EKG Acquisition, Lead Placement, and Troubleshooting
About 28% of the exam covers the mechanics of actually obtaining a tracing: where each electrode goes, how the machine is calibrated, how the patient is prepared and positioned, and how to recognize and fix the artifacts that make a tracing unreadable. This is the domain where an inch of misplacement changes what the physician sees, so the anatomic landmarks are worth memorizing exactly. Everything here assumes the standard settings of 25 mm/sec paper speed and 10 mm/mV standardization unless a deliberate, documented change is made.
The 12-Lead: Ten Electrodes, Twelve Views
A 12-lead EKG uses ten electrodes: four on the limbs and six on the chest. The four limb electrodes generate the six frontal-plane leads, which are the bipolar leads I, II, and III and the augmented leads aVR, aVL, and aVF. The six chest electrodes generate the precordial leads V1 through V6, which look at the heart in the horizontal plane. The chest positions are found by locating the sternal angle, sliding laterally to the second rib, and counting down the intercostal spaces. Place V1, V2, and V4 by counting spaces, then fill in V3 as the midpoint between V2 and V4, then place V5 and V6 horizontally in line with V4 rather than by counting ribs, because the chest wall curves downward as you move laterally.
Continuous Monitoring: 3-Lead and 5-Lead Systems
Continuous monitoring answers a different question than a diagnostic 12-lead. It watches rhythm over time rather than capturing a detailed one-time picture, so it uses fewer electrodes placed on the torso where they will not be dislodged by movement. A 3-lead system uses white, black, and red electrodes on the right shoulder, left shoulder, and left lower chest, producing leads I, II, and III; lead II is usually chosen for display because it shows P waves clearly. A 5-lead system adds a green ground and a brown chest electrode, allowing all six limb leads plus one modified chest lead. Telemetry sends the same signal by radio to a central station so an ambulatory patient can be watched continuously.
Machine Settings, Calibration, and the Standardization Mark
The rectangular mark printed at the start of a tracing is the standardization mark, and at normal sensitivity it is 10 millimeters tall, meaning 1 millivolt produces a 10 millimeter deflection. This is what allows a tracing taken in one facility to be compared with one taken years earlier somewhere else. The horizontal axis is time at 25 mm/sec, so a small box is 0.04 second, a large box is 0.20 second, five large boxes are one second, and 1500 small boxes pass in a minute. Both settings can be deliberately changed, and both changes must be documented, because an undocumented change causes an interpretation error rather than a technical one.
Skin Preparation, Positioning, and Patient Comfort
Most bad tracings are made before the machine is ever switched on. Good contact requires clean, dry, oil-free skin: clip excess hair at the electrode site only, wipe with alcohol, let it dry completely, and lightly abrade with dry gauze to lower skin resistance. Do not apply lotion, do not place an electrode over broken skin, a wound, a scar, or a bony ridge, and do not use electrodes whose gel has dried. The standard position for a resting 12-lead is supine with arms relaxed at the sides and legs flat and uncrossed. If a patient cannot lie flat, raise the head of the bed to a tolerable position and document it, because position affects amplitude and axis. Explaining the procedure in plain language is a technical step as much as a courtesy, since a tense patient generates tremor artifact.
Artifact and Troubleshooting
Three artifacts account for most unreadable tracings, and each has a distinctive appearance that points directly to its cause. A wandering baseline drifts slowly up and down and comes from poor electrode contact, lotion or oil on the skin, cable tension, or deep respiration. Somatic tremor is irregular jagged spiking from muscle movement, shivering, tension, or a movement disorder. Sixty-cycle AC interference is a uniform fuzzy thickening of the baseline caused by nearby alternating current. A fourth pattern, artifact confined to a single lead, points to one loose electrode or one damaged lead wire, since each lead uses a different combination of electrodes. Lead reversal is a separate category: it produces a clean but wrong tracing, which is why it is the most dangerous of all, and the classic sign is global inversion of P wave, QRS, and T wave in lead I from a right arm and left arm swap.
Last updated: July 2026