ECG introduction Flashcards
Define ECG
It is the summation of all of the action potentials of the heart.
Label the regions of the cardiac conduction pathway
Does the SA depolarisation show up on an ECG?
No
What is happening in the “P” region of the ECG?
- Atrial Depolarisation
- The first “deflecton” on the ECG
- Typically small, low amplitude
What is happening at the highlighted pont of the ECG?
Delay of signal at the AV node
GIves times for Atria to contract, and for ventricles to fill
What is happening at “Q”?
- conduction through the bundle branches of the ventricles
What is happening at QRS?
Ventricular depolarisation
Should be nice and narrow
What is represented by the box on this ecg?
Plateau phase of repolarisation - this is the ST Segment
What is happening at this stage of the ECG?
Rapid repolarisation of ventricles
Unipolar and bipolar leads are both used in ECG. List which leads these are, and what the difference is
Unipolar:
- Measuring electrical activity at a single point
Bipolar:
- Measuring electrical activity between two points
Unipolar leads:
- aVR, aVL, aVF, V1-6
Bipolar lewads:
- Limbs 1, 2 and 3
What is an ECG lead?
- Electrical vector - the angle and direction of the electrical impulse of the heart - basically an imaginary line between 2 electrodes
- Not the wire attached to the patient
How many electrodes are attached to a patient for ECG? List where they are attached
List the 12 ECG leads and what they all measure
Chest leads
V1 – Septal view of heart
V2 – Septal view of heart
V3 – Anterior view of heart
V4 – Anterior view of heart
V5 – Lateral view of heart
V6 – Lateral view of heart
Other leads
Lead I – Lateral view (RA-LA)
Lead II – Inferior view (RA-LL)
Lead III – Inferior view (LA-LL)
aVR – Lateral view (LA+LL – RA)
aVL – Lateral view (RA+LL – LA)
aVF – Inferior view (RA+LA – LL )
Limb leads are providing the coronal view of the heart
Chest leads are providing the transverse view of the heart
Study electrode placement
Note how limb electrodes can be put ALL distally or ALL proximally. Neither is better than the other
Thinking about bipolarlimb leads, where positions are measuring between which anatomical points? Note how the positive electrode relies on the negative electrode as a reference point
The unipolar limb leads essentially measure an average of what the bipolar limb leads are measuring.
Different leads are providing different views of the heart
V1 - V4 = Anterior
Inferior = 2,3 and VF
Lateral = 1, AVL, V5, V6
In MI, abnormality can be related to the coronary artery involved. For example, left anterior descending blockage is likely to show up as abnormality in the Anterior ECG.
A singal moving towards a lead will give a positive signal, anyhing moving away will give negative signal.
What is meant by the QRS?
This is very simply a way of generalising the direction of the depolarisation of the heart. Best way is to look at QRS wave and see if there is any deviation.
Systemic approach for an ECG
- Ask for clinical context of the patient
- Check, data, time and Patient detaisl
- Assess technical quality (paper speed - 25mm/s, gain - 10mV/mm) , are there any weird artifacts?
- Identify P/QRS/T wave - i.e. can you find them, are they in the right order? - use the rhythm strip which is typically the three readings at the bottom of an ECG
- Measure the heart rate, although generally measured by machine)
- Check the ECG intervals (usually machine is accurate)
- Determine the QRS axis by looking at the limb leads
- Look at P/QRS/T morphology - basically look at the 12 leads individualls
Do not rely on automatic interpretation
Looking at Old ECGs is very helpful
Determining heart rate - there are two main ways. What are they?
- count the number of large squares between each QRS complex - divide 300 by that number
- Alternatively, count the number of QRS complexes across ECG and multiply by 6
ECG intervals are an important factor when diagnosing an ECG. Give typical values in ms for PR interval, QRS and QT interval and describe what these ECG phases are
PR interval (delay at AV node) < 1 large square, <200ms
QRS (ventricular < 3 small squares, <120ms. Duration of QRS complex is very important, tells you whether the signal used the ventricular conduction systen. Anything higher suggests it did not. Broad QRS is really bad.
QT interval (ventricular depolarisation AND repolarisation) < 11 small squares, <440ms
mm of Big and small squares on ECG?
Large = 200ms
Small = 40ms
In degrees, what is a typical QRS axis? How do we check this is OK?
- 30 to + 90 degrees
- Check limb leads 1 and 2 first - if the QRS is predominantly positive, then you’re porbably fine
