Learn it well Demo Chapter 2: Coordinating the Action Potential with the Surface EKG

From the upper portion of Fig. 1 follow the Cardiac Action Potential. At the Resting potential (Phase 4 of the Action Potential) where the charge is -90 mV it is clear that it has a negative polarity. Depolarization occurs when this negative charge is overcome by movement of mostly sodium ions and fewer calcium ions into the cell during Phase 0 making the charge +25 mV (a bit of overshoot) that is corrected to a charge of approximately 0 mV with closure of the sodium channels in Phase 1. This change in charge from -90 mV to 0 mV from Phase 4 through Phase 1 is true depolarization (or loss of polarity).

During Phase 2 calcium ions enter the cell as potassium ions exit and during Phase 3 potassium ions exit the cell, ultimately regaining a negative charge of -90 mV in Phase 4. This is called Repolarization and indeed the cell is again polarized.

Adhesive electrodes placed on the skin can detect electrical activity of the heart when attached to an electrocardiograph (the instrument that records the electrocardiogram or EKG). The electrocardiograph inscribes a positive (upward) deflection on the EKG tracing when a wave of positive (+) charges passes the location of the electrode on the skin.

The surface electrocardiogram (EKG) in the lower portion of Fig. 1 has the same timing from Q to R to S to T and corresponds with the timing from Phase 0 to Phase 4 on the Cardiac Action Potential. The EKG is the sum of all of the action potentials in the heart at any instant. The QRS represents Depolarization and the ST segment and T wave represent Repolarization. The PR will be discussed in Chapter 3 and the QT interval in Chapter 17.

Refractory Period: Once the cardiac action potential initiates there is a period of time that the cardiac cell is unable to initiate another action potential. This period of time is slightly shorter than the action potential duration (Fig. 1) and is called the refractory period. If a stimulus occurs during the refractory period it cannot be conducted. The refractory period is cardio-protective as it can prevent different forms of reentrant tachycardia.

The refractory period varies in different locations of the heart (i.e.: AV node or ventricular cells).

The refractory period and the QT interval on the EKG have approximately the same duration and can be influenced by electrolyte disturbances and side effects from drugs such as certain cardiac antiarrhythmic drugs, antibiotics, antidepressants and others. This is addressed in Chapter 17.

Examples in electrocardiography where the refractory period applies include blocked APCs (Fig. 18), functional (rate-related) aberrant intraventricular conduction (Fig. 92) and determining why an implanted ventricular pacemaker impulse that falls in the refractory period fails to capture (Fig. 86).