Lecture 5 - Cardiac Electrophysiology

Question 1

What are the 5 components of the SPECIALIZED conduction system of the heart?

Answer 1

1. SA Node
2. AV Node
3. Bundle of His
4. Bundle Branches
5. Purkinje Fibers

• pumps from ENDO to EPICARDIUM (end = inside)

Question 2

What do the following components of the heart mirror in the Electrocardiogram (EKG):

1. Atrial Activation
2. Ventricular Activation
3. Ventricular Recovery

Answer 2

1. P-R
2. QRS
3. T

Question 3

What equations represent the Theoretical and ACTUAL membrane potentials?

Answer 3

1. Nernst is theoretical

2. Goldman-Katz is ACTUAL

Question 4

Define the following for the Na/K Pump:

1. net current (inward or outward)
2. Metabolic energy?
3. inhibited by?
4. MAIN FUNCTION

Answer 4

1. net OUTWARD (3 Na+ out, for every 2 K+ in)
2. ATP!
3. Digitalis (in a flower)
4. Maintain Na/K gradient across membrane (used later for depolarization - AP!)

Question 5

Define the following for the Ca2+ Pump:

1. net current (inward or outward)
2. low or high intracellular Ca?
3. driven by what gradient?

Answer 5

1. 3 Na+ in, for 1 Ca2+ out = net INWARD
2. LOW intracellular calcium (high outside) –> pumps CA out
3. driven by Na (sodium) gradient

Question 6

What kind of relationship between Ek (equilibrium K) & extracellular K does the nernst equation predict? How does Vm relate to this? At which values ESPECIALLY?

Answer 6

Linear relationship!

• Vm deviates from that predicted by Nernst Equation
• especially at LOW extracellular K+ (low K+ means that the cell hyper polarizes of Vm plateaus)

Question 7

What are 2 reasons for the deviation of Vm from that predicted by the Nernst Equation?

Answer 7

1. Small Na+ influx
2. Decrease in K+ permeability
   = Anamolous Rectification

Question 8

Why is the Left Ventricle 3x thicker than Right ventricle?

Answer 8

resistance is higher in systemic regulation (aorta) than in pulmonary system

• more resistance = increased pressure = difficult t push blood from ventricle into Aorta

Question 9

What is the primary pacemaker of the heart? Conduction?

Answer 9

SA Node + AV node = pacemaker (small diameter = poor conduction)

Bundle of his/purkinje = CONDUCTION (large cells,)

Question 10

What is the term for a decrease in K+ permeability (IK1) when either the electrical or chemical driving force on K+ is increased?

What are two ways this can occur?

Answer 10

Anamolous Rectification!

1. decrease in extracellular K+ (K high inside usually - moves OUT)
   - if too little on outside cell could hyper polarize
   - increase threshold for AP to be reached
2. Depolarization of membrane

Question 11

At negative voltages to Ek, where does K+ flow? At voltages positive to Ek where does K+ flow?

Answer 11

1. negative to Ek –> K+ flows out
2. at more positive voltages, K+ outflow is REDUCED (flows out & then anomalous rectification occurs)
   = flows out during phase 4, and anomalous rectification occurs during phase 2

• K+ permeability decreases and depolarization is decreased

Question 12

Define 3 important criteria for HYPERKALEMIA(high extracellular K+):

1. K+ permeability(increase/decrease)
2. K+ gradient (increase/decrease)
3. more +/- RMP? (Resting Membrane Potential)

Answer 12

1. increases K+ permeability
2. decreases K+ concentration gradient across membrane
3. more POSITIVE RMP

Question 13

Define 3 important criteria for HYPOKALEMIA (low extracellular K+):

1. K+ permeability(increase/decrease)
2. K+ gradient (increase/decrease)
3. more +/- RMP? (Resting Membrane Potential)

Answer 13

1. Decreases K+ permeability
   (less moving in)
2. Increases K+ gradient on membrane
3. Little to NO CHANGE on RMP due to INWARD RECTIFICATION

Question 14

What order are the components of the heart activated?

Answer 14

1. Atrium activated by SA node
2. Atrial Muscle
3. AV node activated
4. Bundle of His activated next
5. Ventricular Muscle
6. Ventricular Muscle recovery

Question 15

What does the x axis and y axis of the EKG represent?

Answer 15

x = Time

y = VOLTAGE

Question 16

Which EKG interval represents the AV Nodal Conduction time, which carries impulses from the Atria to the Ventricle?

Answer 16

P-R interval

Question 17

The RMP of the heart is the same as a nerve and is dependent on which ion?

Answer 17

dependent on K+ (potassium)

• diffuses out, leaves negative charges (inside of cell negative) Na/K pump restores this

Question 18

Calcium is low or high within the cell?

Answer 18

LOW in the cell

Question 19

How does Hyperkalemia & Hypokalemia usually occur? What is the normal range of K?

Answer 19

HYPERKALEMIC = result of kidney failure

HYPOKALEMIC = diuretics, cause loss of potassium

• normal is between 3-5millimolar

Question 20

Describe what occurs during the following phases:
0
1
2
3
4

Answer 20

0 - Na+ channels open, Membrane approach Ena

1 - Na+ channels inactivate, K+ channels(Ito) open

2 - Ca channels open, and K+ conductance (Ik1) decreases = INWARD RECTIFICATION

3 - delayed activation of K+ channels (Ik) and background Ik1 conductance increases again

4 - background K conductance (Ik1) is high, delayed Ik channels closed (deactivated)

• Ca channels closed and Na channels recover from inactivation but remain closed

Question 21

Which phase represents inward rectification?

Answer 21

Phase 2 on the graph,

Ca open, and K (Ik1) channels decrease in conductance

Question 22

What is the effect of TTX on Purkinje fiber action potential?

1) Fast Na channels
2) slow Ca channels
3) plateau
4) repolarization

Answer 22

1. Fast Na channel (phase 0 ) blocked by TTX
2. slow Ca channels unaffected, responsible for phase 0 upstroke!!
3. plateau (phase 2 ) unchanged
4. repolarization (phase 3 ) unchanged

Question 23

What channels are blocked by Tetrodotoxin? Which channels replace the function of the blocked channels?

Answer 23

1. Fast Na channels!!
2. Calcium channels responsible for upstroke! (phase 0)
   - Ca influx (ca low inside) can depolarize the membrane = SLOW RESPONSE ACTION POTENTIAL
   - no affect on other channels

Question 24

What do all fast responses contain?

Answer 24

SLOW responses

(Na and Ca activated)

• fast responses can become slow responses

Question 25

What changes to the conductance result from TTX?

Answer 25

- slow action potential since Na channels blocked - CA slow channels are unaffected THUS CONDUCTANCE IS SLOWED DRAMATICALLY

Question 26

If all fast responses were changed to slow response AP's, what would result in the heart?

Answer 26

Could not conduct or contract normally, resulting in CARDIAC ARREST

Question 27

If only a small portion of the heart changed from fast to slow response because of local damage (infarct), what is the result?

Answer 27

1. Conduction is SLOWED in the DAMAGED REGION ONLY | 2. slow conduction = ARRHYTHMIA

Question 28

What are slow response tissues? Fast response?

Answer 28

1. SA, AV node | 2. ATRIAL, Bundle of His-Purkinje, VENTRICULAR

Question 29

How long is the duration of a fast response vs a slow response?

Answer 29

Fast response has a LONG duration - 200 - 400 millisecond - slow response = 100-200 milliseconds

Question 30

Do SA & AV node have slow or fast conduction?

Answer 30

SLOW conduction

Question 31

What determines the configuration of the QRS complex?

Answer 31

HIS PURKINJE SYSTEM activating the VENTRICLES - ventricle depolarization - if not conducting through HIS PURKINJE then would lead to arrhythmia

Question 32

During a premature beat, is the Action potential normal? What happens to the QRS complex?

Answer 32

YEs, ap is normal since heart is at normal RMP - BUT QRS is different = different route of conduction (not end to epic = SOWER) - QRS is SLURRED = not normal conduction time

Question 33

What changes the amplitude from higher to lower in an EKG?

Answer 33

- route of conduction is more parallel to the lead | = LARGER AMPLITUDE

Question 34

When the permeability of the cell is closer to ENa of sodium, what does the graph look like? If more similar to Ek?

Answer 34

1. UPSTROKE of AP when membrane more permeable to Na | 2. At rest more permeable to Ek and during REPOLARIZATION (negative values)

Question 35

If more current is able to flow down a cell rather than OUT, does the space constant increase or decrease?

Answer 35

INCREASES Space COnstant - reaches node and causes AP and another AP

Question 36

In cardiac tissue, space constant depends on what 2 things?

Answer 36

1. Nexal Connections (gap junctions) 2. SIZE of cell (Parking cells larger than AV nodal cells) - less nexal = increased resistance = SLOWER CONDUCTION and a SMALLER space constant

Question 37

When do gap junctions change?

Answer 37

HEALING OVER - increase in internal resistance due to a decrease in number of open gap junctions caused by: 1. INCREASE in Ca (cytosolic/intracellular) 2. increase in H+ ions - during MI

Question 38

In hyperkalemia , how does the QRS complex change? Conduction?

Answer 38

- it depolarizes all cells (lose Na channels), RMP more POSITIVE 1. SLURRED QRS 2. SLOW CONDUCTION (lose Na channels)

Question 39

In ISCHEMIA , how does the QRS complex change? Conduction?

Answer 39

- lack of blood flow causes a shift in RMP to more + values 1. SLURRED QRS 2. SLOWED CONDUCTION

Question 40

When RMP becomes more + in an area and conduction slows dramatically, what can occur?

Answer 40

- conduction can block and go in an aberrant direction = ABNORMALLY SLOW AP 1. VF or 2. Re-Entry of Excitation can occur

Question 41

A longer AP can lead to what? (slower conduction)

Answer 41

1. Torsades de Pointes | 2. EAD

Question 42

What does the AV node refractory period depend on?

Answer 42

TIME (calcium channels are time dependent)

Question 43

If a person is in AFIB, what drug can be given?

Answer 43

CALCIUM CHANNEL BLOCKER | - slow down ventricular rate through AV NODE (which filters SA node)

Question 44

What is valsalva maneuver? How can it be used to stop an arrhythmia?

Answer 44

1. Forced expiration on a closed GLOTTIS | 2. activates VAGAL STIMULATION - stop an arrhythmia

Question 45

If conduction slows enough, what can it cause?

Answer 45

a BLOCK