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Electrophysiology of the Heart
Electrophysiology of the Heart by Dr. Gibbons
37
Medical
Professional
09/29/2011

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Term
the pathway of electrical conduction of the heart
Definition
SA node--> depolarize the ventricles--> AV node--> through the AV bundle-->to the left and right bundle branches--> to the purkinje fibers-->to the cells of the ventricles
Term
What occurs during the AV node bottleneck delay?
Definition
the ventricles fill with blood
Term
what physically causes the AV nodal delay?
Definition
decreased numbers of gap junctions in the AV node, also the fibrous tissues between the atria and ventricles act as a insulator
Term
Where is cardiac impulse conduction the fastest?
Definition
the purkinje fibers
Term
the fast conduction of the purkinje fibers promotes what?
Definition
coordinated ventricular contraction
Term
Where the cardiac impulse is conducted the slowest?
Definition
AV node
Term
the ion that causes the plateau period during ventricular AP
Definition
Ca
Term
the ion that causes promotes repolarization during ventricular AP
Definition
K
Term
WHat cases the large spike during ventricluar AP
Definition
Fast Na channels
Term
THese occur during which phase
 Slow voltage-gated Ca++ channels (L-type) open  Ca++ influx
 Voltage-gated K+ channels close  reduce K+ efflux,
however ungated K+ channels still open
 Inward Ca++ current is balanced by outward K+ current
 Ca++ influx  contraction + release Ca++ from SR
Definition
Phase 2 plateau period of ventricular contraction
Term
What occurs during the Phase 1 dip of ventricular AP
Definition
Partial repolarization due to close Na+ channels and opens
of K+ channels
Term
The conduction of the action potential through the heart is slowest in the
Definition
AV Node
Term
A patient received a heart transplant from a healthy donor and it is routine that
the surgeon leaves the transplanted heart unconnected to the
patient’s autonomic nervous system. What do you predict will be the
patient’s resting heart rate when fully recovered from the operation?
Definition
100-110 beats/min
Term
What causes the phase 2 in ventricular action potenital
Definition
 Slow voltage-gated Ca++ channels (L-type) open  Ca++ influx
 Voltage-gated K+ channels close  reduce K+ efflux,
however ungated K+ channels still open
 Inward Ca++ current is balanced by outward K+ current
 Ca++ influx  contraction + release Ca++ from SR
Term
what causes phase 1 (dip) in ventricular AP
Definition
partial repolariztion due to the closing of Na channels and the opening of K channels
Term
Phase 3 repolarization of ventricular AP
Definition
Ca++ channels close  stop Ca++ influx
Voltage-gated K+ open  K+ efflux
Term
Phase 4 AP is caused by (rest/ reload)
Definition
Stable resting membrane potential due to high K+ permeability
Term
why can't cardiac musle go into tetani
Definition
because the absolute and the relative refractory preriod are almost as long as the AP
Term
what is a common mechanism for arrhythmia
Definition
abnormal waves of conduction through the heart
Term
Long QT syndrome leads to
Definition
ventricular tachycardia and sudden death
Term
What can the physician do to prevent sudden death in these patients with Long QT syndrome ?
Definition
put in a pacemaker
Term
What symptoms is a patient with ventricular tachycardia likely to experience?
Definition
light headed, heart flutter, decreased pump efficiecny
Term
for which part of the heart is the resting potential losest to threshold
Definition
SA node
Term
the phases of SA node AP
Definition
Phase 0: upstroke phase
Phase 3: repolarization
Phase 4: spontaneous depolariztion and pacemaker potential
Term
what happens during phase 0 of SA node AP?
Definition
slow voltage gated Ca channels T-type open and calcium enters the cell
Term
what happens at phase 3 of SA node AP?
Definition
voltage gated K channels open and K leaves the cell
Term
what occurs during phase 4 of SA node AP
Definition
there are multiple mechanisms
1. FUnny sodium current
2. Calcium current
3. Potassium current
Term
hyperpolarization-activated Na channels open causing Na to enter and is modulated by cAMP, ACH, and NE
Definition
funny sodium current
Term
L-type and T-type channels cause an inward calcium current SR-mediated ----- channels
Definition
calcium currents
Term
decreased K conductance promotes depolarization
Definition
Potassium current
Term
If your patient does not have a normally functioning ‘funny’ (If) current (hyperpolarization-actived sodium channel); what do you expect the patient’s resting heart rate to be ?
Definition
there would be a decreasein heart rate because less Na channels are available to cause depolarization
Term
Why does the SA node set the pace or rate of electrical impulses through the heart ?
Definition
because it has the most rapid phase 4 depolarization
Term
Given the intrinsic rate of the SA node; what keeps the normal heart rate 60-90 bests/minute ?
Definition
the vagus nerve of the parasympathetic autonomic nervous system
Term
An Atrial Tachycardia is causing too rapid a ventricular rate and causes fainting.
What site along the conduction pathway is the best target to slow the ventricular response?
Definition
the best place to control is the AV node because there are multiple autorhymic sites in the atria
Term
Based on your knowledge of the cellular physiology of SA and AV node Phase 4 depolarization ; what do you predict may happen to the patient’s A-V conduction when you administer a drug for an atrial tachyarrhythmia that:
Definition
blocks nodal L-type calcium channels ?
reduces nodal cAMP levels ?
Term
Patient symptomatic due to rapid ventricular response to atrial tachyarrhythmia.
Application of Physiology Principles
Therapy -- further delay A-V node conduction
Definition
the target would be the Phase 4 channels

Adenosine (reduces cAMP) this reduces Na influx frim the funny channels and enhances K efflux

Verapmil block L tpye calcium channels
reducing Ca influx
Term
What is the mechanism underlying the effect of parasympathetic nerve activation and acetylcholine release on the SA node rate ?
Definition
Ach slows the heart rate by activating the G-protein coupled muscarinic receptor to modulate several pathways in the SA nodal cells:
Decreases Na+ influx via If ‘funny current’
(If current reduced by Ach-induced decrease in cAMP)
Decreases Ca + + influx
Increases K + efflux
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