ECG generation Flashcards
what is the resting membrane potential in cardiac cells
-90 mV
how is a normal ECG produced
- depolarisation an repolarisation of cells produce an electrical current in oposite directions
- dipoles exist within the myocardium (currents flow between poles as consequence of ion movement
- ## a minimum of 2 leads (usually 3) on the blimbs of patient measures the electric charge and compares them to give an ECG
what is magnitude of deflection and what is it determined by
height or depth of the waves
determined by:
- direction of depolarisation
- amount of muscle mass that is depolarising (i.e. atria have less deflection than ventricles b/c have less muscle mass)
- which lead you are using or where the electrodes are connected
- other factors such as insulation (fluid or fat surrounding the heart)
where do you want to place ECG leads on dogs/cats
- left forelimb (below elbow)
- right forelimb(“ “)
- left hindlimb (above hock)
rarely placed on the chest unless trying to pick up more subtle details (diagnostic for DCM)
where do you place ECG leads on horses/cows
typically just use a base-apex lead arrangement
why dont you see atrial repolarisation in an ECG
magnitude of deflection is determined by muscle mass
- since atrial mass is less than ventricular mass, dont see because ECG picks up ventricular, but would be somewhere between QRS
what does the P wave indicate
depolarisation of the atria.
the direction of the resulatant vector is normally such that the P wave is positive
- direction of resultant vector is normally such that the P wave is positive and points to the left side of the animal during depolarisation
how do you calculate HR from an ECG
the number of small boxes on the ECG paper between two R waves can be counted and divided into 3,000 (for a paper speed of 50 mm/s) or 1500 (25 mm/s).
(i.e 1500/# of boxes between 2 R waves)
What does the QRS complex indicate
depolarisation of the ventricles
what does the ECG tell us
the electrical activity that intiates contractions NOT contractions themselves
why is it possible to produce an ECG from surface electrode monitoring (why can we pick up electircal impulses produced in the heart in the skin of the limbs)
how are electircal impulses produced in the heart picked up in skin
because a large number of cells are electrically active simultaneously, the extracellular currents become so strong that it is possible to record voltage differences between electrodes on the surface of the body (electrical currents spread into tissues surrounding the heart)
What does the QRS complex indicate
depolarisation of the ventricles
depolarisation wave is propogated from the atria, through the AV node and onto the vntricles
- depolarisation of the AV node occurs between the P wave and the beginning of the QRS complex
- the Q wave: the septum is the first part of the ventricles to depolarise resulting in the vector being small and the deflection being negative
- the R wave: from the septum, the depolarisation wave spreads through the prukinje fibres to the inside of the myocardium. during most of the the depolarisation, the resultant vector points downwards from right to left and the deflection is therefore positive
- The S wave: the base of the left ventricle is the last part of the ventricles to depolarise. the direction of the resultant vecotr results in a negative deflection
What does the T wave indicate
ventricular repolarisation
biphasic - could be positive or negative, both normal. (would be abnormal in humans, be careful about googling)
what can different sizes of ECG complexes tell you about heart size
- P-wave may be taller with right atrial enlargement
- Pwave may be wider with left atrial enlargement
- R wave may be taller with left ventricular enlargment
- S wave may be deeper with right ventricular enlargement
what does paroxysmal tachycardi mean
heart rate increases abruptly
