The cardiac cycle is comprised of three events: depolarization, refractory period and repolarization. Basically the heart beats, after which it cannot do anything for a brief period of time, and then it recharges for another beat. The resting phase of a cardiac cycle is called the refractory period. Events that occur in the resting phase of a cardiac cycle center on cellular activity.
Polarized Myocardial Cell
Heart cells are called myocardial cells. Linda Wright, RN, CCRN, explains that heart cell membranes have special channels that control the flow of sodium, potassium and calcium through the cell membrane. Sodium, potassium and calcium are electrolytes that are responsible for the myocardial cell’s ability to contract, or beat.
A polarized myocardial cell is one that is rested and fully charged. All electrolytes are in the correct place. Sodium is outside surrounding the cell, and potassium and calcium are inside the cell. For myocardial cells to beat properly, there must be balance between these electrolytes. When a cell is polarized, it has the ability for an action potential. Simply put, it has the capacity to beat.
Once a signal is received, the myocardial cell contracts. This is called depolarization. Donna D. Ignatavicius, MS RN, and M. Linda Workman, Ph.D., authors of “Medical-Surgical Nursing: Critical Thinking for Collaborative Care,” explain that during this process, the sodium, potassium and calcium switch places by crossing the cell membrane through the appropriate channels. On an electrocardiogram, depolarization is shown by a large spike on a rhythm strip. This spike is referred to as the QRS of the heart beat. Following this spike is a smaller bump, which is called the T wave. The T wave represents the refractory period.
The refractory period, or resting phase of a cardiac cycle, is made up of two parts. The absolute refractory period is the time frame immediately after the myocardial cells depolarize until about halfway through the resting phase. Richard Klabunde, Ph.D., explains that on the ECG, the absolute refractory period begins right after the QRS and ends halfway through the T wave. During this period the myocardial cells are not capable of any activity.
The relative refractory period is the last part of the T wave. The sodium-potassium channels are opening, and the electrolytes are just beginning to move across the cell membrane again. During this time, if an electrical impulse reaches the cell, it may cause a contraction; however, the beat would be an abnormal one because the cell is not fully charged yet.
Repolarization is the completion of the relative refractory phase. In other words, the electrolytes are moving across the cell membrane to where they need to be; sodium moves back out of the cell, and potassium and calcium moves back in to the cell. The channels close and the myocardial cell is once again polarized.