Week 3: Basic Electrocardiograms Flashcards
Describe the conduction sequence of the heart
- The sinoatrial node has the fastest automaticity (meaning it has the greatest amount of slow sodium pacemaker channels) meaning it has the steepest pacemaker potential meaning it depolarises first in a healthy heart. Thus, it is known as the pacemaker
- The internodal pathway takes the electrical activity from the sinoatrial node to the atrioventricular node. Meanwhile interatrial pathways allow the spread to go from the right atrium to the left atrium so that both atria pump at once for efficiency
- At the same that the atria are contracting, the AV node (which has had its delay) is depolarised, the depolarisation travels though the bundle of his and in the bundle branches it moves from left to right (this gives the negative deflection in an ECG, Q wave). It is simultaneously still travelling down the left side (this gives the start of the R wave) depolarising the purkinje fibres depolarise. It then moves from apex and the free wall. - goes from endocardial surface to epicardial surface (gives the down stroke of the R)
- It then travels back up from the apex to the base of the heart (moving away from lead 2) (epi to endcardium)
- The T wave is the ventricles repolarising
Describe what each wave on an ECG represents
- The atria is the first to depolarise. This is known as the P wave
- There is then a pause, or isoelectric line which allows the atria to contract
- The depolarisation of the base through to the apex is known as the Q wave. This spreads from left to right down towards the apex
- The apex of the ventricle all the way up towards the base is the next to depolarise, this is known as the R wave
- There is then the depolarisation of the final portion of the ventricular free wall, this is known as the S wave
- There is then full ventricular depolarisation (meaning an isoelectric line) when the ventricles contract (no currents are flowing)
- There is then repolarisation from apex to base (and epicardium to endocardium) which is known as the T wave
- The heart is then at the 4th phase, at an isoelectric line
How do you calculate the heart rate in an ECG
- Count the number of small squares between each QRS complex
- If each small square is equivalent to 0.04 seconds, we then multiply the number of squares by 0.04
- This will give us the time between each QRS complex
- We then take this time, multiply 60 by it, which would give us the number of beats per minute
What does the X and Y axis of an ECG represent?
y = mm/sec and x = mm/mV
What is the isoelectric line and why is it important?
The baseline present in an ECG, this is present when the heart is fully depolarised or fully repolarised - there are no currents
What does the Q and S waves represent and why are they in opposite directions?
Q - bundle branches depolarising, when the depolarisation moves from left to right
S - depolarisation of the final ventricle wall - endo to epicardial surface. Apex to free wall
What is happening in the heart during the P-R segment and what would it tell your if the P-R segment was longer or shorter?
Time when the atria are fully depolarised and are in contraction.
If PR was linger then there is a delay in normal conduction through AV node. - negative domotropic
Where is the repolarisation wave of the atria on the
ECG trace from a healthy person?
hidden by the R wave of the QRS complex
what is the difference between an electrode and a lead and how many are there of each?
electrode = patch put opn the patients skin
lead = ‘view’ of the heart - seen from positive electrode
10 electrodes and 12 leads
Explain the 12 leads in an ECG - include the positions of the positive and negative electrodes
lead 1 - let arm (+) to right arm (-) looks at hear form the left
Lead 2 - right arm (-) to left leg(+) - looks at from apex to base
Lead 3 - left leg (+) to left arm (-) - looks from towards apex of heart up towards base on right side
aVR: upper right from base to apex
aVL: Looks at upper left side of heart
aVF: looks at inferior wall of heart
The aV chest leads use one of the leads a positive and average the other two as a zero reference point.
The additional 6 chest leads allow to view the heart in a horizontal plane
Why should you double check that the calibration is correct for each ECG you record from your patients
values changed can lead to misdiagnosis - might mimic dysrhytmias
What would an ECG look like for leads 1 and aVF if you had left axis deviation
(HINT: its deviating towards the left side of heart, use the cardiac axis)
upward deflection of lead 1 and downward deflection of aVF
What would an ECG look like for leads 1 and aVF if you had extreme axis deviation?
(HINT: its deviating towards the left side of heart, use the cardiac axis)
both downward deflection
What would an ECG look like for leads 1 and aVF if you had no deviation
(HINT: its deviating towards the left side of heart, use the cardiac axis)
both upward deflection
