Cardiac Conduction & ECG Genesis Flashcards
can specialized conduction cells contract and relax
no - can only generate action potentials
what happens if the SA and AV nodes fail
the bundles of His and Purkinje fibers have SLOW pacemaking ability
can generate 20-40 BPM
do cardiomyoctes have pacemaking ability
no - can only conduct current based on RMP and sodium channels
is conduction velocity the same across regions of the heart
no
SA node: slow - maintains rhythm
internodal: fast conduction
AV node: slow - allows adequate atrial filling before conducting signal to the ventricles
His bundle/purkinje - fast conduction for efficient contraction
where is the SA node located
junction where cranial vena cava enters right atrium
SA node conduction
spontaneous
FAST phase 4 to depolarization rate
what does ANS innervation of the SA node control
heart rate
how does the AP get conducted throughout the atrium
internodal tracts spread signal to:
1. left atrial myocytes
2. AV node
are atrial myocyte action potentials strong or weak
weak and short
function of the AV node
“gatekeeper”
controls and slows conduction to ventricles to maximize atrial filling
ONLY site that conducts atrial –> ventricular signaling
what structures can penetrate the fibrous skeleton of the heart
bundle of His & AV node
what does ANS innervation of the AV node control
conduction velocity
how does the AP get conducted throughout the ventricles
AV node –> His bundle –> L & R bundle branches –> Purkinje fibers –> ventricular myocytes –> epicardium and septum –> apex to base
are ventricular myocyte APs strong or weak
strong and long
left vs right bundle branches
L: 2 fascicles –> branch extensively
R: trabeculae septomarginalis (moderator band) spreads signal to the R ventricular wall
electrocardiogram (ECG)
graphic representation of summed electrical activity of the heart using electrodes on the body surface
what does an ECG measure
heart rate, rhythm, and conduction of electrical activity
chronotropy and dromotropy
chronotropy
initiating APs
affects HEART RATE
dromotropy
conducting APs
affects CONDUCTION VELOCITY
what does an ECG not measure
cardiac function
inotropy and lusitropy
inotropy
contractility of the muscle
lusitropy
relaxation of the muscle
what does the size and shape of the ECG waveform depend on
- direction of the current
- amount of tissue the current travels through
- speed (conduction velocity)
current direction
the direction that the current spreads throughout the myocardium in relation to the + and - pole of the leads
how does the waveform appear when the current moves toward the positive electrode
positive waveform
how does the waveform appear when the current moves parallel to the positive electrode
small positive/negative waveform
how does the waveform appear when the current moves away from the positive electrode
negative waveform
what are ECG vectors
the average of the direction of current conduction across all cells
if current spreads in different directions in different cells –> ECG will display the AVERAGE of those directions
how many ECG electrodes are there
3 +/- 1 ground electrode
RTL: white
LTL: black
LPL: red
RPL: green (ground electrode)
bipolar leads
uses one positive and one negative electrode
setup lies within frontal plane to make a triangle around the heart
lead I
RTL: -
LTL: +
current moves straight across from RTL –> LTL
lead II
MOST COMMON
RTL: -
LPL: +
current moves diagonal from top right to bottom left
lead III
LTL: -
LPL: +
current moves straight down left side
augmented unipolar leads
single positive electrode lead in the frontal plane
averages all other electrodes to form the negative pole
records 1/2 voltage –> machine amplifies (aV)
what are the unipolar leads
aVR: positive RTL
aVL: positive LTL
aVF: positive LPL
base-apex lead
lead system used in large animals; primarily evaluates leads I and II
R jugular furrow: -
left apex: +
QRS complex is NEGATIVE in health
does SA node depolarization show on ECG
NO - too small; assume it occurs before the P wave
P wave
atrial myocyte depolarization
(rounded, slow)
PQ interval
AV nodal and His bundle branch/purkinje fiber depolarization
(flat line between P and Q waves)
**ALL specialized conduction tissue has depolarized within PQ interval
QRS complex
ventricular depolarization
Q: first negative deflection
R: first positive deflection
S: negative deflection after R; is NOT always present
T wave
ventricular repolarization
(rounded wave)
what is type A purkinje system
conduction occurs in three phases
starts at septum –> spreads to apex –> travels up to base
purkinje system is NOT deeply penetrating
causes a POSITIVE wave front
what species have a type A purkinke system
dogs, cats, humans
what is a type B purkinje system
conduction occurs in two phases
starts at septum –> spreads to base in rapid succession
purkinje system is deeply penetrating
causes a NEGATIVE wavefront
what species have a type B purkinje system
hoses, ruminants, pigs, birds
what is the most positive bipolar lead on a dog ECG
lead II (similar to aVF unipolar lead)
what is the least positive bipolar lead on a dog ECG
lead I
