Lecture 5 - Cardiac Electrophysiology Flashcards
What are the 5 components of the SPECIALIZED conduction system of the heart?
- SA Node
- AV Node
- Bundle of His
- Bundle Branches
- Purkinje Fibers
- pumps from ENDO to EPICARDIUM (end = inside)
What do the following components of the heart mirror in the Electrocardiogram (EKG):
- Atrial Activation
- Ventricular Activation
- Ventricular Recovery
- P-R
- QRS
- T
What equations represent the Theoretical and ACTUAL membrane potentials?
- Nernst is theoretical
2. Goldman-Katz is ACTUAL
Define the following for the Na/K Pump:
- net current (inward or outward)
- Metabolic energy?
- inhibited by?
- MAIN FUNCTION
- net OUTWARD (3 Na+ out, for every 2 K+ in)
- ATP!
- Digitalis (in a flower)
- Maintain Na/K gradient across membrane (used later for depolarization - AP!)
Define the following for the Ca2+ Pump:
- net current (inward or outward)
- low or high intracellular Ca?
- driven by what gradient?
- 3 Na+ in, for 1 Ca2+ out = net INWARD
- LOW intracellular calcium (high outside) –> pumps CA out
- driven by Na (sodium) gradient
What kind of relationship between Ek (equilibrium K) & extracellular K does the nernst equation predict? How does Vm relate to this? At which values ESPECIALLY?
Linear relationship!
- Vm deviates from that predicted by Nernst Equation
- especially at LOW extracellular K+ (low K+ means that the cell hyper polarizes of Vm plateaus)
What are 2 reasons for the deviation of Vm from that predicted by the Nernst Equation?
- Small Na+ influx
- Decrease in K+ permeability
= Anamolous Rectification
Why is the Left Ventricle 3x thicker than Right ventricle?
resistance is higher in systemic regulation (aorta) than in pulmonary system
- more resistance = increased pressure = difficult t push blood from ventricle into Aorta
What is the primary pacemaker of the heart? Conduction?
SA Node + AV node = pacemaker (small diameter = poor conduction)
Bundle of his/purkinje = CONDUCTION (large cells,)
What is the term for a decrease in K+ permeability (IK1) when either the electrical or chemical driving force on K+ is increased?
What are two ways this can occur?
Anamolous Rectification!
- decrease in extracellular K+ (K high inside usually - moves OUT)
- if too little on outside cell could hyper polarize
- increase threshold for AP to be reached - Depolarization of membrane
At negative voltages to Ek, where does K+ flow? At voltages positive to Ek where does K+ flow?
- negative to Ek –> K+ flows out
- at more positive voltages, K+ outflow is REDUCED (flows out & then anomalous rectification occurs)
= flows out during phase 4, and anomalous rectification occurs during phase 2
- K+ permeability decreases and depolarization is decreased
Define 3 important criteria for HYPERKALEMIA(high extracellular K+):
- K+ permeability(increase/decrease)
- K+ gradient (increase/decrease)
- more +/- RMP? (Resting Membrane Potential)
- increases K+ permeability
- decreases K+ concentration gradient across membrane
- more POSITIVE RMP
Define 3 important criteria for HYPOKALEMIA (low extracellular K+):
- K+ permeability(increase/decrease)
- K+ gradient (increase/decrease)
- more +/- RMP? (Resting Membrane Potential)
- Decreases K+ permeability
(less moving in) - Increases K+ gradient on membrane
- Little to NO CHANGE on RMP due to INWARD RECTIFICATION
What order are the components of the heart activated?
- Atrium activated by SA node
- Atrial Muscle
- AV node activated
- Bundle of His activated next
- Ventricular Muscle
- Ventricular Muscle recovery
What does the x axis and y axis of the EKG represent?
x = Time
y = VOLTAGE
Which EKG interval represents the AV Nodal Conduction time, which carries impulses from the Atria to the Ventricle?
P-R interval
The RMP of the heart is the same as a nerve and is dependent on which ion?
dependent on K+ (potassium)
- diffuses out, leaves negative charges (inside of cell negative) Na/K pump restores this
Calcium is low or high within the cell?
LOW in the cell
How does Hyperkalemia & Hypokalemia usually occur? What is the normal range of K?
HYPERKALEMIC = result of kidney failure
HYPOKALEMIC = diuretics, cause loss of potassium
- normal is between 3-5millimolar
Describe what occurs during the following phases: 0 1 2 3 4
0 - Na+ channels open, Membrane approach Ena
1 - Na+ channels inactivate, K+ channels(Ito) open
2 - Ca channels open, and K+ conductance (Ik1) decreases = INWARD RECTIFICATION
3 - delayed activation of K+ channels (Ik) and background Ik1 conductance increases again
4 - background K conductance (Ik1) is high, delayed Ik channels closed (deactivated)
- Ca channels closed and Na channels recover from inactivation but remain closed
Which phase represents inward rectification?
Phase 2 on the graph,
Ca open, and K (Ik1) channels decrease in conductance
What is the effect of TTX on Purkinje fiber action potential?
1) Fast Na channels
2) slow Ca channels
3) plateau
4) repolarization
- Fast Na channel (phase 0 ) blocked by TTX
- slow Ca channels unaffected, responsible for phase 0 upstroke!!
- plateau (phase 2 ) unchanged
- repolarization (phase 3 ) unchanged
What channels are blocked by Tetrodotoxin? Which channels replace the function of the blocked channels?
- Fast Na channels!!
- Calcium channels responsible for upstroke! (phase 0)
- Ca influx (ca low inside) can depolarize the membrane = SLOW RESPONSE ACTION POTENTIAL
- no affect on other channels
What do all fast responses contain?
SLOW responses
(Na and Ca activated)
- fast responses can become slow responses
What changes to the conductance result from TTX?
- slow action potential since Na channels blocked
- CA slow channels are unaffected THUS
CONDUCTANCE IS SLOWED DRAMATICALLY
If all fast responses were changed to slow response AP’s, what would result in the heart?
Could not conduct or contract normally, resulting in CARDIAC ARREST
If only a small portion of the heart changed from fast to slow response because of local damage (infarct), what is the result?
- Conduction is SLOWED in the DAMAGED REGION ONLY
2. slow conduction = ARRHYTHMIA
What are slow response tissues? Fast response?
- SA, AV node
2. ATRIAL, Bundle of His-Purkinje, VENTRICULAR
How long is the duration of a fast response vs a slow response?
Fast response has a LONG duration - 200 - 400 millisecond
- slow response = 100-200 milliseconds
Do SA & AV node have slow or fast conduction?
SLOW conduction
What determines the configuration of the QRS complex?
HIS PURKINJE SYSTEM activating the VENTRICLES
- ventricle depolarization
- if not conducting through HIS PURKINJE then would lead to arrhythmia
During a premature beat, is the Action potential normal? What happens to the QRS complex?
YEs, ap is normal since heart is at normal RMP
- BUT QRS is different = different route of conduction (not end to epic = SOWER)
- QRS is SLURRED = not normal conduction time
What changes the amplitude from higher to lower in an EKG?
- route of conduction is more parallel to the lead
= LARGER AMPLITUDE
When the permeability of the cell is closer to ENa of sodium, what does the graph look like? If more similar to Ek?
- UPSTROKE of AP when membrane more permeable to Na
2. At rest more permeable to Ek and during REPOLARIZATION (negative values)
If more current is able to flow down a cell rather than OUT, does the space constant increase or decrease?
INCREASES Space COnstant
- reaches node and causes AP and another AP
In cardiac tissue, space constant depends on what 2 things?
- Nexal Connections (gap junctions)
- SIZE of cell
(Parking cells larger than AV nodal cells)
- less nexal = increased resistance = SLOWER CONDUCTION and a SMALLER space constant
When do gap junctions change?
HEALING OVER
- increase in internal resistance due to a decrease in number of open gap junctions caused by:
1. INCREASE in Ca (cytosolic/intracellular)
2. increase in H+ ions - during MI
In hyperkalemia , how does the QRS complex change? Conduction?
- it depolarizes all cells (lose Na channels), RMP more POSITIVE
- SLURRED QRS
- SLOW CONDUCTION (lose Na channels)
In ISCHEMIA , how does the QRS complex change? Conduction?
- lack of blood flow causes a shift in RMP to more + values
1. SLURRED QRS
2. SLOWED CONDUCTION
When RMP becomes more + in an area and conduction slows dramatically, what can occur?
- conduction can block and go in an aberrant direction
= ABNORMALLY SLOW AP
1. VF or
2. Re-Entry of Excitation can occur
A longer AP can lead to what? (slower conduction)
- Torsades de Pointes
2. EAD
What does the AV node refractory period depend on?
TIME (calcium channels are time dependent)
If a person is in AFIB, what drug can be given?
CALCIUM CHANNEL BLOCKER
- slow down ventricular rate through AV NODE (which filters SA node)
What is valsalva maneuver? How can it be used to stop an arrhythmia?
- Forced expiration on a closed GLOTTIS
2. activates VAGAL STIMULATION - stop an arrhythmia
If conduction slows enough, what can it cause?
a BLOCK
