Cardiac Excitation And Contraction

Question 1

Cardiac output split between atria and ventricles (CO)

Answer 1

Atria/ventricles = 10/90

Question 2

P-wave (EKG)

Answer 2

The first ‘bump’ on the EKG

Represents the wave of depolarization across the atria

Signal starts at SAN —> hits all parts of the atria and converges at the AVN

Fibrous ring keeps the conduction from traveling to the ventricles…and only to the AVN

I.e. the signal conduction can only get to the ventricles through the AVN

Question 3

Role of AVN

Answer 3

Slows down conduction

‘Electrical filter’ to protect the ventricles

Structurally it can do this because…

1. Fewer gap junctions
2. Low expression levels of Na+ channels

THEREFORE Ca2+ is the dominant current, which has a smaller amplitude

Question 4

Flat line immediately after P-wave

Answer 4

Signal is limited to the small area of the AVN…

So small that cannot detect electric potential on machine

Question 5

QRS interval

Answer 5

Time taken for ventricular depolarization

AVN —> bundle of His (Purkinje Fibers)

Very very fast

Flow of conductance: endocardium —> epicardium

Question 6

Gap junctions / intercalated discs in muscle cells

Answer 6

Highly permeable

Protein = connexons

Open condition = low [Ca2+] and normal pH

Closed = high [Ca2+] and low pH

—> mechanism to protect neighboring cells…so dangerous environment conditions don’t spread uncontrollably

Question 7

Relationship between ATP levels and gap junction conformation

Answer 7

Low ATP —> increase [Ca2+] —> lower pH —> closed channels

Question 8

T-wave

Answer 8

After QRS interval

Repolarization of ventricles

Flow: epicardium —> endocardium (reverse of depolarization)

—> due to the fact that APs have a shorter duration in the epicardium

Why T-wave is upright

Question 9

What does a large P-wave indicate (amplitude)

Answer 9

Large Atria

= congestive heart failure

Question 10

Abnormally wide QRS interval =

Answer 10

LV hypertrophy

Question 11

If T-wave too high

Answer 11

Hyperkalcemia

Question 12

If QT (ventricular repolarization) is too prolonged…

Answer 12

Rish for death!!! AAAHHHHHH

Question 13

Regional differences in AP duration

Answer 13

Purkinje > ventricular&raquo_space;> atrial

Question 14

Regional differences in phase 1 notch

Answer 14

Epicardial&raquo_space; endocardial

Question 15

Regional differences in unstable diastolic potential

Answer 15

SAN > AVN > purkinje

They have to ability to spontaneous depolarized

Question 16

Ionic basis of cardiac action potential

Answer 16

1. K+ outflow (I-k1)
2. Na+ influx (I-Na)

(Then most Na+ channel close)

3. Some transient outflow of K+ (I-to)

(Then Ca2+ channels start to open)

4. Ca2+ influx

(Ca2+ influx (+) K+ outlfow —> ‘plateau’ in graph ‘#2’)

5. I-k activates outflow —> rapid repolarization

Question 17

Relationship between conduction velocity and Na+ current

Answer 17

Direct relationship

Question 18

Steps 0 —> 4 of cardiac AP

And their corresponding responsible current

Answer 18

0 = I-Na+ (influx)

1 = ‘notch’ = I-to (outflow)

2 = I-Ca2+ (influx)

3 = I-K (outflow)

4 = RP = I-K1 (@ E-k)

Question 19

Effects of voltage and time on Na+ availablity

Answer 19

At -40mV = all channels inactivated

Cannot initiated AP until atleast 50% of channels are rested (RRP) - not ideal thou