Electrocardiography (ECG)

Introduction

• The ECG is a basic investigation that is widely used in medicine
• You are guided through a module based on your objectives.
• This package teaches you the important fundamentals of recording a 12-lead ECG and labeling the ECG.
• Numerous patient cases are provided for evaluation while your understanding of material will be tested at the end of the module

Normal activation of the heart

• The resting potential is maintained by ionic gradients across the cells with a higher sodium concentration outside and higher potassium concentration inside
• Following a stimulus, a rapid inward movement of sodium occurs causing a loss of the negative internal potential. This is called as depolarization.
• This is followed by repolarization, a slower process than depolarization where the potassium moves out of the cell

Cardiac conduction system

• The sinus node is the normal pacemaker of the heart.
• It is located in the posterior part of the right atrial wall at the junction of the right atrium and the superior vena cava.
• The sinus node is made up of three types of cells; P cells, transitional cells and the Purkinje’s cell.
• The P cells have the highest degree of automaticity. The transitional cells transmit impulses from the P cells to the Purkinje cell which in turn transmit to the atrial myocardium and the internodal conduction pathways.
• The sinus node has rich innervation from both sympathetic and parasympathetic nerves.
• The SA node is capable of generating independent impulses from its specialized tissues
• The impulse is then spread through the atria after a brief delay to the specialized tissues of the heart designed for rapid transmission of electrical activity such as atrioventricular node, bundle of His and its right and left branches to the ventricular myocardium.
• At the AV node there is a brief delay of the impulse before traveling further.
• The ventricles are activated initially from the lower part of the interventricular septum.

Septal depolarization

• The activation begins from the left subendocardium and spread to the right from the left bundle.
• At the same time, a smaller activation of the septum occurs by the right bundle

Free wall depolarization

• Next the free wall of both ventricles are depolarized from endocardium to epicardium
• The last area to be depolarized is the basal area of the ventricles

Repolarization

• When the heart is fully depolarized, there is no electrical activity for a brief period. This is represented by the ST segment.
• The repolarization begins from the endocardium and spreads to the epicardium
• Atrial depolarization gives rise to the P wave.
• Ventricular depolarization gives rise to the QRS complex.
• Ventricular repolarization gives rise to the T wave
• Since the cells of the heart are linked close to each other, depolarization beginning in one area spreads rapidly through the entire heart

Theory of electrocardiography

• Electrical potentials produced by the heart are a sum total of the minute amounts of electricity generated from individual cardiac muscle cells.
• If the current flows in the same direction as axis of lead, a positive deflection is recorded
• If the current flows in the opposite direction as axis of lead, a negative deflection is recorded
• If the current flows in a perpendicular direction as axis of lead, then the recording shows an equiphasic deflection
• If no current flows, then the recording shows no deflection
• The magnitude of deflection is measured in millivolts (mV)
• The duration of the deflection is measured in seconds.
• The distance between two thin vertical lines is 0.04 seconds
• The distance between two thick vertical lines is 0.2 seconds
• The distance between two thin horizontal lines is 1mm
• The paper speed is usually 25 mm per second but can be increased to 50mm per seconds if the heart rate is rapid.
• The ECG is calibrated before commencement. Often 1cm (10 mm) deflection represents 1 mV

Lead system

• Lead I records the potential between left arm and right arm electrode. the left arm (L) is the positive lead and the right arm (R) is the negative lead.
• Lead II records the potential between right arm and left leg. The left leg is the positive lead and right arm is the negative lead
• Lead III records the potential between left arm and left leg. The left leg is the positive lead and the left arm is the negative lead
• Leads aVR, aVL and aVF are the augmented limb leads
• The leads aVR, aVL and aVF are unipolar leads.
• The letter V is used to indicate an unipolar lead.
• The letter 'a' comes from "augmented" and means that the tracing of the complex is amplified. aVR is an augmented unipolar lead of the right arm, aVL from the left arm and aVF from the left leg.
• Although an unipolar lead is mentioned, this does not mean that the trace is made by one electrode. Whenever these tracings are made the electrocardiograph, by means of an automatic circuit, calculates an average potential of the leads involved. This average potential serves as reference point for the unipolar leads. In fact one registers between a specific electrode (the lead) and this reference point (the average potential, also called the Central Terminal). The difference between two unipolar limb leads correspond to a bipolar limb lead:
• Lead I = aVL - aVR.
• Lead II = aVF - aVR.
• lead III = aVF - aVL

V1         4th interspace – right border of sternum V2         4th interspace – left border of sternum V3         Halfway between V2 and V4 V4         Fifth intercostal space on left mid-clavicular line V5         Left anterior axillary line at the horizontal level of V4 V6         Left mid-axillary line at the horizontal level of V4

Recording an ECG (electrocardiogram)

Notes on recording an electrocardiogram

Items

         Be familiar with the equipment

•  Patient cable
• Limb electrode
• Chest electrode
• Electrode cream
• Recording paper
• Power cord
• Thermo-pen
•  Ground lead

Controls and connections

Location of ECG

ü  Should be located in a warm and quiet room free from vibration and dust

ü  Should not be used near high voltage and heavy duty power lines or near powerful electrical instruments (X ray or ultrasonic equipment)     

Grounding

ü  Grounding the instrument assures safety of both the patient and operator and avoid shock

Power cord connection

Follow instruction on equipment

Recording paper check

ü  Make sure that there is sufficient amount of recording paper loaded

Check on

ü  Sensitivity adjustment

ü  Paper speed

ü  Filter

Patient cable connection

            Have power off when connecting the patient cable to the instrument

Electrode placement

ü  Clean the patient’s skin with alcohol

ü  Apply a spot of ECG cream to the skin and rub in it

ü  Spread the cream thinly on the surface of the limb electrodes

ü  Place the electrodes on the skin  

ü  Fasten or clip on the electrodes but not too tightly

Chest electrode

ü  Clean the patient’s chest with alcohol where the chest electrodes are to be placed

ü  Apply a spot of cream on each of the chest position

ü  Connect the chest cable tips firmly to the corresponding chest electrodes

ü  Apply cream to the rim of the chest electrode cup

ECG recording

Eliminating AC interference

AC interference

           Check on,

 

ü  Is the metal part of the examination bed grounded properly?

ü  Is the patient touching the wall or metal object?

ü  Is another person touching the patient?

ü  Is the patient still wearing glasses or jewellery?

ü  Are the electrodes and patient cable correctly mounted and connected?

ü  Is there a powerful electrical instrument being used nearby?

   EMG artifact

ü  Is the examination room comfortable?

ü  Are the electrode straps fastened firmly but not too tightly?

ü  Is the patient relaxed?

ü  Is a comfortable bed being used?

Wandering baseline

ü  Are electrodes being held firmly in place?

ü  Are patient cable tips connected to electrodes firmly?

ü  Are both electrodes and patient’s skin clean?

ü  Is a sufficient amount of cream applied to the electrodes?

ü  Is wandering baseline being caused by the patient’s body movement or breathing?      

Lead connection

• Bipolar limb leads (lead I, II, III)
• Unipolar augmented limb leads (aVr, aVl & aVf)
• Unipolar precordial chest leads (V1 – V6)

Nomenclature of QRS terminology

When there are several positive and negative deflections, the large amplitudes are denoted by capital letters and the small amplitudes are denoted by small letters.

• Q or q wave – first negative deflection preceding the R wave
• R or r wave – first positive deflection
• S or s wave – first negative deflection following the R wave
• R’ – first large positive deflection after the R wave
• S’ – first large negative deflection after S wave
• r’ – first small positive deflection after R wave
• s’ – first negative deflection after S wave
• QS – totally negative complexes

Artefacts in the ECG

• The ECG tracing can be generally distorted by the following factors
• Technical factors in the recording (AC interference)
• Patient factor
• Technical factors in the recording (AC interference);
• Alternating current
• An AC interference has a regular saw tooth peaks with a typical AC frequency of cycles per second
• Patient factors;
• Muscular tremor (cold room, too narrow couch, Parkinsonism)
• The pattern is of irregular deflections
• Patient movements (deep breathing, poor electrode contact with skin)

Identification data should be recorded in the ECG;

• Name and sex
• Ward
• Date and time of recording
• Correct lead if not automatically labeled