ECG Flashcards
Explain how the heart directs an electrical current
SAN sets the rhythm because it depolarises the fastest. SAN is located in the right atria and then the wave of excitation is passed to the AVN. The AVN imposes a delay before sending the current to the bundle of His. It then passes down to through the septum to the apex and then up the ventricle walls via the purkinje fibres - rapid spread of depolarisation
Why is the bundle of His essential
Valves contain fibrous rings made out of dense cartilage tissue. This means the current cant pass from the atria to the ventricles because they are electrically insulating and to do so require the bundle of His. -Allow conduction from atria to ventricles
Describe repolarisation of the heart
Repolarisation happens in the opposite direction to depolarisation -starts at the the last cell to be depolarised and goes backwards up the septum
Discuss the membrane potentials during a single heart beat
At rest = positive charge on outside
During depolarisation = negative charge on outside
During repolarisation = turns back to positive charge on outside
Discuss how an EXG detects the changing membrane potential
During depolarisation in the direction (towards) the positive electrode of the ECG shows an upward deflection. But once fully depolarised shows flat line because no current flow. Then during repolarisation away from the positive electrode so also shows upstroke because the charge is becoming positive.
How can the diction of depolarisation affect the ECG
At a direct angle shows full upstroke but if at an oblique angle to all of the changing membran potential is picked up and the upstroke is smaller. If its directly opposite will be a downstroke because
Height/ depth of deflection depends on the angle the wave is coming toward the electrode
What are the 4 shapes of the different combination of depolarisation , repolarisation and direction of the current flow
Depolarisation wave towards pos electrode = upstroke
Depolarisation wave away from positive electrode = downward stroke
Repolarisation wave towards pos electrode = downward stroke
Repolarisation wave away from positive electrode= upstroke
Explain the P wave
= depolarisation of the atria. Small depolarisation toward electrode. Only shallow deflection because direction of depolarisation is around the atria walls toward the AVN
Explain the flat line between p and q
Due to delay at AVN and passing of current to bundle of His
Explain the q wave
Muscle from left to right depolarises in the septum. Downward deflection because moving obliquely away
Explain the R wave
Large upward deflection because depolarisation is moving directly toward the electrode towards the apex of the heart
Explain the S wave
Movement of depolarisation upwards to base of ventricle away from electrode
Explain the T wav
Repolarisation moving in opposite direction - away from the electrode
Explain the overall shape of PQRST
P= depolarisation of atria QRS= depolarisation of ventricle T= repolarisation
Explain the positioning of the electrodes during an ECG
10 electrode
4 on limbs and 6 on chest.
Limbs = Ride (red + right) Your (yellow -) Green (green -) Bike(black neutral). Must be placed on bone - shows the heart in the vertical plane
chest- V1-V6, all placed in the intercostal spaces - shows the horizontal view
which electrodes are best for looking at lateral wall of left ventricle
AvL and I - here able to see whether a myocardial infarction was due to occlusion of the left coronary artery
which electrodes are best at looking at the inferior aspect of the heart
III, aVF and II. best for detecting muscle necrosis due to occlusion of right coronary artery
explain the views of the different electrodes
look at pic for this BUT III, aVF and II show inferior view, aVL and I show lateral view and aVR shows medial view. V1-V6 all show horizontal plane, see whats directly opposite the electrode.
what leads show what
lateral = I, aVL,V5 and V6
inferior -II, III, aVF
septal (right ventricle and septum) - V1 and V2
anterior (apex and anterior surface of ventricles and septum)- V4 and V3
why do the different leads show different patterns in their ECG’s
because the angle/direction of depolarisation changed relative to the electrode
how many squares is 1 second and how many is 1 min
1 sec= 5 large squares
1min = 3000 large squares
how to calculate heart rate from an ECG
count the number of boxes between the R intervals. then divide 300 by the number of boxes. (because thats 1 min) so 300/4= 75 bpm
how to calculate an heart rate from an irregular heart beat
if its irregular then count how many QRS there are in 6 seconds and then times by 10 to get 60 boxes= 1 min
what is normal PR interval and whats wrong if this is delayed
3-5 small boxes. prolonged = delayed conduction through AV and bundle of HIs
what is normal QRS interval and what if this is delayed
normalsly <3 small boxes. if its prolonged then not spreading through purkinje fibres
normal QT interval and what if its delayed
it varies with heart rate - gets longer in bradycardia and shorter in tachycardia so its hard to know what its caused by but the upper limit would be 11 boxes
what decides a sinus rhythm
if its regular, 60-100bpm, presence of P waves, P waves upright in leads I and II, PR interval normal, P followed by QRS and QRS followed by P
