The Electrical Conduction System:
Cardiac muscle has 2 unique properties that predispose it to arrhythmias: automaticity and gap junction transmission. Cardiac smooth muscle cells, unlike skeletal muscle cells, exhibit spontaneous depolarization and are able to transmit electrical signals from one cell to another via gap junctions. This property is beneficial because it allows the heart to depolarize and propagate the electrical impulse without the need for central nervous system initiation.
The interiors of myocytes or heart muscle cells are negative or “polarized” at rest. The release of free calcium (Ca2+) ions into the interior of the myocytes causes the interior of the cell to become positive or “depolarized” and stimulates them to contract. The cell-to-cell conduction of depolarization through the myocardium is carried by fast-moving sodium (Na+1) ions. Repolarization is the electrical phenomenon that occurs immediately after depolarization, when the interiors of the myocytes once again become negative due to outflow of potassium (K+) ions from the myocytes. Repolarization occurs so that the myocytes can recover their resting negative charge and be depolarized again.
The SA (sinoatrial) node is the hearts dominant pacemaker and its pacing activity is known as “sinus rhythm”. It initiates a wave of depolarization that spreads outward, stimulating the atria to contract. It is located in the upper posterior wall of the right atrium and initiates depolarization in regular intervals. Each depolarization wave generated by the SA node spreads through both atria and appears as a p-wave on the EKG. The p-wave represents atrial depolarization and contraction. This simultaneous contraction of the atria forces the blood they contain to pass through the AV valves (mitral and tricuspid) into the ventricles. These AV valves (tricuspid and mitral) prevent the backflow of blood into the atria from the ventricles and electrically insulate the atria from the ventricles, leaving the AV node as the sole conduction pathway between the atria and ventricles. The AV node is just above and continuous with a specialized conduction system that distributes depolarization to the ventricles very efficiently. When the wave of depolarization enters the AV node, depolarization slows, producing a brief pause and allowing time for the blood in the atria to enter into the ventricles. This slow conduction through the AV node is carried by calcium ions. After the wave of depolarization is conducted through the AV node, it shoots rapidly through the ventricular conducting system beginning with the HIS bundle and right and left bundle branches which are bundles of rapidly conducting “purkinje fibers” that spread out just beneath the endocardium and directly depolarize ventricular myocytes. Depolarization of the ventricles produces a QRS complex on the EKG.
Atrial fibrillation is considered to be a reentry arrhythmia or a disorder of impulse transmission. Reentry requires at least 2 conduction pathways with a variable block in one of the pathways. If the 2 pathways for conduction have similar conduction velocities (speeds), the electrical impulses will merge distally (downstream) and no arrhythmia will occur. If an event (premature atrial contraction, etc) occurs at the right time to make on the these 2 conduction pathways refractory (still depolarized or positive, not repolarized or negative), the impulse will be blocked in that pathway and not able to travel through. Meanwhile, the electrical impulse that traveled down the other conduction pathway, may continue back up the previously refractory pathway, which is now repolarized and ready to conduct and the arrhythmia will sustain itself.
Phew! that is a ton of high tech mumbo jumbo right? Especial for people in Scranton, PA.
Rest assure, the next couple are a little toned down…