W6 - Altitude Flashcards
1
Q
Describe change to barometric pressure with altitude?
A
Decreases exponentially as altitude increases
Approx halved every 5450m ascent
2
Q
What is change to PO2 at altitude?
A
Alveolar and arterial PO2 fall
Causing hypoxic hypoxia
3
Q
Why is decrease in arterial PO2 greater than decrease in ambient PO2?
A
Inspired air is always saturated with water vapour (47 mmHg)
4
Q
What is PO2 of moist inspired air at 5450m?
A
PO2 = 380-47 x 0.21 = 70 mmHg
5
Q
What is PO2 at Mt Everest summit?
A
Altitude 8848m, P=253mmHg
PO2 = 43 mmHg
6
Q
Describe pressure and altitude experienced during air travel.
A
Modern aircraft travel at altitude of 10-11km
Passengers not exposed to this
Cabin is maintained at pressure equivalent to altitude of 1900-2500m
7
Q
What happens if cabin pressurisation system fails?
A
Rapid decompression
Hypoxia is so severe - causes unconsciousness and death
Useful consciousness lasts for 40s
8
Q
What is AMS?
A
Acute mountain sickness
9
Q
When does AMS occur?
A
Rapid exposure to altitudes 3000-6000m
10
Q
What are symptoms of rapid exposure to 4000m?
A
Hypoxia causes cerebral hypoxia
Deterioration of sensory acuity, judgement, speed of response, manual dexterity
11
Q
What are the 4 steps of oxygen cascade?
A
Alveolar ventilation
Pulmonary oxygen diffusion
Transport of O2 in blood
Tissue diffusion
12
Q
Describe change in PO2 during oxygen cascade?
A
Decrease in PO2 at each stage
Total pressure gradient from inspired air to mixed venous blood of 120 mmHg
13
Q
What is effect of acclimitisation to altitude on O2 cascade?
A
Decrease in slope of oxygen cascade so mixed venous PO2 is little changed
14
Q
What is LAN?
A
Lower altitude native
15
Q
What is HAN
A
High altitude native
16
Q
Is acclimitisation the same in LAN as HAN?
A
Acclimitisation seen in longer term visitors is different from acquired acclimitisation seen in person born and bred at altitude
17
Q
What causes decrease in slope of O2 cascade niacclimitisation?
A
Reduction in PO2 difference between inspired and alveolar air
18
Q
What is effect of altitude > 3000m on newly arrived visitor?
A
Hyperventilation
As alveolar PO2 falls to 65 mmHg
19
Q
What altitude does hyperventilation reach maximum?
A
6000m
20
Q
What causes hyperventilation at altitude?
A
Stimulation of peripheral chemoreceptors of carotid and aortic bodies by fall in arterial PO2
21
Q
What is effect of hyperventilation?
A
Increases alveolar ventilation
Reduces fall in PaO2, PalvO2
Increased loss of CO2
Decrease PaCO2 and PalvCO2
Respiratory alkalosis, arterial pH increase
22
Q
What causes respiratory alkalosis?
A
Hyperventilation
23
Q
When does arterial pH rise?
A
Hyperventilation causes decrease in PCO2 in alveolar and arterial blood
24
Q
What is effect of hyperventilation on CSF?
A
PaCO2 causes fall in PCO2 in CSF
Inhibits ventilation
25
What are the competing influences during initial exposure to hypoxic conditions
Fall in PaO2 stimulating ventilation
Fall in PaCO2 (drop in [H+]) inhibiting ventilation
26
Why is increase in ventilation not seen below 3000m altitude?
Competing influences of PaO2 and PaCO2 both dropping
27
What happens at altitudes between 3000-6000m to ventilation?
Drive to ventilation from peripheral chemoreceptors is greater than brake from central chemoreceptors
PaO2 falls to 60mmHg
28
How long does it take for secondary rise in ventilation to reach maximum increase?
1-2 weeks
Stays as long as visitor stays in altitude
29
What compensates for the decrease in PaCO2?
Arterial respiratory alkalosis is compensated for by renal excretion of HCO3-
Too slow to account for rate at which ventilation rises
30
What happens to sensitivity to CO2 once acclimitisation has occurred?
Respiratory system is more sensitive to CO2 so respiratory centres respond to lower PCO2
Respiratory system behaves as through been reset to run at lower PaCO2
31
Compare ventilation rate between HAN and LAN.
HANs have ventilation rate that is 20% less than in acclimatised visitors
32
Why do HAN have lower ventilation rate?
Reduced pulmonary work decreases energy and O2 demands
HAN uses less energy consuming methods to compensate for hypoxic conditions - efficient pulmonary gas exchange
33
What is effect of increased ventilation on O2?
Increases PaO2 and incr driving pressure for oxygen transfer to pulmonary capillary blood
34
How is pulmonary O2 diffusing capacity increased?
Increase alveolar surface area in contact with functioning pulmonary capillaries
35
Describe pulmonary diffusing capacity in HAN?
Capacity in HAN is greater at rest (20-30%)
Much reduced PaO2 - PalvO2 gradient
36
What are characteristics of HANs?
Barrel shaped chest with incr lung volumes (esp residual volumes)
Alveoli are larger and greater in number than LANs of similar stature
37
Describe pulmonary blood volume in HANs.
Increases in pulmonary blood volume
Increased diffusing capacity
38
Describe pulmonary arterial pressure in HANs.
Increase in pulmonary arterial pressure leads to opening of larger number of pulmonary capillaries = recruitment
39
Describe erythrocyte count in HAN and LAN visitor.
Increase in erythrocyte count = polycythaemia
40
What is polycythaemia?
Increase in erythrocyte (RBC) count
41
What is impact of altitude on haemoglobin?
Concentration in Hb rises due to incr in erythrocyte count
Small proportion of rise is due to small fall in plasma volume
Increases linearly up to 3700m but then increases more rapidly
42
What causes increase in Hb?
Increased secretion of EPO resulting from decrease in PaO2 detected by kidneys
43
Describe timing of Hb increase.
Lag between EPO and RBC production
RBC does not start to rise till 3-5d of altitude exposure
Takes weeks to develop
44
What is effect of increased Hb on oxygen?
Increases oxygen carrying capacity of blood
Ensures arterial oxygen content is maintained at/above value at sea level
If SaO2 falls below 70%, arterial oxygen content declines
45
What is SaO2?
Arterial oxygen saturation
Saturation of haemoglobin
46
What happens to CO and circulation in visitors at altitude during first few hours/days?
Increase in HR
Variable stroke volume resulting in +- increase in cardiac output
47
Cardiac output formula
CO = HR x SV
48
What is effect of cardiac output on visitors at altitude after several days?
CO at rest is equal to/slightly above that at seal level
HR remains elevated, SV decreased
49
Describe blood flow distribution in visitors at altitude?
Redistribution of blood flow so that vital organs receive larger proportion of CO than at sea level
Distributed away from gut and skin
Coronary blood flow increase on first exposure to altitude (vasodilatory effect of hypoxia)
50
What happens to coronary blood flow after 10 days at altitude by LAN?
Coronary blood flow fell by 30%
Oxygen uptake by myocardium maintained by 28% increase in O2 extraction
51
Describe coronary blood flow in HAN?
Diminished by 30%
Partly compensated by increased vascularisation of myocardium
52
What is P50 of O2-Hb curve at sea level?
50% haemoglobin saturated at PO2 of 26-28 mmHg
53
Describe change in O2-Hb curve at altitude of acclimitised visitor or HAN.
Right shift of O2-Hb curve
Rise in P50 (31mmHg after 3 days of altitude)
54
What is impact of right shift in O2-Hb curve?
Aids unloading of oxygen from blood in tissues
Attributed to increased concentration of 2,3-BPG in erythrocytes (induced by respiratory alkalosis)
Hinders O2 loading
55
Describe the studies that indicate respiratory alkalosis is not fully compensated for at altitude.
Blood remains slightly alkaline
Cause decrease in P50 = left shift of O2-Hb curve
Some studies so no change in P50
56
How does oxygen move from capillary blood to mitochondria?
Simple diffusion
Rate is determined by PO2 gradient and distance, SA over which diffusion occurs (Fick's law)
57
What happens to tissues at altitude?
Increased capillarisation of tissues
Aids oxygen diffusion to tissues by decreasing diffusion distance
58
Describe density of functional capillaries when acclimatised at altitude.
Increased density of functional capillaries observed in mammalian species acclimatised at altitude especially in cerebral cortex, myocardium, skeletal muscle
59
Describe relationship between capillary density, muscle fibre and body weight.
Inverse relationship between capillary density and muscle fibre CSA
Linear relationship between muscle fibre CSA and body weight
Therefore decreased body weight = increased capillary density
60
How do some animals adapt rather than acclimatise?
Compensation for hypoxia by increasing extraction of oxygen from blood at tissues
More efficient utilisation of oxygen
Left ward shift of O2-Hb curve
61
What is the main adaptive process which allowed ascent to extreme altitudes?
Increase in ventilation
62
Describe relationship between CO and O2 consumption during exercise at altitude.
Normal linear relationship between CO and O2 consumption
Maximal cardiac outputs are decreased
VO2max never reaches value obtained at sea level
Diffusion limitation across alveolar capillary membranes
Increasing altitude = progressive fall in SaO2 during exercise
63
Describe lactate paradox in HANs during maximal exercise at altitude.
During maximal exercise at altitude, rise in blood lactate concentration is not as great as in maximal exercise at sea level
In HAN, lactate paradox persists for 6 weeks at sea level suggesting developmental/genetic change
Decreased blood lactate due to decreased release of lactate by exercising muscle
64
What does alveolar hypoxia induce?
Induces vasoconstriction in small arteries/arterioles of pulmonary circulation
Leads to incr in pulmonary vasc resistance = pulmonary arterial hypertension
65
Which type of people have mild pulmonary hypertension at rest and marked at exercise?
Long term visitor at altitude and HANs
66
Describe children born at altitude.
Generally low birth weight, reduced in body size and growth rate at all postnatal ages
Due to hypoxia, poor nutritional status, socio-economic conditions
67
Why do children born at altitude and HANs don't have reduced lung size?
Growth of lungs is stimulated by hypoxia
Larger lung volume and vital capacity = advantage of oxygen diffusing capacity
Children of sea level ancestry raised at altitude also have increased lung capacity
68
Describe effect of altitude on foetus.
Predicted: foetus at altitude will suffer hypoxia
Maternal hyperventilation & incr Hb concentration found during pregnancy so arterial oxygen content was as high as pregnant women at sea level
69
What changes are visible in placental morphology at altitude?
Increased vascularity, shortening of diffusion distances, reduction in oxygen diffusion gradient between mother and foetus
70
What hormones are secreted during hypoxia?
Stress hormones; adrenal glucocorticoids, catecholamines
71
Describe thyroid hormone levels of long term visitors and HANs.
Increased T4, T3 levels
72
Describe impact of altitude on aldosterone and ADH.
Increased RBC and blood volume result in decreased secretion of aldosterone (decr Na+) and vasopressin/ADH (decr H2O)
Increased natriuresis (Na+ secretion) and diuresis (water secretion)
73
List the 4 pathophysiological changes that can happen at altitude.
Acute mountain sickness
High altitude pulmonary oedema
High altitude cerebral oedema
Chronic mountain sickness
74
When does AMS occur and what are symptoms?
People who ascend to \>2500m rapidly
Headache, nausea, vomit, insomnia, poor appetite, muscle weakness
Appears in 8-24h but can be delayed by 4d
75
What is prevalence of AMS?
3000m, 30% show symptoms
\>4500m, 75% show symptoms
76
When do AMS symptoms decline?
3-7 days
77
What is primary stimulus to AMS?
Hypoxic conditions
78
Describe role of fluid retention in AMS.
On ascent, increased secretion of ADH + adrenal corticoids = fluid retention
Blood shunted from periphery, accumulation of fluid in lungs, GIT, brain
Organs become mildly oedematous
79
How do you prevent AMS?
Avoiding too rapid ascent;
300m/day for 3000-4270m
150m/day for 4270m +
Drugs: acetazolamide (carbonic anhydrase inhibitor), dexamethasone (synthetic glucocorticoid)
80
How do glucocorticoids prevent AMS?
Correcting respiratory alkalosis which develops on first exposure to altitude allowing acclimatisation to occur more rapidly
81
What is the remedy for AMS?
Descent to lower altitude or breathe oxygen
82
What are symptoms of pulmonary oedema?
Dyspnoea (difficult breathing), dry cough
Severe: foaming pink sputum is coughed up
Can be fatal
83
What are predisposing factors for HAPO?
Rapid ascent
Subjects who exercise in cold
Frequent in younger subjects
84
What causes HAPO?
Hypoxic vasoconstriction of pulmonary arteries/arterioles = pulmonary hypertension
Increased filtration of fluid at pulmonary capillaries
Incr in pulmonary capillary permeability and inflammatory response
85
What is treatment for HAPO?
Oxygen therapy
Descent to lower altitude
86
What is HACO?
High altitude cerebral oedema
87
Describe HACO?
Potentially fatal
Develops within hours/days in individuals with AMS
Increased intercranial pressure = coma, death if untreated
88
What are symptoms of HACO?
Severe weakness and fatigue, confusion, impaired mental processing, breathlessness, unbalanced walk, possibly loss of consciousness
89
What is incidence of HACO?
1% of people exposed to altitudes over 2700m
90
What causes HACO?
Movement of fluid and protein from vascular component across BBB due to cerebral vasodilatation and increased capillary pressure
Increased CSF volume distorts brain structures (esp white matter)
91
What is Monge's disease?
Chronic mountain sickness
92
What population gets Monge's?
HAN, middle-aged men particularly
93
What is treatment for Monge's?
None except move to lower altitude
94
Describe difference between AMS and Monge's.
Monge's is not form of prolonged AMS
Symptoms are associated with changes akin to patient losing acclimatisation to altitude
95
What happens in Monge's disease?
Minute ventilation falls = fall in PaO2
Stimulates erythropoeisis so haematocrit rises to high values (80%) with cyanosis
Pulmonary arterial hypertension becomes exaggerated
96
What is the theory of what causes Monge's disease?
Not fully understood, maybe related to ageing process
