Link between Thyroid dysfunction and AF

Question 1

Name the 2 mechanisms that drive AF

Answer 1

1. Formation of a re-entrant rhythm
2. Increase in focal activity
   both of these mechanisms are required for AF to occur

Question 2

What is a re-entrant rhythm

Answer 2

Rhythm where the action potential, rather than passing through a group of muscle cells once and depolarising them and allowing them to contract, the action potential passes around the group of cells and continually stimulates them to generate action potentials
- can be local or global (Wolff-Parkinson white syndrome)

Question 3

What is required for a re-entrant rhythm to occur

Answer 3

1. changes in conduction velocity
2. shortening of refractory period

Question 4

what is increased focal activity

Answer 4

abnormal automaticity
- cells that normally can’t generate action potentials can suddenly do so
- some cells can generate multiple action potentials

Question 5

What are the main cells which show automaticity

Answer 5

AVN and SAN

Question 6

Describe the process of a normal rhythm (diagram)

Answer 6

1. normally an action potential that is generated from SAN would come down and pass around the non conducting tissue and will eventually meet up at point 3 and cancel out
   - this is because the cells have just been excited by the action potential and have a refractory period, so can’t be excited again
   - so one action potential at the top will stimulate muscle cells to generate a single action potential

Question 7

describe the process of a re-entrant rhythm (diagram)

Answer 7

1. under conditions where the refractory period of tissue is shortened and there are conduction abnormalities, this can lead to an action potential coming down point 1, passing along the bottom and passing up slowly through point 2
2. when it comes out the other side, it reaches a tissue that is recovered and is excitable again, so the action potential can go around and that can lead to a cell generating multiple action potentials
   - each of these depolarisations will be allowing the muscle cells to contract

Question 8

What 3 components need to match up in order to allow AF to occur

Answer 8

Refractory period
conduction velocity
distance around re-entrant loop

Question 9

How would we aim to disrupt AF occuring

Answer 9

need to change refractory period and conduction velocity

Question 10

what can be caused by abnormal automaticity

Answer 10

1. Early afterdepolarisation
2. Delayed afterdepolarisation

Question 11

how does focal activity affect heart rate

Answer 11

Can increase sympathetic activity which increases heart rate by noradrenaline
or decrease parasympathetic activity which slows heart rate by Ach

Question 12

What are early afterdepolarisations caused by

Answer 12

caused by an increase in action potential width (refractory period)

Question 13

What are delayed afterdepolarisations caused by

Answer 13

caused by Ca2+ overload

Question 14

Give an example of a drug that can caused delayed afterdepolarisation

Answer 14

digoxin

Question 15

Give an example of a drug that can decrease automaticity and slow heart rate

Answer 15

Calcium channel blockers, beta blockers

Question 16

Explain the pharmacological link between hyperthyroidism and AF

Answer 16

1. An increase in sympathetic activity drives abnormal automaticity
   - mechanism by which T3, the biologically active thyroid hormone can predispose patients to AF
2. T3 also decreases action potential duration which decreases refractory period of the tissue
3. T3 increases the expression of a particular type of voltage gated K+ channel, allowing more K+ to leave
   - bringing action potential back down to its resting membrane potential much quicker

Question 17

Describe the pathway where T3 can drive focal activity

Answer 17

1. Increased sympathetic tone
2. T3 results in increased sensitivity to B1 adrenergic and decreased sensitivity to M2 muscarinic receptors

Question 18

Describe the pathway where T3 can drive re-entrant rhythms

Answer 18

1. Decreased Heart rate variability
2. mediated by the vagolytic effects of T3

Question 19

What would be demonstrated at the site of focal activity

Answer 19

demonstration of increased automaticity in pulmonary vein cardiomyocytes and increased incidence of supraventricular depolarisations

Question 20

what is meant by remodelling

Answer 20

causing changes in heart muscle cells

Question 21

What does ERP stand for

Answer 21

Effective refractory period

Question 22

What 3 ways can remodelling occur

Answer 22

1. ERP changes
2. Conduction slowing
3. atrial fibrosis

Question 23

what is meant by conduction slowing

Answer 23

speed at which the action potential can pass through

Question 24

what occurs in atrial fibrosis

Answer 24

muscle cells get smaller but connective tissue fills gaps
- connective tissue isn’t conductive

Question 25

what is the main link between arrhythmia and hyper/hypothyroidism

Answer 25

Both have the potential to contribute to AF

Question 26

What is Hyper/hypothyroidism associated with in the link to AF

Answer 26

Hyperthyroidism is associated with AF
Hypothyroidism is associated with increasing cardiovascular risk factors, as well as subclinical and diagnosed CVD, both of which are thought to predispose patients to AF

Question 27

Why is hypothyroidism a major risk factor for coronary artery disease

Answer 27

Can’t produce as many LDL receptors, so plasma LDL levels are elevated

Question 28

What does TSH stand for

Answer 28

Thyroid stimulating hormone

Question 29

What would TSH and T4 levels be like in hyperthyroidism

Answer 29

TSH would be low and T4 would be high

Question 30

What would TSH and T4 levels be like in primary hypothyroidism

Answer 30

TSH would be high and T4 would be low

Question 31

What would TSH and T4 levels be like in secondary hypothyroidism

Answer 31

TSH would be low and T4 would be low