101014 mechanisms of arrhythmia

Question 1

supraventricular tachycardia

Answer 1

abnormal tachy which requires participation of either atrial or AV nodal tissue

when chaotic, it’s referred to as atrial fib and not SVT

Question 2

ventricular tachycardia

Answer 2

abnormal tachy originating in ventricle or His-PUrkinje system

by def, doesn’t require involvement of either atrium or Av NODE

when chaotic-referred to as ventricular fib

Question 3

mechanism of overdrive suppression

Answer 3

in spontaneously active pacemaker cells, hyperpolarizing current of NaK-ATPase is offset by pacemaker current If. however, in passively activated slower pacemaker cells, If is much smaller leading to net hyperpolarization. hyperpolarization leads to slow recovery after overdrive of pacemaker cells by faster foci.

Question 4

how does sympathetics affect automaticity?

Answer 4

beta stimulation leads to increasing the open probability of pacemaker current If

also makes threshold potential more negative

Question 5

mechanisms of cellular tachycardia

Answer 5

enhanced automaticity
abnormal automaticity-abnormal impulse formation in tissue not normally capable of pacemaker activity

triggered activity (due to oscillations in membrane potential). can be triggered by preceding tachy or brady

Question 6

what is early afterdepolarization promoted by

Answer 6

conditions that prolong the AP (QT prolongation on ECG)

Question 7

delayed afterdepolarization is promoted by

Answer 7

conditions which lead to HIGH INTRACELLULAR CALCIUM

promoted by catecholamines
inhibited by calcium ch blockers

mechanism of idiopathic VT
mechanism of digitalis toxicity

Question 8

conduction block

Answer 8

occurs when a propagating wavefront encounters tissue which is unexcitable

permanent or fixed-all impulses fail
intermittent-ability to conduct is variable and changes with time
functional-block may be present only at critical rates which are faster than tissue refractory period will accomodate

Question 9

block in the AV node or His bundle can result in interruption of AV conduction–what are the different degrees?

Answer 9

1st degree-delay w/o failure of conduction
2nd degree-some but not all beats fail to conduct
3rd deg-no propagation from atrium to ventricle (also known as complete heart block)

Question 10

how is reentry different from automatic and triggered tachycardias?

Answer 10

automatic and triggered tachycardias are cellular phenomena, but reentry occurs in myocardial tissue composed of many myocytes working in sequence

Question 11

how does reentry occur?

Answer 11

with presence of abnormal conduction (conduction block and slowed conduction)

Question 12

requirements for reentry

Answer 12

two distinct paths for propagation
slowed conduction in at least one path
unidirectional block (tissue capable of conduction in one but not the opposite direction)

Question 13

ex of reentry

Answer 13

paroxysmal SVT (in normal pts)
WPW
ventricular fibrillation (in ischemic heart tissue, there's scars)

Question 14

all arrhytmias arise from

Answer 14

altered impulse formation or impulse conduction