12 Lead Interpretation

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12 Lead Interpretation
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Objectives

• Ischemia, injury and infarction
• ECG complex review
• J point
• ST segment
• STEMI recognition

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Ischemia to Infarct

• Infarction is an evolving process
• As the infarct evolves ECG changes may occur
  quickly or more gradually
• These changes may be seen on the 12 lead as they
  evolve
• Acquiring more then one 12 lead may be necessary
• A normal 12 lead does not rule out acute
  myocardial infarction

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The ECG Complex

• The J point is the point where the S wave ends
  and the ST segment begins
• ST elevation is measured after the J point
• The ST segment is compared to the base line
• The base line or isoelectric line is found at the
  bottom of the calibration bar

J point
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ST Segment

• Starts with the J point
• Ends with the beginning of the T wave
• Elevation or depression of the ST segment is
  measured 0.08 seconds (2 small squares) to the
  right of the J point

ST segment
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PR Segment

• Begins at the end of the P wave
• Ends at the beginning of the QRS
• When determining the isoelectric or baseline find
  the PR interval of 2 consecutive complexes, draw
  a line using a straight edge and measure ST
  elevation from this line this is the most
  accurate way to determine if the ST segment is
  elevated

Isoelectric line
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TP Segment

• Begins at the end of the T wave
• Ends at the beginning of the P wave
• Can be used as a back up to the PR interval to
  determine the baseline when assessing ST
  elevation
• Not as accurate as the PR interval

TP segment
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Hyperacute T-wave

• As an acute myocardial infarction develops
  various 12 lead ECG changes occur
• Initially the 12 lead ECG may show tall or
  hyperacute T-waves signifying cardiac ischemia
  may only be present for a short time after
  ischemia has begun (5 to 30 minutes)
• Paramedics may not see this change as many
  patients wait for at least 30 minutes to call EMS

T-wave
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ST segment elevation

• Usually seen with in the first few hours after
  the onset of symptoms
• Changes may be very subtle or pronounced
• Any elevation in 2 contiguous leads is
  significant
• ST segment elevation greater than 1mm or 2mm in
  precordial leads (V1 through V6)
• 1mm 1 small square on the ECG paper
• For more information on ST segment abnormality
  click on this link
• http//www.madsci.com/manu/ekg_st-t.htm

2mm or 2 small squares
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Tombstones

• Pronounced ST segment elevation may appear as
  tombstones
• Tombstones are a result of the fusing of the ST
  segment and T wave

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Pathological Q wave

• Indicate a loss of viable myocardium
• May develop 1 to 2 hours after the onset of
  symptoms but can take anywhere from 12 to 24
  hours to develop
• Abnormal Q waves are at greater then one third of
  the R wave height deep and greater then 1mm (or 1
  small square) wide
• Q waves may be visible in a patient without
  infarct but will not meet the parameters to be
  considered abnormal

Greater than 1mm wide
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Reciprocal Changes

• Are seen as ST depression in the opposite leads
  from where the ST elevation is seen
• Leads II, III and aVF are opposite to Leads I,
  aVL, and all of the V leads
• Therefore, if there is ST elevation in leads II,
  III and aVF any ST depression (if present) would
  be seen in leads I, aVL and any of the V leads

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Reciprocal Changes

• ST segment depression seen in the opposite leads
  from ST segment elevation
• Highly sensitive as an indicator of acute MI
• Frequently seen in larger infarctions

ST elevation
Reciprocal ST depression
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The 12 Lead Printout
Calibration bar (the isoelectric line is
indicated by the bottom of the calibration bar)
Paper speed indicates how fast the paper moves (5
larger squares 1 second)
On the Zoll 12 lead printout there is a break in
the ECG to indicate where the tracing changes to
the next lead
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Practice
Locate the J point in each of the above
complexes Click the mouse to check your location
The J point is at the end of the S. This is
located where the upstroke of the S changes to
become horizontal.
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Practice
Isoelectric Line
Isoelectric Line
Isoelectric Line
Isoelectric
Elevated
Depressed

• Locate the J point in each of the above complexes
• Identify ST segment abnormalities
• Click the mouse to check your answers

• The red dot shows the J point
• The red line shows the ST segment 0.08 seconds (2
  small squares) from the J point

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Normal 12 Lead

• Notice where the J point is for one complex in
  each lead
• Also look at the ST segment for one complex in
  each lead
• This an example of a 12 lead that the Zoll E
  series will generate

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Practice ECG 1

• Look for the J point and ST elevation in the
  above 12 lead
• Look for reciprocal changes
• Which lead groups are involved?
• What area of the heart is involved?
• Which coronary artery feeds this area of the
  heart?

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Answer ECG 1

• Leads II, III, aVF all have ST elevation, these
  are contiguous inferior leads
• Leads I, aVL, V2, V3 and V4 all have ST
  depression signifying reciprocal changes
• These changes are consistent with an acute
  inferior MI
• In 90 of the population the inferior aspect of
  the heart blood supply is via the right coronary
  artery

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Practice ECG 2

• Look for the J point and ST elevation in the
  above 12 lead
• Look for reciprocal changes
• Which lead groups are involved?
• What area of the heart is involved?
• Which coronary artery feeds this area of the
  heart?

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Answer ECG 2

• ST elevation in Leads I, aVL, V2, V3, V4, V5
• Lead II and III show reciprocal changes as well
  as evidence of an old MI (in the form of a
  pathological Q wave)
• Lateral leads (I, aVL and V5) as well as Anterior
  and septal lead groups (V2, V3 and V4)
• Anterior and Lateral are the main areas of the
  heart involved
• Left anterior descending and the left circumflex
  arteries supply this area of the heart

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Practice ECG 3

• Look for the J point and ST elevation in the
  above 12 lead
• Look for reciprocal changes
• Which lead groups are involved?
• What area of the heart is involved?
• Which coronary artery feeds this area of the
  heart?

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Answer ECG 3

• ST elevation in leads II, III and aVF
• Reciprocal changes in leads I, aVL, V2 and V3
• Leads II, III and aVF are contiguous inferior
  leads
• This is an acute inferior MI
• This area of the heart is supplied by the right
  coronary artery in 90 of the population

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Practice ECG 4

• Look for the J point and ST elevation in the
  above 12 lead
• Look for reciprocal changes
• Which lead groups are involved?
• What area of the heart is involved?
• Which coronary artery feeds this area of the
  heart?

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Answer ECG 4

• ST elevation in leads I, V2, V3, V4, V5, V6 and
  aVL
• Reciprocal changes shown in leads III and aVF
• V3 and V4 are anterior leads, I, aVL, V5 and V6
  are lateral leads
• This is an acute anteriolateral MI
• Left anterior descending and left circumflex
  arteries supply these areas of the heart

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Practice ECG 5

• Look for the J point and ST elevation in the
  above 12 lead
• Look for reciprocal changes
• Which lead groups are involved?
• What area of the heart is involved?
• Which coronary artery feeds this area of the
  heart?

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Answer ECG 5

• ST elevation in leads II, III, aVF and V6
• Reciprocal changes in leads V2, V3 and V4
• II, III and aVF are inferior leads, V6 is a
  lateral lead and ST elevation must be present in
  at least 2 leads that view the same area of the
  heart
• This is an acute inferior MI
• The inferior area of the heart is supplied by the
  right coronary artery in 90 of the population

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Practice ECG 6

• Look for the J point and ST elevation in the
  above 12 lead
• Look for reciprocal changes
• Which lead groups are involved?
• What area of the heart is involved?
• Which coronary artery feeds this area of the
  heart?

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Answer ECG 6

• ST elevation in leads V1, V2, V3 and V4
• There are no obvious reciprocal changes
• V1 and V2 are septal leads, V3 and V4 are
  anterior leads
• This is an acute anterioseptal MI
• The left anterior descending artery supplies this
  area of the heart

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