The 12-lead ECG is a vital tool for EMT’s and paramedics in both the prehospital and hospital setting. It is extremely important to know the exact placement of each electrode on the patient. Incorrect placement can lead to a false diagnosis of infarction or negative changes on the ECG.

Electrode Placement

Electrode Misplacement

• Up to 50% of cases have the V1 and V2 electrodes in a more superior location, which can mimic an anterior MI and cause T wave inversion.
• Up to 33% of cases have the precordial electrodes (V1-V6) inferiorly or laterally misplaced, which can alter the amplitude and lead to a misdiagnosis.

Electrode Reversal

• RA/LA Reversal: Lead I is inverted, Lead II and III are reversed, aVR and aVL are reversed
• RA/RL Reversal: Lead II shows isolated asystole, aVF and aVR are identical
• LA/LL Reversal: Lead III is inverted, aVL and aVF are reversed

Lead Groups

The ECG leads are grouped into two electrical planes. The frontal leads (Lead I-III, aVR-F) view the heart from a vertical plane, while the transverse leads (V1-V6) view the heart from a horizontal plane.

12-Lead Explained

One of the most common questions regarding a 12-lead ECG is why there are only 10 electrodes. It’s important to fully understand what the term “lead” actually means. A lead is a view of the electrical activity of the heart from a particular angle across the body. Think of a lead as a picture of the heart and the 10 electrodes give you 12 pictures. In other words, a lead is a picture that is captured by a group of electrodes.

Reducing Artifact

The heart’s electrical signal is very small and unfortunately this can be combined with other signals of similar frequency to create artifact. It’s not uncommon for 12-lead ECG’s to have some form of artifact; however, it’s important to try to reduce any interference to ensure an accurate ECG. Below is a list of guidelines that will help reduce artifact when performing ECG’s.

Patient Positioning

• Place the patient in a supine or semi-Fowler’s position. If the patient cannot tolerate being flat, you can do the ECG in a more upright position.
• Instruct the patient to place their arms down by their side and to relax their shoulders.
• Make sure the patient’s legs are uncrossed.
• Move any electrical devices, such as cell phones, away from the patient as they may interfere with the machine.

Skin Preparation

• Dry the skin if it’s moist or diaphoretic.
• Shave any hair that interferes with electrode placement. This will ensure a better electrode contact with the skin.
• Rub an alcohol prep pad or benzoin tincture on the skin to remove any oils and help with electrode adhesion.

Electrode Application

• Check the electrodes to make sure the gel is still moist.
• Do not place the electrodes over bones.
• Do not place the electrodes over areas where there is a lot of muscle movement.

Visit the ECG section to learn how to interpret 12-lead ECG's and identify the different types of heart rhythms.

The PR, QRS, and QT are intervals (a portion of the baseline and at least one wave) that are measured in seconds on an ECG.

PR Interval

The PR Interval is calculated from the beginning of the P wave to the beginning of the QRS complex. It represents the time it takes the heart’s electrical impulse to travel from the atria through the AV node, bundle of His, and left/right bundle branches.

• Normal Duration: 0.12-0.20 seconds (120-200 ms)
• Prolonged PR Intervals represent a conduction delay through the atria or AV node.

QRS Complex

The QRS complex follows the P wave and represents the depolarization (contraction) of the ventricles.

• Normal Duration: 0.06-0.12 seconds (60-120 ms)
• Prolonged QRS complexes represent a conduction defect at the Bundle of His or in the bundle branches.

QT Interval

The QT interval represents the depolarization and repolarization of the ventricles.

• Normal Duration: 0.36-0.44 seconds (360-440 ms)

A sinus rhythm occurs when the SA node acts as the primary pacemaker of the heart.

Normal Sinus Rhythm

• Rate: 60-100 bpm
• Rhythm: Regular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: Normal and equal
• R-R Intervals: Equal
• QRS Complexes: Normal and after each P wave

Sinus Arrhythmia

• Rate: 60-100 bpm
• Rhythm: Irregular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: Normal and equal
• R-R Intervals: Unequal
• QRS Complexes: Normal and after each P wave

Sinus Bradycardia

• Rate: < 60 bpm
• Rhythm: Regular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: Normal and equal
• R-R Intervals: Equal
• QRS Complexes: Normal and after each P wave

Sinus Tachycardia

• Rate: 100-150 bpm
• Rhythm: Regular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: Normal and equal
• R-R Intervals: Equal
• QRS Complexes: Normal and after each P wave

Sinus Arrest

• Rate: 60-100 bpm
• Rhythm: Regular, except during a pause
• P Waves: Identical in shape and before each QRS complex, but are absent during a pause
• PR Intervals: Normal and equal, but are not measurable during a pause
• R-R Intervals: Equal, except during a pause
• QRS Complexes: Normal and after each P wave, but are absent during a pause

An atrial rhythm occurs due to abnormal electrical impulses that start in the atria or around the AV Node.

Atrial Fibrillation

• Rate: < 100 bpm (controlled) > 100 bpm (uncontrolled)
• Rhythm: Irregular
• P Waves: Absent or indiscernible
• PR Intervals: Not measurable
• R-R Intervals: Unequal
• QRS Complexes: Normal

Atrial Flutter

• Rate: 250-350 bpm (atrial)
• Rhythm: Regular
• P Waves: Absent and replaced with a “sawtooth” pattern
• PR Intervals: Not measurable
• R-R Intervals: Equal
• QRS Complexes: Normal

Supraventricular Tachycardia

• Rate: > 150 bpm
• Rhythm: Regular
• P Waves: Absent or indiscernible
• PR Intervals: Not measurable
• R-R Intervals: Equal
• QRS Complexes: Normal

A junctional rhythm occurs when the AV node takes over as the primary pacemaker because either the SA node failed or the AV node blocked the atrial impulse.

Junctional Rhythm

• Rate: 40-60 bpm
• Rhythm: Regular
• P Waves: Absent, inverted or after the QRS
• PR Intervals: Not measurable, unless P wave is inverted and present
• R-R Intervals: Equal
• QRS Complexes: Normal and after each inverted P wave, if present

Accelerated Junctional Rhythm

• Rate: 60-100 bpm
• Rhythm: Regular
• P Waves: Absent, inverted or after the QRS
• PR Intervals: Not measurable, unless P wave is inverted and present
• R-R Intervals: Equal
• QRS Complexes: Normal and after each inverted P wave, if present

Junctional Tachycardia

• Rate: > 100 bpm
• Rhythm: Regular
• P Waves: Absent, inverted or after the QRS
• PR Intervals: Not measurable, unless P wave is inverted and present
• R-R Intervals: Equal
• QRS Complexes: Normal and after each inverted P wave, if present

Read the Junctional Rhythm: A comprehensive overview article to learn more about junctional rhythms.

An atrioventricular (AV) block occurs when atrial depolarizations fail to reach the ventricles or when there is a delay in atrial depolarization conductions.

First Degree AV Block

• Rate: 60-100 bpm
• Rhythm: Regular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: > 0.20 seconds and equal
• R-R Intervals: Equal
• QRS Complexes: Normal and after each P wave
• Key Points: The First Degree AV Block is simply a prolonged delay in conduction. The PR interval, which represents the time it takes the heart’s electrical impulse to travel from the atria through the AV node, is normally between 0.12-0.20 seconds. In this case, the PR interval is > 0.20 seconds and remains constant.

Second Degree AV Block Type I (Mobitz Type I – Wenckebach)

• Rate: 60-100 bpm
• Rhythm: Regularly irregular
• P Waves: Identical in shape and before each QRS complex (when present)
• PR Intervals: > 0.20 seconds and get progressively longer until a QRS complex is “dropped”
• R-R Intervals: Equal when QRS complex is present
• QRS Complexes: Normal and after each P wave, but are absent periodically
• Key Points: The Second Degree AV Block Type I is characterized by a progressively prolonged PR interval. Electrical impulses traveling through the AV node take longer and longer until one impulse is blocked completely.

The next QRS complex on this rhythm strip would be "dropped" and it would look like a pause. The cycle would then restart and the PR intervals would continue to get longer and longer before another QRS complex is "dropped".

Second Degree AV Block Type II (Mobitz Type II – Hay)

• Rate: 60-100 bpm
• Rhythm: Regular, regularly irregular or irregular (based on the ratio of QRS complexes)
• P Waves: Identical in shape and before each QRS complex (when present)
• PR Intervals: Normal and equal or > 0.20 seconds and equal
• R-R Intervals: Equal or unequal
• QRS Complexes: Normal and after each P wave, but are absent periodically
• Key Points: The Second Degree AV Block Type II is when a conduction delay occurs below the AV node, either at the bundle of His or bundle branches. The ECG will show a pattern of conducted P waves (constant PR interval), followed by one or more non-conducted P waves.

Third Degree AV Block (Complete Heart Block)

• Rate: 30-60 bpm (ventricular) 60-100 bpm (atrial)
• Rhythm: Regular
• P Waves: Identical in shape and occurring independently of the QRS complexes
• PR Intervals: Unequal (disassociation between P waves and QRS complexes)
• R-R Intervals: Equal
• QRS Complexes: Normal or > 0.12 seconds (ventricular pace site)
• Key Points: The Third Degree AV Block is indicated by a complete disassociation between the atria and ventricles. The SA node generates an electrical impulse that causes the atria to contract, but the impulse is then blocked and never reaches the ventricles. This causes an accessory pacemaker in the ventricles to initiate and contract the ventricles at their own slower rate. Because two independent electrical impulses occur — the SA node and accessory pacemaker impulse — there is no relationship between the P waves and QRS complexes.