6. ECG basics And Leads Flashcards
SA node
—> generate action potentials (depolarising currents) that are transmitted by channels to certain areas causing muscles to contracts
Impulse conducting system of the heart -steps
- The depolarising (+ve) current is generated in SA node
- Spreads to Atrium and then to the AV node
- From the AV node the +ve current moves to bundle branches depolarising the interventricular septum
- The +ve current then moves to the purkinje fibers depolarising the ventricles (causing ventricles to contract)
Repolaraisation
- The depolarising wave is immediately followed by repolarising (-ve) current
- Atria begin to repolarise when ventricles are depolarising
- Ventricles repolarise before the next wave of depolarisation begins
ECG - what is it
- Graphical representation of electrical activity of the heart
- The electrodes placed in the body captures movement of ions and records it
3 charges
Positive deflection
Negative defection
Isoelectric point
Positive charges
- positive deflection = +ve current moving towards +ve electrode will give a +ve deflection -
- neglative deflection +ve current moving away from +ve electrode will give a -ve deflection
- isoelectrric point/ straight line +ve current moving perpendicular to the electrodes will give isoelectric line
Negative charges
- Positive deflection = -ve current moving towards -ve electrode will give a +ve deflection
- Negative electrode = -ve current moving away from -ve electrode will give a -ve deflection
- Isoelectric point/ straight line = -ve current moving perpendicular to the electrodes will give isoelectric line
Lead II
—-> used as a rhythm strip = as it has a more prolonged time period of 10seconds
• Used to determine rate and rhythm of heart beat
Most commonly used to produce the normal sinus rhythm (NSR) tracing
• -ve electrode placed in right arm and +ve electrode in left leg
Waves
• Pqrst
• Rarely may get a u wave
P wave
• Formed by atrial depolarisation
SA node generate positive charge and sends it down depolarising the atria
• Red arrow is the net movement of the charges towards the av node
• Positive charges move towards positive electrode = positive deflection
P-R interval
In the AV node the positive charges remain ‘ there for 0.1 seconds
• 0.1 s delay in conduction in AV node leading to P-R segment
• +ve charge is not moving!
Isoelectric line
Q wave
—> septal depolarisation
• Positive charges move down into bundle branches • Left bundle branch receives positive charges first = Left bundle branch depolarises the interventricular septum and right bundle branch • Positive charges move away from positive electrode = negative deflection
R wave
ventricular (apex) depolarisation
- Left bundle depolarises interventricular septum and right bundle
- After right bundle is depolarised charges move down and depolarise ventricular apex
- More depolarisation happens in left ventricle than right due to its thicker wall
- So movement of positive charges are more towards left ventricle = movement more towards positive electrode
• Atria begin to repolarise as ventricles are depolarising: but this repolarising wave is ‘hidden’ within the big R wave
S wave
- Positive charges move up to lateral walls o’f ventricles
- Positive charges move away from potisitve electrode = negative deflection
• Purkinje fibers depolarises the walls of the ventricles
S- T segment
- Short delay before ventricular repolarisation
- Isoelectric line
- Entire ventricular myocardium is depolarised and is not repolarised yet.
- No net movement of ions
T wave
—> ventricular repolaraisation
• Repolarization = all positive charges become negative ○ Negative ions move away from positive electrode and up towards negative electrode = negative reflection
- Negative charge moving towards the negative electrode giving a positive deflection
- QT interval: time taken for ventricular depolarisation and repolarisation
Rare u wave
• Due to repolarisation of papillary muscles
ECG electrodes
- Clinical ECG recording typically uses 4 limb electrodes and 6 chest electrodes
- Leads are also called views
The three bipolar limb leads
–> 3 limb leads in the legs or arms, organised in a triangle
• Leads I, II, and III can be represented schematically in terms of a triangle, called Einthoven’s triangle
Lead 1
- -ve electrode in right arm
- +ve electrode in left arm
- Looking at the heart from slightly above = High lateral wall of left ventricle
Lead 2
- -ve electrode in right arm
- +ve electrode in left leg
- Looking at the heart from the base – inferior rv and lv
Lead 3
- -ve electrode in left arm
- +ve electrode in left leg
- Looking at the heart from the base – inferior rv and lv
What is the view from a lead
Look at the +ve lead to determine where info is connecting
Lead I waves
• Same waves as lead 2 but lower amplitude
Lead Il waves
• Same waves as lead 3
Lead Ill waves
• Same waves as lead 2 but lower amplitude
Lead equation
• The bipolar leads are related by the ‘ equation: Lead I + Lead III = Lead II
Good way to check if electrodes are in the right place
Lead 2 waves should look like lead 1 and 3 waves together
How leads view heart
• The area of heart covered by the leads:
—-> ecg focuses on the ventricles
– Lead I: High lateral wall of left ventricle
– Lead II and III: Inferior portion of right and left ventricle
As lead 2 and lead 3 have the same view, they will have same wave view but just at different amplitudes
3 Augmentated unipolar limb leads
aVR - right arm -
aVL - left arm
aVF - foot
aVR
- Augmented vector right
- -ve electrode on left arm
- -ve electrode on foot
- +ve electrod in right arm
- Look from the +ve electrode – augmented view from the right side
- Gives a triangle
- Waves are an opposite pattern
aVL
- +ve elctrodre on the left arm
- It is looking from the left side like lead 1 - it has the same wave pattern as lead 1 (2 and 3)
- Location is looking at high lateral wall of left ventrical like lead 1
aVF
• +ve foot
* Same wave patternas lead 2 (1 and 3) * As it is looking from same direction as lead 2 – inferior wall of ventricles)
aVR waves
The area of heart covered by the aVR:
– Right ventricle and basal septum
• ECG tracing waves will be the opposite of lead I, II and III
Almost reflected
All waves are interved
Pqrst
aVL waves
• The area of heart covered by the aVL:
– High lateral wall of left ventricle
• ECG tracing similar to lead I, II and III
aVf waves
• The area of heart covered by the aVF:
– Inferior wall of the heart
• ECG tracing similar to lead I, II and III
Six precordial chest leads - what are they
—> these leads are unipolar- have only one positive electrode
V1
– V1: Right 4th intercostal space
V2
– V2: Left 4th intercostal space
V4
– V4: Left 5th intercostal space, midclavicular line
V5
– V5: Left 5th intercostal space, anterior axillary line (where the clavicle is ending)
V6
– V6: Left 5th intercostal space, mid axillary line (middle of armpit)
V3
– V3: Between V2 and V4
Six precordial chest leads - which parts of the heart are covered
– V1-V2: Right ventricle
– V2-V3: Basal septum
– V2-V4: Anterior wall of the heart
– V5-V6: Lateral wall of the heart
Six precordial chest leads - the waves
Same waves pqrst
– R-wave increase in size from V1- V6
• V1 has small r wave
• V2 has large r wave
– S-wave decrease in size from V1-V6
• V1 has large s wave
• V6 has small s wave
