CORN 201: Introduction to 12 Lead ECGs and Axis

Sylvia van Eijk

Humber College

What information can a 12 lead ECG provide?

• Ischemic events

• MIs

• Bundle branch blocks

• Anterior and posterior fascicular blocks

• Axis determination

• Hypertrophy

• Ectopy vs abberrancy

When should we do a 12 lead ECG?

• Analyze a cardiac rhythm

• Patient symptoms: e.g. chest pain, SOB

• Suspected MI

• Electrolyte imbalances

• Digoxin Toxicity

Einthoven’s ECG machine

12 lead ECG

• Electrical recording of the heart

• Conduction system creates electrical impulses

• Majority of cells in the body generate an electrical signal that can be measured

Normal 12 lead ECG

Electrical forces and vectors

Cardiac vector:

• Vector describes electrical impulses of cells

• Electrical current is generated by depolarization and repolarization

• Vectors show strength and direction of the electrical impulse

• Depicted as an arrow

Cardiac Vector

Cardiac Vectors

• Add up when going in the same direction

• Cancel each other out if going in opposite directions

• Sum of all vectors in ventricle = electrical axis

• Determination of QRS vector

Cardiac Vectors

12 lead ECG leads

Six limb leads:

• 3 standard limb leads: I, II, and III

• 3 augmented leads: aVR, aVL, and aVF

Six precordial leads: V 1-6

• 15 lead ECG: V7-9

12 lead ECG leads

Positive electrode:

• Attached to extremities or designated areas of chest

Negative electrode:

• Single electrode attached to an extremity or an ‘indifferent’ zero reference point – the central terminal

12 lead ECG leads

• Standard limb leads are bipolar

• Positive and negative electrodes are attached to the extremities

• Detect electrical activity between 2 electrodes

Standard limb leads

Lead I

Lead II

Lead III

Augmented limb leads

• Unipolar

• aVR, aVL, aVF

• Positive electrode on one of three extremities

• Negative electrode: central terminal

Augmented leads and axes

Lead Axis

• Each lead measures the difference in electrical potential between the positive and negative electrodes (or CT)

• Hypothetical line joining the poles of a lead is the ‘axis’ of the lead (vector)

Lead Axis

Frontal and Horizontal Planes

• 3 standard limb leads and 3 augmented leads measure electrical activity of the heart in the 2-dimensional frontal plane

Frontal and horizontal planes

Einthoven’s Triangle

Einthoven’s Law

• Lead I + Lead III = Lead II

Triaxial reference figure for leads I, II, III

Triaxial reference figure for augmented leads

Hexaxial reference figure

• Combines the triaxial reference figures for the standard and augmented limb leads

Precordial leads

Six unipolar leads:• V1,V2, V3, V4, V5, V6

• Use positive electrode and central terminal

Precordial lead placement

Right sided chest leads

Precordial lead axes

The heart in 3 dimensions

Facing Leads

Review Questions

A ________ lead represents the difference in electrical potential between 2 electrodes:

• Unipolar

• Bipolar

• Central

• Terminal

Review questions

The placement of the V4 positive chest electrode is:

A. Left side of the sternum in the 4th ICS

B. Midclavicular line in the 5th ICS

C. Anterior axillary line at the 5th ICS

D. Midaxillary line at the 6th ICS

Review questions

The ____________ surface of the heart is viewed by ECG leads II, III, and aVF.

A. Lateral

B. Anterior

C. Inferior

D. Posterior

Cardiac vectors and the QRS axis

QRS axis

Points to remember:

• QRS axis points to the left and downward

• QRS axis most important

• P and T axis rarely determined

Currents, vectors, and axes

• Parallel to lead axis positive or negative deflection

• Perpendicular to lead axis no deflection

• The greater the magnitude, the larger the deflection

Direction of flow of electrical currents

Hexaxial reference figure

• Identify direction of the QRS axis

• Use the 6 limb lead axes

• Spaced 30 degrees apart around the wheel: 0 to 180

• Divided into 4 quadrants

Hexaxial reference figure

Normal and abnormal QRS axes

Causes of abnormal axis

• Ventricular enlargement

• Ventricular hypertrophy

• Bundle branch block

• Fascicular block

Left axis deviation

Disorders that may lead to LAD:

• Systemic hypertension, aortic stenosis

• Ischemic heart disease

• Other disorders affecting LV

• PVCs and VT of RV origin

• Late inferior MI

Right axis deviation

• COPD, PE, congenital heart disease

• Other disorders that cause severe pulmonary hypertension

• RBBB

• Normal in infants and young adults (up to 120°)