L8: Heart Rhythmicity & EKG Flashcards
timing of signal conductance from SA to ventricles
SA — > AV 0.3
delayed 0.9
delayed in bundles 0.4
reaches ventricles 0.16 after origin
factors that cause slow/delayed conduction
small cell size
low amplitude
slow depolarization rates
SA node resting potential and threshold
- 55 to -60
- 40mV
fast Na channels: rest to AP
inactive at rest
inactivation gate closes when less < -55
activation gate opens at -40
inactivation gate closes 100-150msec after opening
slow Na channels: rest to AP
are open/leaky at rest
causes slow depolarization/repolarization
K+ channels: rest to AP
open when Na/Ca channels inactivate +20
nodal cells slowly repolarize
channels remain open
efflux of K+
ventricular fiber resting potential
-85 to -90
sinus rhythm
SA generating AP
ectopic focus
any other region of the heart generating an action potential besides the normal SA AP
how can the AP and heart rate be modulated?
vagus nerve - ACH
sympathetic - Norepinephrine
vagus nerve stimulation
mainly to SA/AV nodes
activates ACH
response to vagus nerve stimulation
decrease SA rate and excitability of SA/fibers
increase permeability of fiber membranes to K+ ions
K+ efflux - cell becomes more negative
results in hyperpolarization
hyperpolarization numbers
normal -65 to -70
hyper -55 to -60
a substance that causes a decrease in heart rate
negative chronotropic effect
a substance that causes an increase in heart rate
positive chronotropic effect
sympathetic stimulation
to all parts of the heart
mainly ventricles
neurotransmitter: norepinephrine
response to sympathetic stimulation
stimulates beta-1 adrenergic receptors
increase depolarization rates
increase permeability of fiber membranes to Na and Ca
leak into cell = more easily excitable
result of sympathetic stimulation
increased HR
increased contractile power of ventricles
atrial depolarization and contraction
P wave
phase 0 of atria
ventricle depolarization and contraction
QRS complex
phase 0 of ventricles
ventricle repolarization
occurs at the end of T wave
phase 3 of ventricles
atrial repolarization
hidden by QRS-T waves
how does an EKG differ from the recordings of TM potentials?
EKG is a graphic representation of electrical activity in cardiac muscle tissue
TM potential recorded from an axon
when does deflection from 0 occur in an EKG
when current is flowing between regions of heart
current only flows when regions are partly polarized
no potential is ever recorded when regions are fully polarized or depolarized
under what conditions does current flow or not flow?
current flows negative to positive
only flows when regions are partly de-/re-polarized
P-Q interval
also called P-R
P to beginning of QRS
0.16sec
Q-T interval
0.35 sec
___ bipolar limb ____
3 bipolar limb leads
I
II
III
limb lead l
neg = right wrist pos = left wrist
looks at heart from right to left
vector = 0 degrees
limb lead ll
neg = right wrist pos = left ankle
looks at heart from upper right to apex
vector = 60 degrees
limb lead lll
neg = left wrist pos = left ankle
looks at heart from upper left to apex
vector = 120 degrees
current in the heart flows from ____ to ____ primarily from base to apex, until ….?
from negative to positive
until the very end
list the recordings of each lead as negative or positive?
all 3 record positive
what is Einthoven’s triangle?
the triangle formed by the 3 bipolar limb leads
if the electrical potentials of any two of the three bipolar limb leads are known at any given instant, the third can be determined by summing the first two.
Einthoven’s law
define vector
an arrow that points in the direction of the electrical potential generated by flow of current
arrow always points towards positive
length of arrow is determined by size/strength of current
voltages readings: effect of direction in relation to triangle
vector is perpendicular to axis of lead
–EKG is very low
same axis
–entire voltage recorded
horizontal vector = horizontal
summated vector of the generated potential at a particular instant
instantaneous mean vector
QRS vector
mean electrical axis of heart
+59 degrees
average vector during depolarization thru ventricles
abnormal ventricular conditions that cause deviation from mean
change in position of heart in chest hypertrophy bundle block fluid in pericardium pulmonary emphysema
most common cause of a high voltage EKG
hypertrophy of the ventricle
when does a high voltage EKG occur
when the sum of the voltages of all the QRS complexes in the 3 leads in >4mV
causes of decreased voltage EKG
conditions surrounding the heart
cardia myopathies
normal QRS timing
0.06 to 0.08
QRS timing in hypertrophy
also dilation/prolonged
0.09 to 0.12
conditions causing bizarre patterns of QRS
destruction of cardiac muscle and replaced with scar tissue
multiple small local blocks in the conduction impulses of the purkinje system
different cardiac abnormalities cause part of the heart to remain depolarized all the time
currents of injury
causes of currents of injury
mechanical trauma
infectious processes
ischemia**
most common cause of currents of injury
ischemia
reference point for analyzing current of injury
J point
the 0 line on the vertical axis
