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

ecg/15/15x/1509x.htm

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 !