Electrocardiography
Voltage produced when charges are in motion.
Two types of leads – bipolar and unipolar (+)
12 Lead ECG has 3 bipolar leads (I, II, III) and 9 unipolar leads (aVR, aVL, aVF) and (V1-V6). 10 electrodes
Bipolar leads (limb leads) record potential difference between 2 electrodes. 4th electrode on right leg serves as an electrical ground. Positive poles of these leads lie inferiorly and to the left.
6 leads: limb leads (I, II, III) and augmented leads (aVR, aVL, aVF) are in coronal plane and are derived from the four limb electrodes.
Augmented leads fill gaps between limb leads. Lead aVR stands opposite from others, superior and to the right.
The precordial leads (V1-V6) are in horizontal plane. They point positively towards the chest. add 3D.
A positive vector towards a positive lead will have a positive deflection on ECG.
Cardiac repolarization is opposite of cellular depolarization (positive T wave). Due to repolarization delay (out to in) away from positive electrode.
Charge reverse in endocardium
PR interval is time between beginning of P wave and beginning of QRS complex. Time through AV node (0.2 sec in adult)
QRS Complex – (0.08 sec)
Initial negative = Q
Initial positive = R
Negative after R = S
ST segment – no signature – phase 2 – very important.
T wave is repolarization. Beginning of QRS complex to T wave is QT interval, a measure of AP duration (.44 sec) (should be less than 1/2 RR interval). T wave abnormalities are common.
SAN depolarization gives positive deflection in I and aVF
As it passes from left ventricle to right ventricle, you get negative Q wave in lead I and a positive R wave in aVF.
Endocardium > Epicardium gives a positive R wave in I and aVF
Depolarization of left lateral wall is last, gives R wave in I and negative S wave in aVF
Repolarization is superior and to the right giving T wave a positive deflection in I and avF
Thin lines are .04 sec, thick lines are .20 seconds (25mm/sec). Each lead displayed for 2.5 seconds.
Right Ventricle is tough to see.
- Inferior wall of left ventricle – II, III, aVF
- Lateral – I, aVI, V5, V6
- Anterior – V1-V4
- Posterior – V1, V2, V8, V9
- Right Ventricular – RV1-RV6
