Normal pacemaker sinus node of the heart: regulation of excitation and conduction
The sinus node controls the heart's rhythm because its rhythmic discharge rate is faster than any other part of the heart. Therefore, the sinus node is almost always pacing the heart normally.
Generation and transmission of cardiac impulses through the heart, we note that normal impulses arise in the sinus node.
In some abnormal states, this is not the case. Other parts of the heart may also exhibit intrinsic rhythmic stimulation in the same way that sinus nodal fibre does; This ability is especially true in the AV node and Purkinje fibre.
AV nodal fibre, when not stimulated from some external source, discharge at an intrinsic rhythmic rate of 40 to 60 times per minute, and Purkinje fibre discharge at a rate of approximately 15 to 40 times per minute. These rates contrast with the normal sinus node rate of 70 to 80 beats per minute.
Why in this case does the sinus node and not the AV node or the Purkinje fibre regulate the rhythm of the heart? The answer comes from the fact that the discharge rate of the sinus node is significantly faster than the spontaneous self-excited discharge rate of either the AV node or the Purkinje fibre. Each time the sinus node discharges, its impulses are conducted into both the AV node and the Purkinje fibre, which also discharge electrically stimulating their cell membranes. However, the sinus node discharges again before either the AV node or the Purkinje fibre can reach its own threshold of self-excitation.
Thus, a new impulse from the sinus node that discharges both the AV node and the Purkinje fibre before self-excitation can occur in either of these sites.
As such, the sinus node controls the heart's rhythm because its rhythmic discharge rate is faster than any other part of the heart. Therefore, the sinus node is almost always pacing the heart normally.
Sometimes some other part of the heart develops a rhythmic discharge rate that is much faster than the sinus node. For example, this growth sometimes occurs in the AV node or in the Purkinje fibre when one of them becomes abnormal. In both cases, the pacing drive of the heart varies from the sinus node to the AV node or the excitatory Purkinje fibre. In more rare conditions, a site in the atrial or ventricular muscle develops that is prone to overstimulation and becomes a pacemaker.
A pacemaker located elsewhere than the sinus node is called an “ectopic” pacing. An ectopic pacemaker causes an abnormal sequence of contractions of different parts of the heart and can cause significant impairment of cardiac pumping.
Another cause of pacing shift is obstruction of the conduction of cardiac impulses from the sinus node to other parts of the heart. This new pacing drive then occurs most often at the AV node or in the transverse portion of the AV bundle en route to the ventricles.
When atrioventricular obstruction occurs, that is, when cardiac impulses fail to pass from the atria into the ventricles through the AV node and bundle system, the atria continue to beat at the normal rhythmic rate of the sinus node, during when a new pacemaker normally develops in the Purkinje system of the ventricles and the ventricular motor muscles at a new rate somewhere between 15 and 40 beats per minute. After sudden blockage of the AV bundle, the Purkinje system does not begin to emit its intrinsic rhythmic impulses until 5–20s later because, prior to obstruction, the Purkinje fibres have been “overdone” loaded” by rapid sinus impulses and consequently is in an inhibited state. During those 5-20 seconds, the heart fails to pump blood, and the patient faints after the first 4-5 seconds because of a lack of blood flow to the brain. This delay of the heartbeat is called Adams - Stokes syndrome.