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Purkinje fibers also have the ability of firing at a rate of 20–40 beats per minute if upstream conduction or pacemaking ability is compromised. [9] In contrast, the SA node in normal state can fire at 60-100 beats per minute. [9] In short, they generate action potentials, but at a slower rate than the sinoatrial node. [9]
An impulse (action potential) that originates from the SA node at a relative rate of 60–100 bpm is known as a normal sinus rhythm. If SA nodal impulses occur at a rate less than 60 bpm, the heart rhythm is known as sinus bradycardia. If SA nodal impulses occur at a rate exceeding 100 bpm, the consequent rapid heart rate is sinus tachycardia ...
The left and right bundle branches, and the Purkinje fibers, will also produce a spontaneous action potential at a rate of 30–40 beats per minute, so if the SA and AV node both fail to function, these cells can become pacemakers. These cells will be initiating action potentials and contraction at a much lower rate than the primary or ...
Purkinje fibres: 20–40 bpm; The potentials will normally travel in order SA node → Atrioventricular node → Purkinje fibres Normally, all the foci will end up firing at the SA node rate, not their intrinsic rate in a phenomenon known as overdrive-suppression. Thus, in the normal, healthy heart, only the SA node intrinsic rate is observable.
The AV node's normal intrinsic firing rate without stimulation (such as that from the SA node) is 40–60 times/minute. [13] This property is important because loss of the conduction system before the AV node should still result in pacing of the ventricles by the slower pacemaking ability of the AV node.
If the AV node were blocked, the atrioventricular bundle would fire at a rate of approximately 30–40 impulses per minute. The bundle branches would have an inherent rate of 20–30 impulses per minute, and the Purkinje fibers would fire at 15–20 impulses per minute.
The slope of phase 0 on the action potential waveform (see figure 2) represents the maximum rate of voltage change of the cardiac action potential and is known as dV/dt max. In pacemaker cells (e.g. sinoatrial node cells), however, the increase in membrane voltage is mainly due to activation of L-type calcium channels.
The first finding is that junctional rhythms are regular rhythms. This means that the time interval between beats stays constant. The next normal finding is a normal QRS. Since the impulse still travels down the bundle of His, the QRS will not be wide. Junctional rhythms can present with either bradycardia, a normal heart rate, or tachycardia. [9]