ECG: The Electro Cardio Gram Flashcards
Describe the conducting system of the heart?
Specialised cells are able to generate action potentials and conduct the impulse very rapidly to all subendocardial regions of the ventricles, resulting in depolarisation of myocytes (ventricles depolarise from endocardium to epicardium), enabling coordinated contraction of the atria and ventricles.
Why does the Sino Atrial Node (SAN) set the rhythm of the heart?
It has the fastest rate of depolarisation.
Where does an impulse pass after being derived from the SAN and why is this necessary?
The AtrioVentricular Node (AVN) is continuous with the Bundle of His, which is the only conducting pathway between the atria and ventricles, as impulses can’t travel through the fibrous ring, which the muscles are attached to.
Where are the right and left bundle branches and what are the Purkinje fibres?
The right and left bundle branches lie subendocardially in the interventricular septum.
The purkinje fibres are fine branches of the BoH, where there’s rapid spread of depolarisation throughout the ventricular myocardium (4m/s).
In what order to the components of the ventricles depolarise?
Myocytes of the interventricular septum are depolarised first, then the apex and r and left free walls, with the base of the ventricles the last to depolarise - repolarisation happens in reverse order.
What does an ECG do?
It records changes on the extracellular surface of cardiac myocytes during the wave of depolarisation and repolarisation, from the surface of the body, using electrodes pasted on the skin.
What changes in the cardiac myocytes produce a dip on an ECG?
A wave of depolarisation travelling away from the electrodes view or a wave of repolarisation towards it.
What changes in the cardiac myocytes produce a peak on an ECG?
A wave of depolarisation towards and electrode or a wave of repolarisation away from an electrode produces a peak?
Why might there be a straight line on an ECG?
If the myocytes are isoelectric, even if they are all depolarised.
What difference does it make now directly a wave of depolarisation is travelling towards an electrode?
If a wave is travelling directly to (instead of obliquely towards) a view, there will be a taller complex. A wave at 90 degrees to an electrode will show a biphasic (little up then little down) or no complex.
What is the first electrical event in the heart and how does this show on an ECG?
The SAN depolarisation, nut there’s insufficient signal to register on a surface ECG.
Describe the depolarisation of the atria and its effect on the ECG.
Depolarisation spreads along the atrial muscle fibres and intermodal pathways (throughout L and R atria), downwards and to the left (towards the AVN). This produces a small upward deflection of the p wave, as it’s travelling towards the recording positive electrode.
What follows the p wave on an ECG and why?
There is delay at the AVN with conduction slowed down, so there’s time for the atrial contraction to fill the ventricles - the signal is very small, meaning an isoelectric flat line (fibrous ring between atria and ventricles means no direct contact between their myocytes).
What is the point of the fibrous ring between the atria and ventricles?
The fibrous ring is only crossed by the Bundle of His, so depolarisation can only travel down it and is thereafter rapidly conducted by the His-Purkinje system (also contributing to the isoelectric segment from AVN depolarisation).
What, on an ECG, is a q wave and how is it produced?
Depolarisation of the myocardium, first to do so being the muscle in the interventricular septum, which does so from left to right, producing a small downward deflection, the q wave, as the wave is moving obliquely away.
What is the result on an ECG of depolarisation of the apex and free ventricular wall and why?
A large upward deflection, known as the R wave, because depolarisation is travelling directly towards the electrode and a large muscle mass means more electrical activity (correspondingly taller as the LV hypertrophies).
What is the S wave on an ECG and what is it caused by?
A small downward deflection to finish the QRS complex, caused by the depolarisation finally spreading up to the base of the ventricles. It is moving away, but not directly.
Ventricular repolarisation begins on the epicardial surface and spreads in the opposite direction to depolarisation, producing a medium upward deflection on an ECG, because the repolarisation is moving away from the electrode. What is this called?
The T wave.
All components of an ECG reading may not be seen in all leads, but generally, what shows atrial and what shows ventricular depolarisation?
The P wave is atrial depolarisation and the QRS complex is ventricular depolarisation.
Recording an ECG requires ____ electrodes: ___ on the limbs and ____ on the chest, connected by cables to the machine. They give ___ views to heart aka ________.
10 4 6 12 'Leads'
Leads looking at the inferior surface of the heart are best for looking at problems there e.g. Muscle necrosis from an inferior MI. Which leads look at the inferior heart?
Leads II, III and aVF look at the inferior surface of the heart.
Where do leads I and aVL look and so which problems are they most equipped to detect.
Leads I and aVL look at the left side of the heart and are the best limb leads for looking at problems in the lateral left ventricle.
From which planes do the different leads view the heart?
The 6 limb leads (from 4 limb electrodes), give views in the vertical plane, whereas the chest leads (from 6 electrodes), give 6 views of the heart in the horizontal plane.
Which are the septal leads, facing the right ventricle and septum?
Leads V1 and V2 are the septal leads.
