Physiological Basis of ECG Part 1 Flashcards
how many electrodes and readings are placed on the skin for an ECG
9 electrodes (12 readings)
what specifically does the Electrocardiogram detect
It detects a difference = deflection in the read out that can be negative or positive
if there is no difference = No deflection in the read out
what part of the cell is the ECG measuring
changes of electrical activity of Cardiac muscle tissue - depolarization and repolarization
Measures the extracellular potential
why does ECG not report the differences between the atria and ventricles
because the atria and ventricles are isolated to insulation from the fibrous skeleton of the heart
what is the axis of the ECG
x axis = time
y axis = voltage difference
how does the ECG tracing deflector work when there is a depolarization to a no depolarization to a repolarization to being repolarized
Depolarization starts (difference between portions) = leads to a signal
once cells are all depolarized there is no difference = no signal
repolarization starts (there is a difference) = a signal
once all cells are polarized there is no difference = no signal
how does the average current flow in regards to negative and positive
negative toward the base of the heart
and with positivity towards the apex
what is the path of depolarization in the heart
SA node Atrial muscle AV node Common bundle bundle branches purkinje fibers ventricular muscle ends at the walls on the sides of the heart in the ventricle
p wave
SA node causes atria to depolarize from right to left
represents phase 0 of AP in atrial muscle
all cells of the atria are depolarized at end of p wave
PR interval
the delay between all of the atria being depolarized to the start of the ventricle being depolarized
QRS interval
Ventricles depolarize generally from right to left, from apex to base, from interior to exterior
represents the phase 0 of AP in ventricular muscle
at the start of the ST interval, all cells in the ventricle have become polarized
ST segment
Action potential phase 2 delays for the repolarization of the ventricles
no difference on the ECG
T wave
Ventricles repolarize generally from the left to the right, base to the apex (opposite order of depolarization)
Represents phase 3 (repolarization) of AP in ventricular muscle
U wave
not typically observed but is a down wave after the T wave
can be alot of different things:
- delayed repolarization of purkinje cells
- prolonged repolarization of mid-myocardial cells
- after potentials from mechanical forces in ventricular walls
- repolarization of papillary muscle
what is the difference between segments and intervals in an ECG
Segment: represents the duration of a single event on the ECG
Interval: represents the duration of two or more events
how are the grids on an EKG measured
Horizontally: 5 large boxes equal 1 second
Vertically: 2 large boxes equal 1.0 mv
time between dark lines = .2 sec
time between light lines = 0.04 sec
what is the length of PR interval, and ST Interval,
PR interval is about 0.16 ms
ST Interval is about 0.35 seconds
what is the height in mv of the QRS complex, P wave, and T wave
QRS complex: 1 - 1.5 mv
P wave: 0.1 - 0.3 mv
T wave 0.2 - 0.3 mv
why do we use 12 leads
Each lead displays voltage differences in the myocardium from different perspectives by different electrodes
What are the standard bipolar limb leads
1: right arm to left arm
2: RA to leg (LL)
3: LA to LL
what are the augmented limb leads
aVF = augmented vector foot
-(RA+LA) to foot
aVR = augmented vector right
-(LL +LA) to RA
aVL = augmented vector left
-(LL + RA) to LA
what are the chest or pecordial leads
V1-V6
V1 and V2 mainly negative
V4, V5, V6 mainly positive
they are not assigned an axis
instead assigned a region of the heart and each lead acts as its own positive electrode (moving anterior/lateral)
when is a lead positive or negative
as AP spreads toward the positive lead of an elctrode set a positive will be observed
Einthovens Triangle
each limb has a + and - polarity and voltage between
Lead I + = LA
Lead II + = LL
Lead III + = LL
