Action Potential, Basic EP Concepts, & Antiarrhythmics

Question 1

Polarization

Answer 1

Difference in charge between sides of a membrane

Question 2

Polarized cell

Answer 2

Intracellular side is more negative

Question 3

Depolarized cell

Answer 3

Intracellular side is more positive

Question 4

Influx

Answer 4

Flow into the cells

Question 5

Efflux

Answer 5

Flow out of the cell

Question 6

Excitability

Answer 6

A response to a stimulus out of proportion to the strength of that stimulus

Question 7

Current

Answer 7

The flow of charged particles across a membrane

Question 8

Voltage

Answer 8

Difference in positive charges from one side of a membrane to the other
-example: -80mV means fewer positive charges on the inside of the cell

Question 9

Sodium

Answer 9

-Predominates extracellulary
-Equilibrium potential +60 mV
-Reversal potential is around +55 mV

Question 10

Potassium

Answer 10

-Predominates intracellularly
-Equilibrium potential -90mV
-Reversal potential is around -95mV

Question 11

Gap junction

Answer 11

Low resistance intercellular connections that allows depolarization of neighboring cells

Question 12

Ion

Answer 12

An atom or molecule with a net electric charge due to the loss or gain of one or more electrons

Question 13

Anion

Answer 13

Negatively charged ion

Question 14

Cation

Answer 14

Positively charged ion

Question 15

Nernst Equation

Answer 15

Calculates the equilibrium potential for a specific ion across a membrane based on its concentration inside vs outside the cell

Question 16

Calcium

Answer 16

Equilibrium potential +120 mV

Question 17

Chloride

Answer 17

Equilibrium potential -70 mV

Question 18

Wedensky effect

Answer 18

-When a large suprathreshold stimulus causes a following stimulus which would normally be subthreshold to result in the initiation of an action potential
-Can lead to inaccurately low threshold testing results

Question 19

Action potential phase 0

Answer 19

Depolarization

-Na+ channels open
-Abrupt influx of extracellular Na+ ions into the myocyte
-Causes membrane potential to go from -90mV to +20mV

Question 20

Action potential phase 1

Answer 20

Initial/brief repolarization

QRS complex

-Change in polarity initiates closure of Na+ channels and opens K+ channels
-K+ ions already in the cell begin to leak out through a newly opened channel, initiation the cell transition back to a negative state

Question 21

Action potential phase 2

Answer 21

Plateau

ST segment

-Ca++ channels open
-Ca++ channels open at same membrane potential as K+ channels, but their activation is slower (K+ is 1ms, CA++ is 10-20 ms)
-The counterbalance between the slow inward Ca++ and fast outward K+ mediates the plateau phase

Question 22

Action potential phase 3

Answer 22

Repolarization

T wave

-Ca++ channels close
-K+ channels open, allowing potassium to flow out of the cell, becoming more negative
-Cell is vulnerable to depolarization during this phase

Question 23

Action potential phase 4

Answer 23

Quiescent period

After T wave, before P wave

-Normal distribution of Na+ and K+ is restored
-Sodium potassium pump helps to maintain the resting membrane potential by actively transporting Na+ ions out of the cell and K+ ions into the cell
-Return to resting membrane potential, which is almost flat
-Cell is repolarized (strongly polarized), ready for depolarization again
-Heart rate is controlled by varying the slope of the phase 4 action potential

Question 24

Absolute refractory period

Answer 24

-Period of time in which the action potential of the cell cannot be depolarized regardless of the strength of the stimulus
-Occurs during the depolarization phase and initial part of repolarization phase
-Primarily due to inactivation of voltage gated sodium channels
-Even if very strong stimulus is applied, no new action potential can be generated because sodium channels are blocked

Question 25

Effective refractory period

Answer 25

Longest coupling interval that fails to capture myocardial tissue or fails to conduct an electrical impulse

Question 26

Relative refractory period

Answer 26

Period where it’s possible for the neuron to produce another action potential but it requires a much greater stimulus to reach the threshold

Question 27

SA node

Answer 27

Resting potential: -40 to -60 mV Action potential amplitude: 60 to 70 mV

Question 28

AV node

Answer 28

Resting potential: -60 to -70 mV Action potential amplitude: 70 to 80 mV

Question 29

Purkinje fibers

Answer 29

Resting potential: -90 to -95 mV Action potential amplitude: 120 mV

Question 30

Ventricular muscle

Answer 30

Resting potential: -80 to -90 mV Action potential amplitude: 110 to 120 mV

Question 31

Class 1a antiarrhythmics

Answer 31

Sodium channel blockers - moderate degree of sodium channel blocking Meds: Quinidine, procainamide, disopyramide Pacing threshold: increase Defibrillation threshold: increase EKG changes: increase QRS, increase QT

Question 32

Class 1b antiarrhythmics

Answer 32

Sodium channel blockers - weak degree of sodium channel blocking Meds: Lidocaine, mexiletine Pacing threshold: no change Defibrillation threshold: increase EKG changes: decrease QT

Question 33

Class 1c antiarrhythmics

Answer 33

Sodium channel blockers - strong degree of sodium channel blocking Meds: Flecainide, propafenone Pacing threshold: increase Defibrillation threshold: increase EKG changes: increase QRS (QT), prolong PR

Question 34

Class II antiarrhythmics

Answer 34

Beta blockers Pacing threshold: no effect or increase Defibrillation threshold: no effect

Question 35

Class III antiarrhythmics

Answer 35

Potassium channel blockers Meds: sotalol, Tikosyn, amiodarone, dronaderone Pacing threshold: —Sotalol/Tikosyn: no effect —Amiodarone/dronaderone: increase Defibrillation threshold: —Sotalol/Tikosyn: decrease —Amiodarone/dronaderone: increase EKG changes: Prolong QT

Question 36

Class IV antiarrhythmics

Answer 36

Calcium channel blockers Pacing threshold: increase Defibrillation threshold: no effect

Question 37

Use dependence

Answer 37

Increased drug effect at higher heart rates Occurs with class 1 antiarrhythmics Example of why we get exercise stress test for people on flecainide

Question 38

Revere use dependence

Answer 38

-Greatest lengthening of the action potential duration occurs at slow heart rates, with a shortening of the action potential duration with shorter cycle lengths -Short-long-short intervals may result in a prolongation of the APD following the long R-R interval, leading to instability of repolarization. -Can result in a significant risk of QT prolongation, TdP and VF Occurs with class III antiarrhythmics

Question 39

Volume of distribution

Answer 39

The theoretical volume into which a drug is distributed to maintain the plasma concentration at steady state IV only drugs have a smaller volume of distribution Amiodarone will have a VD greater than the circulating blood volume because it is highly lipophillic VD is decreased in heart failure, shock → drug toxicity

Question 40

First pass metabolism

Answer 40

Concentration is significantly reduced by the time the drug reaches systemic circulation after one pass through the portal vein/liver This accounts for IV only or requirement for high oral dose compared to its IV dose Examples: lidocaine, propranolol

Question 41

Chronotropy

Answer 41

Heart rate

Question 42

Dromotropy

Answer 42

Speed of conduction

Question 43

Inotropy

Answer 43

Force of contraction

Question 44

Starting antiarrhythmics for ICD patients

Answer 44

-Need to increase the VT detection cycle by 30-50 ms so VT doesn’t slow below the cutoff rate and remain undetected