Cardiac Electrophysiology

Question 1

Sketch a typical action potential in a ventricular muscle and a pacemaker cell, labeling both the voltage and time axes accurately.

Question 2

Describe how ionic currents contribute to the four phases of the cardiac action potential.

Question 3

Explain differences in shapes of the action potentials of different cardiac cells (particularly pacemaker vs non-pacemaker cells).

Answer 3

Pacemaker cells have a slower conduction time than non-pacemaker cells

this is important to prevent tetanus in cardiac muscle

Pacemakers have unstable resting membrane values which allows the pacemaker activity

Question 4

What does the shape of the cardiac action potential depend on?

Answer 4

Ca, Na, K

Question 5

Explain what accounts for the long duration of the cardiac action potential and the resultant long refractory period.

Answer 5

The long refractory period is the result of the rapid depolarization, followed by a plateau phase, and repolarization

Force development occurs n the diastole phase which prevents the heart from being able to contract again (safegaurd)

Question 6

What is the advantage of the long plateau of the cardiac action potential and the long refractory period?

Answer 6

It allows the cell to fully relax and fill before the next electrical event can occur

Question 7

Contrast the duration of the action potential and the refractory period in a cardiac muscle, a skeletal muscle, and a nerve.

Answer 7

• Cardiac
  
  • longest action potential (200-400ms)
  • calcium ions are involved in phase 2
  • More negative resting membrane potential
• Skeletal
  
  • AP and RP are very short (2-5 ms)
  • Application of another stimulus before the contraction phase has ended will lead to more calcium release and an increase in force development
  • ARP extends through the max tension development of muscles
• Nerve
  
  • Shortest action potential (1 ms)
  • Depolarization phase is caused by fast opening Na channels (same in muscle)

Question 8

Sketch the temporal relationship between an action potential in a cardiac muscle cell and the resulting contraction (twitch) of that cell.

Question 9

On the basis of that graph, explain why cardiac muscle cannot remain in a state of sustained (tetanic) contraction.

Answer 9

You can’t have tetanus in cardiac muscle because force development is largely completed within the RP of the cell membrane

Valuable safeguard for heart b/c no blood can be pumped during tetanus

relaxation phase is jut as important as contraction phase in heart

Question 10

Explain the ionic mechanism of pacemaker automaticity and rhythmicity, and identify cardiac cells that have pacemaker potential and their spontaneous rate.

Answer 10

Cardiac Cells with pacemaker potential are SA and AV nodes

SA and AV nodes do not have fast Na channels

Question 11

Identify neural and humoral factors that influence their rate.

Answer 11

• Sympathetic stimulation (norepinephrine) or increased circulating catecholamines (epinephrine) increased the slope of prepotential
  
  • in SNS there is an increase in permeability of Na and Ca and a decrease in K permeability
  • uses B1 receptors
• Parasympathetic (acetylcholine) - hyperpolarization 0 decreases the slop of prepotential - decrease in heart rate
• Temperature affects the prepotential slope
  
  • Fever- increases the slope of prepotential - increased heart rate

Question 12

How does the SNS affect ion permeability?

Answer 12

• in SNS there is an increase in permeability of Na and Ca and a decrease in K permeability

Question 13

What receptors does the SNS use?

Answer 13

B1 receptors

Question 14

How does the Parasympathetic NS affect ion permeability?

Answer 14

Increase in K permeability

decrease in Na and Ca permeability

Question 15

What receptors does the Parasympathetic NS utilize?

Answer 15

ACh and Muscarinic receptors

Question 16

How does fever affect heart rate?

Answer 16

It increases the slope of prepotential

increases heart rate

Question 17

Beginning in the SA node, diagram the normal sequence of cardiac activation (depolarization) and the role played by specialized cells.

Question 18

Predict the consequence of a failure to conduct the impulse through any of these areas.

Answer 18

Failure to contract = heart failure

Question 19

Explain why the AV node is the only normal electrical pathway between the atria and the ventricles.

Answer 19

It is the only region where impulses from the atria can get to the ventricles

other pathways would be faster which could lead to problems

Question 20

Explain the functional significance of the slow conduction through the AV node.

Answer 20

Slow conduction produces delay in ventricular excitation (allows time to fill)

Question 21

What does the delay in ventricular excitation cause by slow conduction through the AV node cause?

Answer 21

The delay makes it possible for the atria to contract and contribute their 10-20% to the ventricular volume prior to the ventricular depolarization and contraction

Question 22

Describe factors that influence conduction velocity through the AV node.

Answer 22

• Autonomic Nervous System
  
  • Parasympathetic Nervous (Vagal) - decreases conduction velocity in AV node
  • Sympathetic - increases conduction velocity

Question 23

Discuss the significance of overdrive suppression, ectopic pacemaker and reentry including the conditions necessary for each to occur.

Answer 23

• Reentry:
  
  • occurs when an excitation wave reexcites some region through which it has recently passed
  • reentry circuits can be either random or ordered
  • unidirectional block is necessary for reentry
• Overdrive suppression
  
  • higher frequency of SA nodal firing suppresses other pacemaker sites
  • specifically SA is dominant to AV but if needed AV can take over
• Ectopic Pacemaker
  
  • Can cause additional beats or take over the normal pacemaker activity of the SA node
  • can lead to tachycardia or bradycardia
  • produces a resting rate much slower than the SA Node

Question 24

Contrast the sympathetic and parasympathetic nervous system influence on heart rate and cardiac excitation in general.

Answer 24

• SNS
  
  • greater effect on heart rate
  • it releases hormones that accelerate heart rate
• Parasympathetic NS
  
  • releases acetylcholine to slow heart rate

Question 25

Identify which arm of the autonomic nervous system is dominant at rest.

Answer 25

Parasympathetic → “rest and digest”

Question 26

Discuss ionic mechanisms of these effects on pacemaker cells.

Question 27

Phase 0

Answer 27

Depolarization of SA nodal cell The chemical and electrostatic forces both favor the entry of Na⁺ into the cell through fast Na⁺ channels to generate upstroke

Question 28

Phase 1

Answer 28

The chemical and electrostatic forces both favor the efflux of K⁺ through i_w channels to generate early, partial repolarization

Question 29

Phase 2

Answer 29

During the plateau, the net influx of Ca⁺⁺ through Ca⁺⁺ channels is balanced by the efflux of K⁺ through i_K, i_Kl, i_w channels

Question 30

Phase 3

Answer 30

The chemical forces that favor the efflux of K⁺ through i_K, i_Kl, i_w channels predominate over the electrostatic forces that favor the influx of K⁺ through these same channels

Question 31

Phase 4

Answer 31

The chemical forces that favor the efflux of K⁺ through i_K, and i_Kl channels exceed very slightly the electrostatic forces that favor the influx of K⁺ through these same channels

Question 32

What are the requirements for Reentry?

Answer 32

Unidirectional Block the ARP of the reentered tissue must be shorter than the propagation time around the reentry loop

Question 33

What does diltiazem do to cardiac action potential and contractile force?

Answer 33

It blocks L type calcium channels Phase II gets longer Plateau phase gets shorter

Question 34

When does the heart have the highest permeability to K⁺?

Answer 34

At rest

Question 35

What would happen if the Sodium Potassium pump is inhibited by 50%?

Answer 35

The cell would hypopolarize and be more excitable The RMP would become less negative

Question 36

What happens in a normal heart when 2 action potentials meet?

Answer 36

The APs collide and cancel out

Question 37

What is a mechanism of reentry?

Answer 37

1. injury that causes a unidirectional block 2. conduction tissue is damaged and prevents further transmission of impulse down the conduction pathway 3. not blocked impulse continues through the pathway like normal but is not blocked where APs normally meet 4. The AP continues down its own pathway like normal as well as down and up the other pathway in the reverse direction 5. it travels through the block and reenters the tissue it came from

Question 38

What can reentry act as?

Answer 38

its own pacemaker

Question 39

What phase does the P wave match with on the electrophysiology graph?

Answer 39

none; it goes with atrial depolarization

Question 40

What phase does the QRS wave match with on the electrophysiology graph?

Answer 40

Phase 0

Question 41

What causes the QRS wave?

Answer 41

the depolarization of cardiomyocytes

Question 42

What phase does the T wave match with on the electrophysiology graph?

Answer 42

Phase 3

Question 43

What causes the T wave?

Answer 43

repolarization of the cardiomyocytes

Question 44

Which of the following statements regarding the autonomic nervous system is correct: A. Parasympathetic activity increases the membrane permeability to K+ B. Sympathetic innervation decreases the slope of the pacemaker potential C. Sympathetic innervation decreases the membrane permeability to Na+ and Ca++ D. The heart will stop beating if it is denervated E. The heart rate will decrease significantly if the heart is denervated

Answer 44

A. Parasympathetic activity increases the membrane permeability to K+

Question 45

Which of the following is most responsible for phase 0 of the SA nodal cell? A. Pacemaker channels B. Voltage-gated Ca²⁺ channels C. Tetrodotoxin-sensitive Na+ channels D. K⁺ leak channels E. Cl^- channels

Answer 45

B. Voltage-gated Ca²⁺ channels

Question 46

Which one of the following would increase the rate of phase 4 depolarization in an SA nodal cell? A. An increase in SA nodal K⁺ conductance B. Stimulation of cardiac Muscarinic receptors C. Stimulation of the vagus nerve D. Increasing levels of circulating epinephrine

Answer 46

D. increasing levels of circulating epinephrine

Question 47

In the normal heart, premature re-excitation of myocytes is prevented by:

Answer 47

The long absolute refractory period of the myocardial cells

Question 48

During atrial fibrillation, the rate of ventricular depolarization depends on:

Answer 48

The refractory period of cells in the AV node

Question 49

What is atrial fibrillation?

Answer 49

a lot of unorganized electrical activity in the atria

Question 50

What separates the atria and the ventricles electrically?

Answer 50

AV node

Question 51

Why is it important that myocytes are excited in sequence order?

Answer 51

to obtain a normal heart rate

Question 52

What does the long absolute refractory period of the myocardial cells do?

Answer 52

it ensures the heart can't be excited too early to undergo sustained contraction

Question 53

What happens if the heart undergoes sustained contraction?

Answer 53

it is not an effective pump

Question 54

How do circulating levels of epinephrine increase heart rate?

Answer 54

Epinephrine binds to a β₁ receptor ⇣ causes an increase in sodium leak and calcium leak ⇣ increases heart rate

Question 55

What does it mean to increase the rate of phase 4 depolarization in the SA nodal cell?

Answer 55

Make slope steeper so it depolarizes more quickly and reaches threshold faster

Question 56

What does stimulation of cardiac muscarinic receptors do?

Answer 56

It makes the slope flatter because it increases K⁺ permeability

Question 57

What does stimulating the vagus nerve do?

Answer 57

it is the same as releasing ACh binds muscarinic receptors → opens K⁺ channels → increases K⁺ permeability → decreases slope of prepotential (slows heart rate)

Question 58

What causes depolarization in ventricular muscle and atrial muscle cells?

Answer 58

Fast - Na+ channels

Question 59

What happens when voltage-gated Ca²⁺ channels open?

Answer 59

they allow the SA nodal cells to depolarize when the cell reaches threshold

Question 60

What would happen if the parasympathetic and sympathetic innervations were cut in the heart?

Answer 60

heart rate would increase

Question 61

What would happen if the heart as denervated?

Answer 61

SA node will serve as a pacemaker and still beat

Question 62

How does parasympathetic activity increase membrane permeability to K⁺?

Answer 62

1. it releases ACh which binds to muscarinic receptors 2. K+ channels open up 3. increases permeability to K⁺ 4. decrease the slope of prepotential (slows heart rate)

Question 63

What does sympathetic innervation do to the slope of the pacemaker potential?

Answer 63

it releases NE, which binds the β₁ receptor and increases the slope of the prepotential

Question 64

How does sympathetic innervation affect membrane permeability to Na⁺ and Ca⁺?

Answer 64

it increases membrane permeability to Na⁺ and Ca⁺ slope is steeper and increases heart rate