The San Diego Fire Fighters & Paramedics
12 Lead E.C.G. Field Diagnosis Made Easy
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 definative 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
- All cells membranes in the body are charged.
- In a polarized state (resting) the membrane carries a net +
charge. (results from the difference of intra and extra cellular electrolyte
Depolarization: The shift in electrolytes
which reverses the charge
Repolarization: Return to the resting
Helpful hints and rules to reading the 12
- Always be sure to check for an inverted or negative QRS complex in
AVR. This will ensure that the lead were correctly connected appropriately and
your recordings shold be accurate.
- Analize and evaluate the right chest leads. V1 & V2 will reveal
more than any other two contigeous leads. This is where you check for a Bundle
Branch Block, Anterior and Posterior wall infarctions, and "R" wave
- When checking your axis, always focus on Leads I & AVF. Be sure
to check for Bundle Branch Block. Axis vectors are inaccurate in their
- When checking for signs of infarct, omit AVR. It is of no diagnostic
value due to the fact that it misrepresents pathological Q waves and obscures
- Acute Myocardial Infarction cannot be positively identified in the
pressence of LBBB!!!
It is prudent to suspect it per the patients
presentation: however, serum enzyme tests among other things are needed to make
- As a rule, standard criteria for diagnosing AMI dictates that ST
elevation of 1 mm. or more in the presence of pathological Q waves in 2 or more
contigous leads is sufficient.
Three kinds of leads:
- Standard Limb Leads
- Augmented Lead
- Precordial Leads
Einthovens Triangle represents the leads that we all use with our
monitors on a regular basis:
1) Standard Limb Leads
The hexaxial view
- Lead I: The positive lead is above the left brest or on the left
arm and the negative lead is on the right arm.
Records the difference of
potential between the Left arm and Right arm.
- Lead II: The postive lead is on the left abdomen or left thigh and
the negative lead is also on the right arm.
Records the difference of
potential between the left leg and the right arm.
- Lead III: The postive lead is also on the left abdomen or left
lower lateral leg but the negative lead is on the left arm.
difference of potential between the left leg and the right arm.
Leads I, II & III & AVR, AVL, and AVF.
- I: Left Chest
- II: Left Upper Quadrant
- III: Right Upper Quadrant
- AVR: Right lateral arm
- AVL: Left lateral arm
- AVF: Right lateral lower leg
2) Augmented Leads
- The four limb leads go on the four extremities as follows:
- The upper extremities need placement of the electrodes on the
area of the lateral humoral aspect of the arms.
- The lower extremities need placement of the electrodes on the
lateral lower legs near the lateral mallelous.
- Lead aVR faces the heart from the right shoulder and is
oriented to the cavity of the heart.
- Lead aVL faces the heart from the left shoulder and is oriented
to the Left Ventriacle.
- Lead aVF face the heart from the left hip and is oriented to
the inferior surface of the Left Ventricle.
3) Precordial Leads
- Six Precordial Electrode Placement:
potential in the horizontal plane. Each lead is positive.
- The major forces of depolariztion move from right to
- V1 and V2 are negative deflections.
- V3, V4, V5 and V6 become more positive ( peak positive is
V3 or V4 ).
- V1 - fourth intercostal, right strernal border.
- V2 - fourth intercostal, left sternal border.
- V3 - equal distance between V2 and V4.
- V4 - fifth intercostal, left mid clavicular line.
- V5 - anterior axillary line, same level with V4.
- V6 - mid axillary line, same level with V4 and V5.
The precordial views make up a cross section view of the heart
in a transverse horizontal plane projecting a view across the AV
Einthovens 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 veiws 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?????
we only need 10 electrodes palced.
( 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 consistutes activation of the cardiac
response team at the hospital by reporting the field daignosis of
- Category one: AMI that clearly meets the criteria.
- 1 mm or more of ST elevation in the inferior leads (II, III, AVF)
with reciprocal changes in the lateral leads (I, AVL, V5, V6)
- Reciprocal changes not necesssary to make the diagnosis.
- Category two: The following will result in your
reporting the specific findings of concern that may or may not result in the
Cardiac Response team.
- 1 mm of ST elevation in the anterior leads. (V1-V4)
- Injury/Infarct pattern in the presence of LBBB with cardiogenic
The following situation will result in the 12 Lead ECG being
reported as "normal". No subsequent activation of the cardiac response
- Category Three: No patterns of ischemia or
Other Signals to use as a
- Tackycardia: (heart rate above 100) indicates damge to
the left Ventricle and an "anterior" or "lateral" infarct. The Left Circumflex
and or Left Decending Coronary Artery is occluded.
Visable elevation in the CHEST LEADS: V-3, 4, 5, &
- Bradycardia: (heart rate below 60) indiactes damage to
the Right Ventricle and an "inferior" or "posterior" infarct. The Right
Coronary Artery is occluded.
Elevation in the
LIMB LEADS: II, III, & AFV.
Systematic Infarct Recognition
- Assure that aVR is priamarily negative.
- Rule out a :eft Bundle Branch Block (LBBB) in V1 and or V2...
Verify in V6.
- Check all leads for patterns of ischemia, injury, infarction and
AMI diagnosis criteria: 1mm. or more of ST elevation
in 2 or more contiguous leads.
Anterior wall reqires 2mm. or more of
ST elevation (V1-V4)
"ST Depression indicates
|Anterior - Septal
||V1, V2, V3, and V4 -- 0.2mV or
more in leads
||V1, and V2 -- 0.2mV or more in
||II, III, and aVF -- 0.1mV or
more in 2 leads
||I, and aVL -- 0.1mV or more in 2
||V5, and V6 -- 0.1mV or more in 2
Diagrams below indicate which part the heart is being
affected and what lead would show the changes.
|Region of ST
||Region of ST Depression
|Inferior (leads II, III,
||Anterior (leads V1-V3 or lateral
lead 1. aVL)
|Lateral ( leads I, aVF, V5,
||Inferior ( leads II, III,
12 lead rapid assessment
- Verify aVR is negative
- Assess rate and rhythm
- Axis determination - Leads I and aVF
- Conduction abnormalities:
- LBBB - seen in V1
Drugs or Electrolytes
- Ischemia, Injury, Infarct signs:
- T-wave inversions
- ST segment elevation
- Significant Q waves
- Acute MI pattern:
- ST elevation in V1, V2, V3, V4
Inferior: ST elevation in II, III, aVF
- ST depression in II, III, aVF
Lateral: ST elevation in I, aVL, V5, V6
- ST depression in V1, V2, V3, or I, aVL
Septal wall: ST elevation in I, aVL, V1, V2
- ST depression in II, II, aVF
Posterior: tall and wide R waves and ST depression in V1, V2
Right Ventricular: ST elevations in V4R, V5R, V6R
- (5 additional right chest wall electrodes placed on the chest in
the same positions as the precordial leads)
- Clinical pressentaion
- Treatment plan
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 infaction 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
Think of it like this: An Xray film is placed
behind the heart at the area between the Ventrical Septum. The X-Ray machine
shoots the picture from the anterior heart directly above the film. and the
film catures the image. We would be looking at the area of the heart at the
Septal region which would be in ECG terminology V3.
- The infract area will have no electrical current. The ST
segment will be depressed
- The injured area will have ST elevations and will release
Cardiac Muscle enzimes. These enzimes are CK, MyoGlobin and Troponin
- The ishemic area may have ST elevations and ECG cahnges to
include PVC's, PAC's and PJC's.
The 12 Lead Photograph
Simulatneous aquisition 2.5 seconds per view, 10 seconds
for a complete study
12 lead ECG; a real time vedio recording of the hearts
This record indicates a "septal / anterior
12 lead rapid
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...
||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
- If ST elevation is diffuse and unassociated with Q wavesor
reciprocal ST depression, consider pericarditis
- Location of injury can be determined in same manner as infarct
- Usually associated with reciprocal ST depression in other
- 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
Table below shows what the ECG would look like in the Vector
where the heart is being affescted. 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.
- There are elevations ( 1 mm )in two contegous (connecting) leads:
Leads adjacent to eachother...
- There is at least one lead with
- If the Q wave is more than 1/3 the size
of the R wave...
- ST elevation without abnormal Q wave
- Usually associated with occlusion of the left anterior
decending branch of the left coronary artery (LCA)
- ST elevation with/without abnormal Q wave
- May be a component of a mutiple-site infarction
- Usually associated with abstruction of the left circumflex
- ST elevation with/without abnormal Q wave
- Usally associated with right coronary artery (RCA)
- Usually accompanies inferior MI due to proximal acclusion of
- 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: reprofussion,
adequate IV fluids for right heart filling, and pacingf to maintain A-V
synchrony if necessary
- Tall, broad (>0.04 sec) R wavr and ST depression in V1 and
V2 (reciprocal changes)
- Frequestly associated with inferior MI
- Usually associated with obstruction of RCA and or left
circumflex coronary artery
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 "Rabit 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 BUNEDLE BRANCH BLOCK (V-1, V-2, & V-5,
Infarction associated with a
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.
R wave is recorded, then the infarct is extreamly acute. There is no electrical
activity of the ventricle durring polarization and contraction.
A LBBB may result from an acute myocardial infarction (AMI), but field
paramedics cannot dianose 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
abnoemally 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
- LBBB is usually associated with an Inferior wall AMI when an AMI is
- LBBB is usually associated with hypertensive ishemia or primary
Diagnosing the Bundle Branch Block:
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 deflextion),
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.
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 refered to as
the "evolution of Myocardial Infarction."
It is offen suggested that the first observable evolutionary change is
the ischemia we associate with T-wave inversion or ST segmnet
depression. Then, onto what is refered 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 developement
of a pathological Q wave. This Q-wave comfirms the
diagnosis of MI.
This evolution is not precise, however. Often times the T-wave may
invert in the presence of ST segmnet elevation durring the end of the
hyperacute phase. In any event... the mose critical observation should be the
recognition of ST elevation in 2 or contiguos leads. This is most important to
paramnedic in the pre-hospital phase because the developement 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
Use Lead I, II, and aVF to diagnois Axis
Vectors and Axis
- Vector: A quantity of electrical force that has a known magnitude
- Axis: A hypothetical line which joins the poles of a lead which
measure electrical force.
- Mean Cardiac Vector: The avaerage of all the instananeous vectors.
( AKA mean electrical axis ).
- The postion of the mean cardiac vector provides information
about the electrical "position" of the heart, and is influenced by the
relationship of the heart within the chest, as well as by the anatomy of the
For pre-hospital purposes, the axis is either
"normal" or "not normal."
- Normal Deviation:
- The QRS deflection is upright or positive in I and either aVF or
- A normal axis means the QRS axis falls between 30 and 90 degress
in the chest. The heart is lying in an angle between these parameters.
- Right Axis Deviation:
- The QRS is downward or negatively deflected in I and positive in
aVF or Lead II.
- The heart is lying in an angle lower the 30 dgress in the
Can be normal in young adults or "thin peolpe."
abnormal in peolpe who have a block in the posterior division of the left
Can imply delayed activation of the right ventricle ( as seen
in RBBB ) or Right Ventricular enlargement.
Ventricular enlargemnet and hypertrophy. C.O.P.D. Pulmonary Embolism,
Congenital heart Disease, Inferior wall MI.
- Left Axis Deviation:
- The QRS is uright or postively deflection in I and negative in
aVF or Lead II.
- The heart is lying in an angle greater than 90 degress in the
Can be normal in the presence of acites, abdominal tumors,
pregnancy or obesity.
Abnormalities are due to Left Ventricular
enlargement or a Left anterior hemiblock.
Pathology: Left ventricular
enlargement, and hypertrophy, Hypertension, Aortic Stenosis. Ischemic Heart
Disease. Inferior wall MI.
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 noraml; 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 difiicult 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.
- Eric Yeargain, Paramedic
- San Diego Paramedic Association field handbook.
- Palomar College, San Marcos, CA. Paramedic Program
to S.D. Medic 12 lead for
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phone # 1-800-527-0145
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