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Basics of Electrocardiography

Dr. Badri Paudel GMC

Outline 1.  Review of the conduction system

2.  ECG leads and recording

3.  ECG waveforms and intervals

4.  Normal ECG and its variants

5.  Interpretation and reporting of an ECG

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Electrocardiography

•  A recording of the electrical activity of the heart over time

•  Gold standard for diagnosis of cardiac arrhythmias

•  Helps detect electrolyte disturbances (hyper- & hypokalemia)

•  Allows for detection of conduction abnormalities

•  Screening tool for ischemic heart disease during stress tests

•  Helpful with non-cardiac diseases (e.g. pulmonary embolism or hypothermia

• Pericarditis and Chamber hypertrophy 4/18/12 badri@gmc 3

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.

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Recording ECG

William Einthoven

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Basics

" ECG graphs: – 1 mm squares – 5 mm squares

" Paper Speed: – 25 mm/sec standard

" Voltage Calibration: – 10 mm/mV standard

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ECG Graph Paper •  Runs at a paper speed of 25 mm/sec •  Each small block of ECG paper is 1 mm2 •  At a paper speed of 25 mm/s, one small block equals 0.04 s •  Five small blocks make up 1 large block which translates into 0.20 s (200 msec) •  Hence, there are 5 large blocks per second •  Voltage: 1 mm = 0.1 mV between each individual block vertically

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ECG Paper: Dimensions 5 mm

1 mm

0.1 mV

0.04 sec 0.2 sec

Speed = rate

Voltage ~Mass

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ECG Leads Leads are electrodes which measure the difference in electrical potential between either:

1. Two different points on the body (bipolar leads)

2. One point on the body and a virtual reference

point with zero electrical potential, located in the center of the heart (unipolar leads)

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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.

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Recording of the ECG: Leads used: •  Limb leads are I, II, II. • Each of the leads are bipolar; i.e., it requires two sensors on the skin to make a lead. •  If one connects a line between two sensors, one has a vector. •  There will be a positive end at one electrode and negative at the other. •  The positioning for leads I, II, and III were first given by Einthoven. Form the basis of Einthoven’s triangle.

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Types of ECG Recordings

�  Bipolar leads record voltage between electrodes placed on wrists & legs (right leg is ground)

�  Lead I records between right arm & left arm

�  Lead II: right arm & left leg

�  Lead III: left arm & left leg

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Standard Limb Leads

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Standard Limb Leads

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Augmented Limb Leads

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All Limb Leads

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Where do those chest stickers go?

Ø  Make sure to “feel” for intercostal space – don’t just use your eyes! 4/18/12 badri@gmc 18

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Precordial Leads

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……and the FEMALES �  Not all nipple lines are

created equal

�  Measure intercostal spaces to be accurate in electrode placement �  All 12 leads measured from

same electrode placement

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Lead Placement in the Female �  Avoid placing electrodes on top of breast tissue

�  Use the back of the hand to displace breast tissue out of the way to place electrode �  Avoids perception of “groping” �  Can ask the patient to move left breast out of way.

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Precordial Leads

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Summary of Leads

Limb Leads Precordial Leads

Bipolar I, II, III (standard limb leads)

-

Unipolar aVR, aVL, aVF (augmented limb leads)

V1-V6

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Arrangement of Leads on the EKG

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Anatomic Groups (Septum)

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Anatomic Groups (Anterior Wall)

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Anatomic Groups (Lateral Wall)

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Anatomic Groups (Inferior Wall)

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Anatomic Groups (Summary)

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Heart & 12 – Lead Strip Correlation

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12 – Lead Strips Remember: Every lead is like a “camera angle”

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12 – Lead Strips cont. Imagine your strips broken into groups like this…

I

II

III

aVL

aVF

V1

V2

V3

V4

V5

V6

aVR

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Elements of the ECG: •  P wave

•  Depolarization of both atria; •  Relationship between P and QRS helps distinguish various cardiac arrhythmias

•  Shape and duration of P may indicate atrial enlargement

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•  PR interval: •  From onset of P wave to onset of QRS

•  Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)

•  Represents atria to ventricular conduction time (through His bundle)

•  Prolonged PR interval may indicate a 1st degree heart block

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• QRS complex:

•  Represents ventricular depolarization

•  Larger than P wave because of greater muscle mass of ventricles

•  Normal duration = 0.08-0.12 seconds

•  Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular hypertrophy, MI, electrolyte derangement, etc.

•  Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may represent MI

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T wave: •  Represents repolarization or recovery of ventricles •  Interval from beginning of QRS to apex of T is referred to as the absolute refractory period

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What’s a J point and where is it? �  J point – point to mark end

of QRS and beginning of ST segment �  Evaluate ST elevation 0.04

seconds after J point �  Based on relationship to the

baseline �  Used in assessing ST

elevation

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ST segment: •  Connects the QRS complex and T wave •  Duration of 0.08-0.12 sec (80-120 msec

QT Interval

•  Measured from beginning of QRS to the end of the T wave •  Normal QT is usually about 0.40 sec •  QT interval varies based on heart rate

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�  3 distinct waves are produced during cardiac cycle

�  P wave caused by atrial depolarization

�  QRS complex caused by ventricular depolarization

�  T wave results from ventricular repolarization

ECG

Fig 13.24 13-63

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Elements of the ECG: •  P wave: Depolarization of both atria;

•  Relationship between P and QRS helps distinguish various cardiac arrhythmias •  Shape and duration of P may indicate atrial enlargement

•  PR interval: from onset of P wave to onset of QRS

•  Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)

•  Represents atria to ventricular conduction time (through His bundle)

•  Prolonged PR interval may indicate a 1st degree heart block

•  QRS complex: Ventricular depolarization

•  Larger than P wave because of greater muscle mass of ventricles

•  Normal duration = 0.08-0.12 seconds

•  Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular hypertrophy, MI, electrolyte derangement, etc.

•  Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may represent MI

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ST segment: •  Connects the QRS complex and T wave •  Duration of 0.08-0.12 sec (80-120 msec

T wave: •  Represents repolarization or recovery of ventricles •  Interval from beginning of QRS to apex of T is referred to as the absolute refractory period

QT Interval •  Measured from beginning of QRS to the end of the T wave •  Normal QT is usually about 0.40 sec •  QT interval varies based on heart rate

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Time relationships between developed force and the changes in transmembrane potentials in a thin strip of ventricular muscle

Time

Mechanical event

Electrical event

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QRS waveform nomenclature

R r qR qRs Qrs QS

Qr Rs rS qs rSr’ rSR’

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Localising the arterial territory

Inferior II, III, aVF

Lateral I, AVL, V5-V6

Anterior / Septal V1-V4 4/18/12 badri@gmc 53

Standard sites unavailable �  Patient pathology

Amputation or burns or bandagesà should be placed as closely as possible to the standard sites

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Specific cardiac abnormalities

�  Situs inversus dextrocardiaà right & left arm electrodes should be reversed

pre-cordial leads should be recorded from V1R(V2) to V6

�  RVH & RV infarction:V3R & V4R

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Continuous monitoring �  Bed side:

�  Holter monitoring:

�  TMT: Mason Likar system

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Other practical points �  Electrodes should be selected for maximum

adhesiveness and minimum discomfort,electrical noise,and skin-electrode impedance

�  Effective contact between electrode and skin is essential.

�  ECG :calibration

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�  ECG :paper speed

�  Electrical artifacts:external or internal

external can be minimized by straightening the lead wires

internal can be due to muscle tremors,shivering ,hiccoughs .

�  Supine position

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MODERN  EKG  MACHINE  

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