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Basics of
Electrocardiography
What is an ECG?
An ECG is the recording (gram)
of the electrical activity(electro)
generated by the cells of the
heart(cardio) that reaches the body
surface.
Recording ECG
William Einthoven
Useful in diagnosis of…
Cardiac Arrhythmias
Myocardial ischemia and infarction
Pericarditis
Chamber hypertrophy
Electrolyte disturbances
Drug effects and toxicity
Recording an ECG
Basics
ECG graphs:
– 1 mm squares
– 5 mm squares
Paper Speed:
– 25 mm/sec standard
Voltage Calibration:
– 10 mm/mV standard
ECG Paper: Dimensions5 mm
1 mm
0.1 mV
0.04 sec
0.2 sec
Speed = rate
Voltage
~Mass
ECG Leads
The standard ECG has 12 leads: 3 Standard Limb Leads
3 Augmented Limb Leads
6 Precordial Leads
The axis of a particular lead represents the viewpoint from
which it looks at the heart.
Summary of Leads
Limb Leads Precordial Leads
Bipolar I, II, III(standard limb leads)
-
Unipolar aVR, aVL, aVF (augmented limb leads)
V1-V6
Anatomic Groups(Summary)
Interpretation of an ECG
Steps involved
Heart Rate
Rhythm
Axis
Wave morphology
Intervals and segments analysis
Chamber enlargement
Specific changes
Wave forms
Cardiac Conduction: Cycle
Initiation
Cardiac Conduction: P Wave
Cardiac Conduction: AV Node
Cardiac Conduction: Bundle
Branches
Cardiac Conduction: Purkinje
Fibers
Cardiac Conduction: QRS
Complex
Cardiac Conduction: Plateau
Phase
Cardiac Conduction: T-Wave
Label the ECGP – Wave: Atrial Depolarization.
• Can be positive, biphasic, negative.
QRS Complex: Ventricular Depolarization.
• Q – Wave: 1st negative deflection wave before R-Wave.
• R – Wave: The positive deflection wave.
• S – Wave: 1st negative deflection wave after R – wave.
T – Wave: Ventricular Repolarization.
• Can be positive, biphasic, negative.
Calculating Heart Rate1) Measure Cycle Length (CL).
1) (# small boxes from R – R) (40ms) = CL .
2) Calculate HR
• 60,000/CL = “x” BPM
(20)(40ms) = 800ms
60,000/800 = 75 bpm
(25)(40ms) = 1000ms
60,000/1000 = 60 bpm
(12)(40ms) = 480ms
60,000/480 = 125 bpm
The Rule of 300
# of big
boxes
Rate
1 300
2 150
3 100
4 75
5 60
6 50
Calculating the Heart Rate
The QRS Axis
The QRS axis represents the net overall
direction of the heart’s electrical activity.
Abnormalities of axis can hint at:
Ventricular enlargement
Conduction blocks (i.e. hemiblocks)
The QRS Axis
By near-consensus, the
normal QRS axis is defined
as ranging from -30° to +90°.
-30° to -90° is referred to as a
left axis deviation (LAD)
+90° to +180° is referred to as
a right axis deviation (RAD)
Determining the Axis
The Quadrant Approach
The Equiphasic Approach
Determining the Axis
Predominantly
Positive
Predominantly
Negative
Equiphasic
The Quadrant Approach
1. Examine the QRS complex in leads I and aVF to determine
if they are predominantly positive or predominantly
negative. The combination should place the axis into one
of the 4 quadrants below.
Example 1
Negative in I, positive in aVF RAD
Example 2
Positive in I, negative in aVF Predominantly positive in II
Normal Axis (non-pathologic LAD)
-90°-60°
-30°
0°
aVL
I
30°
60°
aVR
II
90°
120°III
150°
180°
-150°
-120°
aVF
Marked RAD
LAD
RAD
Normal Axis
-30° to +100°
Example 1
Equiphasic in aVF Predominantly positive in I QRS axis ≈ 0°
Example 2
Equiphasic in II Predominantly negative in aVL QRS axis ≈ +150°
Common causes of LAD
May be normal in the elderly and very obeseDue to high diaphragm during pregnancy, ascites, or ABD tumorsInferior wall MILeft Anterior HemiblockLeft Bundle Branch BlockWPW SyndromeCongenital LesionsRV Pacer or RV ectopic rhythmsEmphysema
Common causes of RAD
Normal variant
Right Ventricular Hypertrophy
Anterior MI
Right Bundle Branch Block
Left Posterior Hemiblock
Left Ventricular ectopic rhythms or pacing
WPW Syndrome
The Normal ECG
Normal Sinus Rhythm
Originates in the sinus node
Rate between 60 and 100 beats per min
P wave axis of +45 to +65 degrees, ie. Tallest p waves in Lead II
Monomorphic P waves
Normal PR interval of 120 to 200 msec
Normal relationship between P and QRS
Some sinus arrhythmia is normal
Sinus Arrhythmia
ECG Characteristics: Presence of sinus P waves
Variation of the PP interval which cannot be
attributed to either SA nodal block or PACs
When the variations in PP interval occur in phase with respiration, this is
considered to be a normal variant. When they are unrelated to respiration,
they may be caused by the same etiologies leading to sinus bradycardia.
Normal P wave
Atrial depolarisation
Duration 80 to 100 msec
Maximum amplitude 2.5 mm
Axis +45 to +65
Biphasic in lead V1
Terminal deflection should not exceed 1
mm in depth and 0.03 sec in duration
Normal P wave
PR interval
AV node conduction
From the beginning of P wave to the
beginning of q wave
120-200 ms
Normal QRS complex
Completely negative in lead aVR , maximum positivity in lead II
rS in right oriented leads and qR in left oriented leads (septal vector)
Transition zone commonly in V3-V4
RV5 > RV6 normally
Normal duration 50-110 msec, not more than 120 msec
Physiological q wave not > 0.03 sec
ECG showing qR pattern in lead III
,disappears on deep inspiration q wave
not significant
Mech:shift in the QRS axis
QRS-T angle
The normal t wave axis is similar to the
QRS axis
Normally the QRS-T angle does not
exceed 60 deg
Amplitude of QRS
Depends on the following factors
1.electrical force generated by the
ventricular myocardium
2.distance of the sensing electrode from
the ventricles
3.Body build;a thin individual has larger
complexes when compared to obese
individuals
4.direction of the frontal QRS axis
Normal T wave
Same direction as the preceding QRS
complex
Blunt apex with asymmetric limbs
Height < 5mm in limb leads and <10 mm
in precordial leads
Smooth contours
May be tall in athletes
ST segment
Merges smoothly with the proximal limb of
the T wave
No true horizontality
Normal u wave
Best seen in midprecordial leads
Height < 10% of preceding T wave
Isoelectric in lead aVL (useful to measure
QTc)
Rarely exceeds 1 mm in amplitude
May be tall in athletes (2mm)
QT interval
Normally corrected for heart rate
Bazett’s formula
Normal 350 to 430 msec
With a normal heart rate (60 to 100), the
QT interval should not exceed half of the
R-R interval roughly
Measurement of QT interval
The beginning of the QRS complex is best
determined in a lead with an initial q wave
leads I,II, avL ,V5 or V6
QT interval shortens with tachycardia and
lengthens with bradycardia
Prolonged QTc
During sleep
Hypocalcemia
Ac myocarditis
AMI
Drugs like quinidine,procainamide,tricyclic antidepressants
Hypothermia
HOCM
Advanced AV block or high degree AV
block
Jervell-Lange –Neilson syndrome
Romano-ward syndrome
Shortened QT
Digitalis effect
Hypercalcemia
Hyperthermia
Vagal stimulation
Normal Variants in the ECG
Sinus arrhythmia
Persistent juvenile pattern
Early repolarisation syndrome
Non specific T wave changes
Persistent juvenile pattern
Features of ERPS
Vagotonia / athletes’ heart
Prominent J point
Concave upwards, minimally elevated ST segments
Tall symmetrical T waves
Prominent q waves in left leads
Tall R waves in left oriented leads
Prominent u waves
Rapid precordial transition
Sinus bradycardia
Early Recognition Prevents Streptokinase infusion !
Reporting an ECG
1. Patient Details
“ Whose ECG is it ?!”
2. Standardisation and lead
placement
“Is it properly taken ?”
3. Analysis of Rate, Rhythm and
Axis
4. Segment and wave form
analysis
Final Impression
“ Does the ECG correlate with
the clinical scenario ?”
Thank you !