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”