Heart Physio

Question 1

Quickly remind me what each wave in the EKG means

Answer 1

P wave= atrial activation
Q wave= His, BB, septum activation
R wave= ventricular activation
S wave= late ventricular activation
T wave= ventricular repolarization
U wave= purkinje repolarization
J wave= during ST segment

Question 2

Under what situations might the U wave change?

Answer 2

U wave will increase with hypokalemia

Question 3

What causes the J wave to change?

Answer 3

hypocalcemia and hypothermia increase J wave

Question 4

Describe the 7 phases of the cardiac cycle

Answer 4

1- atrial contraction
2- isovolumetric contraction 
3- rapid ejection
4- reduced ejection
5- isovolumetric relaxation
6- rapid filling
7- reduced filling

Question 5

In order to make the heart contract, what ion travels down the T tubule? What channel must it then pass through to activate the______ receptor? When this is activated, something gets released, what?

Answer 5

Ca travels down the T tubule and enters through the calcium channel (ICa,L)–> activates ryanodine receptor –> releases sarcoplasmic Ca into the cytosol –> initiates contraction

Question 6

What are the 4 heart sounds indicative of?

Answer 6

S1- closure of the mitral and tricuspid valves
S2- closure of the aortic and pulmonic valve
S3- when audible, occurs early in vent. filling (ventricular dilation)
S4- when audible, vibration of the ventricular wall during atrial contraction (vent. hypertrophy)

Question 7

What transporters are imperative to the reduction of intracellular calcium levels?

Answer 7

SERCA: sarcoplasmic calcium ATPaseNCX:Sodium/Calcium exchanger

Question 8

Active tension is dependent on?

Answer 8

action potential duration, which is frequency-dependent

Question 9

What are positive and negative ionotropy?

Answer 9

positive ionotropy = increased contractility (seen with cardiac glycosides)

negative ionotropy = reduced contractility (seen with calcium channel blockers)

Question 10

What is the functional refractory period? and what are the three divisions of this period?

Answer 10

FRP= minimum time period after an AP required for a threshold stimulus to produce a full response again

Absolute/Effective RP (no AP can be initiated), Relative RP (action potential can be initiated but it requires more than usual inward current), Supernormal RP

Question 11

What regions of the heart exhibit fast response AP’s? Slow?

Answer 11

Fast response AP: atrium, ventricle, His-Purkinje

Slow response AP: SA node, AV node

Question 12

T/F: in order to depolarize a cardiac cell, the K conductance must increase?

Answer 12

FALSE: increased potassium conductance hyperpolarizes a cardiac cell

Question 13

Formulas!

Voltage=?
ionic current=?
conductance=?

Answer 13

V=current x resistance = I x R

ionic current voltage = Vm - Eion

Conductance = g = 1/R

ionic current= Gion x (Vm - Eion)

Question 14

What factors affect the threshold?

Answer 14

resting potential (changes in potassium change this)

excitability (sodium affects this)

cell size

Question 15

In terms of Gk and cell excitability, what happens if the [K]o decreases < 5mM? (eg, hypokalemia)

Answer 15

[K]o < 5mM –> Gk decreases –> easier to excite cell

This is because there is less Gk during phase 3 repolarization and phase 4, so net effect is increased excitability.

Question 16

In terms of Gk and cell excitability, what happens if [K]o increases >10 mM? (eg, hyperkalemia)

Answer 16

[K]o > 10mM –> Gk increases, but cell is less excitable due to decreased Na availability
(from notes – I don’t understand…can someone explain this to me?

Here is my guess:
high [K]o –> decreased gradient for K repolarization currents –> cell “freezes” in depolarization (refractory, plateau phase) = less excitable

Question 17

what factors affect the refractory period?

Answer 17

AP duration (proportional to QT interval)
Excitability (Na current availability)
repolarizing potential - K current availability

Question 18

T/F: at rest potassium conductance is 20x greater than sodium conductance?

Answer 18

TRUE

Question 19

what does rectification mean? what is the advantage of rectification?

Answer 19

Rectification= channel conducts current better in one direction

Advantage= rectification (inward) reduces gK (makes it less negative) so it is easier to depolarize the cell

Question 20

Describe the conceptual relationship between equilibrium potential (Ex) and concentration gradient.

Answer 20

Ex is equal and opposite to the driving force of the concentration gradient of X.

E(ion) = 61.5/z x log ( [ion]o / [ion]i )

Question 21

For APs in the atrium, His-Purkinje, and ventricle, describe the ion currents responsible for Phase 0 and Phase 1.

Answer 21

Phase 0: Upstroke
I-Na

Phase 1: Early Repolarization
I-to = transient outward K current
Inactivation of I-Na

Question 22

For APs in the atrium, His-Purkinje, and ventricle, describe the ion currents responsible for Phase 2.

Answer 22

Phase 2: Plateau

Inactivation of I-to

and a balance between…
I-CaL = inward Ca current
I-Kur = ultra rapid outward K current

Question 23

For APs in the atrium, His-Purkinje, and ventricle, describe the ion currents responsible for Phase 3 and Phase 4.

Answer 23

Phase 3: Repolarization
slow inactivation of I-CaL
I-Kr + I-Ks = delayed rectifier outward K current

Phase 4: Resting Vm
I-K1 = background outward K current

Question 24

For APs in the SA & AV nodes, describe the ion currents responsible for Phase 0 and Phase 3.

Answer 24

Phase 0: Upstroke
I-CaL = inward Ca current

Phase 3: Repolarization
I-Kr + I-Ks = delayed rectifier outward K current
slow inactivation of I-CaL

Question 25

For APs in the SA & AV nodes, describe the ion currents responsible for the pacemaker potential (Phase 4).

Why is the “funny current” funny?

Answer 25

Pacemaker Potential/Phase 4

The “funny current”
I-CaT, I-CaL = inward Ca current
I-NCX = 3 Na in / 1 Ca out = net inward positive current

The funny current is funny because it involves an inward Na current turned on by repolarization.