Antiarrhythmic Basics

Question 1

What is cardiac arrhythmia?

Answer 1

Loss of cardiac rhythm

Question 2

What is cardiac excitability?

Answer 2

Ease with which cardiac cells undergo a series of events characterized by:

• -sequential depolarization and repolarization
• -communication with adjacent cells
• propagation of the electrical activity with a normal or abnormal manner

Question 3

What are the fast response and slow response action potentials?

Answer 3

Fast Response (initiate depolarization):
ventricular and atrial muscle as well as purkinje fibers
Slow Response:
Sinoatrial (SA) node and atrioventricular (AV) node

Question 4

The resting potential (80-95) of the cell is set by what?

Answer 4

1. Balance of inward (sodium and calcium) and
2. Outward (potassium) currents and
3. The corresponding equilibrium potentials of these currents
   - -altering the balance of these ions (Via antiarrhythmics) will affect the excitability of the cardiac myocytes

Question 5

There are 5 phases of the fast response action potential. In general what are these phases?

Answer 5

Phase 0: initial rapid depolarization of the cell membrane
Phase 1: Subsequent partial repolarization of the membrane
Phase 2: unique to cardiac muscle is called the plateau
Phase 3: rapid repolarization phase of the action potential
Phase 4: Resting membrane potential

Question 6

Now each card will go through the phases of the fast response action potential. Phase 0, what are the features?

Answer 6

Action potential begins: myocyte is depolarized to a potential of about -50mV
–at this threshold potential voltage gated Na channel open and an influx of Na comes into the cell — resulting in a self reinforcing depolarization of the cell
Now, the same depolarization that cases the channels to open also inactivates the channels several milliseconds later.
Calcium channels also open during depolarization but the inward calcium flux is much slower

Question 7

Phase 1, what are some features?

Answer 7

K+ conductance is responsible for the resting membrane potential and is suppressed during depolarization, therefore impairing repolarization of the tissue.

• -some enhanced K conductance during depolarization which causes opening and then closing of potassium channels that only allow K to exit the cell
• this coupled with inactivation of the Na channels in the membrane causes a temp repolarization of the cell

Question 8

Phase 2, what are some features?

Answer 8

L-type Ca2+ channels open once the membrane is depolarized to about -50mV (positive charge rushes into cell)
–during this phase of the action potential, the hyperpolarizing effects of K efflux are diminished because of inactivation of K channels by membrane depolarization.
The influx of positive current from Ca2+ matches the remaining positive efflux carried by K exiting through a few open K channels. — this causes the membrane potential to remain constant at a positive value and present as plateu.

Question 9

Phase 3, what are some features?

Answer 9

Outward K channels open during phase 3

• -Na and Ca channels because inactivated
• –outward movement bring about rapid repolarization of the cell

Question 10

Phase 4, what are some features?

Answer 10

Inward K channels which permit outflux of K at highly negative membrane potentials keeps the resting membrane potential maintained.
–Na channels become activated

Question 11

While depolarized the cell is resistant to a subsequent depolarizing event called absolute refractory period. What is relative refractory period?

Answer 11

After partial but incomplete repolarization a subsequent depolarization is possible but occurs slowly

Question 12

Moving on to the slow response cardiac action potentials. What are the phases?

Answer 12

Phase 4: spontaneous depolarization which triggers the action potential
Phase 0: depolarization
Phase 3: repolarization

Question 13

Nodal tissues does not contain fast voltage gated Na channels. The action potential is therefore dependent on what?

Answer 13

L-type Ca channels
–the slow opening and closing of these channels upon depolarization creates a phase 0 that is slower than that seen in ventricular cells

Question 14

Once an action potential in the SA node is triggered, the electrical impulse spreads rapidly through the atria and enters the AV node. Conduction through the AV node is slow (allowing time for atrial contraction to propel blood into the ventricles). What happens next?

Answer 14

The impulse then propagates through the His-Purkinge system to all parts of the ventricles

Question 15

Moving on to the basics of Arrhythmias, Arrhythmias can be classified as either supraventricular (Atrial or AV junctional) or Ventricular. What are factors for an arrhythmia?

Answer 15

1. CAD
2. Electrolyte imbalances
3. Changes in your heart muscle
4. Injury following a heart attack
5. Irregular heart rhythms

Question 16

All arrhythmias result from disturbances in impulse formation, disturbances in impulse conduction or both. First what does disturbances in impulse formation mean?

Answer 16

SA role: normal impulse formation
–AV node will take over if SA node is not producing impulses.
two MOA associated with impulse formation: altered automaticity and afterdepolarizations

Question 17

What are factors for why altered automaticity can occur?

Answer 17

Occurs at the SA node and AV node
–due to many factors: beta adrenergic stimulation, hypokalemia, and hypoxia
Sympathetic and Parasympathetic affect the discharge of the pacemaker cells
Automatic behavior at other cardiac sites that ordinarily lack spontaneous pacemaker activity can show enhanced automaticity.
–both enhanced normal (autonomics) or abnormal automaticity lead to arrhythmias

Question 18

Some individuals posses accessory electrical pathways that bypass the AV node. Thus what happens?

Answer 18

The ventricular tissue receives impulses from both the normal and the accessory pathways

Question 19

The second mechanism associated with disturbances in impulse formation is afterdepolarizations, when does this occur?

Answer 19

Normal action potential triggers abnormal depolarizations

–if this reaches threshold, it may give rise to secondary upstrokes that can propagate and create abnormal rhythms.

Question 20

There are two types of afterdepolarizations, early and delayed. What are early depolarizations?

Answer 20

Depolarizations that interrupt phase 2 or phase 3.
–phase 2: inward Ca2+ current
–phase 3: inward Na channels
Can lead to arrhythmias — aka torsade de points
Marked prolongation of the cardiac action potential

Question 21

What are delayed afterdepolarizations?

Answer 21

Interrupt phase 4

–shortly after repolarization is complete

Question 22

Finally the last thing at all arrhythmias result from is disturbances of impulse conduction. Re-entry tachyarrhythmias, what does this mean?

Answer 22

Myocardial ischemia and injury can induce severe ventricular arrhythmias

Question 23

There are various types of Arrhythmias. Each card will go through the types. First are premature atrial contractions, what is this?

Answer 23

Early beats that originate in the atria

–harmless and do not require treatment

Question 24

Premature Ventricular Contractions (PVCs) are the next type of arrhythmias. What are some features?

Answer 24

Most common arrhythmias

• -skipped beats caused by stress, nicotine, exercise, heart disease or electrolyte imbalance.
• harmless

Question 25

Next arrhythmia is atrial fibrillation, what are some features?

Answer 25

Irregular heart rhythm results in the atria contracting abnormally

Question 26

Next arrhythmia is atrial flutter, what are some features?

Answer 26

Caused by one or more rapid circuits in the atrium

–often converts to a-fib

Question 27

next arrhythmia is paroxysmal supraventricular tachycardia, PSVT. What are some features?

Answer 27

Rapid heart rate 
--regular rhythm 
--from above the ventricles 
--begins and ends suddenly 
two main types: accessory path tachycardias and AV nodal reentrant tachycardias

Question 28

Next arrhythmia is ventricular tachycardia (V-tach), what are some features?

Answer 28

Rapid heart rhythm originating from the ventricles

–treatment immediately

Question 29

Next arrhythmia is ventricular fibrillation, what are some features?

Answer 29

Erratic, disorganized firing of impulses from the ventricles
–emergency because the ventricles quiver and are unable to contract or pump blood to the body.

Question 30

The last arrhythmia is sinus node dysfunction, what are some features?

Answer 30

Slow heart rhythm due to abnormal SA node

–results in pacemaker

Question 31

A quick review on EKG. what do the various waves represent?

Answer 31

P wave: atrial depolarization
QRS: ventricular depolarization
T wave: ventricular repolarization
PR: beginning of atrial depolarization to beginning of ventricular depolarization
QT: duration of ventricular depolarization

Question 32

Finally what are some non drug therapies for arrhythmias/

Answer 32

Electrical Cardioversion: Administer a short anesthesia and the electrical shock is delivered that synchronizes the heat
Pacemaker: sends small electrical impulse to the heart muscle to maintain a suitable heart rate
ICD: treats ventricular tachycardia and ventricular fibrillation, delivers energy to heart muscle to cause the heart to beat normal
Catheter Ablation: used to treat PSVTs, atrial flutter, atrial fibrillation and some atrial and ventricular tachycardias