Download Cardio81-ECGPt2

Cardiopulmonary I
Tues., 01/28/03, 11am
Dr. Downey
Corey Fischer
Page 1 of 6
ECG 2
Vectorial analysis of ECG (cont)
 Vector of ventricular depolarization
(-) electrode
on right arm
(+)
electrode
on left arm
Horizontal axis  produced by the limb lead I system
o Determining Normal depolarization vector (A)
resolved into a horizontal component:
--draw a line perpendicular to axis to touch the end of the vector (A)
--deflection will be B-units in this limb system
–use other limb leads to determine resultant vector A

Axis System
Axes system for each limb lead:
I – horizontally
II – downward to the left
III – downward to the left
(clockwise to axis system)
ANGLES  downward is positive
Actual depolarization process is A
o what is measured is the 3 limb lead deflections: B (limb lead I), C (limb
lead II), and D (limb lead III)
o polarity of deflections are also measured: B is (+), C is (+), D is (+)
o determine A by drawing perpendicular lines from the magnitude of
each limb lead (I, II, III) to determine magnitude of A
Cardiopulmonary I
Tues., 01/28/03, 11am
Dr. Downey
Corey Fischer
Page 2 of 6

Change of waveform in QRS complex
1.
2.
3.
4.
1. Atrial depolarization
 begins in SA node and spreads over atria and to the AV node
 produces an upright deflection = P wave
2. Wave enters interventricular septum
 depolarization extends from left to right predominantly = Q wave
 Q wave tells us that there is a depolarization away from the positive
electrode in limb lead I
-away from left arm and to the right arm – thus negative deflection
3. Depolarization thru the ventricles
 greater left ventricular muscle mass = upright deflection of R wave
 as depolarization becomes complete, the R wave rises, and then goes
down
 R wave is at peak when half of the ventricle is depolarized
4. Final left ventricular depolarization
 little bit of depolarization away from (+) electrode (rt arm) = small
negative S wave
Cardiopulmonary I
Tues., 01/28/03, 11am
Dr. Downey
Corey Fischer
Page 3 of 6

Limb Lead Systems
 3 limb lead system
 augmented limb lead
system
 Horizontal plane
depolarization
ECG is a voltmeter with (+)
and (-) input
augmented limb lead R  (+) electrode placed on right arm
 (-) input to ECG is the electrical sum of the left leg and left arm
 thus direction of axis is upward and toward right arm
augmented limb lead L  (+) electrode placed on left arm
 (-) input to ECG is the electrical sum of the left leg and right arm
 thus, direction of axis is upward and toward left arm
augmented limb lead F  (+) electrode placed on left leg
 (-) input to ECG is the electrical sum of the left arm and right arm
 thus, direction of axis is down and left toward left leg
Augmented and 3-lead system use same leads, but different machines
-these 2 systems provide info about depolarization in the vertical plane
Horizontal plane depolarization process
chest leads = VI – V6 (know where they go – in picture)
 (+) input is from each lead VI – V6
 (-) input is from combination of the the 3-limb lead positions
 V6 – on left side of chest produces large upright deflections
(depolarization down and left in ventricles)
 V1 – on right side of chest produces a net negative QRS complex
(depolarization away from (+) V1)
Cardiopulmonary I
Tues., 01/28/03, 11am
Dr. Downey
Corey Fischer
Page 4 of 6
Augmented Lead System


difference in the orientation of the QRS complex in different lead systems
aVR  depolarization away from right arm (-) thus, net negative complex
Axes of 3 limb leads
I=0
II = 60
III = 120
Augmented limb leads
(bisect limb leads)
Determine direction of depolarization with augmented limb leads:
 lead with largest deflection is most parallel with direction of depolarization
ECG and Cardiac Myopathies
1. Ventricular Hypertrophy
 Magnitude and Orientation of Ventricular Depolarization
 -used to determine if a ventricle has hypertrophied
 -more muscle mass to depolarize
 -greater sum of depolarization processes in area of hypertrophy
 -larger amplitudes of QRS complexes
Cardiopulmonary I
Tues., 01/28/03, 11am
Dr. Downey
Corey Fischer
Page 5 of 6
eg/ right ventricular hypertrophy
 due to pulmonary artery stenosis – pulmonary valve inadequate
 right ventricle must generate greater pressure for normal output
 detected on ECG with:
large negative wave in limb lead I
axial deviation to the right
eg/ left ventricle hypertrophy
o due to hypertension (greater pressure in aorta)
o left ventricle must generate greater pressure for output
o detected on ECG with:
left axis deviation
2. Ischemia
Ischemia’s effect on S-T segment
“S-T segment elevation or depression indicates myocardial ischemia”
-clinical lingo, but REALLY:
 during diastole, part of heart is depolarized (with ischemia)
 diastolic period (T  P) will be shifted away from baseline
 baseline (isoelectric line) is set at 0, therefore appears as S -T deviation
3. Cardiac Arrhythmias
Tachycardia = fast heart rate
>100 beats / min
Bradychardia = slow heart rate
<60 beats / min
see examples of ECG in handout
4. Sinus Arrhythmia
Cardiac rhythm changes with time and affects heart rate
Deeper respiration exaggerates sinus rhythm
Sinus rhythm controlled by sympathetic and parasympathetic systems and
by amount of blood entering atria (stretches pacemaker cells)
5. Nodal Block
SA block  No P wave or ventricular waves on ECG
-ventricular depolarization begin later, but still no P waves
AV block  prolonged P-R interval = 1st degree AV block
2nd degree AV block
failure to conduct excitation thru AV node
dropped beat – no QRS with a P wave
rd
3 degree AV block
P waves with very few QRS
Cardiopulmonary I
Tues., 01/28/03, 11am
Dr. Downey
Corey Fischer
Page 6 of 6
6. Premature Contraction
- beats that occur earlier than expected
Atrial  atrial contraction earlier than expected
Ventricular
o PVC (Premature ventricular contraction)
o Different characteristics than normal QRS complex
o Arises from ectopic location in ventricle, and is conducted in
different pattern thru ventricles
-took more time without Purkinje system
o longer QRS duration, and greater amplitude
o multifocal PVCs  occur in different places in ventricle
7. One-Way Conduction
-causes reentry
o enlarged ventricle that conducts in only one direction (ischemic)
o reentry = tissue that has been previously excited is excited again with
same action potential
becomes a continuous circular process of depolarization
types of reentry:
atrial and ventricular flutter
o regular, circular reentry
atrial and ventricular fibrillation
o less organized reentry
o ventricular fibrillation – ventricle can’t
contract
treated with defibrillator