ECG

Question 1

Explain how the heart directs an electrical current

Answer 1

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

Question 2

Why is the bundle of His essential

Answer 2

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

Question 3

Describe repolarisation of the heart

Answer 3

Repolarisation happens in the opposite direction to depolarisation -starts at the the last cell to be depolarised and goes backwards up the septum

Question 4

Discuss the membrane potentials during a single heart beat

Answer 4

At rest = positive charge on outside
During depolarisation = negative charge on outside
During repolarisation = turns back to positive charge on outside

Question 5

Discuss how an EXG detects the changing membrane potential

Answer 5

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.

Question 6

How can the diction of depolarisation affect the ECG

Answer 6

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

Question 7

What are the 4 shapes of the different combination of depolarisation , repolarisation and direction of the current flow

Answer 7

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

Question 8

Explain the P wave

Answer 8

= depolarisation of the atria. Small depolarisation toward electrode. Only shallow deflection because direction of depolarisation is around the atria walls toward the AVN

Question 9

Explain the flat line between p and q

Answer 9

Due to delay at AVN and passing of current to bundle of His

Question 10

Explain the q wave

Answer 10

Muscle from left to right depolarises in the septum. Downward deflection because moving obliquely away

Question 11

Explain the R wave

Answer 11

Large upward deflection because depolarisation is moving directly toward the electrode towards the apex of the heart

Question 12

Explain the S wave

Answer 12

Movement of depolarisation upwards to base of ventricle away from electrode

Question 13

Explain the T wav

Answer 13

Repolarisation moving in opposite direction - away from the electrode

Question 14

Explain the overall shape of PQRST

Answer 14

P= depolarisation of atria 
QRS= depolarisation of ventricle 
T= repolarisation

Question 15

Explain the positioning of the electrodes during an ECG

Answer 15

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

Question 16

which electrodes are best for looking at lateral wall of left ventricle

Answer 16

AvL and I - here able to see whether a myocardial infarction was due to occlusion of the left coronary artery

Question 17

which electrodes are best at looking at the inferior aspect of the heart

Answer 17

III, aVF and II. best for detecting muscle necrosis due to occlusion of right coronary artery

Question 18

explain the views of the different electrodes

Answer 18

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.

Question 19

what leads show what

Answer 19

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

Question 20

why do the different leads show different patterns in their ECG’s

Answer 20

because the angle/direction of depolarisation changed relative to the electrode

Question 21

how many squares is 1 second and how many is 1 min

Answer 21

1 sec= 5 large squares

1min = 3000 large squares

Question 22

how to calculate heart rate from an ECG

Answer 22

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

Question 23

how to calculate an heart rate from an irregular heart beat

Answer 23

if its irregular then count how many QRS there are in 6 seconds and then times by 10 to get 60 boxes= 1 min

Question 24

what is normal PR interval and whats wrong if this is delayed

Answer 24

3-5 small boxes. prolonged = delayed conduction through AV and bundle of HIs

Question 25

what is normal QRS interval and what if this is delayed

Answer 25

normalsly <3 small boxes. if its prolonged then not spreading through purkinje fibres

Question 26

normal QT interval and what if its delayed

Answer 26

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

Question 27

what decides a sinus rhythm

Answer 27

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