hypoxia

Question 1

what is hypoxia

Answer 1

describes a specific environment

specifically the PO2 in that environment

Question 2

what is hypoxaemia

Answer 2

describes the blood environment

specifically the PaO2

Question 3

what is ischaemia

Answer 3

describes tissues receiving inadequate oxygen

eg forearm ischaemia

Question 4

what is the oxygen cascade?

Answer 4

the O2 cascade describes the decreasing oxygen tension from inspired air to respiring cells
fick’s law of diffusion states that flow rate is proportional to the pressure gradient

• structural disease reduces this area
• breathing hypoxic gas reduces this gradient
• fluid in the alveolar sacks increases this thickness

Question 5

Hypoxic stress ?

Answer 5

can be brought on by altitude, exercise and disease e.g. COPD

Question 6

how is the dissociation curve different for oxygen and carbon dioxide

Answer 6

Sigmoid dissociation curve for oxygen, linear for CO2

Question 7

what is altitude cascade?

Answer 7

Altitude cascade: reduces in ambient pressure reduces oxygen and gradient - harder to maintain homeostasis

Question 8

challenges to oxygen cascade:

Answer 8

Challenges to cascade:
Alveolar ventilation:
V/Q matching: if blockage in respiratory tree and are not ventilating but are perfusing then will not gain oxygen and will drop
Diffusion capacity: thickness of exchange surface will reduce oxygen gain
Cardiac output: need high Q to move blood to tissues

Question 9

Challenges of high altitude:

Answer 9

Challenges of high altitude:
Hypoxia: less oxygen in ambient air
Thermal stress: -7OC per 1000m; high wind-chill factor
Solar radiation: less atmospheric screening and reflection off snow
Hydration: water loss humidifying inspired air and induced diuresis
Dangerous: windy, unstable terrain, hypoxia-induced confusion and malcoordination

Question 10

Response to altitude:

Answer 10

Still 21% O2 but at lower partial pressure, decreasing PA/PaO2, which activates peripheral chemoreceptors (as opposed to central control using CO2)
Increased SNS outflow increases ventilation to increase alveolar oxygen and oxygen loading
Increased SNS will increase Q (HR/SV) to increase oxygen loading (and tissue delivery)
Hyperventilation leads to hypocapnia, reducing central drive to breathe, reducing ventilation and hence oxygen loading
CO2 loss increases pH, shifting ODC to left, increasing affinity decreasing oxygen unloading
Alkalosis produced by increased pH detected by carotid bodies, increasing bicarbonate secretion (and causing kidneys recover/save/manufacture acid) to normalise ODC and increasing oxygen unloading
Low blood oxygen increases erythropoietin production, increasing RBC production and oxygen loading
Other changes: oxidative enzyme/mitochondrial numbers increase to allow for greater oxygen utilisation to produce energy; small 2,3-DPG increase, causing shift to right and increased oxygen unloading

Question 11

prophylaxis

Answer 11

Prophylaxis
Acclimation: stimulated by artificial environments to lead to artificial acclimatisation (e.g. Hyperbaric chamber/breathing hypoxic gas)

Question 12

acetazolamide

Answer 12

Acetazolamide: carbonic anhydrase inhibitor, accelerates slow renal compensation to hypoxia induced hyperventilation

Question 13

native highlanders adaptations

Answer 13

Barrel chest: larger TLC, more alveoli and greater capillarisation
Increased Hct: greater oxygen carrying capacity
Larger heart: to pump through vasconstricted pulmonary circulation
Increased mitochondrial density: greater utilsation at cellular level