Ectopic pacemaker

An ectopic pacemaker or ectopic focus is an excitable group of cells that causes a premature heart beat outside the normally functioning SA node of the human heart. Acute occurrence is usually non-life threatening, but chronic occurrence can progress into ventricular fibrillation.

Etiology
Ectopic pacemakers can occur within healthy hearts in response to various stimulating events, such as:
 * Elevated sympathetic nervous system input
 * Overstimulation from drugs such as caffeine.
 * Cardiac Ischemia (particularly ventricular ischemia) - The membranes of apoptotic (dying) cells become "leaky" and cause surrounding tissue to become hyperkalemic or hypercalcemic (high concentration of potassium/calcium), causing random excitation.

Physiology
An ectopic pacemaker can reside within a part of the conduction system of the heart, or within the muscle cells of the atria or ventricles. When an ectopic pacemaker initiates a beat, premature contraction occurs. A premature contraction will not follow the normal signal transduction pathway, and can render the heart refractory or incapable of transmitting the normal signal from the SA node.

Diagnosis
On an ECG, the QRS complex will be abnormally shaped. It can be perceived as a skipped beat on both the ECG and through normal pulse-taking.