ECG Flashcards
what happens in an ECG
track the electrical impulse as it travels across the heart using electrodes on the chest surface
how do the impulses appear on the graph
an impulse traveling towards the electrode results in a positive deflection
an impulse travelling away results in a negative deflection
electrode placement
standard ecg has 12 leads
6 on the chest called V1-6
4 limb electrodes - right arm (red), left arm (yellow), left leg (green) and right leg (black)
chest electrode placements
v1 - 4th intercostal space, right sternal angle
v2 - 4th intercostal space, left sternal angle
v3 halfway between v2 and v4
v4 over the apex ( 5th ICS, mid clavicular line)
v5 - same level as v4 but on the anterior axillary line
v6 - at the same level as v4+5 but on the mid axillary line
incorrect placement can affect the ECG readings
placing them at these specific locations provides useful information across the heart
these leads look at the transverse plane
placement of limb leads
look at the coronal plane
bipolar leads - 1,2,3
unipolar leads - aVL, aVR, aVF
what does limb leads 1-3 look at
lead 1 - signals travelling from right arm to left are
lead 2 - signals from right arm to left leg
lead 3 - left arm to right leg
what do leads aVL, aVF and aVR look at
aVL - heart to left shoulder
aVF - heart to feet
aVR - heart to right shoulder
what are the most important leads to look at in a 12 lead ECG
limb lead 2 is normally long so you can determine the rhythm
using leads 1,2 and 3 to calculate deviation
if leads 1 and 2 are positive then axis is normal
if leads 2 and 3 are negative then patient has left axis deviation
if lead 1 is positive and lead three is positive then patient has right axis deviation
first thing to do when reading an ECG
check for calibration
check for reference pulse (rectangular wave on left hand of paper) normal calibration the wave should be 10mm tall, this is equal to 1mV
paper speed should be 25mm/s
describe the 3 waves produced during cardiac cycle
p wave - caused by atrial depolarisation
QRS complex - caused by ventricular depolarisation
t wave - ventricular repolarisation
describe the QRS complex
Q wave - first downward deflection
r wave - first upward deflection
s wave - downward deflection after first upward deflection
describe the intervals
pr interval - time impulse takes to conduct through bundle of His (0.12-0.2s)
qrs duration - time taken for ventricular depolarisation (<0.12s)
st segment - start of ventricular repolarisation (<0.44s m, <0.46s f)
what do the intervals tell us
st elevation - MI, pericarditis, repolarisation abnormalities
st depression - ischaemia, left ventricular strain
prolonged pr interval - conductive tissue disorder
shortened pr interval - impulse is bypassing the AV node (wolff-parkinson-white syndrome)
qrs duration - can show right bundle block or left bundle block
what is left ventricular hypertrophy
too much muscle in the left ventricles
represented by abnormally tall r waves in v5 or v6 and deep s wave in v1
what is right bundle branch block in v1
no change in initial impulse (small r wave)
impulse depolarises LV by itself (s wave)
RV depolarised late by impulse through muscle (r’ wave)
rsr’ patter (m shape on ecg trace)
left bundle branch block in v1
initial deflection altered since it travels left to right now (q wave has negative deflection)
RV depolarises unopposed (may produce small r wave)
travels across septum to depolarise LV (deep s wave)
appears as a w on trace
if patient has lbbb then st segments are uninterpretable
how to calculate heart rate from an ECG
if cardiac rhythm is regular count the number of squares between r waves
rate = 300/number of large squares between r waves
if rhythm is irregular - use rhythm strip at the bottom of 12-lead ecg
this is a 10sec recording of the heart, therefore rate = number of QRS complexes x 6
what is bradyarrhythmia
any abnormality of cardiac rhythm resulting in a slow heart rate (<60bpm)
could be caused by heart block or slow AF
what is tachyarrhythmia
any abnormality of cardiac rhythm resulting in a fast heart rate (>100bpm)
could be due to SVT, uncontrolled AF/flutter or VT
what is 1st degree av block
regular rhythm
pr interval of more than 0.2s and is constant
usually does not require treatment
causes - ischaemic heart disease, conduction system disease, can be seen in healthy kids or athletes
what is 2nd degree av block (mobitz 1)
irregular rhythm
pr interval continues to lengthen until a qrs complex is missing (non-conducted sinus beat)
pr interval is not constant
rhythm is usually benign unless associated with underlying pathology (MI)
what is 2nd degree av block (mobitz 2)
irregular rhythm
qrs complexes may be wide (>.12s)
non-conducted sinus impulses appear at irregular intervals
rhythm is somewhat dangerous as the block is lower in the conduction system
may cause syncope or may deteriorate into complete heart block
its appearance in the setting of an acute MI identifies a high risk patient
treatment - pacemaker
causes - IHD, fibrosis of the conduction system
what is 3rd degree av block
complete heart block
atria and ventricles beat independent of each other (av dissociation)
qrs complexes have their own rhythm, p waves have their own rhythm
may be caused by inferior MI, and its presence worsens the prognosis
may cause syncopal symptoms or angina, especially if ventricular rate is low
treatment - pacemaker
what is narrow complex tachycardia
qrs duration of <.12s
uncontrolled atrial fibrillation of flutter
atrial tachycardia
atrioventricular nodal reentry tachycardia
atrioventricular reentrant
what is broad complex tachycardia
qrs duration >.12s
ventricular tachycardia
ventricular fibrillation
