L7 Cardiac Refractory Periods

Question 0

Relative refractory period

Answer 0

channels are partially recovered so a strong enough stimulus could make an AP

VULNERABLE PERIOD

Question 1

Absolute refractory periods

Answer 1

Channels are inactive and cannot make an AP

Question 2

Refractory periods are due to

Answer 2

the voltage and time dependence of Na and Ca channels

Ca takes longer, which is why slow response channels have a longer refractory period (RP)

Question 3

Ventricular muscle refractory period on an EKG is..

Answer 3

the last segment of the T wave, the down slope

Question 4

R on T

Answer 4

A premature ventricular beat during the last part of the T wave, during the RP of the previous AP.
After one R on T, subsequent PVCs result from re-entry

Can lead to Torsades de Pointes

Question 5

AV node refractory characteristics

Answer 5

AV node uses slow response channel. This channel has a long refractory period that prevents fast and irregular impulses like a-fib from the SA node to pass through to the vents. it’s like a filter

Question 6

What determines ventricular rate and rhythm during a-fib?

Answer 6

In a-fib, the atria are beating irregularly, and this overwhelms the AV node with impulses. The AV node can’t filter them all out, so some random, irregular beats make it through and cause contraction of the ventricle.

Question 7

How do you slow the HR of a person in a-fib?

Answer 7

Increase vagal stimulation to increase the refractory period and slow HR

• valsalva maneuver
• plug carotid sinus to simulate high BP
• cold press on face to stimulate diving receptors

Question 8

Interval-duration relationship

Answer 8

The HR determines the duration of the AP.
Systole decreases slightly to conserve some diastole so the heart can still fill completely. Diastole shrinks more tho

On EKG, QT interval decreases, PR interval stays the same

AP duration = QT interval

Question 9

Prolonged QT syndrome

Answer 9

Definition: abnormal prolongation of QT interval (the AP duration)
Causes: bradycardia, hypokalemia (makes hard to get AP), anti-arrhythmic drugs, congenital lesions in Na or K channels.

Can initiate a torsades de pointes, polymorphic tachyarrhythmia. This is when the conduction in the ventricles is not going thru his/purk but going around the heart in a sphere pattern
Could also be due to EADs