|Author(s)||J.S.S.G. de Jong|
|Moderator||J.S.S.G. de jong|
|some notes about authorship|
The pacemaker cells in the sinus node are not the only cells in the heart that can depolarize spontaneously. Actually all cardiomyoctyes have this capacity. The only reason why the sinusnode 'rules' is that it is the fastest pacemaker of the heart. From sinusnode to ventricle all healthy cardiomyocytes can function as a ectopic pacemaker. Ectopic pacemaker activity can originate from the atria (60-80 bpm), AV-node (40-60 bpm) and the ventricles (20-40 bpm). So, as the sinus rate drops (e.g. during atrial infarction), other cells can take over. The configuration of ectopic beats or extrasystoles, as seen on the ECG, reveals its origin, whether they are atrial, nodal or ventrical.
|Celltype||Frequency||QRS width (*)|
|SA node (not ectopic)||60-100 bpm||narrow|
|AV Nodal ectopic pacemaker||45-50 bpm||narrow|
|His bundle||40-45 bpm||narrow|
|Bundle branch||40-45 bpm||narrow or wide|
|Purkinje cells||35-40 bpm||wide|
|Myocardial cells||30-35 bpm||wide|
(*) QRS width can only be narrow if the conduction system downstream is normal (i.e. no bundle branch block)
Compensatory or noncompensatory pause
The duration of the interval following a premature beat can help in the differentiation of a atrial or ventricula premature beat.
Non-Compensatory pause, following an atrial premature beat
If a sinusbeat is followed by an atrial premature beat, the electrical activity of the atrial premature beat can enter the sinusnode and reset its timing. If the sinusrate was for example 60 /min, the reset pacemakercells will restart firing after 1 full second has passed. The time from the extra beat to the next QRS complex in this example is thus 1 second. The sequence "normal sinus beat" - "atrial premature beat" - "normal sinus beat" will therefore shorter than the 3 seconds it would have taken if the premature beat had not occured. This is called a non (fully) compensatory pause. Sometimes the electrical activity of the APB cannot enter the sinusnode due to refractoriness of the sinus node (if the interval between previosu sinusbeat and APB is short) or due to electrical block to enter the sinus node (as in sinus nodal disease).
Fully compensatory pause, following a ventricular prematur beat
The electrical activity of a ventricular premature beat is usually not conducted through the AV node towards the atria. The sinus node will not be reset. If the next sinus beat finds the AV nodal tissue still refractory (and thus not conducting) there will be a pause until the next sinus beat follows. The interval between the premature ventricular beat and the next sinus beat will be longer (longer than 1 second in the above sample of a heart rate of 60 / min). This is called a fully compensatory pause. Using a calliper the preceding sinus rate can be tracked beyond the ventricular premature beat. This is a telltale sign of a ventricular premature beat without retrograde conduction.
Parasystole is the phenomenon that results from two competing pacemakers in the heart. Usually a supraventricular and nodal / ventricular pacemaker compete. For example sinusrhythm interfered by ventricular ectopic beats. The basic rhythm in this example would be sinusrhythm with e.g. a rate of 60/min. Interfering with this rate ectopic ventricular beats can be seen. Parasystole is diagnosed when the ectopic beats have a constant RR interval, e.g. 1500ms, resulting in a rate of 20 ventricular ectopic beats per minute. Fusion beats are often present as the two rhythms are dissociated.