Lecture 8 - The Electrocardiogram Flashcards
describe the spread of excitation over the normal heart
- ap generated by pacemaker cells in san
- electrical activity spreads over the surface to the avn where there is a 120ms delay
- excitation spreads down septum via right and left bundle branches
- spreads over ventricular myocardium from endothelial to epicardial surface until all ventricular cells are depolarised
- after 280ms cells begin to repolarise
- repolarisation spreads from epicardial to endocardial surface of ventricles
describe the signal recorded by an extracellular electrode near a myocardial cell
only record changes in membrane potential
two signals per systole
what are the rules regarding signal recording by a positive recording electrode?
depolarisation towards electrode = upward
depolarisation away from electrode = downward
repolarisation towards electrode = downward
repolarisation away from electrode = upward
the more muscle depolarising and the more directly towards the electrode, the bigger the amplitude
describe the form of a signal recorded by a single electrode towards the apex of the heart
- atrial depolarisation produces small upward deflection - small because little muscle, upward because toward
- 120ms delay
- excitation spreads halfway down septum then out across axis of heart - small downward deflection - downward because away, small because not directly away
- depolarisation spreads through ventricular muscle along axis left to septum - large upward deflection - upwards because towards, large because direct
- depolarisation spreads up to base of ventricle - small downward deflection - downwards because away, small because not direct
- ventricular contraction 280ms
- repolarisation spreads through ventricular myocardium (out to in) - medium upward deflection - upwards because away, medium because timing in cells is dispersed
what does the p wave represent?
atrial depolarisation
what does the q wave represent?
septal depolarisation spreading to ventricle
what does the r wave represent?
main ventricular depolarisation
what does the s wave represent?
end ventricular depolarisation
what does the t wave represent?
ventricular repolarisation
where should limb leads be placed?
one on each limb red (ride) - right upper yellow (your) - left upper green - left lower blue (bike) - right lower
where should chest leads be placed?
v1 - red - 4th intercostal space right of sternum
v2 - yellow - 4th intercostal space left of sternum
v3 - directly between v2 and v4
v4 - 5th intercostal space at midclavicular line
v5 - level with v4 at left anterior axillary line
v6 - level with v5 at left midaxillary line (under mid point of armpit)
what view of the heart to limb leads give?
vertical
what view of the heart do chest leads give?
horizontal
how is heart rate calculated from a rhythm strip for a regular heart beat?
300/(number of little squares in R-R interval)
how is heart rate calculated from a rhythm strip for a regular heart beat?
use a larger interval than R-R
multiply number of beats in 10 seconds by 6
