ECG, info and interpretation Flashcards
What does an ECG meausre
- the electrical activity generated by the heart as it contracts
Outline the route of the cardiac condution system
What kind of cells is the cardiac conduction system made up of?
- Modified cardiac myocytes that can conduct electricity
- Not nerves
True or false: ECGs are picking up the electrical activity of the cardiac condution system
- false
- cardiac condution system has such little energy that it would be hard to pick up
- ECGs are picking up the depolarisation of the cardiac myocytes (that the conduction system penetrates)
How many leads are used in a 12 lead ECG?
10
Where do all the leads go for a 12 lead ECG
- There are six chest electrodes:
- V1: 4th intercostal space at the right sternal edge.
- V2: 4th intercostal space at the left sternal edge.
- V3: midway between the V2 and V4 electrodes.
- V4: 5th intercostal space in the midclavicular line.
- V5: left anterior axillary line at the same horizontal level as V4.
- V6: left mid-axillary line at the same horizontal level as V4 and V5.
- There are four limb electrodes:
- Red (RA): Anywhere between the right shoulder and right elbow
- Yellow (LA): Anywhere below the right torso and above the right ankle
- Green (LL): Anywhere between the left shoulder and the left elbow
- Black (RL) (earth electrode): LL (Left Leg) - Anywhere below the left torso and above the left ankle
additional notes
- The limb leads can also be placed on the upper arms and thighs. However, there should be uniformity in your placement. For instance, do not attach an electrode on the right wrist and one on the left upper arm.
- For female patients, place leads V3-V6 under the left breast.
- Do not use nipples as reference points in placing electrodes for both men and women as nipple locations vary from one person to another.
looking at lead 2, what is the P wave expressing?
- Depolarisation of the atria; electrical impulses traveling from the SA node to the AV node
looking at lead 2, what is the short flatline after the P wave expressing? (aka P-R interval)
- The AV node is a (relatively) slow conductor (delays impulses by approximately 0.09s), this is important to ensure that the atria have ejected their blood into the ventricles first before the ventricles contract)
- This delay/ moment of slow conduction translates to a lack of direction of AP, therefore the positive ECG lead II can’t pick the AP up and therefore it shows a breif flat line
looking at lead 2, what is the negative Q wave depression expressing? And why is it negative?
- IV septum depolarisation
- It’s negative because..
- Before the IV septum depolarises, the impulse travels down from the AV node
- when the impulse travels down, it doesn’t equally perferate the left and right bundle branches: it innervates the LBB first, and then the LBB shoots the AP over to the RBB
- the net vector direction of this AP impulse from LBB to RBB is away from the positive lead II electrode and therefore it presents as a negative deflection on the ECG
looking at lead 2, what is the R wave expressing?
- ventricular depolarisation
- ie the myocytes of the apex of ventricular myocardium all depolarising and contracting
looking at lead 2, what is the S wave expressing? And why is it negative?
- the depolarisation at the bases of the ventricles
- it’s negative because, unlike the R wave ventricular depolarisation, the direction of depolarisation is away from the lead II and therefore it presents as a negative deflection
looking at lead 2, what does the S-T segment represent? Why does the S wave return to ‘neutral’ and why is it isoelectric?
- The entire ventricular myocardium is depolarised and is not repolarising yet
- Therefore there is no net movement of electricity
- Therefore there is nothing for the ECG electrodes to pick up
looking at lead 2, what is the T wave represent? Why is it a positive deflection?
- Ventricular myocardium repolarisation
- remember: depolarisation is a positive charge, repolarisation is a negative charge, so..
- Even though the direction of ‘charge movement’ is away from the positive electrode II, the ‘charge’ is of repolarisation (negative charge), not depolarisation (positive charge), therefore the negative repolarisation charge is moving away from the positive elctrode and this has an oppisite effect than if a positive depolarising charge was moving away from the electrode (negative deflection); therefore it’s a positive deflection
- TL;DR: negative repolarisation charge moving away from electrode has same affect as positive depolarisation charge moving towards electrode; ie positive deflection
a negative and positive electrode = a lead (ie. a view of the heart)
is the positive or the negative electrode the ‘eye’ or the ‘exploring electrode’?
ie. which side, pos or neg, is the perspective from?
- pos
- ‘put your eye where the pos electrode is and look towards the neg’
name all of the leads that cover the axial plane of view
the 6 chest leads
name all of the lead that cover the frontal plane
- 3 bipolar limb leads (I, II, III)
- 3 augmented uni-polar limb leads (AVF, AVR, AVL)
draw out the lead directions of all the 6 limb leads
see picture
outline which parts of a normal ECG reflect which parts of the cardiac electrical system
