Lead Systems Flashcards
ECG lead positions
Identify the angle of Louis (place your fingers at bottom of throat and move them down to feel boney lump. Move fingers to the right and feel the 2nd intercostal gap)
V1 – positioned in the 4th intercostal space, right sternal border.
V2 – positioned in the 4th intercostal space, left sternal border.
V3 – In-between V2 and V4
V4 – positioned in the 5th intercostal space, mid-clavicular line.
V5 – positioned on the same horizontal line as V4, anterior axillary line.
V6 – positioned on the same horizontal line with V4, mid axillary line.
Where are limb leads placed?
The limb leads should be placed at least 15cm away from the heart. Ideally limb leads should be placed on the ankles and wrists.
What is Einthoven’s triangle?
Einthoven’s triangledescribes the relationship between the limb leads and electrodes. It is an imaginary formation of three limb leads in a triangle used inelectrocardiography, and is formed by the RA, LA and LL electrodes.The shape forms an inverted equilateral triangle with the heart at the center.
3 bipolar limb leads: Lead 1 looks at the voltage difference between the RA and LA. Lead 2 between RA and LL, and Lead III between LA and LL.
Electrical connections for the leads are such that the potential in lead II equals the sum of potentials sensed in leads I and III
I + III = II
Then you have your 3 unipolar limb leads –aVr, aVl and aVf.
Which leads are bipolar, augmented unipolar and unipolar?
With the use of these 10 electrodes – 12 leads can be derived.
Bipolar limb leads have a negative and positive electrode and measure the potentials between the two electrodes.
3 augmented unipolar leads have a single positive electrode on right and left arms and left leg.
6 unipolar chest leads – look at the heart from the front.
Components of cardiac conduction system
1- The Sino-Atrial Node (pacemaker) (upper wall of RA, at junction where SVC joins)
2- The Atrio-Ventricular Node (interatrial septum by coronary sinus, centre of Koch’s triangle)
3- Bundle Of His
4 -Left Bundle Branch
5- Right Bundle Branch
6 -Purkinje Fibres
Describe cardiac conduction system
The sino-atrial node generates the electrical impulse that travels through heart causing contraction.
The electrical signal travels from cell to cell through the atria causing them to contract, then continues on to AV node.
AV node delays stimulus, before relaying it on to the ventricles.
Impulse is passed to the bundle of His and on to the right and left bundle branches.
Both bundles eventually terminate in individual purkinje fibres.
Contraction occurs in the ventricles.
Optimal haemodynamic function occurs.
What is a lead?
The term lead refers to the ECG obtained as a result of recording the difference in potential between a pair of electrodes.
ECG picks up impulses generated by polarisation and depolarisation of the cardiac muscle, translating it into a waveform.
Two Types of Leads
1) Bipolar Leads;
-Leads I, II and III - Einthoven’s Triangle
2) Unipolar Leads;
-V1-V6 - Wilson’s Central Terminal
- AVR, AVL, AVF - Augmented limb leads (Goldberger)
Bipolar Leads
- 2 electrodes based at 2 different sites
- The leads register the difference in potential between the 2 sites
- The potential at either electrode is not known – only the difference between them is measured.
- 1 electrode designated as positive input
- Negative electrode is subtracted from the potential at the positive electrode to generate the bipolar potential
Which are the bipolar limb leads?
I, II & III
Describe the way the difference in voltages between the two extremities (limb leads)
Output is the potential difference between 2 limb leads
Unipolar Leads
- Measure absolute electrical potential at 1 site
- Needs a reference site to do this.
- Reference site requires potential to be 0
- Reference site may be location far away from active electrode or special electrode configuration
- Unipolar recording therefore is potential sensed by single electrode at 1 site in relation to the reference (0) potential
What is Wilson’s Central Terminal
Wilson, in an attempt to obtain a central terminal with a “zero potential”, connected all three limb electrodes through 5000Ω resistances to form the indifferent electrode
These three electrical potentials added together give a resultant potential very close to zero, a reference for the whole ECG.
Kirchoffs Voltage Law - the directed sum of potential differences (voltages) around any closed loop is zero -> I + II + III = 0
How are unipolar chest leads formed?
Measure absolute electrical potential at 1 site
The exploring electrode, on the chest wall (V1 - V6), is connected to the positive pole on the galvanometer of the ECG and the negative to Wilson’s central terminal.
Thus obtaining the 6 unipolar chest leads, V1 - V6.
What are Goldberger’s augmented leads?
Wilson CT worked but voltages were small
- A modification to Wilson’s central terminal was made by Goldberger whereby the connection from the limb under study to the central terminal was omitted.
- This modification enlarged or augmented the voltage of the ECG and the leads so derived are referred to as “a” leads
aVR, right arm lead
aVL, left arm lead
aVF, left foot lead
Augmented Leads of Goldberger - aVR
RA as exploring electrode at positive pole
LA and LL as indifferent electrode at negative pole
Augmented Leads of Goldberger - aVL
LA as exploring electrode at positive pole
RA and LL as indifferent electrode at negative pole
Augmented Leads of Goldberger - aVF
LL as exploring electrode at positive pole
RA and LA as indifferent electrode at negative pole
What are the lead configurations on a 12-lead ECG?
I, II, III - Einthoven’s bipolar limb leads
aVR, aVF, avL - Unipolar augmented Limb leads (Goldberger’s)
V1-6 - unipolar chest leads (Wilson’s central terminal)
From which position is each lead viewing the heart
Lead I - Lateral LV wall
Lead II - Inferior/diaphragmatic region
Lead III - Inferior/diaphragmatic region
aVR - Cavity lead (cavity of heart from right shoulder therefore negative as ECG signal going away from lead)
aVL - High lateral LV wall
aVF - Inferior/diaphragmatic region
V1 & V2 - lie close to RV
V3 & V4 - lie close to IVS
V5 & V6 - face LV lateral wall but are separated by distance.
V4 - usually lies at the cardiac apex.
The size of the R wave increases from V1 - V6 - R wave progression
