The San Diego Paramedics

12 Lead Cardiography

12 Lead E.C.G. Field Diagnosis Made Easy

rev. 09-03-97

Knowledge of the anatomy of the heart, the physiology of the heart, and the electrocardiography of the heart make a paramedic an
Emergency Cardiac Care Provider.

As primary point of care field paramedics, we have been taxed with the new challenge of diagnosing 12 lead cardiographs in the field and making a definitive diagnosis of acute myocardial infarction. This overwhelming task has been at sometimes difficult to manage and to complicate matters, we do not use this skill regularly enough to maintain the skills.

So, I hope the following information is helpful...

The 12 lead EKG measures electrical potential

Depolarization: The shift in electrolytes which reverses the charge

Repolarization: Return to the resting state

Helpful hints and rules to reading the 12 lead...

Three kinds of leads:

Electrode Placement:

Einthoven's Triangle represents the leads that we all use with our monitors on a regular basis:

1) Standard Limb Leads

The hexaxial view

Leads I, II & III & AVR, AVL, and AVF.

2) Augmented Leads

3) Precordial Leads

The precordial views make up a cross section view of the heart in a transverse horizontal plane projecting a view across the AV Node.

Einthoven's Triangle and the four limb leads make up the "HEXAXIAL VIEW!" This view is a vertical/frontal-posterior - ventral/dorsal plane making a star with 6 points intersecting through the heart in a flat frontal plane across the patients chest.

The PRECORDIAL views are used to make up the other six views of the heart for a total of twelve views.

So, adding this up: lead I, II and III, lead AVR, AVL, AVF, and the 6 precordial leads equals 12 leads... RIGHT?????

Correct, however, we only need 10 electrodes placed.
( 4 on the limbs and 6 on the chest )
The cardiac monitor uses the four Limb Leads to make up Lead I, II, III & AVR, AVL, AVF; six views...

12 lead Quick Triage

The following situation constitutes activation of the cardiac response team at the hospital by reporting the field diagnosis of AMI!

The following situation will result in the 12 Lead ECG being reported as "normal". No subsequent activation of the cardiac response team.

Other Signals to use as a diagnostic tool:

Systematic Infarct Recognition Approach

AMI diagnosis criteria: 1mm. or more of ST elevation in 2 or more contiguous leads.
Anterior wall requires 2mm. or more of ST elevation (V1-V4)
Caution: LBBB

Lead Groups
Anterior - Septal V1, V2, V3, and V4 -- 0.2mV or more in leads
Posterior V1, and V2 -- 0.2mV or more in leads
Inferior II, III, and aVF -- 0.1mV or more in 2 leads
High Lateral I, and aVL -- 0.1mV or more in 2 leads
Low Lateral V5, and V6 -- 0.1mV or more in 2 leads
"ST Depression indicates Angina"

Diagrams below indicate which part the heart is being affected and what lead would show the changes.

Reciprocal Changes

Region of ST Elevation Region of ST Depression
Anterior (leads V1-V4) Inferior (true posterior)
Inferior (leads II, III, aVF) Anterior (leads V1-V3 or lateral lead 1. aVL)
Lateral ( leads I, aVF, V5, V6) Inferior ( leads II, III, aVF)
True Posterior Anterior (leads V1-V3)

12 lead rapid assessment

Electrical Current:

Electricity always flows from positive to negative. The electrical current should flow from negative to positive in the normal healthy heart. So, if this pattern is disrupted by a "detour" or as in the heart, "an infarct" or "injury" the ECG recording will indicate the abnormal flow of current. With an infarcted heart, the electrical current flows opposite of where it is expected to flow. Hence, the elevated or depressed ST segment . For instance, an inferior infarction will show an elevation in lead II, III and aVF. The normal flow would be "isoelectrical" and the ST segment would be equalized or level. But, since the flow is going backwards around the damaged heart muscle, we see an elevation on the record. It is this precise measurement that can dictate exactly where the infarct is located. If the ST segment is elevated in V2, V3 and V4, the infarct is anterior. These views are looking at the front or anterior area of the heart muscle. The current is flowing toward the positive electrode on the patients chest. When the current is disrupted, it will show as an elevation in the ST segment versus an isoelectric reading.

Think of it like this: An X-ray film is placed behind the heart at the area between the Ventricle Septum. The X-Ray machine shoots the picture from the anterior heart directly above the film. and the film captures the image. We would be looking at the area of the heart at the Septal region which would be in ECG terminology V3.

The 12 Lead Photograph
Simultaneous acquisition 2.5 seconds per view, 10 seconds for a complete study
I aVR V1 V4
II aVL V2 V5
12 lead ECG; a real time video recording of the hearts electrical function.

This record indicates a "septal / anterior Infarct."

If you can comprehend which way the current is expected to flow in The HEXAXIAL VIEW and The PRECORDIAL VIEW of the heart, then you can diagnose which area is effected if it is an abnormal flow...

See the information below.

12 lead rapid interpretation
Common ECG Formation
Iscehmia=Inverted T waves
  • Inverted T wave is symmetrical
  • T waves are usually upright in leads I, II, and V2-V6
Injury=Elevated ST segment
  • Signifies an acute process; ST returns to baseline with time
  • If ST elevation is diffuse and unassociated with Q waves or reciprocal ST depression, consider pericarditis
  • Location of injury can be determined in same manner as infarct location
  • Usually associated with reciprocal ST depression in other leads
Infarction=Q wave
  • Small Qs may be normal in V5, V6, I and aVL
  • Abnormal Q must be one small square (0.04 sec) wide
  • Also abnormal if Q-wave depth is greater than one-third of QRS height in lead III

Making the accurate Field Diagnosis:

Table below shows what the ECG would look like in the Vector where the heart is being affected. All other areas would look normal, without elevation or depression. unless there is an "old MI." In that case, the prior damage would show up as a depressed segment.
Anterior Infarction
  • ST elevation without abnormal Q wave
  • Usually associated with occlusion of the left anterior Descending branch of the left coronary artery (LCA)

Lateral Infarction
  • ST elevation with/without abnormal Q wave
  • May be a component of a mutiple-site infarction
  • Usually associated with obstruction of the left circumflex artery

Inferior Infarction
  • ST elevation with/without abnormal Q wave
  • Usally associated with right coronary artery (RCA) occlusion

Right Ventricular Infarction
  • Usually accompanies inferior MI due to proximal occlusion of the RCA
  • Best diagnosed by 1 - 2 mm ST elevation in lead V4R
  • An important cause of hypotension in inferior MI recognized by jugular venous distension with clear lung fields
  • Aggressive therapy is indicated, including: reperfusion, adequate IV fluids for right heart filling, and pacing to maintain A-V synchrony if necessary

Poterior Infarction
  • Tall, broad (>0.04 sec) R wave and ST depression in V1 and V2 (reciprocal changes)
  • Frequently associated with inferior MI
  • Usually associated with obstruction of RCA and or left circumflex coronary artery

Pathological Q waves:

If the Q wave ( the first downward "negative" deflected wave ) is more than 1/3 the size of the R wave ( the first upward deflected "positive" wave ) it is pathological and indicative of an A.M.I.

If no R wave is recorded, then the infarct is extremely acute. There is no electrical activity of the ventricle during polarization and contraction.

Bundle Branch Block
In Bundle Branch Block, the firing of the Ventricles does not occur simultaneously as it should (It occurs in series instead of parallel). Conduction reaches a block in one of the branches (in the cardiac septum) and refers it to the opposing branch to be conducted completely. It is then when conduction jumps the Intra-Ventricular Septum to ultimately conduct to the remaining blocked Bundle Branch. It is because of this that you see two different distinctly separate QRS complexes over-lapping one another. Hence, the "Rabbit Ear" and "RSR pattern." Remember, the QRS complex will always be at least .12 in width and posses abnormal morphology. ALWAYS CHECK RIGHT AND LEFT CHEST LEADS FOR BUNDLE BRANCH BLOCK (V-1, V-2, & V-5, V-6)
Infarction associated with a
Left Bundle Branch Block

A LBBB may result from an acute myocardial infarction (AMI), but field paramedics cannot diagnose AMI in the presence of LBBB. The presence of LBBB negates meaning ful interpretation of other EKG criteria

A LBBB pattern prior to the onset of clinical findings of AMI with marked reduction in voltage of the QRS complex may offer clues to the diagnosis of an infarction.

LBBB obscures the pattern of AMI since the initial QRS vector is abnormally directed in a LBBB pattern. It will obscure the infarction vector and abnormal Q waves will not appear. The most diagnostic feature of AMI is the abnormal direction of the initial 0.04 sec of the QRS vector (ie; the abnormal Q wave).

Diagnosing the Bundle Branch Block:

Right or Left???

The last 0.04 seconds of deflection on the QRS complex is used to determine the direction of the block.

In V1 or MCL1, if the QRS duration is greater that 0.12 seconds (usually 0.14 - 0.20 seconds) and the last 0.04 second segment of the complex is pointing down (negative deflection), the block is LEFT.

If the last 0.04 seconds of the QRS complex is pointing up and is positively deflected, the block is RIGHT.

Infarct Recognition

Some Additional Tips...

Certain easily identifiable ECG changes that are observed in the presence of cardiogenic chest pain, reveal some strong presumptive evidence toward the positive diagnosis of AMI. This pattern of changes is referred to as the "evolution of Myocardial Infarction."

It is often suggested that the first observable evolutionary change is the ischemia we associate with T-wave inversion or ST segment depression. Then, onto what is referred as the hyperacute phase. In the hyperacute phase of the MI, (usually the first few minutes) the T-wave may simply increase in height, and/or the ST segment becomes elevated. The finale phase is the acute phase. In the acute phase, (usually the first hour or more) the ST segment elevation is accompanied by the development of a pathological Q wave. This Q-wave confirms the diagnosis of MI.

This evolution is not precise, however. Often times the T-wave may invert in the presence of ST segment elevation during the end of the hyperacute phase. In any event... the most critical observation should be the recognition of ST elevation in 2 or contiguous leads. This is most important to paramedic in the pre-hospital phase because the development of the Q-wave may take hours and could easily be missed in the field.

Eventually, the ST segment will return to its baseline and the T-wave resumes its normal position, leaving only the Q-wave as evidence that an infarction has occurred. Recent research and studies have produced 95% accuracy in field diagnosis by paramedics. Perhaps some reasons would include other indications for ST changes. They would include simple angina, drug effects, and electrolyte imbalance.

Axis Deviation:

Use Lead I, II, and aVF to diagnose Axis Deviation

Vectors and Axis

For pre-hospital purposes, the axis is either "normal" or "not normal."

As stated above. the electrical current should flow to the positive lead. If it does not flow in a positive direction, the heart is pointing toward the upper right or the left. So, if the QRS is negative in aVF, the heart is pointing more to the left than normal; hence, Left Axis Deviation. If the QRS is negative in Lead I, the heart is pointing more to the right than normal; hence, Right Axis Deviation.

This is very complicated and difficult to explain in this forum. If you need info on AXIS deviation or 12 lead diagnosis, please send E-Mail and information will be provided by E-Mail or conventional postage.

By, Mitch Mendler E.M.T. paramedic, San Diego Paramedic.


to S.D. Medic 12 lead for more information...

More information and educational Software from these companies:

Genentec Inc.
phone # 415-225-1000
Armus Cardiology Educational Software

phone # 1-800-942-7687
Little Brown Publishing Company
phone # 1-800-527-0145

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