Cardiac Electrophysiology Flashcards
What is excitable tissue?
nerves and muscle
How do excitable tissue communicate?
through AP
What is an action potential?
brief, rapid, large change in membrane potential
What are the 2 types of APs in heart muscle?
- pacemaker cell AP
- contractile cell AP
What is pacemaker cell AP?
cells can fire AP on their own
What is contractile cell AP?
cells cannot fire AP on their own
What is a contractile cell?
ventricular myocyte
What is EK?
-90 mV
What is ENa?
+60 mV
What is ECa?
~ +60 mV, or more positive
What heart cells have contractile cell AP?
99% of heart cells
- most cells are contractile – in ventricles especially (but atrium also has them)
- much lower amount of conduction tissue
What does the stable resting membrane potential depend on?
K+
What does the sharp fast rising phase of AP depend on?
Na+
What does the plateau phase of AP depend on?
Ca2+ and K+
What is the absolute refractory period?
second AP can’t be fired while membrane potential is changed (during previous AP) – must wait until membrane potential returns to resting
What regulates the absolute refractory period?
Na+ state
- Na+ channels inactivate when AP reaches peak
- as long as Na+ cannot be fired, next AP cannot be fired
What is a muscle twitch?
muscle contraction when Ca2+ enters
What is tetanus?
sustained muscle contraction evoked when APs are emitted at very high rate
Compare the duration of a muscle twitch to duration of AP.
equal duration
Can twitches in skeletal muscles be summated?
yes
Can twitches in cardiac muscles be summated?
no – therefore, cannot tetanize heart
Why is tetanization of the heart bad?
if muscle contracted and was able to fire more APs, muscle will be constantly contracted
heart needs to beat regularly to fill and pump
What is the SA node?
(tissue) cells that can contract/fire on their own
Pacemaker Cell Action Potential – SA Node
- slight depolarization
- Na+ enters slowly (first rise)
- few different channels potentially responsible for allowing Na+ entry - depolarization
- Ca2+ enters through Ca2+ channels
- don’t open as quickly - hyperpolarization
- K+ enters through open K+ channels
What are cells in the heart connected by?
gap junctions (ion channels)
What happens once AP is initiated?
current produced moves across heart, causing contraction of atria and then ventricles
What determines heart rate?
rate of pacemaker APs (SA node)
What happens to HR when slope of pacemaker potential is increased?
increase rate
What happens to HR when slope of pacemaker potential is decreased?
decrease rate
What is the primary way to increase/decrease heart rate?
ANS release of:
- noradrenaline/adrenaline from SNS
- acetylcholine from PSNS
What do sympathetic nerves do?
increase slope of pacemaker potential to speed up heart rate
SNS
What does noradrenaline and adrenaline do?
increases slope of pacemaker depolarization
Where is noradrenaline from?
nerve
Where is adrenaline from?
adrenal gland
What does PSNS do?
decreases slope of pacemaker potential to slow down heart rate
PSNS
What does acetylcholine do?
- decreases slope of pacemaker depolarization (takes longer to reach threshold before Ca2+ channels open and AP fires)
PSNS
What receptors does acetylcholine act on?
muscarinic (M2) receptor on SA node cells
SNS
What receptor does noradrenaline and adrenaline act on?
𝛽1-adrenergic receptor on SA node cells
SNS
Noradrenaline and Adrenaline in Nodal Cells
- receptor
- result
receptor:
- 𝜷1 receptors
result:
- increase phase 4 slope
- increase heart rate
PSNS
Acetylcholine in Nodal Cells
- receptor
- result
receptor:
- muscarinic M2 receptors
result:
- decreases phase 4 slope
- decreases heart rate
SNS
Noradrenaline and Adrenaline in Ventricular Muscle Cells
- receptor
- result
receptor:
- 𝜷1 receptors
result:
- increase Ca2+
- increase stroke volume
PSNS
Acetylcholine in Ventricular Muscle Cells
- receptor
- result
no direction action
SNS
Noradrenaline in Vascular Smooth Muscle Cells
- receptor
- result
receptor:
- 𝛼1 receptors
result:
- increase Ca2+
- vasoconstriction
SNS
Adrenaline in Vascular Smooth Muscle Cells
- receptor
- result
receptor
- 𝜷2 receptors
result:
- decrease MLCK activity
- vasodilation
PSNS
Acetylcholine in Vascular Smooth Muscle Cells
- receptor
- result
limited direct action
What is phase 4?
diastolic depolarization / slow depolarization / pacemaker potential
When does phase 4 occur?
occurs once heart relaxes… eventually leads to next contraction
What are the conduction tissues of the heart? (5)
- SA node
- AV node
- bundle of His
- left and right bundle branches
- Purkinje fibres
Pathway of AP
- Starts at SA node
- Passes through AV node
- Passes through Bundle of His
- slows down when travelling here to regulate timing of contraction – allows atria to contract before ventricles - Passes through left and right bundle branches (large pieces of conduction tissue)
- Passes through Purkinje fibres in right and left ventricles
- moves from cell to cell via gap junctions - Ventricle contracts
How can AV node cells produce AP?
- AP arrives from SA node (60-100 beats/min)
- if not, it self-depolarizes (40-50 beats/min)
How does the AV node compare to SA node?
has similar properties to SA node
beats on its own
Does AV node or SA node beat faster?
on their own: AV node beats slower
in the heart: both beat at same rate
How does the SA node and AV node beat at same rate (60 bpm) in the heart?
- SA node fires before AV node
- AV node becomes stimulated to fire before it would normally fire itself
When is the only time AV node would beat on its own?
if something happens to SA node (ie. in disease) – would be acting as pacemaker
What is an electrocardiogram (ECG)?
using recording electrodes placed on body surface to produce tangible record of electrical changes
method of measuring wave of depolarization and repolarization occurring in heart (starting from SA node)
- pattern of electrical depolarization (and repolarization) repeats itself over and over again
What do bipolar limb leads do?
measure wave of depolarization
What is Lead I?
right arm (-) to left arm (+)
What is Lead II?
right arm (-) to left leg (+)
What is Lead III?
left arm (-) to left leg (+)
Einthoven’s Triangle
–
When does ECG pen deflect upwards?
when depolarization moves to positive end of lead
When does ECG pen deflect downwards?
when depolarization moves away from positive end of lead
What is the P wave?
atrial depolarization
- wave of atrial depolarization travels downward and left
- as it moves down, leads pick it up as upward deflection (atrial depolarization)
What is the QRS wave?
ventricular depolarization
- more complex than atria (takes place in several stages)
In ventricle, which cells repolarize first?
last cells to depolarize are first to repolarize
wave of repolarization is in opposite direction of depolarization
What is depolarization?
upstroke of AP in ventricle
picked up outside of heart by leads
What is repolarization?
down phase of ventricular AP
when it’s going back down to RMP
What is the T wave?
ventricular repolarization
- wave of depolarization moves away from positive end of lead
- ECG pen deflects upward
- produces relatively simple waveform on ECG
What are the 2 things that have changed when repolarization begins?
- direction of change in membrane voltage (depolarization → repolarization) that’s picked up from outside
- changed direction – depolarization towards positive end = positive deflection
**if repolarization and direction changes, it’s still in upwards direction
What is a positive deflection?
depolarization towards positive end
3 Frontal Leads – Perspectives
–
What is a regular (normal) heart rate?
60-100 beats/min
What is paper speed?
25 mm/s
What is the time that each large sqaure represents?
0.2 seconds
What is the rate of normal sinus rhythm?
~75 beats/min [300/4 on paper]
between 60-100 beats/min
What is the rhythm of normal sinus rhythm?
regular
Are there P waves in normal sinus rhythm?
yes
- all facing upwards
- 1:1 with QRS (every QRS is preceded by P wave)
What is sinus bradycardia?
elongated period between beats
What is sinus tachycardia?
fast beating, not much time between beats
What is paroxysmal atrial fibrillation?
- atrial depolarization and repolarization are disorganized
- AV node is receiving random rapid volleys of APs
- APs proceed to ventricle depending on whether AV node is refractory or not
What is the danger of paroxysmal atrial fibrillation?
- hypotension: high irregular ventricular rate will decrease heart’s cardiac output
- embolisms: blood flow within atria becomes stagnant and can produce clots
