Revision 8: The Electrocardiogram Flashcards
describe the spread of excitation over normal heart from SAN -> AVN -> ventricles
SAN->AVN (P) - depol. spreads over atria to AVN, which delays the depol. for about 120ms
Conduction over heart (QRS): activity spreads down eptum over vent. myocardium, from inside (endocardial surface) to outside (epicardial surface) until all cells are depolarised
Repolarisation (T): after ~280ms cells start to repol., spreads in opposite direction to depol. (outs->ins)
Rules w/ electrodes measuring the ECG
1 depol. heading towards -> upward signal
2 repol. heading towards -> downward signal
(and vice versa for both)
3 amplitude depends on:
a) how much muscle is depol.
b) how directly towards the electrode excitation is moving
Desc. an ECG viewed from below looking towards apex
P: atrial depol.: small upwards as little muscle but moving directly towards
P-Q interval: AVN delays conduction for ~120ms
Q: septal depol. spreading to ventricles: small downwards as moving indirectly away
R: main vent. depol.: large upwards as directly towards and involving lots of muscles
S: End of vent. depol.: small downwards as moving indirectly away
S-T interval: after about 280ms repol. begins
T: ventricular repolarisation: long med. upwards as moving away but not all cells are repol. at once
Atrial repol. is lost in the QRS complex
placement of Leads I-III
I: +ve top left, -ve top right -> from left
II: +ve bottom left, -ve top right -> from bottom left
III: +ve bottom left, -ve top left -> from below
placement of 12 lead ECG
Chest:
V1: 4th intercostal space to right of sternum
V2: 4th intercostal space to left of sternum
V3: directly between V2 and 4
V4: 5th intercostal space at midclavicular line
V5: directly between V4 and 6
V6: level with V5 at left midaxillary line (i.e. directly under armpit midpoint)
Limb: Ride Your Green Bike (clockwise looking posterior to anterior starting right upper limb)
state single electrode views of Leads I, II, III and aVF, aVR and aVL
how to calculate HR from ECG rhythm strip, also normal PR interval value
all rhythm strips run at 300 big squares/minute, so HR=300/(no. or squares in R-R interval)
for irregular beats, more than 1 R-R interval is needed eg No. of beats in 10 sec.s X 6
normal PR interval is about 120-200ms
identify problem
Ventricular ectopic beat: ventricular cells gain pacemaker activity and contract early
the ECG is wider and taller
can occur every other/3rd/4th etc beat, or in couplets/triplets etc
in this rhythm strip it occurs after every other beat
identify problem
atrial fibrillation: p wave is absent and replaced by irregular fibrillation waves
as no regular stimulus reaches the AVN, ectopic pacemakers are needed for ventricular contraction
identify problem
ventricular fibrillation: uncoordinated contraction of ventricles, it instead ‘quivers’ rather than contracting properly
explain different types of heart block
heart block is a communication problem between atria and vent.s
1st degree: P-R interval elongates beyond avg. 200 ms, there is conduction delay through AVN but signal reaches vent.s
2nd degree Type I: P-R is erratic, it continually elongates until eventually a QRS is skipped, ‘resetting’ the heart beat, some but not all atrial beats reach the vent.s
2nd degree Type II: normally a constant P-R, conduction sometimes fails to pass through AVN/bundle of His, not all atrial cont.s reach vent.s
Complete 3rd degree: atrial contractions are normal, NO electrical conduction through to vent.s, ectopic vent. pacemaker needed but beats are normally slower
Bundle branch: lengthens and changes the shape of the QRS complex, many variations depending on location of block
identify problem
1st degree heart block: P-R is elongated, conduction is dleayed through AVN but all signals reach vent.s
identify problem
2nd degree Type I heart block: P-R is erratic and continually elongates until eventually a QRS is skipped, which ‘resets’ the pattern, some but not all atrial beats reach ventricles
identify problem
2nd degree Type II heart block: P-R is constant but excitation sometimes fails to pass through AVN/bundle of His so not all atrial cont.s reach vent.s
identify problem
complete 3rd degree heart block: atrial contracts as normal but there is NO conduction between atria and vent.s -> ectopic ventricular pacemaker is needed but beats are normally slower
in the picture, the P wave is completely independent of the rest of the ECG, and there are no Q waves, as there is no conduction between atria and ventricles
identify problem
bundle branch heart block
lengthens and changes shape of QRS, many variations depending on location of block
Cardiac axis and sinus rhythm
the general direction (or vector) of the depolarisation of the heart muscle
normal rhythm of the heart, ie every P wave is followed by a QRS complex
