39 - High Altitude Physiology

Question 1

What is the effect on PaO2 at high altitudes?

Answer 1

Less paO2
Blood through lungs is less saturated
Pulmonary hypoxia and hypoxaemia

Question 2

Where are hypoxia detectors

Answer 2

Carotid bodies

Question 3

What causes decrease in ventilation at high altitudes

Answer 3

Excess blow of CO2
Causes alkalosis at central chemoreceptors
inhibits the increase in respiratory drive

Question 4

When is the hypoxic drive from carotid bodies significant?

Answer 4

PO2 below 60mmHg

Question 5

How is hypoxia worsened with rapid ascent to high altitude?

Answer 5

PaO2 in alveoli is low - pulmonary circulation VASOCONSTRICTS in hypoxia so worsens
= pulmonary artery hypertension

Question 6

When is acclimatisation rapid and how long does it take?

Answer 6

to 2000m - usually within a day a or two

Question 7

What happens up to 6000m

Answer 7

Fully acclimatised

well, reasonable appetite, normal sleep

Question 8

What happens above 7000m

Answer 8

Hypoxia present
Tired
lethargy
Hard to walk

Question 9

What happens above 7500m

Answer 9

Death Zone
Severe hypoxia
Physiological damage

Question 10

What are the 3 mechanisms in acclimatisation

Answer 10

1 - metabolic acidosis
2 - Increase in erythrocyte number
3 - Reduced pulmonary vascular resistance

Question 11

What causes the metabolic acidosis

Answer 11

Retention of acid

Increased excretion of bicarbonate in the kidney

Question 12

What causes respiratory alkalosis at high altitude

Answer 12

low PaO2
increased breathing rate
respiratory alkalosis
High pH
inhibits central chemoreceptors
Breathing decreases
Hypoxaemia

Question 13

How does the body acclimatise to oppose the respiratory alkalosis

Answer 13

Kidneys decrease proton excreting ATP-ase in the kidney tubules so kidneys decrease renal excretion of acid and increase excretion of bicarbonate
- restores PH to normal

Question 14

How is EPO production increased in acclimatisation? What is the result of this?

Answer 14

Hypoxaemia stimulates interstitial cells in the kidney
raise epo production
= increases the haematocrit
= increase the oxygen carrying capacity of the blood

Question 15

Why is there a limit to maximum haematocrit?

Answer 15

Increase in haematocrit increases blood viscosity
Increases the pulmonary vascular resistance
can cause pulmonary arterial hypertension and RHF

Question 16

Why does pulmonary vascular resistance fall?

Answer 16

Reduced hypoxic vasoconstriction

collateral circulations open up between pulmonary arteries and vens

Question 17

What mediates the decrease in hypoxic vasoconstriction?

Answer 17

Increased synthesis of nitric oxide in the pulmonary endothelium

Question 18

What is AMS

Answer 18

Acute Mountain Sickness

= The first sign that something is wrong

Question 19

HACE

Answer 19

High Altitude Cerebral Edema. Increased IC pressure. Can follow on from AMS if not treated. A serious neurological condition; fatal if not treated

Question 20

HAPE

Answer 20

High Altitude Pulmonary Edema. Equally serious pulmonary condition which can follow on from AMS.

Question 21

Signs and symptoms of AMS

Answer 21

Headache
Poor Sleep
Tiredness
Loss of appetite, nausea, vomiting
Dizziness

Question 22

How do you diagnose AMS

Answer 22

The signs and symptoms are scored 0-3 and a score of more than 3 is needed for diagnosis

Question 23

Would you get AMS between 1500-2000m

Answer 23

Unlikely

may get mild illness

Question 24

Would you get AMS at 2500m

Answer 24

1 in 5 people have some symptoms if they ascend from sea level within a day - most will acclimatise in a day

Question 25

Would you get AMS at 5000m

Answer 25

Everyone will be temporary ill if they ascend within a few hours to 5000m acclimatisation occurs but takes several days or more

Question 26

Treatment for AMS

Answer 26

IF mild then rest and no further ascent IF severe then descent, oxygen, acetazolamide 250mg tds 3 times a day Dexamethasone 4mg qds 4 times a day

Question 27

How do you prevent AMS

Answer 27

``` Slow ascent (<300m per day over 3000m) Avoid unnecessary exercise Acetazolamide 250g ```

Question 28

What does acetazolamide do

Answer 28

Carbonic anhydrase inhibitor - increased bicarbonate excretion producing metabolic acidosis compensates for the alkalosis caused by hyperventilation

Question 29

What is the effect of acetazolamide

Answer 29

less acid is secreted in the urine. The blood thus becomes acidified and respiration is stimulated. = Speeds up acclimatization process (read over the paragraph on this in notes, make sure u understand I cba to make qs about it)

Question 30

Why does High altitude cerebral oedema occur?

Answer 30

ATP decreases, Na pumps not working so Na leaks into nerve cells – pulls water = cell swelling

Question 31

Symptoms of brain swelling

Answer 31

Ataxia, nausea/hallucination, confusion, coma

Question 32

Treatment for HACE

Answer 32

Descend immediately, acetazolamide, oxygen, dexamethasone, hyperbaric chamber

Question 33

How does acetazolamide treat HACE

Answer 33

Reduces formation of CFS so reduces intracranial pressure

Question 34

How does high altitude pulmonary oedema occur

Answer 34

Pulmonary arterial hypertension - raised arterial and capillary pressure - fluid leaving blood and entering alveoli - worsens gas exchange

Question 35

Signs and symptoms of HAPO

Answer 35

Dysponea, reduced exercise tolerance, blood stained sputum, crackles on auscultation of chest

Question 36

Treatment of HAPO

Answer 36

``` Descend immediately Sit patient upright Oxygen Nifedipine Hyperbaric chamber Viagra - sildenafil ```

Question 37

What is the action of nifedipine

Answer 37

Calcium channel blocker | Prevent the pulmonary arteries contracting

Question 38

What is the action of the hyperbaric chamber

Answer 38

increases the PaO2 of oxygen to improve oxygenation of blood - reduces hypoxic vasoconstriction

Question 39

What is the action of viagra - sildenafil

Answer 39

Pulmonary hypoxic vasoconstriction due to lack of NO - slows down the breakdown of cyclic GMP (vasodilator) - increases cyclin GMP levels viagra relaxes pulmonary arteries and stops PAH and improves oxygenation of blood