BASIC 12 LEAD ECG INTERPRETATION

Dr. Jeffrey Elliot Field, HBSc. DDS, Fellow, American Dental Society of AnesthesiaDiploma, the National Dental Board of Anesthesia.

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Objectives To gain a cursory understanding of 12 lead

ECG’s

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Basic 12 Lead Interpretation

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12 lead ECG’s give you the opportunity to look for all the arythmias we have studied in a 360 degree view of the heart with the added bonus of being able to diagnose and localize myocardial infarctions ( i.e. areas of muscle damage)

Remember to look at all leads to rule out all of the arrhythmias and abnormalities we have studies thus far.

In particular look first at Leads I , II, and III as these will be the most familiar to you .

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In a 12 lead ECG 10 electrodes/leads attached to the patient.

Myocardial InfarctionMyocardial infarctions can be categorized as

follows:-Q-wave MI-Non Q-wave MI

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Q-Wave Myocardial InfarctionThis is the classic presentation for MI’s. The developing MI is seen as ST segment

elevation followed by deepening Q-waves in the leads where ST segment elevation was 1st seen.

The ECG changes are accompanied by elevated cardiac enzymes and markers and of course physical signs and symptoms of an MI ( chest pain ,nausea ,vomiting , etc)

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Non Q-Wave Myocardial InfarctionIn this case you get classic signs and

symptoms symptoms of an MI(i.e. elevated cardiac enzymes and markers and of course physical signs of an MI ( chest pain ,nausea ,vomiting , etc)

But non of the usual ECG changes ( i.e. ST segment elevation and deepening Q-waves). In fact sometimes the only clue on the ECG are inverted T-waves.

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Cardiac Enzyme changes and Markers for MIThere are 4 markers for cardiac enzymes as

follows:CK-MB isoenzymeCK-MB isoformsMyoglobin Troponin T or Troponin I

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Relative Advantages and Disadvantages of the Various cardiac Markers

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CK-MB Isoenzyme

Myoglobin CK-MB Isoforms

Troponin ( Tor I)

Specific for cardiac muscleDamage

No No No Yes

Sensitivity= early rise ( 1-6 hours after damage

Yes YES Yes Yes

Sensitivity=sustained elevation after damage

No No No Yes

Sensitivity= will detect reinfarction soon after the initial episode

Yes No Yes No

Diagnostic use verified by clinical studies

Yes Yes Yes Yes

Prognostic Use verified by clinical studies

No No No Yes

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CK-MB Isoenzyme

Myoglobin CK-MB Isoforms

Troponin ( Tor I)

Score 3 yes 3 No

2 Yes4 No

3 Yes3 No

5 yes1 No

Therefore the best test overall is Troponin T or I. But these will not detect reinfarction and therefore more than one test is required.

The current recommendation is to combine one of the CK-MB tests with one of the Troponin tests.

It should be noted that high troponin levels post MI correlate with poor outcomes.

Finally please note CK-MB also rises in unstable angina ( damaged cells that will recover ) as well as MI ( damaged cells that won’t recover) so it won’t differentiate between unstable angina and an MI

Localizing Myocardial infarctionsAnterior( blockage of left anterior descending

artery) –look for ECG changes in leads V1-V4Inferior ( blockage of right coronary artery or

less commonly right circumflex)- look for ECG changes in leads II, III, and AVF

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Anterior MI

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Inferior MI

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Localizing Myocardial infarctions continuedLateral Infarction ( blockage of circumflex or

diagonal branch of the LAD)-look in leads V5, V6 and AVL

Posterior (blockage of right coronary artery or circumflex) –look for mirror image changes to anterior in V1-V4 (i.e. ST depression and dominant R-wave).

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Posterior MI

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Lateral MI

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