Anectopic pacemaker, also known asectopic focus or ectopic foci, is a group ofexcitable cells that causes apremature heart beat known as anectopic beat, outside the normally functioningSA node of theheart. It is thus acardiac pacemaker that isectopic, producing anectopic beat. Acute occurrence is usually non-life-threatening, but chronic occurrence can progress intotachycardia,[1]bradycardia orventricular fibrillation.[2] In a normal heart beat rhythm, the SA node usually suppresses the ectopic pacemaker activity due to the higher impulse rate of the SA node. However, in the instance of either a malfunctioning SA node or an ectopic focus bearing an intrinsic rate superior to SA node rate, ectopic pacemaker activity may take over the natural heart rhythm.[3] This phenomenon (an intrinsically slower pacemaker activity being unmasked by failure of faster pacemaker tissue 'upstream') is called an escape rhythm, the lower rhythm having escaped from the dominance of the upper rhythm. As a rule, premature ectopic beats (i.e. with a shorter than the prevailing preceding R-R' interval) indicate increased myocyte or conducting tissue excitability, whereas late ectopic beats (i.e. with a prolonged preceding R-R' interval) indicate proximal pacemaker or conduction failure with an escape 'ectopic' beat.
Isolated ectopic beats frequently cause no symptoms, although the most common symptom is the perception of a 'missed beat'. This occurs because the person notices the prolonged gap between the early (ectopic) beat and the next normal beat.
Ectopic pacemakers can occur within healthy hearts in response to various stimulating events, they can be caused byautomaticity or triggered activity, such as:
CardiacIschemia (particularly ventricular ischemia) – the membranes of apoptotic (dying) cells become "leaky" and cause surrounding tissue to becomehyperkalemic orhypercalcemic (high concentration of potassium/calcium), causing random excitation.[7]
They can also occur within unhealthy hearts, caused by:
An ectopic pacemaker can reside within a part of theelectrical conduction system of the heart, or within themuscle cells of theatria orventricles. 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. Location of the pacemaker can also change its effect on the SA node and its rhythm. An ectopic pacemaker located in the atria is known as an atrial pacemaker and can cause the atrial contraction to be faster.[10] An ectopic pacemaker situated near theAV node and theseptum is known as ajunctional pacemaker.[11] The pacemaker that is operating in the ventricles is known as the ventricular.[12] Other such ectopic pacemakers can even lie within thepulmonary vein and thoracic vein walls.[13][14]
On anECG, theQRS complex will be abnormally shaped when looking at ventricular ectopic activity, often it occurs earlier with an absent P wave. It can be perceived as a skipped beat on both the ECG and through normal pulse-taking.[12]During atrial ectopic activity where the P wave is normally rounded can be inverted or peaked. However the QRS complex and T waves appear relatively normal.[10]Conversely, during junctional ectopic activity the P wave is frequently absent or can be hidden in the QRS complex.[11]
^Tveito, Aslak; Lines, Glenn T. (2008). "A condition for setting off ectopic waves in computational models of excitable cells".Mathematical Biosciences.213 (2):141–50.doi:10.1016/j.mbs.2008.04.001.PMID18539188.
^abcRozanski, GJ (1991). "Atrial ectopic pacemaker escape mediated by phasic vagal nerve activity".The American Journal of Physiology.260 (5 Pt 2): H1507–14.doi:10.1152/ajpheart.1991.260.5.H1507.PMID2035673.
