Lecture 5 Flashcards
What is an ECG?
It is an electrocardiogram that records the electrical activity from he heart vii the body surface. The electrodes are on the body surface.
Describe the P wave?
Atrial depolarisation. Relatively small compared to the QRS. Atrial have smaller muscle mass compared to the ventricles, thus a smaller amplitude. It has the same duration as the QRS complex.
Describe the P-R segment?
The AV node delays activation of the ventricle - isoelectric (at the level of the baseline). There is no flow of current/no change in voltage. This reflects the time taken to activate the ventricles.
Describe the QRS complex?
Ventricular depolarisation. The QRS is activated by the fast conduction system, thus a fast duration. The ventricles have greater muscles than the atria, thus a greater amplitude. The atrial cells also depolarise in this complex, but you don’t see this.
Describe the S-T segment?
The myocytes and the ventricles are sitting at their plateau. This phase is isoelectric - there is no current.
Describe the T wave?
Ventricular repolarisation. Complicated event.
Describe wavefront dipoles?
When the tissue is at rest the inside of the cells is negative and the outside is positive. When the wavefront travels along, the cells become depolarised and the inside becomes positive and the outside becomes negative. Because on the outside there is both positive and negative charge, there will be a travel of current (as it wants to equalise) int he extracellular space - ECG records the extracellular space. So in an ECG we are recording the difference in the current and the direction. We can represent this as a dipole and represent it as a vector. The direction of the dipole is the movement of the wavefront, the size of the vector is the strength of the dipole.
Describe dipole projection?
If the dipole is going away from the electrodes, you will only see the projection. If the dipole is perpendicular to the electrode you will not see anything as the projection is zero.
Describe a dipole in a volume conductor?
The electrodes record the difference in voltage. the measured potential depends on the following:
1. Magnitude of change (dipole).
2. Orientation of dipole and electrodes.
3. Distance between dipole and electrodes.
The dipole (the heart) is generating an electric field on the body surface - record it via electrodes.
Describe the QRS complex in terms of ECG interpretation?
What happens is you have activation starting at the left base of the septum (where normal activations tarts). The projection on the lead line is pointing to the negative electrode, thus you will get a negative deflection (Q). You then get activation of the bundle branches and the sum of the dipoles you will get a positive deflection (R). You then get activation (depolarisation) of the ventricles and you have a negative deflection (S).
Describe T wave in terms of ECG interpretation?
The electrical process of repolarisation is opposite to depolarisation. The AP duration of the myocytes on the endocardium is longer than the AP of the epicardium. Activation starts at the inside surface and moves outwards, but repolarisation starts at the outside surface and moves inwards (this happens because the AP is a lot longer). This is why the T wave has a positive deflection.
How many leads are used for the ECG?
Use 12 different connections of electrodes - 12 lead ECG.
Describe bipolar leads?
Measure the difference between two electrodes. Compare one electrode directly with another. Use Lead I, II, and III. Use RA (right arm), LA (left arm) and LL (left leg). Potentials are recorded between the combinations of RA, LA and LL. This is the equivalent of connecting electrodes to the corners of an equilateral triangle - known as Einthoven’s triangle. Lead I - comparing LA with RA. Lead II - comparing LL with RA. Lead III - comparing LL with LA.
Describe unipolar leads?
Comparing a single electrode to a constant reference. Unipolar limb leads - VR, VF, VL. The zero is the point in the middle of the triangle. There are now 6 directions in which you can look at the heart dipole from. There are unipolar chest leads (V1-V6) which look at the heart in a horizontal plane.
Describe the 12-lead ECG?
The leads are all looking at the heart (the exact same thing) yet each are looking at different point of views.
