Cardiac Refractory Periods

Question 1

Explain the interval-duration relationship.

Answer 1

The duration of the action potential varies with the cycle length (the interval between
successive beats). For example, as heart rate increases (corresponding to a decrease in interval), the action potential duration (systole) decreases.

Question 2

Which phase of the cardiac cycle shortens more with increased HR, systole or diastole?

Answer 2

Diastole! We see a large change in the amount of time between beats. Systole only shortens a little due to a more vigorous contraction.

Question 3

What is torsades de pointes?

Answer 3

Also known as polymorphic ventricular tachycardia, this is a condition in which the Q-T interval is abnormally prolonged, possibly resulting from the development of early afterdepolarizations (EADs).

Question 4

Which part of the EKG corresponds to the action potential duration?

Answer 4

Q-T interval

Question 5

Explain the R on T phenomenon.

Answer 5

This is when a premature beat (R wave) occurs during the relative refractory period (T wave) of the previous beat. It can eventually lead to ventricular tachycardia.

Question 6

Describes an often lethal disruption of heart rhythm that occurs as a result of a blow to the area directly over the heart (precordial region) at a critical time during the cycle of a heart beat (i.e., relative refractory period), causing cardiac arrest.

Answer 6

commotio cordis

Question 7

Commotio cordis is a form of ________ and NOT due to ______ or _______.

Answer 7

ventricular fibrillation; mechanical damage to the heart muscle itself; heart disease

Question 8

What are two possible causes of prolonged Q-T syndrome?

Answer 8

• acquired causes (bradycardia, hypokalemia, drugs)

- congenital causes (genetic lesions in Na+ and/or K+ channels)

Question 9

In slow responses, _______ outlasts ________.

Answer 9

refractoriness; action potential duration