Basics of Electrocardiography for Technicians (Class note)

What is an ECG?

Recording of the electrical activity heart.

ØGraph of voltage versus time 

Basics

ECG graph:

Ø1 mm Small squares

Ø5 mm Large squares

Paper Speed:

Ø25 mm/sec standard

Voltage Calibration:

Ø10 mm/mV standard

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 centre of the heart (unipolar 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).
V2	In 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 line
V5	Anterior axillary line at the same level as V4
V6	Midaxillary line at the same level as V4 and V5

Arrangement of Leads on the ECG

Normal standardization

—1 mV=10 mm

—Will 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

Specific cardiac abnormalities

—dextrocardiaà right & left arm electrodes should be reversed

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

Artefacts on ECG

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

—ECG tracing is affected by patient’s motion.

—

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

—

—May lead to:

ØAltered diagnosis, treatment, outcome of therapy and legal liabilities

Reducing Artefacts during an ECG

Patient Positioning

—Supine or semi-Fowler’s position.

ØIf 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.

Skin Preparation

—Dry the skin if it’s moist or diaphoretic.

—

—Shave any hair that interferes with electrode placement.

Øensures 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.

Electrode Application

—Check 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 Rate

—Rhythm

—Axis

—Wave morphology

—Intervals and segments analysis

—Specific changes (If any)