Physiology 7: Electrophysiology of the Heart

Question 1

What is different about AV/bundle branches (from SA node) that causes slowed conduction and therefore allows increased ventricular filling ?

Answer 1

Decreased number of gap junctions ! Slows conduction through these two areas.

Question 2

Is conduction through the Purkinje system slow or fast ?

Answer 2

Fast !

Question 3

On an EKG which interval correlates to the time it takes the impulse to go from the SA node through the bundles ?

Answer 3

PR interval (P is atrial depol, QRS is ventricular depol)

Beginning of P to Beginning of QRS

Question 4

Ions that have voltage gated channels include…

Answer 4

Na+
Ca++
K+

Question 5

Funny Na+ Channels (I-f) lead to…

Answer 5

Pacemaker activity in the SA, AV and Purkinje fibers

Question 6

Fast Na+ Channels (I-Na+) lead to …..

Answer 6

rapid depolarization in atrial and ventricular myocytes and Purkinjie fibers

Question 7

Calcium Channels (I-Ca++) lead to

Answer 7

Rapid depolarization in Sa and AV nodal cells, triggers contraction in all myocytes

Question 8

Potassium channels (I-k+) lead to

Answer 8

repolarization in all cardiomyocytes

Question 9

Describe Phases 0 of the Myocyte AP

Answer 9

Phase 0 –> Rapid depolarization due to Voltage gated Na+ Channels opening leading to Na+ influx

Question 10

Describe Phases 1 of the Myocyte AP

Answer 10

Na+ channels close while K+ and Cl- channels open leading to a short but strong repolarization (will not repolarize completely due to Phase 2 Ca++ influx)

Question 11

Describe Phases 2 of the Myocyte AP

Answer 11

Ca++ open leading to a slower repolarization from k+ channels. This is the plateau portion of the AP.

Question 12

Describe Phases 3 of the Myocyte AP

Answer 12

Ks and Kr channels open and Ca++ channels close leading to de/hyperpolarization.

Question 13

Describe Phases 4 of the Myocyte AP

Answer 13

Kr and Ks channels close while Na+ channels activate but are still closed…. this is resting membrane potential

Question 14

What does hyperkalemia do to resting membrane potential ? How does this effect AP propagation ?

Answer 14

Hyperkalemia will raise the resting membrane potential

This,however, does not make the cell hyperexcitable due to the inactivation of Vgated Na+ channels at this higher membrane potential (fail to reset). This will cause decreased ability to propogate and AP in myocytes.

Question 15

Describe Phases 4 of the Nodal AP

Answer 15

Phase 4 represents depolarization

The first portion in slow due to I-f Na+ channels opening.
This is then precipitated by T-type calcium channel opening and finally the opening of L-Type Ca++ leading to full depolarization.

Question 16

Describe Phases 0 of the Nodal AP

Answer 16

Primarily the influx of calcium from L-Type Channels

Question 17

Describe Phases 3 of the Nodal AP

Answer 17

Closing of L-Type Ca++ channels coupled with opeing of K+ channels –> Re-polarization.

Question 18

Fastest Phase _______ will lead to overdrive suppresion by that tissue.

Answer 18

4 (Phase 4 =depolarization in SA/AV nodes.)

Question 19

Explain how hypokalemia leads to automaticity

Answer 19

Steps for hypokalemia induced automaticity:

EKbecomes more negative (Nernst equation prediction)

A more negative EKcauses the maximum diastolic Vm to become more negative

A more negative diastolic Vm activates more If, which produces enhanced phase 4 depolarization

A low [K+]oreduces IK1; fewer K channels are open in hypokalemia

In Short: Hyperpolarization turns on the I(f) channel  Na+ influx. (Phase 4 current will be stimulated, especially in the Perkinje system  Ectopic pacemaker)

Question 20

Ultra Rapid K+ Channels —>

Answer 20

Lead to shorter AP in Atria than in other myocytes due to faster depolarization ( Phase 1 and 2…these channels shorten both)

Question 21

Absolute Refractory Period (ARP)

Answer 21

– absolute, no action potential can be fired, Na+ channels inactivated

Question 22

Relative Refractory Period

Answer 22

relative, stimulation triggers an action potential, but not all the Na+ channels are ready to go, lower rate of rise of AP

Question 23

Supranormal period

Answer 23

less than normal signal can trigger an action potential (hyper-excitable)

Question 24

What may occur from stimulation of the relative refractory period ?

Answer 24

Premature contraction–> decreased CO due to decreased pre-load.

Question 25

Class IA AA drugs: Increase or decrease ARP ?

Answer 25

Increase ARP

Question 26

Class IB AA drugs: Increase or decrease ARP ?

Answer 26

Decrease ARP

Question 27

Class IC AA drugs: Increase or decrease ARP ?

Answer 27

NO EFFECT !

Question 28

Class III AA drugs : Increase or Decrease ARP ?

Answer 28

Increase

Question 29

Class IV AA drugs : Increase or Decrease ARP ?

Answer 29

Decrease

Question 30

Delayed After Depolarization are caused by excess Ca++. What can cause excess Ca++ ?

Answer 30

Digitalis intoxication

Excessive catecholamine stimulation–> increase Ca++ entry

Question 31

Sympathetic molecule activation of HCN channels lead to which kind of Na+ current ?

Answer 31

I-f

Question 32

Parasympathetic effect of nodal action potentials lead to activation K+ channels as well as Gi Receptors. What will these two do respectively ?

Answer 32

K-Ach channels lead to increased K+ current out of cells

Gi will decrease cAMP and stop I-f (less marked depolarization)

Question 33

Brugada syndrome

Answer 33

Mutation in the fast acting Na+ Channel leading to loss of function

Increased risk of ventricular tachyarrhythmia

Prolonged conduction interval

Slowed conduction due to prolonged phase 0

Na+ channel blockers may evoke/aggravate symptoms

Treatment: implantable cardioverter-defibrillator (ICD)

Question 34

LQT1 and LQT2 – ventricular arrhythmias that occur during….

Answer 34

exercise or stress

Question 35

LQT3

Answer 35

occur during rest

Question 36

What channels mutations are involved in Long QT intervals ?

Answer 36

KCNQ1 (Ks) - impair
KCNH2 (Kr) - impair
SCNA (Na+) - enhance

Question 37

Long QT intervals can lead to …

Answer 37

Torsades de Pointes

Question 38

What drugs can be given to help alleviate symptoms of Long QT syndromes ?

Answer 38

Beta Blockers

Question 39

There was a lot on channelopathies that we did not get to because of Raymond’s whole spiel about his awful podcasts.

Answer 39

Yupp had to put something on the back of this one