12 Lead ECG.
1. Recording.
2. Normal Features.
Aims and Objectives.
• Review and perform correct technique to
record a 12 lead ECG.
• To understand related concepts to ECG
electrode position and functionality.
• To understand the features of a normal
ECG.
• To create a checklist to differentiate
normal from abnormal ECG morphology
and rhythm.
Early ECG recording.
Important Rules.
• A wave of depolarisation travelling toward a positive electrode
results in a positive deflection in the ECG trace.
• A wave of depolarisation travelling away from a positive electrode
results in a negative deflection.
• A wave of repolarisation travelling toward a positive electrode results
in a negative deflection.
• A wave of repolarisation travelling away from a positive electrode
results in a positive deflection.
• A wave of depolarisation or repolarisation travelling perpendicular to
an electrode axis results in a biphasic deflection of equal positive
and negative voltages (i.e no net deflection).
• The instantaneous amplitude of the measured potentials depends
upon the orientation of the positive electrode relative to the mean
electrical vector.
• The voltage amplitude is directly related to the mass of tissue
undergoing depolarization or repolarisation.
Recording a 12 lead ECG.
• Standard limb leads
(bipolar leads).
– Record potential difference
between two electrodes.
– Lead I (+ive electrode left
arm and -ive on right arm).
– Lead II (+ive electrode left
leg and -ive on right arm)
– Lead III (+ive electrode on
left leg and -ive on left
arm).
– Einthoven Triangle equilateral triangle with
heart at centre.
Limb Lead Axis.
• Positive electrode for
lead I is at 0 degrees
(horizontal plane).
• Positive electrode for
lead II is at +60
degrees.
• Positive electrode for
lead III is at +120
degrees.
• Known as Axial
Reference System.
ECG Appearance Limb Leads.
• Wave travelling at +60 degrees greatest
deflection in lead II.
• Wave travelling at +120 degrees greatest in
lead III.
• If travelling at +90 degrees similar deflections
seen in leads II and III.
Augmented Limb Leads (Unipolar).
• Single positive electrodes referenced against
a combination of other limb electrodes.
• Positive electrodes located at aVR (right
arm), aVL (left arm) and aVF (left leg).
• Same electrodes as for leads I, II and III (ECG
machine automatically converts signals for these leads).
• Coupled with bipolar leads make 6 limb leads
of the ECG.
• Record activity along a single 'frontal plane'
relative to the heart - used to determine the
cardiac axis.
Augmented limb lead axis.
• aVL at -30 degrees to lead
I.
• aVR at -150 degrees to
lead I.
• aVF at +90 degrees
relative to lead I.
• e.g. wave deflected
towards +90 degrees aVF
shows greatest positive
deflection.
• OR e.g. wave deflected
towards +60 degrees then
aVR will show greatest
NEGATIVE deflection.
ECG Appearance Augmented
Leads.
• For a normal axis ECG at +60 degrees.
• Greatest deflection positive aVF (+90
degrees).
• Greatest negative aVR (-150 degrees).
Chest Leads (Unipolar).
• 6 positive electrodes placed across the
chest to record electrical activity of
different regions of the heart.
• Records perpendicular to the frontal plane.
• Labelled V1 - V6.
• Record using same rules as previous +ive deflection if impulse travelling toward
that lead, -ive deflection if away.
Chest Lead Position Related to
Heart.
• Leads V1 and V2
view antero-septal
region.
• V3 and V4 view
antero-apical region.
• V5 and V6 view
antero-lateral region.
ECG appearance Chest Leads.
• Depolarisation
normally occurs from
left to right (IV
septum).
• Largest muscle mass
(LV) produces largest
action potential.
• Leads overlying this
region show greatest
deflection.
QRS appearances V1-V6.
Position of Limb ECG Electrodes.
Chest Lead Electrode Position.
V1 – fourth intercostal space at the
right sternal edge
V2 – Fourth intercostal space at the left
sternal edge
V3 – Midway between V2 and V4.
V4 – Fifth intercostal space in the midclavicular line
V5 – Left anterior axillary line at the
same horizontal level as V4
V6 – Left mid-axillary line at the same
horizontal level as V4 and V5
ECG Paper.
• Standardised paper.
• Speed 25mm/sec (1 small square 0.04sec
– large square 0.2 sec).
• Amplitude 1 mV (10mm to 1 mV).
Recording.
•
•
•
•
•
Relaxed.
Comfortable – supine.
12 lead ECG.
Rhythm strip (which leads?).
Special cases (wheelchair, amputations,
emergency).
Somatic Tremor.
•Check the patient is relaxed and
not moving or twitching (check
fingers and toes).
•If the patient is cold, cover them
with a blanket.
Interference.
• Baseline wander.
•Check electrode (sticker) contacts are
secure and correct.
•Check the patient is relaxed and still.
•Patient perspiration can cause bad
connections, dry and clean the skin and
replace the sticker. It may be necessary
to shave hair off male patients.
Key References.
• Society for Cardiological Science and
Technology. (2006). Clinical Guidelines by
Consensus: Recording a Standard 12 lead
ECG.
• AHA / ACC / HRS Scientific Statements.
Recommendations for the Standardisation
and Interpretation of the ECG. Journal of
the American College of Cardiology, 49
(10).
ECG Recording Practical.
ECG Recording Workbook.