The Heart's Electrical System and ECG Setup

This is Unit Twenty-Two of Medical Assistant Prep. Units Nineteen through Twenty-One drew and handled blood. This unit turns to another common test, the electrocardiogram: what the heart's electrical system does, and how to set up a twelve-lead recording. Carry one idea through this lecture. The quality of a tracing is decided before the record button is pressed, by preparation, accurate landmarks and a still, relaxed patient, and the tracing itself is the provider's to interpret. Two constructed setups at the end show that idea. By the end you will be able to trace the heart's conduction pathway. To name the waves and read the paper's timing. To place limb and chest electrodes by landmark. And to record a clean tracing and hand it to the provider.

Six entries on the heart's electrical system: what an ECG records, the pathway, pacemakers, waves and the paper.

Electrical activity.

MAS 526 What an Electrocardiogram Records

An electrocardiogram, or ECG, records the heart's electrical activity through electrodes placed on the skin. The electrodes pick up the small electrical changes of the heart muscle and print them on special graph paper or show them on a monitor. Recording the tracing can be your task; interpreting it belongs to the provider.

SA node to ventricles.

MAS 527 The Conduction Pathway

Each heartbeat follows an electrical pathway through the heart. The signal starts in the sinoatrial node in the right atrium, passes to the atrioventricular node, then down the bundle of His and the left and right bundle branches into the ventricles. A problem anywhere along this path can change the tracing.

The SA node's rate.

MAS 528 The Heart's Natural Pacemaker

The sinoatrial, or SA, node is the heart's natural pacemaker. One reference gives its normal rate as sixty to one hundred beats a minute; another says it fires roughly seventy to eighty times a minute on its own, before nerves adjust the rate. Rates outside the normal range are reported, not explained.

Slower and slower.

MAS 529 Backup Pacemakers

If the SA node fails, other parts of the heart take over as slower backup pacemakers. The atrioventricular node is the first backup; if every other backup fails, fibers in the ventricles pace the heart at only twenty to forty beats a minute. A very slow rate can mean the normal pacemaker is failing; report it at once.

P, QRS, T.

MAS 530 The Main Waves

A normal tracing shows a repeating pattern of waves: the P wave, the QRS complex and the T wave. The P wave reflects the atria firing; the larger QRS complex reflects the ventricles firing and contracting; the T wave reflects the ventricles recovering. Learn the names so you can describe what you see, not so that you can diagnose it.

Boxes measure time.

MAS 531 Reading the Graph Paper

ECG paper is a grid of small and large boxes that measure time. Each small box, one millimeter wide, equals four hundredths of a second; each large box, five millimeters wide, equals two tenths of a second. Time measurements depend on the machine's standard settings, so use the settings the practice has configured.

Five entries on leads and preparation: twelve views, which leads see which part, explaining the test, and preparing patient and skin.

Ten electrodes.

MAS 532 Twelve Views From Ten Electrodes

A twelve-lead ECG records the heart's electrical activity from twelve different angles, giving a complete picture. The twelve leads are produced from ten electrodes: four on the limbs and six on the chest. A lead is a view, not a wire. Ten electrodes produce twelve views.

Which leads see where.

MAS 533 Which Leads See Which Part

Each group of leads looks at a different area of the heart. Leads two, three and aVF view the inferior surface; leads one, aVL, V five and V six view the lateral wall; V one and V two view the septum; V three and V four view the anterior wall. Misplaced electrodes change these views, which is why placement matters.

No shock.

MAS 534 Explaining the Test

Explain the ECG to the patient before you begin. It takes only a few minutes; electrodes stick to the arms, legs and chest; the patient lies still; and the machine only records, sending no electricity into the body. Patients often fear a shock; say plainly that there is none.

Safety steps.

MAS 535 Preparing the Patient

Preparation follows the same safety steps as any procedure. Verify the order, identify the patient with two identifiers, check allergies, provide privacy, and enter the patient's information into the machine as required. Note the patient's age, cardiac medications and any chest pain, because the provider will want them with the tracing.

Oil, moisture, hair.

MAS 536 Preparing the Skin

Electrodes need good skin contact to record a clean tracing. Remove oil, moisture and excess hair where electrodes will go; check that the electrodes are not past their expiration date. Poor contact between the electrodes and the skin is a leading cause of a noisy tracing, so take the time to prepare each site.

Six entries on placement: limb electrodes, the chest leads, right-sided leads, adapted placement and telemetry.

On the limbs, not the torso.

MAS 537 Placing the Limb Electrodes

For a twelve-lead ECG, the four limb electrodes go on the limbs, not the torso. They can be placed anywhere on the arms below the armpit fold and anywhere on the legs, away from fatty areas, large muscles and bony points. Moving limb electrodes onto the torso is acceptable for monitoring, but not for a diagnostic twelve-lead.

The fourth intercostal space.

MAS 538 Placing V One and V Two

The first two chest electrodes sit either side of the breastbone in the fourth intercostal space: V one at the right edge of the sternum, V two directly across at the left edge. These two set the level for the rest, since V four goes one space lower and V three between them. Placed a space too high or too low, they shift the whole chest pattern, because every other chest electrode is placed relative to them. Find the fourth intercostal space with care first, and recheck it if the patient moves.

Level across the side.

MAS 539 Placing V Three to V Six

The other four chest electrodes follow landmarks on the left side. Place V four first, in the fifth intercostal space on the midclavicular line, down from the middle of the collarbone. Then V three midway between V two and V four. V six goes on the midaxillary line, down from the middle of the armpit, level with V four, and V five on the anterior axillary line, from the front fold of the armpit, at the same level. V five and V six sit level with V four, not following the ribs upward, so check that line across all three before attaching wires.

Mirrored positions.

MAS 540 Right-Sided Leads

In some situations the provider orders right-sided chest leads. These mirror the standard chest positions on the right side of the chest; for example, V four R goes in the fifth intercostal space on the right midclavicular line. Mark a right-sided tracing plainly as right-sided; the machine's automatic interpretation may not apply to it.

A missing or injured limb.

MAS 541 Adapting Limb Electrode Placement

Because limb electrodes can sit anywhere along the arms and legs, placement can be adapted for a patient with a missing or injured limb. Place the electrode on the remaining part of the limb, following the practice's procedure. Note any non-standard placement on the tracing so the provider can interpret it correctly.

Rhythm over time.

MAS 542 Telemetry Is Different

Telemetry is continuous heart-rhythm monitoring, using fewer electrodes than a twelve-lead ECG. In one common five-electrode system, white goes near the right shoulder, black near the left shoulder, red and green on the lower left and right ribs, and brown at the V one position. Telemetry watches rhythm over time; it does not replace a diagnostic twelve-lead.

Five entries on recording: removing interference, recording and checking, finishing, and two constructed setups.

Phones and cords.

MAS 543 Remove Sources of Interference

Nearby electrical devices can disturb an ECG tracing. Before recording, ask the patient to set aside a cell phone or smartwatch, and move nearby power cords away from the patient and the machine's cables. Never unplug life-support or diagnostic equipment to reduce interference; move cords and set aside personal devices instead.

Check before removing.

MAS 544 Recording the Tracing

Once the electrodes and wires are attached, the machine records the tracing. Ask the patient to lie still, and start the recording; the machine captures the heart's electrical pattern in a few seconds. Inspect the printout for quality before you remove a single electrode, so a repeat recording needs no new setup.

Skin, dressing, provider.

MAS 545 After the Tracing

When a good tracing has been recorded, remove the electrodes gently and clean the skin. Check the skin for redness or irritation, help the patient dress, and give the tracing to the provider as the practice directs. Notify the provider of any abnormality you were told to watch for, and of any symptom the patient reported.

A routine twelve-lead.

MAS 546 Setting Up a Twelve-Lead ECG

A constructed setup follows the checklist. The assistant confirms the order, identifies the patient, asks about adhesive allergies, cardiac medications and chest pain, and explains that he must lie still and that no electricity enters his body. Gowned and lying with arms at his sides, he sets aside his phone and watch, and she moves a lamp cord away. She cleans the sites, trims hair, checks the electrodes' date, and places the limb electrodes away from bony points. V one and V two go in the fourth intercostal space at the sternal edges, V four in the fifth at the midclavicular line, V three between, and V five and V six level with V four. She attaches the labeled wires and records. The printout is clean, so she removes the electrodes, checks his skin and gives the provider the tracing. Every shortcut would have shown up as noise or misplaced views.

Cold and frightened.

MAS 547 A Cold, Nervous Patient

A constructed case pays off the opening idea. An older woman arrives shivering on a cold morning, says she hates tests, and grips the table edge. Shivering and tension can put fuzz on a tracing, so the assistant prepares for both. She offers a warm blanket and adjusts the room, explains slowly that the test is painless and sends no electricity, rests the patient's arms at her sides and puts a pillow under her knees. She cleans lotion from each site, checks every electrode is firm, and has the phone set aside. She waits until the patient's breathing settles and her hands relax, then records. The tracing is clean the first time. Preventing artifact took less time than troubleshooting, and spared an anxious patient a second or third attempt.

Look at what you can now do. You can trace the heart's signal from the SA node to the ventricles, name the backup pacemakers, the waves, and the timing on the paper. You can explain how ten electrodes give twelve views, and prepare the patient and the skin. You can place the limb electrodes and the six chest electrodes by landmark, adapt placement when needed, and tell telemetry from a twelve-lead. And you can remove interference, record a clean tracing and hand it to the provider. Unit Twenty-Three turns to recording quality, artifact, and the rhythms the provider reads.