L6-7: Cardiac Ventricular Action Potential & Channelopathies

Question 1

How long does cardiac ventriculation last?

Answer 1

~200ms

Question 2

Name the 5 phases of cardiac ventriculation

Answer 2

Depolarisation, early repolarisaton, plateau phase, rapid repolarisation and RMP (polarised state)

Question 3

What occurs in phase 0 of cardiac ventriculation?

Answer 3

Depolarisation

• fast acting Na+ chnnels open the inward Na+ dependent channels
• Na+ moves through pore and into cell
• Lasts a few ms

Question 4

What occurs in phase 1 of cardiac ventriculation?

Answer 4

Early repolarisation

• fast acting Na+ channels are closed but K+ channels are open so there’s a transient outward current due to movement of K+ out of the cell
• this turns around initial depolarisation and initiates repolarisation

Question 5

What occurs in phase 2 of cardiac ventriculation?

Answer 5

Plateau phase

• a subset of Ca channels open (L type) and Ca flows from outside in
• allows external environment to come in relatively close contact with interior aspects of cell (SR)
• stimulates release of Ca from cell and SR receptors responf to influc of Ca, increasing intracellular Ca (Ca induced Ca release)

Question 6

What occurs in phase 3 of cardiac ventriculation?

Answer 6

Rapid repolarisation

• Ca ion channels closed and Ca used up in contraction
• Remaining Ca pumped out of cell via Na+/K+ exchanger or taken back up into SR via SRC ATPase

Question 7

What occurs in phase 4 of cardiac ventriculation?

Answer 7

RMP (polarised)

• Na+ mainly in extracellular fluid & K+ in intracellular fluid
• Pump maintains this gradient

Question 8

What is an arrythmia

Answer 8

Any deviation from the heart’s normal sinus rhythmn

Question 9

What can cause a heart arrythmia?

Answer 9

• abnormal impulse formation (delayed repolarisation)
• enhancing automaticity (spontaneous impulse initiation causing ectopic beats)
• re entry mechanism

Question 10

Distinguish between bradychadia & tachycardia

Answer 10

Bradychadia refers to HR <60

Tachycardia refers to HR <100

Question 11

What is a cardiac channelopathy?

Answer 11

A disorder of heart rhythm caused by abnormal genes/ structure and function of ion channels

Question 12

What is ‘Long QT syndrome’?

Answer 12

Syndrome where there are repolarisation abnormalities caused by mutations in genes which encode K (&Na) ion channels

Question 13

Name features of Long QT syndrome

Answer 13

• has several varities and can be inherited
• can lead to fatal heart rhythm
• ventricular tachycardia can occur

Question 14

Explain the mechanism of Long QT syndrome

Answer 14

Early after depolarisation (EADs)

• K+ can’t easily leave cell so repolarisation slowered and there could be another depolarisation happening
• triggered beats cause Toursades de pointes

Question 15

Name features of ‘Brugada Syndrome’

Answer 15

• tends to affect middle aged men
• causes sudden cardiac arrest in sleep
• caused by abnormalities in Na+ leading to loss of function in SCNSH channel

Question 16

How may Brugada syndrome appear in an ECG?

Answer 16

Coved- type ST segment elevation in anterior percordial leads (V1 to V3)

Question 17

Name features of CPVT (Catecholaminergic polymorphic ventricular tachycardia)

Answer 17

• can lead to fatal heart rhythm
• ventricular tachycardia can occur
• ECG APPEAR NORMAL
• higher estimated fatallities & breakthrough rates during conventional beta blocker therapy

Question 18

Explain the mechanism of CPVT

Answer 18

Abnormal impulse generation that occurs after normal activity

• known as delayed after depolarisation (DADs)
• problem with receptors: spontaneous Ca release and build up leads to Ca being pumped out but Na getting in due to Na/Ca exchanger triggering depolarisation

Question 19

What treatments can be given for channelopathies, give examples?

Answer 19

• LQT: beta blockers & avoidance of specific triggers
• CPVT: beta blockers, decreased sympathetic innervation by cutting left cardiac sympathetic nerves & implant defibrilator (ICDs)
• BrS: ICD