Lecture 11: ECG

Question 1

Dipole

Answer 1

Small positive current from (+) region around resting cell to (-) region around AP cell

Question 2

Voltage vector

Answer 2

Aka mean electrical vector. Sum of individual charge differences in heart at given moment

Question 3

Factors determining voltmeter reading

Answer 3

1. Distance from electrode
2. Mass of tissue (bigger strip, bigger dipole)
3. Orientation (vector toward (-) electrode = negative deflection; vector reading strongest when parallel, 0 when perpendicular)

Question 4

Notes for heart voltage vectors

Answer 4

-Heart points down + left
-L vent. is thicker
-Lead axis = line between electrodes
-Lead direction is (-) to (+)

Question 5

Einthoven leads

Answer 5

Aka bipolar/standard limb leads
I, II, and III

Question 6

Interpreting leads (depolar. vs repolar.)

Answer 6

Question 7

Augmented unipolar leads

Answer 7

aVR, aVL, aVF
Enhance small signals

Question 8

Atrial depolar. reading

Answer 8

P-wave

Question 9

PR interval

Answer 9

Between P-wave start and QRS complex start. Slow conduction thru AV node/proximal conducting branches -> small electrical events

Question 10

Septal depolarization

Answer 10

Q-wave

Question 11

Ventricular depolarization (averaged)

Answer 11

R-wave

Question 12

Late ventricle depolarization (averaged)

Answer 12

S-wave
i.e. up the walls of the ventricle away from apex

Question 13

ST segment

Answer 13

Brief period with no electrical activity as heart is fully depolarized. From end of QRS to start of T

Question 14

Ventricular repolarization

Answer 14

T-wave
Epicardium is last to depolarize but first to repolarize, creating positive deflection in I/aVF

Question 15

Calculating HR from ECG

Answer 15

HR = 60 (sec/min) / R-R interval (sec/beat). Large box = 0.2s, small box = 0.04s