12-Lead ECG & Rhythm Strip

Objective 1.1.1 — Perform a 12-lead electrocardiogram and rhythm strip.

The electrocardiogram (ECG) records the electrical activity of large masses of atrial and ventricular cells as specific waveforms and complexes. A 12-lead ECG views the heart from 12 different angles in both the frontal and horizontal planes — most of those angles look at the surfaces of the left ventricle.

ECGs are used to monitor a patient's:

• Heart rate
• Effects of disease or injury on heart function
• Pacemaker function
• Response to medications (e.g., antiarrhythmics)
• Baseline before, during, and after medical procedures

The heart's electrical activity is observed via electrodes on the skin, connected by cables to the ECG machine.

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Electrodes & Placement

A 12-lead ECG uses 10 electrodes:

• 4 limb electrodes (right arm, left arm, left leg, right leg)
• 6 chest electrodes (V1–V6)

Limb Electrode Placement (Frontal Leads)

• Limb electrodes are usually placed on the wrists and ankles but may be positioned anywhere on the appropriate limb.
• To reduce muscle tension, make sure the patient's limbs are resting on a supportive surface.
• Do not apply electrodes over bony areas, broken skin, joints, scar tissue, burns, or rashes.

Chest Electrode Placement (Horizontal Plane Leads)

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Performing an ECG

Prepare the equipment

• Ensure the machine has adequate ECG paper
• Gather supplies: gloves, gauze, electrodes, skin cleanser
• Connect the ECG cable to the machine
• Inspect for frayed/broken cables or lead wires

Prepare the patient

• Bring the patient into the room, introduce yourself, and explain the procedure
• Have the patient remove garments from the waist up
• Prep the skin to minimize tracing distortion:
  • Briskly rub with a dry gauze pad and skin cleanser (usually soapy water)
  • Shave small amounts of chest hair if needed for good electrode contact
• Remove the backing from each electrode and apply in the appropriate location
• Connect lead wires to the electrodes

Acquire the tracing

1. Coach the patient to relax
2. Adjust the ECG size if necessary
3. Press print, rhythm, or record to capture the tracing
4. Verify the tracing is free of artifact before removing cables and electrodes

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Artifact

Artifact is distortion of the ECG tracing by electrical activity that is non-cardiac in origin.

Artifact can mimic cardiac dysrhythmias — including ventricular fibrillation. Always evaluate the patient first before initiating any medical intervention.

Common causes:

• Patient movement / muscle tremor (shivering, Parkinson's, talking)
• Loose or dried-out electrodes
• Broken or frayed lead wires
• 60-cycle electrical interference from nearby equipment
• Improper skin prep (oil, lotion, sweat)

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ECG Paper Measurements

ECG paper is a grid where the horizontal axis is time and the vertical axis is amplitude (voltage).

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Waveform Deflection from Baseline

The shape of each waveform depends on the direction of the depolarization wave relative to the recording electrode:

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Waveform Interpretation

P Wave

Represents atrial depolarization.

• The first half is recorded as the impulse leaves the SA node and stimulates the right atrium, then reaches the AV node
• Smooth and rounded
• No greater than 2.5 mm / 0.25 mV tall
• No greater than 0.11 seconds wide
• Positive in leads I, II, aVF, and V2–V6

QRS Complex

Represents ventricular depolarization. Because of the left ventricle's greater muscle mass, the QRS predominantly reflects left ventricular activity.

• Q wave — depolarization of the interventricular septum (activates left to right). Begins as a downward deflection.
• R wave — large, upright, triangular waveform; always positive
• S wave — the negative waveform following the R wave
• The R and S waves represent simultaneous depolarization of the right and left ventricles

Normal QRS duration: ≤ 0.11 seconds

T Wave

Represents ventricular repolarization.

At the peak of the T wave, the Relative Refractory Period (RRP) has begun — a stronger-than-normal stimulus during the RRP can produce a ventricular dysrhythmia.

Abnormal T Wave

U Wave

Represents repolarization of the Purkinje fibers.

• Small upright waveform that, when seen, follows the T wave
• Usually less than 0.1 mV
• Negative U waves in V2–V5 may be seen in ischemic heart disease or hypertension

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Segments & Intervals

PR Segment

The line between the end of the P wave and onset of the QRS complex. Represents activation of the AV node, bundle of His, bundle branches, and Purkinje fibers.

PR Interval (PRI)

Begins with the onset of the P wave and ends with the onset of the QRS complex.

ST Segment

Represents the early part of ventricular repolarization — the segment between the QRS complex and the T wave.

ST segment elevation varies and depends on the patient's age, sex, and ECG lead.

J Point

The point where the QRS complex and the ST segment meet.

QT Interval

Represents total ventricular activity — from depolarization (activation) through repolarization (recovery). Varies with age, sex, and heart rate.

Generally, the QT interval should be less than 50% of the R-R interval.

Conditions associated with a prolonged QT:

• Certain drugs (e.g., antiarrhythmics, some antibiotics, antipsychotics)
• Hypocalcemia
• Marked bradyarrhythmias
• Intracranial hemorrhage
• Myocarditis

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Leads

A lead is a tracing of electrical activity between two electrodes. Each lead records the average current flow at a specific time in a portion of the heart.

Standard & Augmented Limb Leads (Frontal Plane)

Formed by leads I, II, III, aVL, aVF, and aVR — they view the frontal plane of the heart and rely on Einthoven's triangle (RA, LA, LL, with RL as ground).

Standard limb leads (bipolar)

Lead II is the most commonly used lead for cardiac monitoring — its electrode orientation most closely resembles the normal pathway of current flow in the heart.

Augmented limb leads (unipolar)

A single positive electrode forms each lead.

Horizontal Plane Leads (V1–V6)

Unipolar chest leads that view the heart as if the body were sliced in half horizontally — directions are anterior, posterior, right, and left.

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Mnemonic — "SALI" (which leads see which wall)

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Video Resources

• 12-Lead ECG Placement
• ECG Interpretation Basics
• 12-Lead ECG Lead Views