Cardiac Electrophysiology Flashcards
cardiac muscle structure?
striated similarly to the skeletal muscle
well developed T-tubule system and sarcoplasmic reticulum
short branched muscle fibres
bound tightly together
embryologically - cardiac muscle?
each muscle developed from one cell - so one nucleus
what is each cardiac myocyte connected to?
each one is electrically connected to another via Gap junctions
what does the intercalated disc allow?
allows the connection of one cardiac myocyte with another so that they form a single organ - working as one
what is within these intercalated discs?
are gap junction channels - important in ensuring the velocity and safety of propagation of impulse
Desmosomes?
allow cardiac myocyte to cope with mechanical stress (stretching and contracting)
Phase 0 of changes that leads to contraction of cardiac myocytes?
depolarisation of the membrane is due to a strong but brief increase in permeability of Na+
Phase 1 of changes that leads to contraction of cardiac myocytes?
the Na+ channels then quickly inactivate and the membrane begins to repolarise
Phase 2 of changes that leads to contraction of cardiac myocytes?
the repolarisation is halted due to opening of voltage gated Ca2+ channels creating a plateau phase - during this phase the influx (Ca2+ and some Na+) and efflux of K+ are in balance
Phase 3 of changes that leads to contraction of cardiac myocytes?
repolarisation then begins with the opening of the K+ channels which is a delayed effect of the preceding depolarisation
Phase 4 of changes that leads to contraction of cardiac myocytes?
this K+ channels are open and maintain a resting membrane potential until another rapid depolarisation occurs
resting potential for SA node fibre?
-55mV (fast Na+ channels are inactivated at this level)
when resting potential is at -55mV, what happens?
at this point, there is a slow leak of cation as the ‘funny channel’ allows Na+/K+ to enter slowly, ‘funny inward current’ - activated by hyperpolarisation
once funny channels open, what happens?
Na+ and K+ enter cell and it slowly depolarises to -40mV
cardiac muscle has how many major ion channels involved in voltage change and what are these?
- “Funny” Na+/K+ (pacemaker)
- Fast Na+
- Slow Ca2+/Na+
- K+ (repolarisation)
What is the refractory period?
it is defined as a period where it is not possible to elicit a new AP immediately regardless of how much the membrane is depolarised - cell is in excitable - absolute refractory period
The bundle of His?
Bundle of conducting muscle cells which penetrate the annulus fibrosus between atria and ventricles in the septum
Divided into right and left bundle branch (RDD and LBB)
The AV node is made up of?
of modified muscle cells located close to the annulus fibrosus in the septum
what does the conduction system allow?
allows for the propagation of the electrical impulse
together with the pacemaker cells the heart also has an extensive conduction system - what does this system allow for?
the more rapid conduction through the entire heart rather than just spreading from muscle cell to muscle cell
the delay in impulse conduction from atria to ventricles
what is the benefit of a rapid conduction system?
it allows the entire ventricle to contract simultaneously as one - allows pressure to build as all ventricle myocytes get ready to contract - increased contractile force leading to increased pressure
why is the delay from atria to ventricle important?
occurs at AV node - AV nodal delay
It allows the ventricle to fill w/ blood before it contracts as one
Annulus fibrosus?
electrical insulator slows down transmission from atria to ventricles and also inhibits re-entry from ventricles to atria
Unipolar extremity lead?
measures difference between mean voltage for two of the extremity electrodes and the voltage of the third one
what do unipolar recordings provide?
provide additional 3 angles of view
why are we missing repolarisation of atria?
it is masked by the massive repolarisation of the ventricles
P wave?
spread of depolarisation through atrium (precedes atrial contraction)
QRS complex?
depolarisation of ventricle (includes atrial repolarisation)
T wave?
ventricular repolarisation
U waves?
repolarisation of papillary muscles or Purkinje fibres
P-R interval?
AV nodal delay
Q-T interval?
ventricular depol and repol - measure of ventricular systole
Where does sinus rhythm originate from?
from the SA node
describe the normal ecg/sinus rhythm?
rhythm originates from SA node
consistent rhythm with no extra beats
R-R distance or P-P distance are regular
Normal ECG
Sinus trachycardia?
still originates from the SA
Narrow QRS complex, RR interval shortened
Could be sympathetic stimulation, exercise, temp, toxin
What does the cardiac rhythm look like during atrial fibrillation?
No P waves
QRS appear random
RR interval random
Baseline is all over
Causes of AF?
vary: genetic, remodelling, size
species or breed predisposition - race horses, eventers, some jumpers
horses can have missed beats under what?
under high vagal tone at rest but this should resolve with light exercise
3rd degree AV block?
P wave no influence on QRS
Both atria and ventricle beat independently
Significant clinical signs - emergency
There is structural abnormalities in the heart
Pacemaker implantation
Species/breed predisposition: CKCS older dogs
