Week 16: (D) The Electrocardiogram

Question 1

What triggers the contraction?

Answer 1

APs normally initiate by the SA

Question 2

How many AP does the SA node fire?

Answer 2

70-80 per min (at rest)

Question 3

How are neighbouring cardiac muscles linked?

Answer 3

gap junctions

Question 4

What does the ECG record?

Answer 4

overall spread of electrical activity through the heart

Question 5

Why an ECG small electrode outside the body pick up small voltages?

Answer 5

At body’s surface as the extracellular space is occupied by the conducting salt solutions of the body tissues

Question 6

What does the size of the potential measured depend on?

Answer 6

size of the extracellular current

Question 7

What are electrical currents generated by the cardiac muscle conducted through?

Answer 7

body fluids

Question 8

Are the electrical currents fired during excitation of atria and ventricles fired synchronously?

Answer 8

YES

Question 9

Is an ECG a direct recording of the actual electrical activity of the heart?

Answer 9

NO, only small part if the electrical activity reaches the body’s surface where it can be recored by electrodes on the skin

Question 10

Is the ECG a recoding of a single AP in a single cell at a single point in time?

Answer 10

NO, represents the overall spread of activity throughout the heart during depolarisation and re-polarisation

Question 11

What does an ECG recording represent?

Answer 11

comparisons in voltage detected by electrodes at two different points on the body surface, not the actual potential

Question 12

what is the specific arrangement of each pair of connections called?

Answer 12

A lead

Question 13

How many leads are used?

Answer 13

12

Question 14

how many limb leads are there?

Answer 14

6

Question 15

How many chest leads are there?

Answer 15

6

Question 16

What are the bipolar limb leads?

Answer 16

Leads 1,2,3

Question 17

Why are they called bipolar leads?

Answer 17

as 2 recording electrodes are used

Question 18

What are the unipolar limb leads?

Answer 18

aVR, aVL, aVF

and all chest leads

Question 19

what does the ECG use the negative pole as?

Answer 19

zero reference

position of the positive pole= point of view

Question 20

What is the zero reference in the unipolar leads?

Answer 20

the average of the 2 opposite limb poles

Question 21

What orientation is the ECG if the depolarisation is toward/away from a lead?

Answer 21

toward= positive (up)
away= negative (down)

Question 22

What has a higher AP, ECG or intracellular?

Answer 22

-Intracellular

1mV compared to 100mV

Question 23

When does the ECG remain at baseline?

Answer 23

• Waves in ECG are only recorded when the p in changing across membranes
• During plateau phase (between QRS and T waves)