Essentials of Internal Medicine – we have followed the same core principles that inspired us in our approach to the last three editions
An electrocardiogram is a graphical record of the change in membrane potential generated during cardiac muscle depolarization and repolarization.
The ECG records the depolarization (stimulation) and repolarization (recovery) potentials generated by atrial and ventricular myocardium and spread all over body. This electric signals are detected by means of electrode (called lead) attached to the extremities and chest wall and are amplified and recorded on a (millimeter) graph paper.
The graph paper is divided into 1 mm2 grid-like boxes. Since the universal ECG paper speed is 25 mm/sec, the smallest 1 mm horizontal division corresponds to 0.04 second with heavier line at interval of 5 small square which is equal to 0.2 second. Vertically the ECG graph measure the amplitude of a specific wave 1 mV = 10 mm with standard calibration.
The conventional ECG is recorded by 12 lead of which 6 are extremity limb lead and 6 are precordial or chest lead. Out of 6 limb leads. Three are bipolar lead [lead-I, leadII, lead-III] and three are unipolar (aVR, aVL, aVF).
Bipolar leads are
• Lead-I → Left arm potential – Right arm potential
• Lead-II → Left leg potential – Right arm potential
• Lead-III → Left leg potential – Left arm potential.
Unipolar limb leads are—aVR, aVL and aVF.
In these unipolar leads potential of right arm, left arm and left leg are recorded against ‘zero’ potential which is made within machine by joining the lead of all four limb and passing it through in resistance.
Standard six precordial lead are V1 – V6 . These are unipolar lead and one electrodes is placed on the following position and the second electrode is zero potential.
• V1 on 4th intercostal space just right of sternum.
• V2 on 4th intercostal space just left of sternum.
• V3 on midway between V2 –V4
• V4 on midclavicular line on 5th space.
• V5 on anterior axillary line on the same plane of V4
• V6 on midaxillary line on the same plane of V4 and V5
• V7 on posterior axillary line on the same plane of V4 V5 V6.
• V8 on posterior scapular line on the same plane of V4 V5 V6 V7
In case of dextrocardia precordial leads are placed on the corresponding position on the right side of the chest and are called V2 R to V6 R respectively