ECG and Arrhythmias Flashcards
when might minor R and L axis deviation be considered normal?
R in tall and thin people
L in short and fat people
normal cardiac axis?
from -30 to + 90 degrees
causes of R axis deviation?
RV hypertrophy, PE
causes of L axis deviation?
conduction defect
normal time of PR interval?
120-200 ms (3-5 small squares)
causes of prolonged PR interval?
1st degree heart block-CAD, digoxin toxicity, acute rheumatic carditis, electrolyte disturbances e.g. hyperkalaemia.
define 2nd degree heart block
intermittent failed spread of depolarisation through the AVN or bundle of His.
name given to type of 2nd degree heart block in which the PR interval progressively lengthens before failure of conduction of an atrial beat, and then a shorter PR interval than the preceding conducted beat?
Mobitz type 1 (Wenckeback phenomenon)
name given to type of 2nd degree heart block in which PR interval relatively constant but occasionally atrial depolarisation without subsequent ventricular depolarisation?
Mobitz type 2
name given to type of 2nd degree heart block in which there are alternate conducted and nonconducted atrial beats, producing 2 P waves for every QRS complex?
2:1 conduction
appearance of 3rd degree heart block on an ECG?
P wave rate regular as atrial contraction normal, but no consistency in PR interval
no relationship between P waves and QRS complexes
QRS complexes abnormally shaped due to abnormal spread of depolarisation from a ventricular focus.
causes of complete heart block (3rd degree)?
MI, usually transient IHD chronic e.g. fibrosis around bundle of His, or block of both bundle branches. aortic valve calcification congenital cardiac surgery/trauma digoxin toxicity infiltration- abscesses, granulomas, tumours, parasites
how can bundle branch block with a sinus rhythm be distinguished from rhythms beginning in the ventricles?
BBB with a sinus rhythm- PR interval constant and normal P waves but this doesn’t happen if rhythm begins in the ventricles.
however, both would show widened QRS complexes= longer time taken for depolarisation to spread through the ventricles.
is RBBB always pathological?
no, often indicates R heart problems but can be found in healthy people with a normal QRS complex duration.
LBBB always an indication of heart disease, usually of LV.
why does a second R wave (R1) follow the S wave in V1 in RBBB?
the RV depolarises after the LV, and so the depolarisation towards V1 now causes an upward deflection following the downward deflection that occurred due to LV depolarisation away from the lead.
this results in a wide and deep S wave following the R wave in V6, and so a wide QRS complex.
what is partial RBBB?
RSR1 pattern in V1 with a QRS complex of normal width
this is a normal variant
explain the appearance of LBBB in V6?
septum cannot be depolarised normally from L to R, so instead depolarises from R to L, producing an upward deflection in V6 as depolarisation moves towards the lead (R wave).
RV then depolarises before L and is of a small muscle mass, so produces a small S wave in V6.
LV then depolarises producing a 2nd R wave in V6.
T wave changes LBBB is assoc. with?
T wave inversion in lateral leads (I, VL and V5-V6), though not necessarily in all of these.
why does the axis usually remain normal with RBBB?
normal depolarisation of LV with its large muscle mass.
when is 2nd degree HB often seen?
acute MI
tment consideration in 3rd degree HB?
always indicates conducting tissue disease
consider temporary or permanent pacemaker
what can cause RBBB?
atrial septal defect
what can cause LBBB?
aortic stenosis
ischaemic disease e.g. acute MI
define sinus rhythm
depolarisation of the heart has begun in the SA node