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mbbs year 1 module 3 circulation and breathing > factors influencing coronary, cerebral, skeletal, skin circulations > Flashcards

factors influencing coronary, cerebral, skeletal, skin circulations Flashcards

1
Q

what are the four different types of special circulation

A

-coronary circulation
cerebral circulation
skeletal circulation
skin circulation

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2
Q

clinical scenario-ischaemic heart disease
a 58 year old man is admitted with acute myocardial infarction
he is transferred for coronary angiography and has a stent inserted to his critically stenosed right coronary artery
prior to discharge he is started on secondary prevention drugs
anti platelet drugs
ace inhibitors
beta blockers
why

A

anti platelet drugs- to prevent rethrombosis
beta blockers- reduce heart rate and increase perfusion time diastolic time, reduce blood pressure and myocardial work
ace inhibitor- to lower the blood pressure to reduce cardiac work

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3
Q

what is the coronary blood flow in ml/100g/min at rest

A

60-80ml/100g/min at rest

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4
Q

what is the oxygen consumption in ml per minute

A

250ml/min

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5
Q

what percentage of oxygen does heart extract at rest

A

heart extracts 70-80% oxygen at rest

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6
Q

what does increased oxygen demand require

A

increased coronary blood flow

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7
Q

by how many times does the coronary blood flow increase during exercise

A

5 times

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8
Q

what is the relationship between supply and demand of oxygen in coronary ischaemia

A

oxygen demand>oxygen supply

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9
Q

what are the 4 determinants of coronary blood flow

A

vasomotor tone
vessel wall diameter
coronary perfusion pressure
perfusion time

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10
Q

which of the following increases coronary blood flow
a) arrhythmia
b)atheroma in coronary arteries
c)hypotension
d)exercise
e)tachyarrhythmia

A

d) exercise
coronary blood flow increases by 5 times in exercise as increased oxygen demand requires increased coronary blood flow

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11
Q

coronary blood flow is lowest when

A

during contraction systole

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12
Q

coronary blood flow is highest when

A

during diastole during relaxation

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13
Q

what does hypertension increase

A

hypertension increases myocardial work

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14
Q

how does hypertension lead to increased myocardial work

A

renin angiotensin aldosterone and sympathetic activity

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15
Q

what does increased sympathetic activity and renin angiotensin aldosterone lead to

A

increased left ventricle hypertrophy

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16
Q

what does left ventricle hypertrophy cause

A

decreased myocardial flow but increased oxygen demand

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17
Q

what do ace inhibitors and beta blockers do to blood pressure and LV hypertrophy

A

reduces blood pressure and hypertrophy.

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18
Q

what is the perfusion time

A

flow occurs during diastole
time in systole is fixed 0.2 seconds
perfusion time depends on heart rate

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19
Q

what increases the perfusion time

A

lower heart rate

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20
Q

how does vessel wall diameter affect coronary blood flow

A

flow proportional to radius
atheroma narrows artery
flow is reduced

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21
Q

what do coronary stents do

A

restore vessel wall diameter to prevent ischaemia

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22
Q

what do antiplatelet drugs do

A

prevent artery/stent occlusion by platelet plug

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23
Q

how does vasomotor tone of large arteries affect coronary blood flow

A

endothelial control
increased flow causes shear stress
normal nitric production
normal vasodilation

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24
Q

how does vasomotor tone of medium arterioles affect coronary blood flow

A

myogenic control
increased blood pressure arteriolar stretch
activates calcium channel
normal vasoconstriction

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25
Q

how does vasomotor tone of small arterioles affect coronary blood flow

A

metabolic control
cardiomyocyte metabolites
co2 adenosine
local vasodilation

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26
Q

what are some factors affecting vasomotor tone

A

-neurohumoral factors
-physical factors
-endothelial factors
-endocrine and paracrine factors
-metabolic factors

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27
Q

what are some neurohumoral factors affecting vasomotor tone

A

noreadrenaline
adrenaline
acetylcholine

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28
Q

what are some metabolic factors affecting vasomotor tone

A

adenosine
po2
pco2,H+,K+

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29
Q

what are some endothelial factors affecting vasomotor tone

A

ET
NO
PGI2
EDHF

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30
Q

what are some endocrine and paracrine factors affecting vasomotor tone

A

angiotensin ii
histamine
bradykinin
txa2
5-HT

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31
Q

what are some physical factors affecting vasomotor tone

A

arterial pressure
coronary pressure
RAP, LVDP,
systolic compression
diastolic compression

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32
Q

how does autoregulation of coronary blood flow work

A

constant balance of oxygen demand and supply
maintains constant flow at physiological blood pressure
allows for compensation example
post stenotic dilation
increased flow in exercise

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33
Q

how do neurohumoral factors affect coronary blood flow

A

sympathetic stimulation increases circulating catecholamines
beta adrenoreceptor activation causes increased heart rate and contractility
increased myocardial work releases co2 adenosine causes vasodilation
alpha adrenoceptor stimulation can cause vasoconstriction

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34
Q

how does ischaemic heart disease work

A

critical coronary artery stenosis: all mechanisms to increase flow are exhausted
supply can no longer meet demand
more than 50% stenosis can result in exertional angina
more than 80% can cause rest pain- unstable angina
angina pain mechanism:
adenosine, bradykinin release due to myocyte ischaemia
stimulates sympathetic afferents that synapse with spinothalamic tract

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35
Q

clinical scenario cerebral circulation
a 24 year old women is admitted with headache and nausea 1 hour after falling and hitting her head
she becomes drowsy confused and hypersensitive
an urgent ct head shows an extradural haematoma
she is intubated ventilated and sedated
her bp is maintained greater than 140 systolic
she is transferred to a neurosurgical theatre she

A

she is intubated ventilated and sedated to control paco2 and po2 and to reduce cmr (cerebral metabolic rate)
her bp is maintained greater than 140 systolic to maintain cerebral perfusion pressure
she is transferred to neurosurgical theatre to evacuate the haematoma and reduce intracranial pressure

36
Q

what is the cerebral blood flow

A

50ml/100g/min

37
Q

what organ has highest oxygen consumption

A

brain

38
Q

why does the brain only tolerate short periods of ischaemia

A

brain is not capable of anaerobic respiration

39
Q

why must cerebral blood flow be maintained for

A

constant delivery of oxygen and nutrients
removal of metabolites

40
Q

what is the cerebrospinal fluid volume

A

150ml

41
Q

what is the blood volume in the brain

A

150ml

42
Q

what is the brain parenchyma volume

A

1400ml 80%

43
Q

explain the cerebral anatomy

A

skull-rigid box of fixed volume
contains brain, cerebrospinal fluid and blood
-brain
-csf
blood
increase volume will increase the intracranial pressure

44
Q

what does cerebral perfusion pressure depend on

A

arteriovenous pressure gradient
cpp= mean arterial pressure- intracranial pressure

45
Q

what are the four determinants of cerebral blood flow

A

-autoregulation
-carbon dioxide and oxygen
-neurohumoral factors
-cerebral metabolism

46
Q

explain the cerebral blood flow graph and how chronic hypertension can change this

A

y axis cerebral blood flow
x axis mean arterial pressure
chronic hypertension shifts the graph to the right

47
Q

what is the main determinant of cerebral blood flow

A

-autoregulation
constant flow at physiological blood pressure
similar to coronary autoregulation
hypertension shifts the range

48
Q

how does co2 affect cerebral blood flow

A

within physiological co2 limits
flow is proportional to co2 concentration
increased pco2 increases h+ions
decreased ph causes direct vasodilation
enhances nitric oxide release
decreases intracellular ca2+

49
Q

within what range of paco2 is co2 concentration proportional to cerebral blood flow

A

30-80mmhg or 3-10 kpa

50
Q

what are physiological co2 limits

A

30-80mmhg

51
Q

how does oxygen affect cerebral blood flow

A

oxygen has no effects on cerebral blood flow within normal limits
hypoxia pao2 <55 mmhg 7.2kpa
causes arterioles to dilate which increases oxygen delivery
direct action on vascular smooth muscle
stimulates adenosine release
cerebral blood flow decreases in hypoxic conditions

52
Q

what does cmr mean

A

cerebral metabolic rate- the rate at which brain utilises metabolic substrates

53
Q

which organ has the highest metabolic requirement

A

brain

54
Q

what does increased metabolism lead to

A

increased metabolism is met by increased blood flow
multiple mediators eg H+ co2, adenosine k+ No

55
Q

local changes within seconds of increased cerebral activity

A

functional hyperaemia-increased blood flow
likely to be nitric oxide mediated

56
Q

how do neurohumoral factors affect cerebral blood flow

A

play a minor role during normal conditions
sympathetic activation leads to vasoconstriction
plays a role in shifting autoregulation curve to the right in hypertension
parasympathetic nerves contribute to vasodilation

57
Q

what is the monro kellie principle

A

the skull is a rigid box
increased volume of one component reduces another
overall increased volume increases pressure
causes systemic hypertension

58
Q

how are brain injuries managed

A

aims to prevent secondary injury
anesthetise and intubate
reduces metabolic demand
controls ventilation to maintain o2 and co2
head up tilt to enhance venous drainage
if brain tissue directly affected by trauma, autoregulation cannot occur

59
Q

if brain tissue directly affected by trauma what cannot occur

A

autoregulation

60
Q

clinical scenario skeletal circulation
a 78 year old man presents to the vascular clinic
he has a 3 year history of worsening pain in the left leg on exertion
he now gets pain even at rest and can barely walk
surgeon lists him for an angioplasty

A

3 year history of worsening pain in the left leg on exertion- intermittent claudication- ischaemia due to inadequate blood flow during exercise
he now gets pain even at rest and cannot walk
critical ischaemia- inadequate blood flow even at rest
angiplasty-balloon dilation of narrowed vessels (coronary or peripheral)
restores blood flow to reduce supply/demand mismatch

61
Q

what are the three determinants of the skeletal muscle circulation

A

neurohumoral control
extrinsic muscle contraction
metabolic control

62
Q

what is the skeletal muscle blood flow

A

1-2ml/100g/min

63
Q

during exercise how much does blood flow increase

A

by 50 fold

64
Q

what is the skeletal muscle blood flow during exercise

A

50-100ml/min/100g

65
Q

why does the blood flow increase need to occur quickly

A

fight or flight response

66
Q

during exercise blood flow to which organ is sacrificed

A

during exercise blood flow to other organs e.g the gut is sacrificed to allow for the increase in skeletal muscle blood flow

67
Q

what does skeletal muscle vascular make a huge contribution to

A

peripheral vascular resistance

68
Q

what is the main determinant of skeletal muscle blood flow at rest

A

sympathetic innervation
contributes greatly to total vascular resistance

69
Q

what do alpha 1 and alpha 2 receptors do

A

vasoconstriction

70
Q

what do beta 2 receptors do

A

vasodilation

71
Q

how does metabolic control affect skeletal muscle blood flow

A

during exercise local metabolic control becomes dominant
functional hyperaemia- increase blood flow associated with metabolic activity
at the start of exercise demand exceeds supply
lactate co2 and h+ accumulate
adenosine is produced and k+ channels open
all contributes to rapid vasodilation

72
Q

why does rapid vasodilation occur under metabolic control of skeletal muscle blood flow

A

to restore oxygen delivery
remove metabolites

73
Q

how does extrinsic muscle contraction affect skeletal muscle blood flow

A

co-ordinated contractions enhance blood flow
skeletal pump mechanism
ensures venous blood is sufficiently cleared
atroventricular pressure difference is maintained
during sustained contraction skeletal blood flow is interrupted or reduced
demand exceeds supply-fatigue

74
Q

what happens when demand exceeds supply

A

fatigue

75
Q

what is claudication

A

peripheral vascular disease
muscle ischaemia on walking
collateral flow sufficient at rest
during exercise demand>supply
pain caused by muscle ischaemia
rest pain-critical ischaemia

76
Q

at normal body temperature what is the skin flow

A

5ml/min/100g

77
Q

what is the skin blood flow in cold environments

A

3ml/min/100g

78
Q

what is the skin blood flow in hot environments

A

7-8ml/min/100g

79
Q

what is the key role of skin circulation

A

thermoregulation

80
Q

what are arteriovenous anastomoses

A

av anastomoses shunt blood from arterioles to venules bypassing capillaries
greatest number of arteriovenous anastomoses in hands toes ears nose lips

81
Q

what is the determinants of skin blood flow

A

neurohumoral control
local metabolic control

82
Q

how does neurohumoral control affect skin blood flow

A

on exposure to cold, peripheral blood cools
this then circulates to thermoregulatory centre in posterior hypothalamus
sympathetic activity is increased
skin arterioles have numerous alpha2 receptors
alpha 2 receptors these constrict and blood is diverted through av anastomoses
reduces superficial capillary blood and therefore heat loss

83
Q

how does local metabolic control affect skin blood flow

A

hot environments- sweat glands activated
activated sweat glands stimulate bradykinin production-vasodilation
increased blood flow through capillary beds-heat loss

84
Q

what is raynauds syndrome

A

excessive sympathetic response to cold
on exposure to cold alpha2 receptors become more sensitive
their vasoconstrictor effect seems to be enhanced

85
Q

what are some key points about circulations

A

in coronary and cerebral circulations autoregulation and local metabolic control are the major factors influencing blood flow
in contrast neurohumoral control has the greatest control in resting skeletal muscle and skin circulations
in all tissues, if oxygen demand exceeds supply by blood flow tissue ischaemia occurs
skeletal muscle contraction enhances blood flow
arteriovenous anastomoses in the skin have a key role in thermoregulation
clinical practice requires knowledge of normal physiology and how it is deranged by disease

86
Q

`

A
87
Q
A

mbbs year 1 module 3 circulation and breathing (10 decks)

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