L9 Mechanisms of Cardiac Arrhythmias

Question 0

Altered automaticity

Answer 0

Altered automaticity is a phenomenon where the pacemaking activity of a pacemaker cell set is altered. As a result, tachycardia, bradycardia, PVC, PAC, can occur 
Tachycardia: 
caused by NE, 
stimulants, 
current of injury from ischemia, 
stretching, 
sick sinus, 
hypokalemia lengthens AP bc less K leaves 

Bradycardia: caused by anti-arrhythmic drugs,
digitalis,
blockers,
ischemia/M.I. messes with conduction system,
aging and fibrosis slows heart tissue conduction abilities

Question 1

Define cardiac arrhythmia (dysrhythmia)

Answer 1

Arrhythmia is any electrical activity of the heart that is not properly regulated by a pacemaker; comes in a faster, slower, or irregular

Question 2

Re-entry of Excitation - ONLY CAUSE OF FIBRILLATIONS (A & V)

Answer 2

THIS IS A TIMING ISSUE 
Requirements for Re-entry:
1. circuit loop
2. unidirectional block
3. slow/delayed conduction in heart tissue (ie RMP, ischemia)

Impulse is blocked one way through damaged tissue, but comes around retrograde and is conducted through the damaged tissue at a SLOW pace and exits the damaged tissue right when the normal tissue has finished its RP. Therefore, upon exit, the impulse sees conductable tissue and excites it, re-starting the loop that never stops

Cause: ischemia/M.I. and congenital bypass tracts (Wolf-Parkinson-White) all enhance this set up of damaged/circuit tissue

Seen on EKG: PAC, PVC, SVTachy, a-flutter, a-fib & v-fib

Question 3

Triggered Activity

Answer 3

Delayed Afterdepolarization - due to high Ca levels and FAST HR
– Ca levels are overloaded in the SR which accidentally releases Ca after the AP. The Na/Ca exchanger tries to remove the excess Ca and the inward current of Na causes a DAD
Causes of DADs: digitalis, ischemia, Catecholamines, rapid HR
PAC, PVC and tachycardia seen

Early Afterdepolarization - due to Long AP duration and SLOW HR
–abnormal reactivation of slow inward Ca current and prolonged QT syndrome can cause EADs
Causes of EADs: anything that lengthens the AP -> ischemia, hypokalemia, slow HR, quinidine.
PVC, PAC, tachycardia seen. Tachy can go to Torsades de Pointes!!!

Question 4

Anti-arrhythmic therapies:

Answer 4

Drugs:

Cardiac ablation: burn the damaged area

Cardioversion: try to depol. all cells simultaneously and get SA to kick back in

Implantable defibrillators: try to depol. all cells simultaneously and get SA to kick back in