2. Electrocardiogram
• Electric current generated by the heart is conducted through body fluids
• A small portion reach the surface of the body, which can be detected and
recorded by electrodes → ECG (not record of single AP)
• ECG → represents the sum of electrical activity in all cardiac muscles
undergoing depolarization and repolarization
• The waves represent comparison in voltage detected by electrodes at 2
different points on the body surface (not actual AP)
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3. ECG Recording
1. Standard limb leads
• Recording electrodes placed on both arms and legs (wrist and
ankle)
• Appendages act as extensions of recording electrodes
• Voltage measurements are made between points that form
equilateral triangle over the thorax (Einthoven’s triangle)
• Any ECG trace is recording of voltage difference measured
between any 2 vertices of Einthoven triangle
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5. ECG Recording
1. Standard limb leads
• Use 2 active electrodes
• Standard limb leads (I, II, III) each record potential difference
between 2 limbs (2 electrodes):
• Lead I → LA (+) & RA (-)
• Lead II → LL (+) & RA (-)
• Lead III → LL (+) & LA (-)
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8. ECG Recording
2. Unipolar limb leads:
• Use exploring electrode connected to an indifferent electrode at zero
potential
• Measure potential difference between exploring & indifferent
electrode
• Two types:
• Augmented limb leads → aVR, aVL, aVF
• Record potential difference between one limb and the other 2 limbs
• Precordial leads/Unipolar chest leads →V1-V6
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9. ECG Recording
• Augmented limb leads
• aVR → b/n RA & the indifferent electrode
• aVL → b/n LA & the indifferent electrode
• aVF → b/n LL & the indifferent electrode
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11. ECG Recording
• Chest leads:
• V1 → 4th intercostal space right sternal edge
• V2 → 4th intercostal space left sternal edge
• To find the 4th space, palpate the manubriosternal angle of Louis)
• Directly adjacent is the 2nd rib, with the 2nd intercostal space directly below
• Palpate inferiorly to find the 3rd and then 4th space
• V3 → halfway between V2 and V4
• V4 → over the apex (5th ICS mid-clavicular line)
• V5 → at the same level as V4 but on the anterior axillary line
• V6 → at the same level as V4 and V5 but on the mid-axillary line
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14. ECG Recording
• Polarity conventions:
• Upward deflection → when voltage difference between 2
electrodes is more positive
• Downward deflection → opposite
• 1cm deflection on the vertical axis → represents a potential
difference of 1mv
• 25mm on the horizontal axis → represents 1 second
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16. ECG Recording
• Polarity conventions (lead-II):
• Upward deflection → indicates an electrical polarity exists between
the LL (+) and the RA electrodes
• Downward deflection → indicates a polarity exists between the
electrodes at that instant, with the left leg electrode being negative
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17. ECG Recording: 12-Lead ECG
• The standard clinical ECG involves voltage measurements recorded
from 12 different leads:
• 3 standard bipolar limb leads → Leads-I, II, III
• 9 unipolar leads:
• 6 are chest electrodes → V1-V6
• 3 are augmented unipolar limb leads (aVR + aVL + aVF) generated
using the limb electrodes:
• 2 of the electrodes are electrically connected to form an indifferent
electrode while the third limb electrode is made the positive pole of the
pair
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18. ECG Recording: 12-Lead ECG
• Electrode placement in 12 lead ECG:
• 6 chest electrodes → V1-V6
• 4 limb electrodes:
• Right arm
• Left arm
• Left leg
• Right leg (neutral)
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19. ECG Recording: 12-Lead ECG
• Augmented unipolar limb leads:
• Lead aVR → recorded from the electrode on the right arm and the
indifferent electrode
• Lead aVL → recorded from the electrode on the left arm
• Lead aVF → recorded from the electrode on the left leg
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21. ECG Recording: 12-Lead ECG
• The standard limb leads + augmented unipolar limb leads record the
electrical activity of the heart as it appears from six different
"perspectives," (all in the frontal plane):
• The axes for leads I, II, and III are those of the sides of Einthoven's
triangle
• While those for aVR, aVL and aVF are specified by lines drawn from
the center of Einthoven's triangle to each of its vertices
• The six limb leads can be thought of as a hexaxial reference system
for observing the cardiac vectors in the frontal plane
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22. ECG Recording: 12-Lead ECG
• Chest leads:
• Precordial/chest leads → V1-V6 → the other 6 leads of the
standard 12-lead ECG
• They are unipolar leads
• These potentials are obtained by placing an additional (exploring)
electrode in six specified positions on the chest wall
• The indifferent electrode is formed by electrically connecting the
limb electrodes
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24. ECG Recording: 12-Lead ECG
• Chest leads:
• When the +ve electrode is placed in position 1 and the wave of
ventricular excitation sweeps away from it, the resultant deflection
will be downward
• When the electrode is in position 6 and the wave of ventricular
excitation sweeps toward it, the deflection will be upward
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29. Electrocardiogram
• Information obtained from ECG:
• Anatomical orientation of the heart
• Relative size of the chambers
• Origin of excitation
• Rhythm and conduction disturbance
• Location, extent and progress of ischemic damage
• Electrolyte disturbance + Influence of drugs
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30. Electrocardiogram
• Major features of ECG:
• P-wave → atrial depolarization
• QRS complex → ventricular depolarization + atrial repolarization
• T-wave → ventricular repolarization
• QT interval → ventricular depolarization + repolarization
• U-wave → repolarization of purkinje fibers (slow HR)
• PR interval → atrial depolarization + AV nodal delay
• PR segment → isoelectric point of depolarized atria
• ST segment → isoelectric point of depolarized ventricles
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36. Electrical Conduction
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AV node
• Many but small cells, thus many
membranes need to be crossed
• Too many gap junctions
• Depolarization dependent on Ca
• Ca2+ channels are slow
conducting
• Less diameter, more resistance
• RMP = - 65mV
Purkinje fibers
• Little but long cells, thus small
membranes need to be crossed
• Little gap junctions
• Depolarization dependent on Na
• Na+ channels are fast conducting
• Large diameter, less resistance
• RMP = - 90mV (more electronegative,
attract cations rapidly)
37. Electrocardiogram
• AV node depolarization
• Too small to be detected
• Ventricular depolarization: 3 stages
1. Septal depolarization (rapid)
2. Major ventricular depolarization
3. Basal depolarization
• Septal part is depolarized by left bundle branch, while the right
bundle branch take the current downward
• Thus, wave of depolarization moves from left & lower part towards
the right & upper portion
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38. Electrocardiogram
• Ventricular depolarization:
• Since septal tissue is small, Septal vector is small & directed
rightward & upward + fast vector
• Inner myocardium depolarized first, then wave of depolarization
moves outward
• Multiple vectors are simultaneously produced (added)
• Left ventricle downward & leftward (stronger)
• Right ventricle downward & rightward (weak)
• Net vector = downward & leftward
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39. Electrocardiogram
• Basal Ventricular depolarization:
• Left: upward & rightward
• Right: upward & rightward
• Net vector: upward & rightward
• During ventricular depolarization, 3 fast vectors are produced since Na+
channels are fast conducting
• During ventricular repolarization, since K+ channels are slow
conducting, the timing of repolarization will overlap and only 1 vector is
produced
• Hence, repolarization of major ventricle will be detected
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40. Electrocardiogram
• P-wave:
• Represents atrial depolarization
• Made up of two separate waves due to right atrial (occurs first) and
left atrial depolarization
• Normal duration = 0.08-0.10sec
• Normal height ≈1.5mm
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41. Electrocardiogram
• P pulmonale
• P wave > 2.5mm
• Occurs due to right atrial hypertrophy
• Causes:
• Pulmonary hypertension
• Pulmonary stenosis
• Tricuspid valve stenosis
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42. Electrocardiogram
• P mitrale
• P wave > 0.08sec (2 small squares) and a bifid shape
• Occurs due to left atrial hypertrophy + delayed left atrial
depolarization
• Causes include:
• Mitral valve stenosis
• LVH
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43. Electrocardiogram
• PR Interval:
• Atrial depolarization + the start of ventricular depolarization
• Measured between the initiation of the P wave to the beginning of
the QRS complex
• Indicates the duration for an action potential to spread through the
atria and the AV node
• Normal duration ≈ 0.12-0.2sec (3-5 small squares)
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44. Electrocardiogram
• PR Interval:
a. Short PR interval (<0.12s) → signify an accessory electrical
pathway between the atria and the ventricles → the ventricles
depolarize early → gives a short PR interval
• NB: Short PR interval occurs in Wolff-Parkinson-White syndrome
where the accessory pathway is called the bundle of Kent
b. Long PR interval (>0.2s) → indicates heart block
• 1st degree block
• 2nd degree block
• 3rd degree block
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45. Electrocardiogram
• PR Interval:
• 1st degree block:
• If there is a constant long PR interval (>0.2s)
• Longer than normal conduction delay at the AV node
• 2nd degree block/Mobitz type I
• PR widens over subsequent beats then a QRS is dropped
• Wenckebach phenomenon → lengthening of the PR interval in
subsequent beats
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46. Electrocardiogram
• PR Interval:
• 2nd degree block/Mobitz type II
• PR is constant then a QRS is dropped
• 3rd degree block
• If there is no discernable relationship between the P-waves and the
QRS complexes
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47. Electrocardiogram
• PR segment
• No voltages are detected on the body surface:
• Atrial cells are depolarized (in their plateau phase)
• Ventricular cells are still resting, and
• The electrical field set up by the action potential progressing
through the small AV node is not intense enough to be detected
• Shortly after the cardiac impulse breaks out of the AV node and into the
rapidly conducting Purkinje system, all the ventricular muscles depolarize
within a very short period of time and cause the QRS complex
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48. Electrocardiogram
• QRS Complex:
• The rapid and large changes in the magnitude and direction of the net
cardiac dipole that exist during ventricular depolarization causes the
QRS complex
• Normal QRS duration ≈ 60-100ms
• Atrial repolarization occurs during the QRS complex
• Atrial repolarization is not evident on the ECG → it is a poorly synchronized
event in a relatively small mass of heart tissue and is completely overshadowed
by the major electrical events occurring in the ventricles at this time
• QRS complex is composed of 3 waves → Q wave + R wave + S wave
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49. Electrocardiogram
• Q-wave:
• Initial ventricular depolarization phase
• Usually occurs on the left side of intraventricular septum which shows:
• A negative component on lead I
• A small negative component on lead II and
• A positive component on lead III
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51. Electrocardiogram
• NB
• It is possible for a given cardiac dipole to produce opposite
deflections on different leads (Q waves appear on leads I and II
but not on lead III…)
• Q wave can be pathological if it is:
• Deeper than 2 small squares (0.2mV) and/or
• Wider than 1 small square (0.04s) and/or
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52. Q-wave
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In a lead other than III or one of the leads that look at the heart from the left (I, II, aVL
aVL, V5 and V6) where small Qs (i.e. not meeting the criteria above) can be normal
53. Electrocardiogram
• R-wave
• Ventricular depolarization when the number of individual dipoles
is greatest and/or their orientation is most similar
• This phase generates the largest net cardiac dipole in the ECG →
ventricular muscles are so numerous and depolarize nearly in
unison
• Such dipole produces a large positive R waves on all 3 limb leads
• The net cardiac dipole is nearly parallel to lead II
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55. Electrocardiogram
• S-wave:
• Occurs near the end of the spread of depolarization through the
ventricles
• Indicates the small net cardiac dipole present at this time
• NB
• S-wave does not necessarily appear on all ECG leads (lead I
below…)
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57. Electrocardiogram
• ST-segment:
• Isoelectric point of depolarized ventricles (no electrical potentials
are measured on the body surface during the ST segment):
• No rapid changes in membrane potential occurs in any of
myocardial cells
• Atrial cells have already returned to the resting phase
• All ventricular muscle cells are in a depolarized state (in their
plateau phase)
• All ECG traces will be flat at the isoelectric (zero voltage) level
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59. Electrocardiogram
• QRS complex
• If the complexes in the chest leads look very tall → consider LVH
• If the depth of the S wave in V1 + the height of the R wave in V6 is >
35mm → LVH is present
• Normal QRS complex < 0.1s (2.5 small squares)
• QRS > 0.1s → suggests ventricular conduction problem → usually
right or left bundle branch block (RBBB or LBBB)
• NB
• BBB can be caused by infarction or fibrosis (related to the ageing process)
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60. Electrocardiogram
• LBBB:
• QRS complex may look like a ‘W’ in V1 and/or an ‘M’ shape in V6
• New onset LBBB with chest pain → consider Myocardial infarction
• NB
• Not possible to interpret the ST segment
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61. Electrocardiogram
• RBBB:
• There may be an ‘M’ in V1 and/or a ‘W’ in V6
• Can occur in healthy people with normal QRS → Partial RBBB
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62. Electrocardiogram
• ST Segment:
• Planar (flat) elevation or depression of ST segment indicates
abnormalities:
• ST elevation → suggest MI/Prinzmetal’s (vasospastic) angina
• ST depression → can represent Ischaemia
• NB
• Myocardial injury or inadequate blood flow can cause ST segment
elevation or depression
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63. Electrocardiogram
• T-wave
• Period of ventricular repolarization:
• Ventricular cells begin to repolarize → a voltage once again appears
on the body surface
• Normally T-wave is positive on lead II → indicates the net dipole
generated during ventricular repolarization is oriented in the same
general direction as that of ventricular depolarization
• Combination of reversed individual dipole polarity and reversed
wavefront propagation pathway during ventricular repolarization would
be a positive T wave
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64. Electrocardiogram
• T-wave
• T-wave is broader and smaller than R-wave → ventricular
repolarization is less synchronous than their depolarization
• At the conclusion of the T wave all the cells in the heart are in the
resting state
• NB
• The last ventricular cells to depolarize are the first to repolarize
• The wavefront of electrical activity during ventricular repolarization
tends to retrace (in reverse direction) the course followed during
ventricular depolarization
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65. Electrocardiogram
• T-wave
• T wave generally shouldn’t be taller than half the size of the
preceding QRS (No definite rule for height)
• Tall T-wave: Causes
• Hyperkalemia + Acute myocardial infarction
• Flat T-wave → may indicate hypokalemia
• Inverted T-wave → may indicate Ischaemia
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66. Electrocardiogram
• QT Interval:
• Roughly approximates the duration of ventricular depolarization
(period of ventricular systole)
• Measured from the start of the QRS to the end of the T wave
• QT interval varies with the heart rate:
• At a HR of 60bpm, the QT interval is < 380ms
• As the HR gets faster, the QT interval gets shorter
• QT interval can be corrected with respect to the HR by using:
• QTc = QT/.RR ( QTc = corrected QT)
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67. Electrocardiogram
• QT Interval:
• Normal range for QTc = 0.38-0.42sec
• Short QTc → may indicate hypercalcaemia
• Long QTc → has many causes
• Long QTc → increases the risk of developing arrhythmia
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68. Electrocardiogram
• U-wave:
• Small rounded, upright wave following T-wave
• Often difficult to see (most easily seen with a slow HR)
• Represents repolarization of purkinje fibers
• Prominent U-waves → sign of hypokalemia, hyperthyroidism
• NB
• No body surface potential is measured until the next impulse is
generated by the SA node
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