B4-089 The Electrocardiogram Flashcards
fast response cardiac action potential occurs in
- cardiomyocytes
- purkinje fibers
slow response cardiac action potential occur in
- SA node
- AV node
use 3 electrodes on both arms and left leg
limb leads
configure Einthove’s triangle, with heart at center
limb leads
- bipolar leads
- recording made between any two vertices of the triangle
limb leads
use same 3 limb electrodes, but compare 2 leads vs. the third
augmented limb leads
- unipolar leads
- called “aV”
augmented limb leads
both limb and augmented limb leads are in the […] plane
coronal
6 unipolar leads that lie on the perpendicular to others (transverse plane)
chest leads
positioned as spokes of a wheel to intersect at a point
hexaxial system
limb/augmented limb
movement of a depolarization wave toward the positive end of a lead produces a […] deflection
positive
movement of a repolarization wave toward the positive end of a lead produces a […] deflection
negative
the height of a wave relects the […] of the current
magnitude
the length of a wave reflects its
duration
the sum of segments and waves
intervals
which interval shows the rate?
R-R
atrial depolarization
p wave
P wave + PR segment
PR interval
ventricular depolarization
QRS complex
corresponds to phase 2 of ventricular action potential
ST segment
ventricular repolarization
T wave
QRS complex + ST segment + T wave
QT interval
firing of the SA node intiates
atrial depolarization
specialized conduction paths to rapidly conduct the impuls from the SA node to the AV node and left atrium
internodal pathways
atrial depolarization is dependent on […] current
sodium
if the sodium current is reduced, the P wave will be
wider and have lower amplitude
the impulse is delayed in the AV node which allows the atria to
contract before the ventricles depolarize
highest conduction velocity in heart
purkinje fibers
the conduction impulse spreads between myocytes via
gap junctions
why will the QRS complex appear different on each lead?
as depolarization spreads through the ventricles, it changes magnitude and direction
sequence of ventricular depolarization
- depolarize atria
- depolarize septum from left to right
- depolarize anteroseptal region of myocardium toward apex
- depolarize bukl of ventricle myocardium, from endocardium to percardium
- depolarize posterior portion of the base of the left ventricle
- corresponds to phase 2 of the ventricular action potential
- recorded along isoelectric line of the ECG
ST segment
represents ventricular repolarization
T wave
represents the entire time required from the onset of ventricular depolarization to repolarization
QT interval
Ca+ current and K+ current
ST segment
average orientation and magnitude of a dipole representing the most intense phase of ventricular depolarization
mean electrical axis MEA
if the QRS is positive in both leads I and aVF, the MEA is
normal
conditions that change MEA
- pregnancy
- ventricular hypertrophy
- infarct
ECG is used to
- measure cardiac function
- detect electrolyte imbalance
- evaluate effective treatment
parasympathetic system […] heart rate
decreases
sympathetic system […] heart rate
increases
mediated by adrenergic receptors
sympathetic
increases heart rate, conduction, and contractility
sympathetic
mediated via cholinergic receptors
parasympathetic
decreases heart rate, conduction, contractility
parasympathetic
increased sympathetic nerve firing to the heart increases
- funny sodium current
- calcium current
- potassium current
norepinephrine
sympathetic activity makes MDP more […] and phase 4 […]
positive
steeper
increased sodium current will cause
contractions to be stronger and briefer
increased potassium current will cause
repolarization to be increased and occur faster
does sympathetic nerve activation change the P wave or QRS complex?
no
effect of increased funny sodium current
increased heart rate
effect of increased L-type calcium current
- increased depolarization
- increased conduction velocity through AV node
- increased contraction strength
effect of increased delayed rectifier potassium current
decreased action potential duration
effect of decreased funny sodium current
slowed depolarization and heart rate
effect of decreased calcium current
reduced depolarization and heart rate
effect of decreased postassium current
extends phase 3 repolarization
slows heart rate
- increased HR
- MDP less negative
- faster phase 4 depolarization
- decreased action potential duration
sympathetic effects (NE) on SA Node
- decreased heart rate
- MDP more negative
- slower phase 4 depolarization
- increased action potential duration
parasympathetic effects (ACh) on SA node
- increased conduction velocity
- faster phase 0 depolarization
sympathetic effects (NE) on AV node
- decreased conduction velocity
- slower phase 0 depolarization
parasympathetic effects (ACh) on AV node
- phase 2 becomes more positive
- action potential duration increases
- contraction strength increases
sympathetic effects (NE) on ventricles
heart rate > 100 bpm
tachycardia
heart rate slower than 60 bpm
bradycardia
group of conditions in which heartbeat is irregular
arrhythmia
limits time of filling between beats
tachycardia
inadequate to support proper cardiac output
bradycardia
causes of arrythmias
- disorders of impulse formation
- disorders of impulse conduction
- both
symptoms of arrhythmias
- palpitations
- lightheadness
- syncope
- SOB
- chest pain
normal sinus rhythm
- rate of 60-100 bpm
- each QRS preceded by P wave
- normal shape of QRS, duration < 120 ms
- no additional waves
- PR < 200 ms
- QT interval less than half the R-R
- originates in SA node
- more bpm than normal
- regular rhythm
- P waves similar
- QRS normal
sinus tachycardia
rapid regular tachycardia that occurs with reentrant activity
paroxysmal tachycardia
- heart rate in 200-350 bpm range
- sinus tachycardia
atrial flutter
- suppression of sinus node
- normal P waves
- normal PR
- normal QRS
- rate <60 bpm
sinus bradycardia
occurs in:
* well conditioned athletes
* during sleep
* hypothyroidism
* vagal stimulation
* medications
sinus bradycardia
signal originates from group of cells outside the SA node
ectopic focus
abnormal action potentials are triggered by a preceding action potential and can result in tachycardia
triggered activity
- “afterdepolarizations”
- occur during phase 2, 3, or 4
triggered activity
- occur during late phase 2/early phase 3
- associated with slow rates of phase 3 repolarization (long QT syndrome)
early afterdepolarizations
EAD
- occur in phase 4 of ventricular myocytes
- high heart rates
- high Ca+ concentrations
delayed afterdepolarizations
DAD
EADs and DADs are secondary to high […]
calcium levels
abnormal depolarization triggers an action potential to spread through myocytes repeatedly
reentrant loop
single depolarization triggered by abnormal depolarizations
premature ventricular depolarization
three requirements for reentry:
- abnormal electical circuit
- slow conduction
- unidirectional block
reentrant loops are more likely when
- conduction velocity is decreased
- duration of action potential is increased
results from the development of multiple reentry loops
fibrillation
- no p waves on ECG
- ventricular rate irregular
- 300-500 bpm
- “bag of worms”
atrial fibrillation
sawtooth baseline
atrial flutter
- prolonged PR interval
- every P waves causes QRS
- caused by reduced conduction velocity
first degree AV block
- multiple P waves precede each QRS
- QRS looks normal
- PR interval progressively lengthens
2nd degree heart block
- P waves not causing QRS complex
- QRS looks normal, indicates ectopic focus
- atrial and ventricular rate completely independent
third degree AV block
AV block
a-fib
atrial flutter
classified as
supraventricular
v tach
branch block
prolonged QT
v fib
classified as
ventricular
