Cardiology Flashcards
on an ecg one large square is how much in time
0.2seconds or 200ms
on an ecg one small square is how long in time
0.04 seconds or 40ms
how many large squares per second in an ecg
5 large squares per second
what is the PR interval and how long should it be
time from the onset of the P wave to the start of the QRS complex
it should normally be 120-200ms
this is 3-5 small squares
draw the ecg axis diagram
normal axis is from what degrees to what degrees
-30 degrees to +90 degrees
left axis deviation will look like what
QRS predominantly negative in VF II and III
QRS predominantly positive in VL and I
right axis deviation will look like what
QRS predominantly negative in I
QRS predominantly positive in III
which leads look at the septum
V3 and V4
what is the transition point
- the v1 lead looks at the heart from the right
- it begins with a small septal r wave as the septum is depolarised left to right initially
- then as the left ventricle depolarises there follows a large downwards S
- the v1 complex is therefore predominantly negative
- the v6 lead looks at the heart from the left
- it begins with a small downwards Q as the septum is initially depolarised left to right
- then there is a large upwards R wave as the left ventricle is depolarised
- the v6 complex is therefore predominantly positive
- the transition point is the point at which the R waves and the S waves are of equal size
- it is usually around v3-v4
what is the significance of the transition point
- it can shift with changing shape of the heart
- with right ventricular hypertrophy the transition point would shift to the left
- this would mean that it would be around v5-v6 rather than v3-v4
- it is as if, seen from the perspective of the feet, the heart has rotated clockwise
what is first degree heart block
it is when the pr interval is longer than 200ms
it’s ‘marked first degree heart block’ if >300ms
what is second degree heart block
- mobitz i (wenckeback)
- progressive lengthening of pr
- followed by a dropped QRS complex
- pr then goes back to being shorter and the cycle continues
- mobitz ii
- PR interval is constant but there is an occasional dropped QRS
- there may be two or three or four P waves for every successful QRS in which case it is given a ratio 2:2 or 3:1 etc
- PR interval is constant but there is an occasional dropped QRS
what is third degree heart block
atrial contraction may be normal but no beats are conducted to the ventricles themselves
ventricular contraction is maintained by ventricular escape rhythms
what does blockage of both bundle branches look like
it has the same effect as a complete block of the his bundle and resembles complete heart block
why does right bundle branch block look like that
- best seen in v1 where there is an RSR pattern
- upwards deflection as septum initially depolarised from left to right
- then s wave as the left ventricle is depolarised as normal
- it takes longer for the right ventricle to be depolarised so there then follows a second R wave as the right ventricle is depolarised after the left
- in v6 there is a wide deep qrs
- initially a q as the septum is depolarised left to right
- then an R wave as the left ventricle is depolarised
- then follows a deep wide s wave as the right ventricle catches up
- right bundle branch block is only significant if the qrs is longer than 120ms
why does left bundle branch block look like that
- if conduction down the left bundle branch fails then the septum is depolarised from right to left
- in V1 this appears as a small q wave
- then the right ventricle is depolarised before the left so there is then an r wave in v1
- subsequent depolarisation of the left ventricle then causes an s wave in v1
- in V6 the initial right to left depolarisation of the septum causes a small r wave
- then follows a small (like a notch) s wave as the right ventricle is depolarised
- then v6 shows another R wave as the left ventricle catches up
- in V1 this appears as a small q wave
in summary how does right bundle branch block appear
best seen in v1
there is an RSR pattern
in summary how does LBBB appear
best seen in v6 where there is a broad qrs with a notched top
the complete picture with a w shape in v1 is often not fully visible
what happens in atrial tachycardia
depolarisation is from an origin in the atria that is different to the sa node
the av node cannot conduct rates fasterr than 200/min so there may be a physiological block
what happens in atrial flutter
if there atrial rate is >250/minute then there is no baseline between p waves
what happens in junctional/nodal tachycardia
the depolarisation originates from the area around the av node
therefore the atria and the ventricles are depolarised at the same time
therefore p wave will be very close to the QRS complex or may be lost within it
what happens in wolff parkinson white syndrome
- some people have an accessory bundle connecting the atria and the ventricles
- there is no AV node delaying contraction
- this provides pre-excitation to the ventricles
- therefore there is a short PR interval and the QRS has a wide slurred upstroke called a delta wave
- the second part of the QRS is normal as the excitation from the his bundle catches up
- the his bundle and the accessory bundle can together form a re-entry circuit that causes tachycardias
what are the 6 questions to ask yourself when identifying abnormal ecg rhythms
- is the abnormality occasional or sustained
- are there p waves
- what is the P:QRS ratio
- are the ventricles contracting regularly or irregularly
- is the QRS a normal shape
- what is the ventricular rate