DR. PALLAB KANTI NATHMBBS, MD (ANESTHESIOLOGY)

CONSULTANT PAIN MEDICINE, ANESTHESIOLOGIST, INTENSIVIST

Basics of Electrocardiography for

Technicians

What will we learn?

1. Basics of the conduction system of heart2. ECG leads and recording methodology3. ECG waveforms and intervals4. Normal ECG and its variants5. Interpretation and reporting of an ECG

What is an ECG?

Recording of the electrical activity heart.Graph of voltage versus time 

Recording an ECG

BasicsBasics

ECG graph:1 mm Small squares5 mm Large squares

Paper Speed:25 mm/sec standard

Voltage Calibration: 10 mm/mV standard

ECG Paper: DimensionsECG Paper: Dimensions5 mm

1 mm

0.1 mV

0.04 sec0.2 sec

Speed = rate

Voltage ~Mass

ECG Leads

Leads are electrodes which measure the potential difference between:

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)

ECG Leads

The standard ECG has 12 leads:

3 Standard Limb Leads

3 Augmented Limb Leads

6 Precordial Leads

Einthoven's triangle 

Precordial Leads

Electrode name Electrode placement

RA On the right arm, avoiding thick muscle.LA On the left arm, avoiding thick muscle.RL On the right leg, lateral calf muscle.LL On the left leg, lateral calf muscle.

V1

In the fourth intercostal space (between ribs 4 and 5) just to the right of the sternum (breastbone).

V2In the fourth intercostal space (between ribs 4 and 5) just to the left of the sternum.

V3 Between leads V2 and V4.V4 5th Intercostal space at the midclavicular lineV5 Anterior axillary line at the same level as V4V6 Midaxillary line at the same level as V4 and V5

Arrangement of Leads on the ECG

Arrangement on an ECG strip

Normal standardization

1 mV=10 mmWill result in perfect right angles at each

corner

Overdamping and Underdamping

Overdamping: When the pressure of the stylus is too firm on the paper so that it’s movements are retarded – deflection fractionally wider and diminished amplitude

Unerdamping: When the writing stylus is not pressed firmly enough against the paper - sharp spikes at the corners

Standard sites unavailable

Amputation/ burns/ bandages

leads should be placed as closely as possible to the standard sites

Specific cardiac abnormalities

dextrocardia right & left arm electrodes should be reversedpre-cordial leads should be recorded from V1R(V2) to V6

Continuous monitoring

Bed side

Continuous monitoring

Holter monitoring

Continuous monitoring

TMT

Artefacts on ECG

Distorted signals caused by secondary internal or external sources, such as muscle movement or interference from an electrical device.

ECG Artefacts

ECG tracing is affected by patient’s motion.

rhythmic motions (shivering or tremors) can create the illusion of arrhythmia. 

May lead to:Altered diagnosis, treatment, outcome of

therapy and legal liabilities

Reducing Artefacts during an ECG

Patient PositioningSupine or semi-Fowler’s position.

If patient can’t tolerate lying flat, do the ECG in a more upright position.

Instruct patient to place arms down by his side and to relax the shoulders.

Patient’s legs should be uncrossed.

Place electrical devices, such as cell phones, away from the patient as they may interfere with the machine.

Reducing Artefacts during an ECG

Skin PreparationDry the skin if it’s moist or diaphoretic.

Shave any hair that interferes with electrode placement. ensures a better electrode contact with the skin.

Rub an alcohol prep pad or benzoin tincture on the skin to remove any oils and help with electrode adhesion.

Reducing Artefacts during an ECG

Electrode ApplicationCheck the electrodes to make sure the gel is

still moist.

Do not place the electrodes over bones.

Do not place the electrodes over areas where there is a lot of muscle movement.

Interpretation of an ECG

Heart RateRhythmAxisWave morphologyIntervals and segments analysisSpecific changes (If any)

Determining the Heart Rate

Rule of 300

10 Second Rule

Rule of 300

Take the number of “big boxes” between neighboring QRS complexes, and divide this into 300. The result will be approximately equal to the rate

Although fast, this method only works for regular rhythms.

The Rule of 300

It may be easiest to memorize the following table:

# of big # of big boxesboxes

Rate (appx)Rate (appx)

11 300300

22 150150

33 100100

44 7575

55 6060

66 5050

Rule of 300

10 Second Rule

As most ECGs record 10 seconds of rhythm per page, one can simply count the number of beats present on the ECG and multiply by 6 to get the number of beats per 60 seconds.

This method works well for irregular rhythms.

QRS axis

The QRS axis represents the net overall direction of the heart’s electrical activity.

Abnormalities of axis can hint at:Ventricular enlargementConduction 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

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.

Using leads I, II, III

LEAD 1LEAD 1 LEAD 2LEAD 2 LEAD 3LEAD 3

NormalNormal UPRIGHTUPRIGHT UPRIGHTUPRIGHT UPRIGHTUPRIGHT

PhysiologicPhysiological Left Axisal Left Axis UPRIGHTUPRIGHT UPRIGHT / UPRIGHT /

BIPHASICBIPHASIC NEGATIVENEGATIVE

Pathological Pathological Left AxisLeft Axis UPRIGHTUPRIGHT NEGATIVENEGATIVE NEGATIVENEGATIVE

Right AxisRight Axis NEGATIVENEGATIVEUPRIGHTUPRIGHTBIPHASICBIPHASICNEGATIVENEGATIVE

UPRIGHTUPRIGHT

Extreme Extreme Right AxisRight Axis NEGATIVENEGATIVE NEGATIVENEGATIVE NEGATIVENEGATIVE

Common causes of LAD

May be normal in the elderly and very obese

High diaphragm during pregnancy, ascites, or Abdominal tumors

Inferior wall MI

Left Anterior Hemiblock

Left Bundle Branch Block

WPW Syndrome

Emphysema

Common causes of RAD

Normal variant

Right Ventricular Hypertrophy

Anterior MI

Right Bundle Branch Block

Left Posterior Hemiblock

WPW Syndrome

Normal Sinus RhythmOriginates in the SA node

Rate between 60 and 100 beats per min

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

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

QRS Complex

Amplitude of QRS

Formed by electrical force generated by the ventricular myocardium

Depends on: distance of the sensing electrode from the

ventricles

Body build - a thin individual has larger complexes when compared to obese individuals

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

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

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

QT Interval

Reporting an ECG

“ WHOSE ECG IS IT ?!”

1. Patient Details

“IS IT PROPERLY TAKEN ?”

2. Standardisation and lead placement

NORMAL OR ABNORMAL?

4. Segment and wave form analysis

“ DOES THE ECG CORRELATE WITH THE CLINICAL SCENARIO ?”

Final Impression

Thank you !