Electrocardiogram

Question 1

ECG

Answer 1

electrocardiogram

tool for evaluating electrical events within the heart

Question 2

how do ECGs work?

Answer 2

when action potentials occur simultaneously in many individual myocardial cells, currents are conducted through body fluids around the heart and can be detected by recording electrodes at the skin surface

Question 3

what different waves represent

Answer 3

P wave corresponds to current flow during atrial depolarisation

QRS complex (0.15s later) - ventricular depolarisation. currents generated in body fluids change direction.

T wave - ventricular repolarisation

atrial repolarisation not evident as it occurs at the same time as the QRS complex

Question 4

lead 1 reference and recording electrode

Answer 4

reference (-) electrode - right arm

recording (+) electrode - left arm

Question 5

lead 2 reference and recording electrode

Answer 5

reference (-) electrode - right arm

recording (+) electrode - left leg

Question 6

lead 3 reference and recording electrode

Answer 6

reference (-) electrode - left arm

recording (+) electrode - left leg

Question 7

augmented limb leads

Answer 7

aVR
aVL
aVF

Question 8

aVR electrode placement

Answer 8

left arm and left leg (-)
right arm (+)

Question 9

aVL electrode placement

Answer 9

right arm and left leg (-)
left arm (+)

Question 10

aVF electrode placement

Answer 10

right arm and left arm (-) 
left leg (+)

Question 11

precordial limb leads

Answer 11

V1, V2, V3, V4, V5, V6

Question 12

precordial limb leads reference (-) electrode

Answer 12

combined limb leads

Question 13

V1 recording (+) electrode

Answer 13

4th intercostal space, right of sternum

Question 14

V2 recording (+) electrode

Answer 14

4th intercostal space, left of sternum

Question 15

V3 recording (+) electrode

Answer 15

5th intercostal space, left of sternum

Question 16

V4 recording (+) electrode

Answer 16

5th intercostal space, centred on clavicle

Question 17

V5 recording (+) electrode

Answer 17

5th intercostal space, left of V4

Question 18

V6 recording (+) electrode

Answer 18

5th intercostal space, under left arm

Question 19

diagnostic value of ECG

Answer 19

provides info on electrical activity of the heart, not mechanical activity

Question 20

summary of Ca2+ activity in the heart

Answer 20

small amount of Ca2+ enters through L-type channels during the plateau
this triggers the release of a large amount of Ca2+ from ryanodine receptors in the SR membrane
Ca2+ activation of thin filaments and cross-bridge cycling leads to generation of force

contraction ends when it’s returned to the SR and ECF by Ca2+ ATPase pumps and Na+/Ca2+ countertransporters

Question 21

absolute refractory period of the heart

Answer 21

period during and following an action potential where an excitable membrane cannot be re-excited

heart cannot generate tetanic contractions

Question 22

mechanisms of the refractory period of the heart

Answer 22

inactivation of Na+ channels

Question 23

time of refractory period in skeletal muscles

Answer 23

absolute refractory period is much shorter (1-2 msec) than duration of contraction (20-100 msec), so a second action potential can be elicited while contraction from the first action potential is occuring

Question 24

length of refractory period in cardiac muscle

Answer 24

due to the prolonged, depolarised plateau the refractory period lasts almost as long as the contraction (250 msec) and muscle can’t be re-excited multiple times during contraction