8: Cardiac Rhythm

Question 1

What 3 things help maintain sinus rhythm?

Answer 1

• Active suppression of other pacemakers
• Co ordinated excitation via specialised conduction system
• Prolonged refractory period in myocardium

Question 2

What is the difference between ARP and ERP?

Answer 2

• ARP = individual cells

- ERP = group of cells

Question 3

What is the supernormal period?

Answer 3

Smaller than normal stimuli can generate propagated action potential

Question 4

The two causes of arrhythmia are disorder of ____ or disorder of ____

Answer 4

• Disorder of impulse formation

- Disorder of impulse conduction

Question 5

Explain the mechanism of re-entrant arrhythmia

Answer 5

• Electrical stimulation of heart during RRP
• Excitation elicits slowly propagating APs
• Gives rise to VT that progresses to VF

Question 6

Which currents are responsible for early and late repolarisation respectively?

Answer 6

I(to) = early

Ikr + Iks = late

Question 7

Activation gates are shut at ___ potential but open with ___ whereas inactivation gates open at ____ but close during ____

Answer 7

Activation gates = shut at RMP but open with depolarisation

Inactivation gates = open at RMP but close with depolarisation

Question 8

What are the 3 things required for a re-entrant circuit?

Answer 8

• Trigger
• Unidirectional block
• Slow conduction

Question 9

Wavelength = ____ x ____

Answer 9

ERP x conduction velocity

250-300 x 1ms

Question 10

What factors influence conduction velocity?

Answer 10

• Cell dimension
• Gap junction density
• Status of Na channels

Question 11

MI causes ___ [K]o and [Na]i and ____, as well as ____ intracellular ATP

Answer 11

Increased
Regional metabolic acidosis
Reduced intracellular ATP

Question 12

Inc [K]o causes
1.
2.
3.

Which results in inhomogenous repolarisation

Answer 12

1. Reduced MP and inactivation of Na channels (slow conduction)
2. Reduced AP duration
3. Activation of ATP dependent K current

Question 13

Reduced intracellular ATP results in

1.

Which leads to after-depolarisations.

Answer 13

1. Reduced Ca-ATPase activity (inc Cai)

Results in spontaneous release of Ca from overloaded SR.

Question 14

What increases risk of re-entry in MI

1. Reduced wavelength
2. Trigger
3. Inc. probability of local conduction block

Answer 14

1. Slow conduction and reduced AP duration
2. Ectopic activation
3. Nonuniform repolarisaiton

Question 15

What are the changes in heart failure that inc. risk of re-entrant activation?

(Atria, NCX, ANS)

Answer 15

1. Atrial dilation (inc path length, stimulate stretch activated ion channels)
2. Atrial fibrosis
3. NCX changes - after depolarisations
4. ANS remodelling influences electrical properties

Question 16

Why does LQTS often seem to be associated with exercise or emotional excitement?

Answer 16

L type Ca channels are sensitive to adrenergic stimulation