Cardiac Electrophysiology Flashcards
What are the 2 kinds of muscle cells in the heart?
- contractile
- conducting
What is the function of the conductive cells?
To rapidly spread APs over the entire myocardium and generate them spontaneously
Describe the conducting sequence as an AP spreads throughout the myocardium
1) SA node
2) atria
3) AV node
4) Bundle of His
5) Purkinje system
6) ventricles
What serves as the pacemaker of the heart?
The SA node
The AP spreads from the SA node to the right and left atria via what?
The atrial intermodal tracts
Slow conduction through the AV node ensures what?
That the ventricles have sufficient enough time to fill with blood before they are activated and contract
What does the term normal sinus rhythm mean?
That the pattern and timing of the electrical activation of the heart are normal
What are the 3 criteria that qualify a sinus rhythm as normal?
1) the AP must originate in the SA node
2) the SA node impulses must occur regularly at a rate of 60 to 100 impulses per minute
3) the activation of the myocardium must occur in the correct sequence and with the correct timing and delays
What is the membrane potential of cardiac cells determined by?
The relative permeabilities to ions and the concentration gradients for the permeant ions
What happens if the cell membrane has a high permeability to an ion?
That ion will flow down its electrochemical gradient and attempt to drive the membrane potential toward its equilibrium potential
The resting membrane potential of cardiac cells is determined primarily by what ions?
potassium
The conductance to K+ at rest is high and the resting membrane potential is close to the K+ equilibrium
What are the 2 mechanisms that can produce a change in membrane potential?
- there is a change in the electrochemical gradient for a permeant ion
- there is a change in conductance to an ion
What is threshold potential?
The potential difference at which there is a net inward current
What are the 3 characteristics of action potentials in the ventricles, atria, and Pukinje system?
- long duration
- stable resting membrane potential
- plateau
How long is the action potential in the SA node, atria, ventricles, and Purkinje fibers?
- SA Node: 150 msec
- Atria: 150 msec
- Ventricles: 250 msec
- Purkinje fibers: 300 msec
The longer the action potential, the _____ cell is refractory to firing another action potential
longer
Describe the plateau that occurs during the APs in the cells of the ventricles, atria, and Pukinje system.
The plateau is a sustained period of depolarization (due to an inward Ca2+ current) which accounts for the long duration of the AP and, consequently, the long refractory periods
Describe the 5 phases of the action potential in a ventricular, atrial, and Purkinje fiber
1) Phase 0, upstroke
2) Phase 1, initial repolarization
3) Phase 2, plateau
4) Phase 3, repolarization
5) Phase 4, resting membrane potential
Upstroke of what ion causes depolarization of the ventricles, atria, and Purkinje system?
Na+
What does upstroke cause?
depolarization
Sodium flows inward during upstroke due to what?
The opening of activation gates on the Na+ channels
At the peak of the upstroke, what value is the membrane potential depolarized to?
about +20 mV
The rate of rise of the upstroke is called what?
dV/dT (volts/second)
What is dV/dT?
The rate of change of the membrane potential as a function of time
dV/dT depends on the value of the resting membrane potential, this dependence is called what?
responsiveness relationship
When dV/dT is greatest, the rate of rise of the upstroke is _____.
fastest
When dV/dT is greatest, the resting membrane potential is most ____, or ___polarized
negative
hyperpolarized
When dV/dT is lowest, the rate of rise of the upstroke is _____.
slowest
When dV/dT is lowest, the resting membrane potential is ___ ____, or ___polarized
less negative
depolarized
dV/dT also correlates with the size of what?
the inward current, typically sodium
Phase 1 in ventricular, atrial, and Purkinje fibers is a brief period of what?
repolarization, immediately following the upstroke
In order for repolarization to occur there must be a net ____ current.
outward
What are the 2 reasons why there is a net outward current during phase 1?
1) the inactivation gates on the Na+ channels close in response to depolarization and the inward Na+ current ceases
2) there is an outward K+ current down the electrochemical gradient caused by a high K+ conductance
What is phase 2 characterized by?
A long period of relatively stable, depolarized membrane potential, called the plateau
How is such a balance of inward and outward currents achieved during the plateau?
There is an increase in Ca2+ conductance which results in an inward Ca2+ current. And in order to balance this inward Ca2+ current there is an outward K+ current
What is the other significant effect of the inward Ca2+ current that occurs during the plateau of the action potential?
It also initiates the release of more Ca2+ from intracellular stores for excitation-coupling
What is phase 3 characterized by?
rapid repolarization to the resting membrane potential
What causes the rapid repolarization that occurs during phase 3?
A combination of a decrease in calcium conductance resulting in a decrease in the inward Ca2+ current and an increase in potassium conductance resulting in an increase in the outward K+ current
By the end of phase 3 what is the membrane potential?
it has returned to resting level of approximately -85 mV
The inward and outward currents are ___ during phase 4
equal
How can the sum of inward Na+ and Ca2+ currents be the same magnitude as the outward K+ current during phase 4?
Even though potassium conductance is very high, the driving force on K+ is low because of the resting membrane potential is too close to the K+ equilibrium potential. While to opposite is true for Na+ and Ca2+. Their conductances are low, but the driving force on them is high.
What are the 3 features that characterized action potentials in the SA node from APs in the atria, ventricles, and Purkinje system?
1) the SA node can spontaneously generate action potentials without neural input (automaticity)
2) the SA node has an unstable resting membrane potential
3) there is no sustained curve in actions potentials in the SA node
In the SA nodal cells, the upstroke is the result of what?
An increase in calcium conductance which results in an inward Ca2+ current
The inward Ca2+ current during upstroke in the SA nodal cells is carried predominantly by what?
T-type Ca2+ channels (“T” for transient)
Describe phase 1 and 2 of the action potential in SA nodal cells
they are absent
What occurs during phase 3 of the action potential in SA nodal cells?
Repolarization due to an increase in potassium conductance resulting in an outward K+ current
What is the significance of phase 4 of the action potential in SA nodal cells?
It is the longest portion of the action potential that accounts for the automaticity of SA nodal cells
During phase 4 what is the most negative value of the membrane potential called and what is its value?
Maximum diastolic potential = -65 mV
The slow depolarization that occurs during phase 4 of the action potential in SA nodal cells is due to what?
The opening of Na+ channels a slow inward Na+ current called I(f)
I(f) or the inward Na+ current during phase 4 of the action potential in SA nodal cells is turned on by what?
Repolarization from the preceding action potential
What does the rate of phase 4 depolarization set?
the heart rate
What cells (other than the cells in the SA node) have the capacity for spontaneous phase 4 depolarization?
latent pacemakers
Latent pacemakers include the cells of what?
- AV node
- bundle of His
- Purkinje fibers
So, why doesn’t the AV node, bundle or His, or Purkinje fibers set the heart rate?
The pacemaker with the fastest rate of phase 4 depolarization controls the HR. The SA node has the fastest rate of phase 4 depolarization, therefore it sets the HR
What is the phenomenon called when the SA node drives the HR and the latent pacemakers are suppressed?
overdrive suppression
What are the 3 conditions in which latent pacemakers take over and become the pacemaker of the heart?
1) if the SA node firing rate decreases or stops completely
2) if the intrinsic rate of firing of one of the latent pacemakers becomes faster than that of the SA node
3) if the conduction of action potentials from the SA node to the rest of the heart is blocked
What is conduction velocity in the heart?
the speed at which APs are propagated within the cardiac tissue
List the myocardial tissues according to their conduction velocities, starting with the slowest and ending with the fastest
- AV node
- ventricles and atria
- Purkinje fibers
The action potentials that originate in the SA node occur at what time?
time zero
It takes ___ msec for the action potential to spread from the SA node, through the atria, AV node, and His-Purkinje system to the farthest points in the ventricles
220
How long does it take for conduction to spread through the AV node?
100 msec (almost half of the total conduction time) because it has the slowest conduction velocity
The physiologic basis for conduction of cardiac action potentials is the spread of ___ currents.
local
What does conduction velocity depend on?
the size of the inward current during the upstroke of the action potential (dV/dT)
Other than the inward current of the upstroke to establish local currents, what does the propagation of the action potential depend on?
The cable properties of the myocardial fibers
What are cable properties of the myocardial fibers determined by?
cell membrane resistance and internal resistance
Because internal resistance is low, myocardial tissue is well suited for ___ conduction.
fast
Does conduction velocity depend on action potential duration?
No
What is excitability?
The capacity of myocardial cells to generate action potentials in response to inward, depolarizing current
What is the physiologic basis for the refractory periods in myocardial cells?
Inactivation gates on the Na+ channels close with depolarization which does not allow an inward current flow, and therefore no upstroke can occur and no action potential can form
What are the 4 refractory periods of the ventricular action potential?
- absolute refractory period (ARP)
- effective refractory period (ERP)
- relative refractory period (RRP)
- supranormal period (SNP)
What parts of the action potential does the absolute refractory period include?
The upstroke, the entire plateau, and a portion of the repolarization
When does the absolute refractory period conclude?
When the cell has repolarized to approximately -50 mV
How long is the effective refractory period?
It is slightly longer than the absolute refractory period
What is the difference between the absolute and effective refractory periods?
Absolute means that absolutely no stimulus is large enough to generate another action potential
Effective means that a conducted action potential cannot be generated
When does the relative refractory period begin and when does it end?
At the end of the absolute refractory period and continues until the cell membrane has almost fully repolarized
The relative refractory period is characterized by what?
A second action potential can be generated, although a greater than normal stimulus is required
When does the supranormal period begin and when does it end?
It begins when the membrane potential is -70 mV and continues until the membrane is fully repolarized back to -95 mV
During the supranormal period the cell is ____ excitable than normal
more
The effects of the autonomic nervous system on heart rate are called _____ effects.
chronotropic
Sympathetic stimulation ____ HR and parasympathetic stimulation ____ HR.
increases
decreases
Positive chronotropic effects are ____ in HR
increases
*sympathetic stimulation
How does sympathetic stimulation increases HR?
Norepinephrine, released from sympathetic nerve fibers, activates beta-1 receptors in the SA node which produces an increase I(f) or the inward Na+ current, which increases the rate of depolarization, and therefore more action potentials are fired
Negative chronotropic effects are ____ in HR
decreases
*parasympathetic stimulation
How does parasympathetic stimulation decreases HR?
ACh, released from parasympathetic nerve fibers, activates muscarinic receptors in the SA node
What are the 2 major effects the parasympathetic nervous system has on the SA node?
- slowing the rate of phase 4 depolarization
- hyperpolarizing the maximum diastolic potential so that more inward current is required to reach threshold less frequently and fires fewer action potentials per unit time
The effects of the autonomic nervous system on conduction velocity are called ____ effects.
dromotropic
Increases in conduction velocity are called ____ dromotropic effects
positive
Decreases in conduction velocity are called ____ dromotropic effects
negative
Sympathetic stimulation ____ conduction velocity and parasympathetic stimulation ____ conduction velocity.
increase
decrease
How is heart block produced?
Occurs when conduction velocity through the AV node is slowed sufficiently (by increased parasympathetic activity or by AV node damage) resulting in action potentials to not conduct at all from the atria to the ventricles
What is an electrocardiogram?
A measurement of tiny potential difference on the surface of the body that reflect the electrical activity of the heart
List the 5 parts of an EKG in order
1) P wave
2) PR interval
3) QRS complex
4) T wave
5) QT interval
What does the P wave represent?
atria depolarization
What is the PR interval?
the time from initial depolarization of the atria to initial depolarization of the ventricles
Increases in conduction velocity through the AV node ____ the PR interval.
Decreases in conduction velocity through the AV node ____ the PR interval.
decrease
increases
What does the QRS complex represent?
depolarization of the ventricles
What does the T wave represent?
repolarization of the ventricles
What does the QT interval represent?
First ventricular depolarization to last ventricular repolarization
What is the ST segment within the QT interval correlated with?
The plateau of the ventricular action potential
____ is measured by counting the number of QRS complexes per minute
Heart rate
In difference to cycle length, what does HR equal?
HR = 1/cycle length
