Cardiac Refractory Periods (L7) Flashcards
Define Effective Refractory Period
Channels responsible for the action potential upstroke are completely inactivated and therefore no action potential can be elicited during this time
Define Relative Refractory Period
Channels responsible for the action potential upstroke are partially recovered and therefore abnormal action potentials can be elicited at this time
What are refractory periods due to?
Voltage and time dependent Na+ (fast response) and Ca2+ (slow response) channels
For a fast response, is it primarily voltage- or time-dependent refractoriness?
Fast response is primarily voltage-dependent refractoriness. Once its repolarized, its ready to go again
For a slow response, is it primarily voltage- or time-dependent refractoriness?
Primarily time-dependent refractoriness. Even after repolarization, its still refractory
How can a premature beat that happens during the relative refractory period lead to further dysrhythmic activity?
The earlier the premature beat is during the relative refractory period, the smaller the premature action potential amplitude and rate of rise is. So the conduction throughout the heart is abnormally slow and can lead to further dysrhythmic activity, as it can lead to reentry of excitation. This is the “vulnerable” period of the heart
What is an R on T and how can this affect the heart?
A premature beat (R wave) that occurs during the relative refractory period (T wave) of the previous beat. This can lead to arrhythmias, or PVCs due to reentry of excitation, as well as torsades de points
What is Commotio Cordis?
Agitation of the heart. When you hit the area above the heart during the t-wave, causes cardiac arrest. Its a form of ventricular fibrillation but not due to mechanical damage to the heart muscle or surrounding organs, and not a result of heart disease
What is Post-repolarization Refractoriness, why does it happen, and what is this mechanism responsible for?
When the refractory period of a slow response is significantly longer than the AV node action potential duration. AKA even though the voltage of the AP fully repolarizes, the cell is still in refractory. This happens because the refractory period of slow Ca2+ channels is more dependent on time, not voltage. This mechanism is responsible for why conduction through the AV node slows when stimulated at higher rates, and it helps prevent rapid ventricular activation during atrial tachy-dysrhythmias (atrial fib or atrial flutter)
In atrial fibrillation, what is determining the rate and rhythm of the ventricular activation?
Its being driven by impulses through the AV node, not an ectopic pacemaker
How do you slow ventricular rate in a patient with atrial fibrillation?
You need to treat with anti-coagulants, because the patient can develop blood clots (since blood is stagnant in the appendages of the atria), as well as with drugs that will lengthen the refractory period and ablate the site of the arrhythmogenesis
What is the Interval-Duration Relationship?
The rate (interval) at which the heart is beating will determine, to some extend, the duration of the cardiac action potential. As heart rate increases, systole decreases (aka action potential duration decreases). This helps restore some of the loss in diastolic (filling) times at higher heart rates
What is Long Q-T syndrome?
Abnormal prolongation of the Q-T interval. Can be acquired, by bradycardia, hypokalemia, and drugs, or can be congenital, due to genetic lesions in the Na+ and/or K+ channels
Define Torsades de Points
Polymorphic ventricular tachycardia, results from conditions in which the Q-T interval is abnormally prolonged. Possibly results from the development of early afterdepolarizations
