Electrocardiogram: Principles, Technique, and Applications

Electrocardiogram: Principles, Technique, and Applications

An electrocardiogram (ECG) is a medical diagnostic technique used to record the electrical activity of the heart. By placing electrodes on the skin, ECG measures and records the weak electrical signals generated by the contraction of the heart muscle. The data obtained is displayed on a graph, allowing doctors to assess heart function and detect abnormalities.

Operating Principle:

When the heart beats, the heart muscle cells generate electrical currents. These currents spread throughout the body and are conducted to the electrocardiograph by electrodes. After amplification, these currents move the recording pens to create a graph of the heart’s electrical activity. This graph is the electrocardiogram.

ECG Equipment:

• Electrocardiograph: The main device for measuring and recording heart electrical signals.

• Electrodes: Small metal discs attached to the skin to collect electrical signals.

• Electrolyte gel: A conductive substance that improves contact between the skin and electrodes.

• Bed: A wooden or electrically insulated bed to avoid interference with the ECG signal.

Types of Electrocardiographs:

• 1-lead machine: Records one lead at a time.

• 3-lead machine: Records 3 leads simultaneously.

• 6-lead or 12-lead machine: Records 6 or 12 leads simultaneously.

Types of Electrodes:

• Electrodes placed on the limbs.

• Electrodes placed on the chest wall.

Electrode Material:

• Silver or tin-plated metal.

Replacing Electrolyte Gel:

If electrolyte gel is not available, it can be replaced with alcohol, saline, or cold water.

Function of Electrolyte Gel:

Electrolyte gel reduces the resistance at the contact point between the skin and electrodes, allowing better conduction of the heart’s electrical signals to the electrodes.

Lead:

A lead is the electrocardiogram image recorded when two electrodes of the recorder are placed at any two points on the body. Numerous leads can be measured.

Types of Leads:

• Standard Leads (Bipolar Limb Leads, Bipolar Peripheral Leads): Both electrodes are probes.

• DI: Right arm and left arm.

• DII: Right arm and left leg.

• DIII: Left arm and left leg.

• The angle formed by the standard leads is 60 degrees.

• Augmented Unipolar Limb Leads (Augmented Unipolar Peripheral Leads): The probe electrode is connected to the limb corresponding to the region to be explored (right arm, left arm, or left leg), and the neutral electrode is the center of the electrical network connecting the other two limbs.

• aVR: Right arm.

• aVL: Left arm.

• aVF: Left leg.

• Precordial Leads: The neutral electrode is the center of the electrical network connecting the 3 limbs: right arm, left arm, left leg.

Right Leg Electrode:

The right leg electrode is a noise-canceling electrode that does not participate in the measurement.

Performing an ECG:

1. Turn on the machine.

2. Apply electrolyte gel and attach electrodes.

3. Enter patient information.

4. Measure 12 leads.

5. Turn off the machine.

Adjustments During Measurement:

• Voltage test: Allows determination of the voltage (amplitude of the wave).

• Paper speed: Allows determination of the wave’s duration or a segment of the wave and heart rate.

Voltage Test:

• Symbol on paper: N, 10mm/mV.

• Height of the test column: 10 small squares.

Paper Speed:

• Symbol recorded on paper.

• Marking point: 2 markers 25 small squares apart.

Long ECG on Lead DII:

A long lead DII is used to:

• Assess heart rate.

• Detect premature beats.

• Calculate heart rate when the rhythm is irregular.

Choosing Lead DII as the Long Lead:

Lead DII is almost parallel and in the same direction as the heart’s electrical axis, allowing determination of the wave height and whether the wave is positive or negative.

Lead Classification:

• Peripheral: The probe electrodes are placed on the limbs. 6 leads: DI, DII, DIII, aVR, aVL, aVF.

• Unipolar: There is 1 probe electrode. 9 leads: aVR, aVL, aVF, V1-6.

Impact of Incorrect Electrode Placement:

Incorrect placement of the electrodes between the right arm and left arm will change the leads:

• DI will be reversed.

• DII and DIII will be swapped.

• aVR and aVL will be swapped.

• Precordial leads will remain unchanged.

Causes of ECG Interference:

• Electrical sources.

• Skeletal muscle contraction, muscle tremors, forceful breathing.

• Poor electrode contact.

• ECG machine.

• Patients with pacemakers.

Significance of the Voltage Test:

Allows the reader to correctly determine the amplitude (mm) or voltage (mV) of the ECG wave.

Correlation Between Amplitude and Voltage:

1mm = 10mV.

Determining Paper Speed:

• Recorded on the ECG paper.

• The distance between 2 markers is 25 small squares.

Paper Speed and Relationship to Time:

Slow paper speed – long time (direct relationship).

Handling an Unclear ECG:

Add more leads.

Right Leg Electrode:

One electrode is placed on the right leg (RF) to prevent interference with the ECG signal; it does not have a measurement function.

Comparison of Lead Types:

• Standard Leads and Augmented Unipolar Limb Leads:

• Similarities: The probe electrodes are placed on the limbs. The electrode colors are the same.

• Differences: Standard leads are bipolar; augmented unipolar limb leads are unipolar. Augmented unipolar limb leads have 1 electrode used for measurement, and the other 2 electrodes are neutral.

• Augmented Unipolar Limb Leads and Precordial Leads:

• Similarities: They both have a neutral electrode. They are both unipolar.

• Differences: Augmented unipolar limb leads are placed on the limbs. Precordial leads are placed in the chest area, using 6 electrodes.

• Standard Leads and Precordial Leads:

• Standard leads are placed on the limbs, do not have a neutral electrode, and use 3 electrodes.

• Precordial leads are placed in the chest area, have a neutral electrode that is the center of the electrical network connecting the 3 limbs, and use 6 electrodes.

Patient Preparation Before ECG Measurement:

• Rest for a while before the measurement.

• Do not use stimulants.

• Encourage the patient to be calm and relaxed.

• Lie still, comfortably, relaxing the muscles, with eyes closed.

• Use sedatives for agitated patients or children.

• The measurement room should have air conditioning.

• Clean the electrode placement areas.

• Remove metal objects, personal belongings, and items that interfere with the signal from the patient.

Paper Speed:

The fixed speed at which the recording paper runs through the recording pens.

Conclusion:

An electrocardiogram is a crucial non-invasive diagnostic tool that helps doctors assess heart function, detect abnormalities, and monitor patients. Understanding the principles, techniques, and applications of electrocardiography is essential to ensure effective diagnosis and treatment of cardiovascular diseases.