined

• The relative refractory period (RRP) begins at the crest of the T-wave and continues until the waveform returns to a resting position on EKG after the depolarization of the atrium, which is denoted by the P-wave deflection, but before the Q-wave shows a negative deflection. The RRP earns its name because it represents a duration in the cardiac depolarization-repolarization cycle where, if given the appropriate stimulus, the cardiac muscle could have enough stored energy potential to cause a second ventricular depolarization (QRS complex) that could then lead to an arrhythmia.

Measuring the Absolute Refractory Period

• In measuring the absolute refractory period, the measuring end of a ruler or the prong of a caliper would be placed at the point in the timing cycle where the R-wave makes a positive deflection from the isoelectric line to the crest of the T-wave. This measurement can vary based on heart rate, but is the sum of the time the R-wave is formed added to the time until the T-wave peaks as expressed in milliseconds.

Measuring the Relative Refractory Period

• In measuring the relative refractory period, the measuring end of a ruler or the prong of a caliper would be placed at the point in the timing cycle where the T-wave crests. Once the T-wave has crested, the resting action potential is charged for the next ventricular (QRS complex). The sum of milliseconds from the T-wave peak to the subsequent moment the Q-wave begins a negative deflection from the isoelectric line is the duration of time the ventricles hold their polarized state.

Phenomenon Associated with the Relative Refractory Period

• An occurrence known as R-on-T phenomenon is directly related to the relative refractory period of the heart. As stated before, the crest of the T-wave signals the RRP period has begun. If a rogue electrical impulse fires during this time, specifically from the ventricles, an R-wave can strike on a T-wave (hence R-on-T) and can cause the heart to fall out of the depolarization-repolarization cycle. Most commonly, this phenomenon will cause ventricular fibrillation, an arrhythmia incompatible with life.