ECG Rhythm Interpretation: Determining QRS Axis in a 12-Lead ECG, Slides of Cardiology

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ECG Rhythm Interpretation

Module VII a

Reading 12-Lead ECG’s

Course Objective

• To systematically analyze a 12-lead

ECG.

Reading 12-Lead ECGs

• The 12-Lead ECG contains information that will assist

you in making diagnostic and treatment decisions in your

clinical practice. In previous modules you learned how to

read and interpret parts of the ECG. Now, we will bring all

that you have learned together so that you can

systematically read and interpret a 12-lead ECG.

• The information will be divided into two modules, VII a

and VII b.

Reading 12-Lead ECGs

The best way to read 12-lead ECGs is to develop a step-

by-step approach (just as we did for analyzing a rhythm

strip). In these modules we present a 6-step approach:

1. Calculate RATE

2. Determine RHYTHM

3. Determine QRS AXIS

4. Calculate INTERVALS

5. Assess for HYPERTROPHY

6. Look for evidence of INFARCTION

If you use the rhythm

strip portion of the

12-lead ECG the total

length of it is always

10 seconds long. So

you can count the

number of R waves

in the rhythm strip

and multiply by 6 to

determine the beats

per minute. Rate?^ 12 (R waves) x 6 = 72 bpm

• In Module II you learned how to systematically analyze a rhythm by looking at the rate, regularity, P waves, PR interval and QRS complexes.
• In Modules III, IV and V you learned how to recognize Normal Sinus Rhythm and the 13 most common rhythm disturbances.
• If you need a refresher return to these modules.

Axis refers to the mean QRS axis (or vector) during ventricular

depolarization. As you recall when the ventricles depolarize (in a

normal heart) the direction of current flows leftward and downward

because most of the ventricular mass is in the left ventricle. We like

to know the QRS axis because an abnormal axis can suggest

disease such as pulmonary hypertension from a pulmonary

embolism.

The QRS axis is determined by overlying a circle, in the frontal

plane. By convention, the degrees of the circle are as shown.

The normal QRS axis lies between -30o^ and +90o.

0 o

30 o

-30o

60 o

-60o

-90o -120o

(^12090) o o

150 o

180 o

-150o

A QRS axis that falls between -30o

and -90o^ is abnormal and called left

axis deviation.

A QRS axis that falls between +90o

and +150o^ is abnormal and called

right axis deviation.

A QRS axis that falls between +150o

and -90o^ is abnormal and called

superior right axis deviation.

How do we know the axis is normal when the QRS

complexes are positive in leads I and II?

The answer lies in the fact that each frontal lead

corresponds to a location on the circle.

0 o

30 o

-30o

60 o

-60o

-90o -120o

(^12090) o o

150 o

180 o

-150o

I

II

avF

avR avL

Limb leads I = +0o II = +60o III = +120o Augmented leads avL = -30o avF = +90o avR = -150o

I

III^ II

0 o

30 o

-30o

60 o

-60o

-90o -120o

(^12090) o o

150 o

180 o

-150o

Since lead I is orientated at 0o^ a wave of depolarization directed towards

it will result in a positive QRS axis. Therefore any mean QRS vector

between -90o^ and +90o^ will be positive.

Similarly, since lead II is orientated at 60o

a wave of depolarization directed towards

it will result in a positive QRS axis.

Therefore any mean QRS vector between

-30o^ and +150o^ will be positive.

I

II

0 o

30 o

-30o

60 o

-60o

-90o -120o

(^12090) o o

150 o

180 o

-150o

Since lead I is orientated at 0o^ a wave of depolarization directed towards

it will result in a positive QRS axis. Therefore any mean QRS vector

between -90o^ and +90o^ will be positive.

Similarly, since lead II is orientated at 60o

a wave of depolarization directed towards

it will result in a positive QRS axis.

Therefore any mean QRS vector between

-30o^ and +150o^ will be positive.

Therefore, if the QRS complex is positive

in both leads I and II the QRS axis must

be between -30o^ and 90o^ (where leads I

and II overlap) and, as a result, the axis

must be normal.

I

II

0 o

30 o

-30o

60 o

-60o

-90o -120o

(^12090) o o

150 o

180 o

-150o

0 o

30 o

-30o

60 o

-60o

-90o -120o

(^12090) o o

150 o

180 o

-150o

… if the QRS is negative in lead I and positive in lead II what is the QRS

axis? (normal, left, right or right superior axis deviation)

QRS Complexes

I

I II Axis

normal

left axis deviation

right axis deviation

II

0 o

30 o

-30o

60 o

-60o

-90o -120o

(^12090) o o

150 o

180 o

-150o

… if the QRS is negative in lead I and negative in lead II what is the QRS

axis? (normal, left, right or right superior axis deviation)

QRS Complexes

I

I II Axis

normal

left axis deviation

right axis deviation

right superior

axis deviation

0 o

30 o

-30o

60 o

-60o

-90o -120o

(^12090) o o

150 o

180 o

-150o

II