Heart Lecture 5: Mechanism of Cardiac Arrhythmias

Question 1

Cardiac arrhythmias result from disturbances in impulse _________ or __________

Answer 1

formation; conduction

Question 2

What are the three mechanisms of cardiac dysrhythmias?

Answer 2

1) altered automaticity (meaning mediated by pacemaker conduction system)
2) re-entry of excitation (ALWAYS pathological - conduction problem)
3) triggered activity (ALWAYS pathological - formation problem)

Question 3

What does automaticity mean?

Answer 3

that the impulse is traveling through the normal, automatic pacing system

Question 4

Tachycardia definition

Answer 4

heart rate over 100bpm

Question 5

Bradycardia definition

Answer 5

heart rate under 60bpm

Question 6

What causes tachycardia?

Answer 6

1) sympathetic nervous activity (NE stimulation)
2) stimulants/amphetamines (like caffeine)
3) ischemia (current of injury)
4) stretching (ventricular aneurysm
5) sick sinus syndrom, fever, hyperthyroidism

Question 7

What causes bradycardia?

Answer 7

1) drugs (anti-arrhythmics, beta blockers, Ca++ antagonists, digitalis)
2) barbituates, anesthetics
3) ischemia or infact
4) sick sinus syndrome
5) aging (fibrosis)

Question 8

Where does re-entry of excitation occur?

Answer 8

Anywhere in the heart

Question 9

What are the 3 requirements for re-entry of excitation?

Answer 9

1) geometry for conduction loop
2) slow or delayed conduction
3) unidirectional conduction block

Question 10

What causes re-entry?

Answer 10

1) ischemia (MOST COMMONly found after MI)
2) infarction (cell death - plugs up part of heart)
3) congenital bypass tracts (Wolf-Parkinson White)

Question 11

Describe the flow of re-entry

Answer 11

1) impulse comes down on tissue
2) hits blockade but AP still occurs right before damaged tissue
3) meanwhile, finds excitable tissue and conducts normally
4) go through damaged region (which has greater membrane potential so some Na+ channels are already activated) and has slow conduction so AP is just barely longer than the one that occurred neighboring it. Therefore, once it ends, the area next to it is excitable again

Question 12

What is Wolf-Parkinson-White?

Answer 12

A congenital abnormality that connects right atria to right ventricle (has functional characteristics of Purkinje fibers - aka rapid conduction and long refractory periods)

Question 13

SVT travels _______ through the AV node and _________ through the bypass tract

Answer 13

anterograde; retrograde

Question 14

What is triggered activity?

Answer 14

random impulse formation

Question 15

What is a delayed afterdepolarization (DAD)?

Answer 15

a transient release of Ca++ from the SR that leads to abnormally elevated intracellular [Ca++] which increases strength of contraction

Question 16

What drug causes DADs?

Answer 16

digitalis

Question 17

What else causes DADs?

Answer 17

elevated catecholamines (increased sympathetic stimulation), rapid heart rate

Question 18

What do DADs look like on EKGs?

Answer 18

PACs or PVCs

Question 19

DADs happen when SR inappropriately releases Ca++ after AP is over. Ca++ is picked up by exchanger so now there is more _______ current

Answer 19

inward (3 Na+ come in, 1 Ca++ goes out)

Question 20

What is Early Afterdepolarization (EAD)?

Answer 20

related to prolongation of AP duration and likely due to an abnormal re-activation of slow inward Ca++ current

Question 21

What causes EADs?

Answer 21

acidosis, hypokalemia, quinidine, slow heart rate

Question 22

What do EADs look like on EKGs?

Answer 22

PACs and PVCs

Question 23

Length of PR interval

Answer 23

120-200ms

Question 24

Length of QRS complex

Answer 24

70-100ms

Question 25

Length of QT interval

Answer 25

250-430ms

Question 26

Tachycardia cycle length

Answer 26

less than 600ms

Question 27

bradycardia cycle length

Answer 27

greater than 1000ms