Cardiac Refractory Periods

Question 1

What is the absolute refractory period?

Answer 1

The time where the channels for the upstroke of the AP are fully inactive.

Question 2

What is the relative refractory period?

Answer 2

The time where the channels for the AP are partly active and abnormal APs can occur.

Question 3

What is the refractory period of fast response Na channels dependent on?

Answer 3

Voltage-dependence

Question 4

What is the refractory period of slow response Ca channels dependent on?

Answer 4

Time-dependence

Question 5

What is R on T phenomena?

Answer 5

R on T phenomena occurs when a premature beat (R wave) occurs during the relative refractory period (T wave) of previous beat. It is an arrhythmia with the beat occurring in a vulnerable period.

Question 6

How do the characteristics of the AV node protect it from rapid atrial rhythms?

Answer 6

The AV node has post-depolarization refractoriness which is a slow response refractory period that outlasts the action potential. Even though the voltage repolarizes, the cell remains refractory, which prevents rapid ventricle activation during atrial fibrillation.

Question 7

During atrial fibrillation, what is deterring the ventricular rate?

Answer 7

The ventricular rate is determined by the AV node refractory period.

Question 8

How do you slow down the heart rate in cases of atrial fibrillation?

Answer 8

Give the patient beta blockers or Ca2+ channel blockers.

Question 9

What is the effect of the heart rate on the interval-duration relationship? How does it affect the QT interval?

Answer 9

The interval-duration relationship implies that the heart rate will determine the length of the cardiac action potential. As heart rate increases the AP duration (systole) will decrease in order to help restore filling (diastole).

The QT interval decreases as the HR increases.

Question 10

What is prolonged QT syndrome and what is the cause of it?

Answer 10

It shows as a lengthened systole and can be acquired through various mechanisms:

• bradycardia
• hypokalemia
• quinidine

It can also be congenital due to malfunctions in the Na/K channels and it can lead to torsades de pointes, which is polymorphic ventricular tachycardia.

Question 11

What is a major factor that can lengthen the action potential?

Answer 11

Hypokalemia. Decreased K+ permeability will prolong the action potential duration.