Cardiac Electrophysiology Flashcards
Sketch a typical action potential in a ventricular muscle and a pacemaker cell, labeling both the voltage and time axes accurately.
Describe how ionic currents contribute to the four phases of the cardiac action potential.
Explain differences in shapes of the action potentials of different cardiac cells (particularly pacemaker vs non-pacemaker cells).
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
What does the shape of the cardiac action potential depend on?
Ca, Na, K
Explain what accounts for the long duration of the cardiac action potential and the resultant long refractory period.
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)
What is the advantage of the long plateau of the cardiac action potential and the long refractory period?
It allows the cell to fully relax and fill before the next electrical event can occur
Contrast the duration of the action potential and the refractory period in a cardiac muscle, a skeletal muscle, and a nerve.
- 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)
Sketch the temporal relationship between an action potential in a cardiac muscle cell and the resulting contraction (twitch) of that cell.
On the basis of that graph, explain why cardiac muscle cannot remain in a state of sustained (tetanic) contraction.
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
Explain the ionic mechanism of pacemaker automaticity and rhythmicity, and identify cardiac cells that have pacemaker potential and their spontaneous rate.
Cardiac Cells with pacemaker potential are SA and AV nodes
SA and AV nodes do not have fast Na channels
Identify neural and humoral factors that influence their rate.
- 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
How does the SNS affect ion permeability?
- in SNS there is an increase in permeability of Na and Ca and a decrease in K permeability
What receptors does the SNS use?
B1 receptors
How does the Parasympathetic NS affect ion permeability?
Increase in K permeability
decrease in Na and Ca permeability
What receptors does the Parasympathetic NS utilize?
ACh and Muscarinic receptors
How does fever affect heart rate?
It increases the slope of prepotential
increases heart rate
Beginning in the SA node, diagram the normal sequence of cardiac activation (depolarization) and the role played by specialized cells.
Predict the consequence of a failure to conduct the impulse through any of these areas.
Failure to contract = heart failure
Explain why the AV node is the only normal electrical pathway between the atria and the ventricles.
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
Explain the functional significance of the slow conduction through the AV node.
Slow conduction produces delay in ventricular excitation (allows time to fill)
What does the delay in ventricular excitation cause by slow conduction through the AV node cause?
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
Describe factors that influence conduction velocity through the AV node.
- Autonomic Nervous System
- Parasympathetic Nervous (Vagal) - decreases conduction velocity in AV node
- Sympathetic - increases conduction velocity
Discuss the significance of overdrive suppression, ectopic pacemaker and reentry including the conditions necessary for each to occur.
- 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
Contrast the sympathetic and parasympathetic nervous system influence on heart rate and cardiac excitation in general.
- SNS
- greater effect on heart rate
- it releases hormones that accelerate heart rate
- Parasympathetic NS
- releases acetylcholine to slow heart rate
Identify which arm of the autonomic nervous system is dominant at rest.
Parasympathetic → “rest and digest”
Discuss ionic mechanisms of these effects on pacemaker cells.
Phase 0
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
Phase 1
The chemical and electrostatic forces both favor the efflux of K+ through iw channels to generate early, partial repolarization
Phase 2
During the plateau, the net influx of Ca++ through Ca++ channels is balanced by the efflux of K+ through iK, iKl, iw channels
Phase 3
The chemical forces that favor the efflux of K+ through iK, iKl, iw channels predominate over the electrostatic forces that favor the influx of K+ through these same channels
Phase 4
The chemical forces that favor the efflux of K+ through iK, and iKl channels exceed very slightly the electrostatic forces that favor the influx of K+ through these same channels
What are the requirements for Reentry?
Unidirectional Block
the ARP of the reentered tissue must be shorter than the propagation time around the reentry loop
What does diltiazem do to cardiac action potential and contractile force?
It blocks L type calcium channels
Phase II gets longer
Plateau phase gets shorter
When does the heart have the highest permeability to K+?
At rest
What would happen if the Sodium Potassium pump is inhibited by 50%?
The cell would hypopolarize and be more excitable
The RMP would become less negative
What happens in a normal heart when 2 action potentials meet?
The APs collide and cancel out
What is a mechanism of reentry?
- injury that causes a unidirectional block
- conduction tissue is damaged and prevents further transmission of impulse down the conduction pathway
- not blocked impulse continues through the pathway like normal but is not blocked where APs normally meet
- The AP continues down its own pathway like normal as well as down and up the other pathway in the reverse direction
- it travels through the block and reenters the tissue it came from
What can reentry act as?
its own pacemaker
What phase does the P wave match with on the electrophysiology graph?
none; it goes with atrial depolarization
What phase does the QRS wave match with on the electrophysiology graph?
Phase 0
What causes the QRS wave?
the depolarization of cardiomyocytes
What phase does the T wave match with on the electrophysiology graph?
Phase 3
What causes the T wave?
repolarization of the cardiomyocytes
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
A. Parasympathetic activity increases the membrane permeability to K+
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
B. Voltage-gated Ca2+ channels
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
D. increasing levels of circulating epinephrine
In the normal heart, premature re-excitation of myocytes is prevented by:
The long absolute refractory period of the myocardial cells
During atrial fibrillation, the rate of ventricular depolarization depends on:
The refractory period of cells in the AV node
What is atrial fibrillation?
a lot of unorganized electrical activity in the atria
What separates the atria and the ventricles electrically?
AV node
Why is it important that myocytes are excited in sequence order?
to obtain a normal heart rate
What does the long absolute refractory period of the myocardial cells do?
it ensures the heart can’t be excited too early to undergo sustained contraction
What happens if the heart undergoes sustained contraction?
it is not an effective pump
How do circulating levels of epinephrine increase heart rate?
Epinephrine binds to a β1 receptor
⇣
causes an increase in sodium leak and calcium leak
⇣
increases heart rate
What does it mean to increase the rate of phase 4 depolarization in the SA nodal cell?
Make slope steeper so it depolarizes more quickly and reaches threshold faster
What does stimulation of cardiac muscarinic receptors do?
It makes the slope flatter because it increases K+ permeability
What does stimulating the vagus nerve do?
it is the same as releasing ACh
binds muscarinic receptors → opens K+ channels → increases K+ permeability → decreases slope of prepotential (slows heart rate)
What causes depolarization in ventricular muscle and atrial muscle cells?
Fast - Na+ channels
What happens when voltage-gated Ca2+ channels open?
they allow the SA nodal cells to depolarize when the cell reaches threshold
What would happen if the parasympathetic and sympathetic innervations were cut in the heart?
heart rate would increase
What would happen if the heart as denervated?
SA node will serve as a pacemaker and still beat
How does parasympathetic activity increase membrane permeability to K+?
- it releases ACh which binds to muscarinic receptors
- K+ channels open up
- increases permeability to K+
- decrease the slope of prepotential (slows heart rate)
What does sympathetic innervation do to the slope of the pacemaker potential?
it releases NE, which binds the β1 receptor and increases the slope of the prepotential
How does sympathetic innervation affect membrane permeability to Na+ and Ca+?
it increases membrane permeability to Na+ and Ca+
slope is steeper and increases heart rate
