How to Interpret ECG: The Basics

Heather A. Crane, M.D.

Attending Physician, Emergency Department

Loma Linda University Medical Center

Goals

Review a systematic approach to ECGs Review the major rhythms Review ischemia and infarct Treatment for various rhythms and ACS

What does the ECG represent?

Vectors of electricity It shows us:

– Rhythm disturbances– Conduction disturbances– Reflects electrolyte status– Reveals damage to myocardium

What does the ECG represent?

Vectors of electricity It shows us:

– Rhythm disturbances– Conduction disturbances– Reflects electrolyte status– Reveals damage to myocardium

How good is the ECG?

It is very specific However, it is not the most sensitive tool,

particularly when ruling out infarct or ischemia

Where do the leads go?

There are limb leads: I, II, III, AVR, AVL, AVF There are precordial leads: V1-V6

Assessing EKGs: The Conduction System

Bachmann’s bundle

Left bundle branch

Posterior division

Anterior division

Purkinje fibersRight bundle branch

Bundle of His

AV node

Internodal pathways

Sinus node

Relationship of the ECG to Anatomy

The picture is too big for this slide

The waves and the intervals

P-wave: represents atrial contraction QRS: represents ventricular contraction T-wave: represents repolarization PR interval: represents the AV node QT interval: ventricular depolarization and

repolarization

The waves and the intervals

P-wave is <.10 seconds wide PR interval is .12-.20 seconds QRS is .06-.10 seconds wide QT is .33-.42 seconds (varies with age and

gender) T-wave should be upright in all leads except

AVR and sometimes III

Determining the Rate

Analyzing the Rhythm

Key questions– Is the rhythm regular or irregular?– Is it fast ( >100) or slow (<60)?– Are QRS complexes and P waves present?– How is the P wave related to the

QRS complex?– Is the QRS complex wide or narrow?– What are the intervals (PR, QT, PP, RR)?

Axis

Is the patients heart enlarged? We look at leads I and Avf to determine axis. The QRS complex should be upright in both. If it is up in Avf and down in I, this is right axis

deviation. If it is down in Avf and up in I, this is left axis

deviation.

Injury, Ischemia, Infarct

Baseline

Ischemia—tall or inverted T wave (infarct),ST segment may be depressed (angina)

Injury—elevated ST segment, T wave may invert

Infarction (Acute)—abnormal Q wave,ST segment may be elevated and T wavemay be inverted

Infarction (Age Unknown)—abnormal Q wave,ST segment and T wave returned to normal

How to approach the ECG

Very Carefully Think: Rate, Rhythm, Axis, Injury, Ischemia,

Infarct This way you don’t overlook things and you

can make sense of even the most chaotic looking ECGs

ECG 1

ECG 2

Comparison ECG

ECG 3

ECG 4

ECG 5

ECG 6

ECG 7

ECG 8

ECG 9

ECG 10

ECG 11

Compare this with ECG 11

Acute MI Localization

aVF inferiorIII inferior V3 anterior V6 lateral

aVL lateralII inferior V2 septal V5 lateral

aVRI lateral V1 septal V4 anterior

ECG 12

ECG 13

ECG 14

ECG 15

ECG 16

ECG 17

ECG 18

ECG 19

ECG 20