Cardiac Arrhythmias Flashcards
What is automaticity?
A cell’s ability to spontaneously depolarize to a threshold voltage to generate an action potential.
What are the normal pacemaker cells?
SA node, AV node, bundle of His, bundle branches, purkinji fibres.
What two factors are responsible for cardiac arrhythmias.
Problems with impulse formation or problems with impulse conduction.
What is the pacemaker current?
An ionic current responsible for phase 4 spontaneous depolarization in pacemaker cells. Channels open when the membrane voltage becomes more negative than approx -50 and allow inward flow of Na+ through slow channels.
What is the threshold potential in pacemaker cells?
Approx -40mV
What is the native and latent pacemakers and what are their firing rates?
Native - SA node: 60-100bpm
Latent - AV node and bundle of His: 50-60bpm
Purkinje system: 30-40 bpm
What 3 factors determine cell firing rates?
- A reduced pacemaker current causes the slope of phase 4 to be less steep.
- A more negative maximum diastolic potential (lowest mV) increases the time to reach threshold potential.
- A change in threshold potential. Less negative = increased time to reach it.
What is the most important modulator of normal sinus mode automaticity?
The autonomic nervous system by:
The SNS increasing the open probability of pacemaker channels which leads to a steeper slope of phase 4 depolarization. Leading to the SA node reaching threshold faster, thus increasing HR.
Additionally the SNS decreasing the threshold potential by ensuring that coltage-sensitive CA++ channels are capable of opening.
How do beta-blockers impact firing of the SA node?
They antagonize the B-adrenergic sympathetic effect therefore decreasing the rate of phase 4 depolarization of the SA node.
How does atropine impact firing of the SA node?
As an anticholinergic (antimuscarinic) it blocks parasympathetic activity and increases the rate of phase 4 depolarization which increases the heart rate.
Which tissue is most sensitive to parasympathetic stimulation?
The SA node and AV node, followed by atrial tissue. The ventricular conduction system is the least sensitive.
Therefore parasympathetic stimulation can slow the sinus rate allowing the pacemaker to shift.
What can cause ectopic beats?
Increased sympathetic stimulation can increase automaticity of latent pacemakers. Additionally, hypoxemia, schema, electrolyte disturbances, and certain drugs (digitalis).
What is the difference between a functional and fixed conduction block?
Functional is due to an area that is still refractory from a previous depolarization.
Fixed is due to an area of fibrosis or scarring.
Two critical conditions for re-entry
Unidirectional block
Slower conduction through the reentry path.
