Physiologic Basis for ECG Flashcards
what does ECG measure?
potential differences between recording electrodes generated by currents from the heart to body surface
ECG measurement?
SUM of all electrical potentials
different ECG leads
view electrical vectors of heart from different angles
what contributes to the rate and rhythm of ECG?
1 - automaticity
2 - conduction system order and timing
3 - functional syncytium
P wave
atrial depolarization
immediately precedes atrial contraction
QRS complex
ventricular depolarization
atria repolarizing simultaneously
immediately precedes ventricular contraction
T wave
ventricular repolarization
ventricular contraction?
remain until after T wave of repolarization
what happens during P wave
begins at SA node, spreads toward AV node
right atrium begins to depolarize before left atrium
what happens during QRS wave?
depolarization at IV septum (Q wave)
AVERAGE current flows from base of heart to apex (R wave)
current reverses flows toward outer wall of ventricles near base (S wave)
what is the Q wave
interventricular septum depolarization
begins at IV septum and endocardial surfaces
what is the R wave
current flows from base of heart to apex
what is the S wave
current reverses, flows toward outer walls of ventricle near the base
first deflection downward?
Q wave
first upward deflection?
R wave
second upward deflection?
R’
first downward deflection following an upward deflection?
S wave
if entire configuration consists solely of one downward deflection?
QS wave
ventricular repolarization?
T wave
- begins in last area that was depolarized
- travels toward base of heart in opposite direction of depolarization
P wave
atrial depolarization
PR interval?
atrial depolarization and AV node delay
QRS?
ventricular depolarization
and atrial repolarization
QT interval?
ventricular depolarization/repolarization
ST segment?
ventricular contraction and ejection
**isoelectric line
RR interval?
heart rate
TP interval
ventricular relaxation and filling
T wave
ventricular repolarization
Units of ECG?
1 small square = 0.04s (1mm)
1 large square = 0.2s
5 large squares = 1s
2 large squares vertically = 1mV
importance of PR interval?
atrial depolarization to ventricular depolarization
-includes delay in conduction that occurs at AV node
important for identifying AV blocks:
primary, secondary, tertiary
what is the normal PR interval
0.12-0.2 seconds
3-5mm on EKG paper
what pathology can be found on PR interval?
AV blocks:
primary
secondary (Mobitz I and II)
tertiary
importance of ST segment?
time from end of ventricular depolarization to start of ventricular repolarization
generally horizontal or gently up-sloping in all leads
deviation from isoelectric line in ST segment?
indicates myocardial ischemia or injury
S-T elevation: MI acute pericarditis LVH hyperkalemia
importance of QT interval
time from beginning of ventricular depolarization to end of ventricular repolarization
all electrical events in the ventricles
heart rate dependent
more time repolarization phase
long QT syndrome?
congenital:
mutant Na+ failure to inactivate
K+ defective repolarization
acquired:
electrolyte imbalance
pharmacologic
normal PR interval?
0.12 - 0.2s
normal QT interval?
less than or equal to .44 s
normal QRS?
0.08 - 0.12s
normal P wave?
0.08 - 0.10s
estimation of heart rate based on number of boxes?
1 - 300 2 - 150 3 - 100 4 - 75 5 - 60 6 - 50 7 - 43 8 - 38 9 - 33 10 - 30
estimation of heart rate tip?
divide 300 by number of large boxes
ex/ 4 large boxes: 300/4 = 75 bpm
second method for heart rate estimation?
three tick marks = 6 seconds
count number of RR intervals per 6 second interval
-multiply by 10
wave or depolarization towards positive electrode?
records positive voltage
wave of depolarization away from positive electrode?
negative voltage
wave of repolarization away from positive electrode?
positive voltage
wave or repolarization toward positive electrode?
negative voltage
waves perpendicular to lead axis?
no net voltage
-isoelectric
wave of depolarization perpendicular to positive electrode?
records biphasic wave (no net voltage)
positive deflection precedes negative deflection
wave of repolarization perpendicular to positive electrode?
records biphasic wave
negative deflection precedes positive deflection
12 leads of EKG?
bipolar standard limb (1+, 1-)
leads I, II, III
unipolar augmented leads (1+, 2-)
leads aVL, aVR, aVF
unipolar chest leads
V1-V6
set up for 12 lead EKG?
2 arm electrodes and 2 leg electrodes
6 chest electrodes
what do limb leads measure?
record electrical activity moving up/down and left/right in the coronal plane
direction lead goes?
to its positive electrode
lead at 0 degrees?
lead I
lead at -30 degrees?
aVL
lead at 60 degrees?
lead II
lead at 90 degrees?
aVF
lead at 120 degrees?
lead III
lead at -150 degrees?
aVR
what is einthoven’s triangle?
RA, LA, LL electrodes
sets up the standard limb leads I, II, III
what makes up lead I?
LA + to RA -
angle of orientation = 0
what makes up lead II?
LL + to RA -
angle of orientation = 60
what makes up lead III?
LL + to LA -
angle of orientation 120
set up for augmented leads?
1 positive electrode
2 negative electrodes
negative electrodes averaged together to create a composite reference lead
aVL lead setup?
RA,LL - to LA +
angle of orientation = -30
aVR lead setup?
LA, LL - to RA +
angle of orientation = -150
aVF lead setup?
RA, LA - to LL +
angle of orientation = 90
mnemonic for frontal plane leads?
Three Little Ones Ran Too Far
what leads give inferior view of heart?
Lead II
Lead III
Lead aVF
what gives left lateral view of heart?
Lead I
Lead aVL
what gives right-sided view of heart?
lead aVR
what do precordial leads measure?
electrical activity moving anteriorly/posteriorly in horizontal plane
placement of chest electrodes?
V1 - 4th intercostal, right of sternum V2 - 4th intercostal left of sternum V3 - between V2 and V4 V4 - 5th intercostal space midclavicular V5 - between V4 and V6 V6 - 5th intercostal space, midaxillary
V1 view of heart?
over right ventricle
V2 and V3 view of heart?
iver interventricular septum
V4 view of heart?
over apex of left ventricle
V5 and V6 view of heart?
over lateral left ventricle
orientation of right and left ventricles?
right - anterior and medial
left - posterior and lateral
anterior view of heart leads?
V2, V3, V4**
left lateral view of heart leads?
I, aVL, V5, V6**
inferior view of heart leads?
II, III, aVF**
right ventricular view of heart leads?
aVR, V1**
