EKG Equipment, Lead Placement, and Tracing Acquisition
Machine settings
Standard 12-lead EKGs run at 25 mm/s paper speed and 10 mm/mV gain; the standardization mark at the start of each row is 10 mm tall and 2 mm wide (0.2 s). Verify calibration before you measure intervals - changing gain alters amplitude but not duration.
Limb and augmented leads
Electrodes go on the right arm (RA), left arm (LA), left leg (LL), and right leg (RL, used as ground). Lead I compares LA to RA, lead II compares LL to RA, and lead III compares LL to LA. The augmented leads aVR, aVL, and aVF view the same vectors from different angles.
Precordial placement
The six chest leads follow a predictable march: V1 at the 4th intercostal space, right sternal border; V2 at the 4th intercostal space, left sternal border; V3 halfway between V2 and V4; V4 at the 5th intercostal space, midclavicular line; V5 at the 5th intercostal space, anterior axillary line; V6 at the 5th intercostal space, midaxillary line. Place electrodes on bare skin, avoiding bone, dense muscle, and breast tissue; clip hair as needed.
Acquisition
Ask the patient to lie still, breathe normally, and avoid talking during the recording. Acquire all leads, check each for artifact and loose electrodes, and repeat the tracing if quality is poor. Label the tracing, document symptoms and events, and keep the patient in view.
Traps
Placing V1 and V2 too high is the most common error and can mimic or hide infarction. Reversing RA and LA inverts lead I and swaps aVR and aVL, giving a negative P and QRS in lead I. Electrode colors differ between AHA and IEC conventions, so identify leads by name under facility policy rather than color alone.
Rule of thumb: 4th intercostal space for V1-V2, 5th for V4-V6 - walk down and around the chest.