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CC2 Class 2
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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°)
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