L17/18 EKG Flashcards
Electrical dipoles
At rest no change in voltage detected (even though has membrane potential)
Depolarizing - differential in voltage detected
Fully depolarized returned to resting conditions
Repolarizing - differential in voltage detected
At rest no differential
Dipole
Pos and neg charge separated by smaller distance which generates local current flow and electrical field
Dipoles close together in space and time can summate
Vectors
Physical quantities with magnitude and direction represented by arrows
Tell what is primary way heart is depolarizing
Cardiac vectors
The resultant sun of all dipoles during cardiac cycle
Electrocardiogram
Record of overall speed of activity throughout the heart during depolarization and repolarization
Record of part of electrical activity induced in body fluids by cardiac impulses that reach body surface (not direct actual electrical activity of heart)
Comparisons in voltage detected by electrodes at two different points on body surface
Not actual potential generated
Does not record potential when muscle is completely depolarized or repolarized
As a positive wave of depolarization within the myocytes flows
Towards a positive electrode, there is a positive deflection recorded on the ekg
Lead arrangements
Limb leads (frontal)
Bipolar leads
I, II, III
Augments unipolar leads
aVR, aVL, aVF
Chest leads (transverse)
Precordial leads
V1-V6
Makes combination that allows to create vector
Gives all different perspective of the heart
Standard limb leads give
Einthovens triangle
Bipolar limb
Frontal plane
Bipolar leads - each lead made up of two active electrodes (+ and -)
+ and - are attached to the arms and legs
Augmented limb leads
Unipolar limb lead
Oriented on frontal plane
Active (exploring) electrode plus an indifferent electrode
Form
Intermediate angles btw standard limb leads
Precordial leads
Unipolar chest leads
6 additional leads consisting of 6 active electrodes positioned on the chest (indifferent electrode is the central terminal)
Positioned perpendicular to the plane of the limbs leads
Oriented to obtain info on the transverse and sagittal planes
Twelve lead system
Gives you 360 of frontal plane and horizontal plane
What info can be obtained from ekg?
Pattern and frequency of events
Rhythm (reg or irreg)
Rate (atrial and ventricular)
Conduction time
Intervals/segments btw waves indicate conduction
Width of waveforms indicates conduction time through an area
Axis determination
Direction of depolarization and repolarization of cardiac structures
Size of chamber
Amplitude is proportional to mass of currently active cells
What an ekg can’t tell you?
Doesn’t tell much about mechanical activity ( contraction, relaxation) except during ventricular fibrillation
Deflections in ekg
- Depolarization of atria
P wave
(SA node not seen because too small) - Flat is AV delay (isoelectric pt)
- QRS complex (ventricles depolarizing) downward deflection, very large upward, another downward deflection (atria is repolarizing simultaneously but masked but QRS
- Upward wave T wave (repolarizing of ventricles) pos because direction of current, traveling alway from electrodes (neg and away = pos)
TP interval
Ventricles completely relaxed and refilling
Waves
Simple upward or downward deflection of ekg
Electrical impulses originating in the SA node produce various waves on the ekg as they spread throughout the heart
Movement away from the baseline in either pos or neg direction
Segments
Period of time btw waveforms
Normally isoelectric
Interval
A period of time that includes waves
P wave
1st wave in cardiac cycle
Depolarization of both atria
Normal duration (0.08-0.10 s)
Enlarged aorta = larger deflection of P wave
QRS complex
Depolarization of both ventricles
Q wave: first neg deflection following the P wave
R wave: first pos deflection following P wave
S wave: a negative wave following R wave
Q,R,S waves may not be present in al QRS
Ventricular hypertrophy - QRS complex will change
T wave
Repolarization of ventricles
T wave inversion (neg T wave) May indicate coronary ischemia or left ventricular hypertrophy
Tall and narrow (peaked/tented) symmetrical T waves may indicate hyperkalemia
Flat T waves may indicate coronary ischemia or hypokalemia
Where is the wave corresponding to atrial repolarization?
Rarely observed
Where are waves of depolarization of SA node and AV node cells?
Events not reflected by any wave in ekg
Mass of tissue is too small
PR and ST segments
PR- Not PQ
Btw end of P and very begging of Q
ST - is isoelectric (can be deflection, pathological)
PR segment
Period btw atria and ventricular depolarization
Isoelectric period btw end of P wave and beginning of QRS complex
Not same as PR interval
Used as baseline to evaluate the ST segment displacement
ST segment
Btw completion of ventricular depolarization and begging of ventricular repolarization
Period btw end of QRS complex and beginning of T wave
Corresponds to plateau of ventricular AP
Normally isoelectric
Does not mean ventricles are at rest
PR interval
Beginning of P wave to beginning of QRS
Represents time for atrial depolarization and the delay through the AV node
Easier to identify
Last about 1 large box (~.2 sec) anything longer start thinking AV nodal delay or conduction block
Denotes conduction of the impulse from the upper part of atrium to the ventricles
QT interval
Measured from onset of QRS complex to end of T wave
Time for ventricular depolarization and repolarization
Approximates the time for a ventricular AP
Denotes the electrical systole of the heart
Very dependent on heart rate
QT interval and heart rate of
QTc: heart rate correlation for QT Interval
QTc=QT/ sq rt R-R
Pathology: long QT syndrome ( timing of depolarization to repolarization of ventriculars prolonged)
Always changing with heart rate!
Ekg and ventricular AP
Phase 0 matches QRS complex
Phase 3 matches T wave
Anytime change in QT interval or ST segment you’d see a change in the plateau phase
P-P interval and R-R interval
Can be used to determine heart rate
P-P atria to atria
R-R ventricle to ventricle
Different but coupled so should be close to each other (if different, they’re not coupled and pathology)
Ventricular is easier to find
Standard calibration of ekg recordings
Horizontally (time)
Tiny box = 1mm~0.04s
Large box =5mm~0.20s
Vertically (voltage)
Small = 1mm ~0.1mV
2 large boxes= 1cm ~1mV
Heart rate equation
Rate (bpm) = beats per 6 sec x 10
6 sec = 30 large boxes
60 sec = 300 large boxes
Quick estimation
HR = 300/# large squares per cycle
From an ekg we typically determine
Vectors/ vector analysis
Net vector: sum of the pos and neg deflections of the waveform
QRS net vector (has pos and neg components, sub neg from pos and obtain net vector)
Determine mean axis of depolarization of ventricles
Used to obtain info about the conduction, pathway, anatomical abnormalities
Expect vector to be pointing downward (because ventricles) and deflect not too far to right or left (implies anatomical abnormality or conduction abnormality)
Mean axis of depolarization
Normal QRS axis - 0-90 degrees
Outside that - mean axis deviation
Left axis deviation
mean QRS axis more neg than 0 degrees
Point too far up and left
Causes:
LV hypertrophy
Obesity (pushes heart up)
Pregnancy (pushes heart up)
Right axis deviation
mean QRS axis more pos than 90 degrees
Points too far down and right
Causes:
RV hypertrophy
LV infarct
Tall, thin body type
Abnormalities in rate
Tachycardia- faster than 100bpm
QRS happen at fast frequency
Bradycardia- slower than 60rpm
QRS occurs less frequently
R to R interval prolonged
Normal: 60-100bpm
Abnormalities in rhythm
Arrhythmia - variation from normal rhythm and sequence of excitation of heart
Ex:
Atrial flutter
Atrial fibrillation (absence of P waves)
Ventricular fibrillation (bizarre waveforms/ unidentifiable QRS)
Heart block
A-V nodal conduction blocks
Partial or complete failure of AV node to spread the AP from atria to ventricles
Fairly common
Duration of PR interval is key
Prolonged PR interval (>200ms)
Actually PR segment but it’s so small so interval is assessed
First degree AV block
Prolonged PR interval (>200ms) due to slowed conduction through AV node or bundle of his
One P wave for every QRS complex
Patients usually asymptomatic
Second degree AV block
Partial dissociation of atria and ventricles
Not every P wave is followed by QRS complex
Roughly 2:1
May require artificial pacemaker
Third degree AV block (complete heart block)
No conduction through AV node
Complete dissociation of atria and ventricles
P and QRS act totally independent of each other
May require artificial pacemaker
