Physiological properties of the heart

Question 1

what is phase 0 a/v depolarisation

Answer 1

• Rapid depolarisation due to ↑ Na+ permeability (gNa+) as fast Na+ channels open

Question 2

what is phase 1 a/v depolarisation

Answer 2

• Start of repolarisation as fast Na+ channels close

Question 3

what is phase 2 a/v depolarisation

Answer 3

• Effect of Ca2+ entry via L-type channels

Question 4

what is phase 3 a/v depolarisation

Answer 4

• Rapid repolarisation as ↑ [Ca2+]i stimulates K+ channels to open and gK+ ↑
• Ca2+ L-type channels close

Question 5

what is phase 4 a/v depolarisation

Answer 5

• Stable resting membrane potential where gK+ exceeds gNa+ by 50:1

Question 6

what is phase 1 SA Node depolarisation

Answer 6

• Gradual dri= ↑ in res>ng membrane poten>al due to ↑ gNa+ as “funny” F-type Na+ channels open and ↓ gK+ as K+ channels slowly close
• “Pacemaker potential”
• Transient (T) Ca2+ channels help with
the “final push”

Question 7

what is phase 2 SA Node depolarisation

Answer 7

• Moderately rapid depolarisation due to Ca2+ entry via slow (L) channels

Question 8

what is phase 3 SA Node depolarisation

Answer 8

• Rapid repolarisation as elevated internal Ca2+ stimulates opening of K+ channels and an ↑ in gK+

Question 9

Sympathetic stimulation of pacemaker

Answer 9

– Noradrenaline acts on β1 receptors to ↑ cAMP production – Increases rate of SAN phase 1 depolarisation
• ↑ gCa2+
• ↑ gNa+ via “funny” channels

Question 10

Parasympathetic stimulation of pacemaker

Answer 10

– Acetylcholine on M2 receptors which ↓ cAMP production – Reduces rate of phase 1 depolarisation
– Hyperpolarises membrane potential to lower starting level
• ↑ extent and dura>on of opening of K+ channels  ↑ gK+

Question 11

Sympathetic stimulation shows

Answer 11

positive chronotropic effect

Question 12

Parasympathetic stimulation

Answer 12

shows negative chronotropic effect

Question 13

why is SAN the intrinsic pacemaker

Answer 13

As SAN has the fastest rate, it is the intrinsic pacemaker
Depolarisation spreads from SAN throughout heart before other regions spontaneously depolarise
If conduction blocked, downstream tissues assume their intrinsic rate

Question 14

when is the biggest delay in transmission

Answer 14

the biggest delay is when it passes AV node

Question 15

Electrocardiogram (ECG)

Answer 15

– Measures electrical activity of the heart over time – Uses multiple electrodes
• Fouronthelimbs
– One is an “earth”, used to remove background noise
– Three used to create virtual “leads” between each pair of electrodes
• Six across the chest
– To give more specific, localised information about areas of the heart

Question 16

trace traction

Answer 16

• Depolarisation moving towards the“positive”causes the trace to go up.
• Depolarisation moving away from the“positive” causes the trace to go down.

Question 17

Size of electrical signals from the heart determined by:

Answer 17

– Current (proportional to tissue mass)

– Direction of signal

Question 18

Observed signal =

Answer 18

E x CosƟ

E = Electrical event
Ɵ = angle between event and ECG lead

Question 19

P wave

Answer 19

– Atrial depolarisation

Question 20

QRS wave

Answer 20

– Ventricular depolarisation

Question 21

T wave

Answer 21

– Ventricular repolarisation

Question 22

Important timing intervals

Answer 22

– P-R interval (0.12-0.2s)
– QRS complex width (0.06-0.12s)
– Q-T interval (0.25-0.35s)

Question 23

Tricuspid

Answer 23

flow anteriorly and medially, i.e. towards the left

Question 24

Mitral,

Answer 24

flow anteriorly and inferiorly towards the apex

Question 25

Pulmonary

Answer 25

flow superiorly and towards the left

Question 26

Aortic

Answer 26

flow superiorly and to the right