Acclimitisation Part 2

Question 1

What are the early responses of the pulmonary circulation to hypoxia?

Answer 1

Diffuse hypoxic vasoconstriction causes direct action on pulmonary vessels independent of hormonal or neural mechanisms

Question 2

How does diffuse hypoxic vasoconstriction effect the circulation?

Answer 2

Increased pulmonary arterial pressure and right ventricle workload

Question 3

How does V/Q change at high altitude?

Answer 3

At sea-level, blood flow greater to the base of the lungs due to increased resistance at apex (and gravity)

At high altitude, decreased resistance at apex increases blood flow to give an even distribution from top to bottom

Question 4

Increased PAP and pulmonary vascular resistance is due to what two mechanisms?

Answer 4

Diffuse hypoxic vasoconstriction & pulmonary arteriolar remodelling

Question 5

What changes occur to the pulmonary vasculature during hypoxia?

Answer 5

Increased SM media, thicker adventitia, inward remodelling

Question 6

What is obliterative vascular remodelling?

Answer 6

Affects 1-5% population, aggresive remodelling of vessels causing increased SM, adventitia and resistance leading to narrowed lumen and right heart failure

Question 7

What are the responses to sustained hypoxia?

Answer 7

Pulmonary vascular remodelling and hypoxic vasoconstriction with sustained PVR lasting up to 2 years

Sustained increase in pulmonary artery pressure causes right ventricular hypertrophy from which most individuals stabilise

Question 8

What is acute mountain sickness?

Answer 8

In susceptible individuals, progressive increase in resistance and PAP leads to right hear failure and death

Question 9

What is polycythaemia and what causes it?

Answer 9

Reduced PaO2 due to high altitude increases RBC production by kidney increasing Hb content of blood and allowing for greater oxygen delivery

Disadvantage: increased blood viscosity which increases resistance

Question 10

What contributes to increased haematocrit?

Answer 10

Early reduction in plasma volume causes early increase in Hct as well as the later increase in erythropoesis

Question 11

Why is haematocrit important for exercise performance?

Answer 11

Decreased Hct reduces exercise performance whereas too high Hct decreases performance

Optimum Hct needed to balance viscosity

Optimum - where viscosity outweighs oxygen carriage

Question 12

Does blood transfusion before ascent increase performance?

Answer 12

Autologous blood transfusion one day before ascent to high altitude (4300m) did not improve exercise performance compared to normal saline

(Sudden increase in Hb not an advantage)

Question 13

What are the affects of reducing Hb on exercise capacity?

Answer 13

Dilution of Hb in acclimitised lowlanders did not reduce maximal oxygen consumption (exercise capacity)

Question 14

What is the effect of respiratory alkalosis on oxygen saturation?

Answer 14

Respiratory alkalosis causes a leftward shift leading to increased affinity of Hb for O2 and higher oxygen content at any given PO2

Question 15

What does acclimitisation do to the Hb-O2 curve?

Answer 15

Left shift means higher affinity of O2 for Hb making it harder to unload O2 to the tissues (disadvantage)

Question 16

How does renal compensation affect Hb-O2 curve?

Answer 16

Leftward shift due to respiratory alkalosis, increased BPG and renal pH correction return curve to normal position

Question 17

How is the cardiovascular system affected by hypoxia?

Answer 17

Immediate effects include tachycardia, increased cardiac output, reduced resistance

Direct vasodilator action on blood vessels partially antagonised by increased SNS activity

Blood pressure maintained at normal levels

Initial responses advantageous to hypoxia onset

Question 18

What are the effects of sustained hypoxia on TPR?

Answer 18

Increased systemic blood pressure and total peripheral resistance

Sustained increase in sympathetic nerve activity antagonises direct vasodilator action of hypoxia

Persists for an extended period after return to sea level (days/weeks)

Question 19

What are the effects of sustained hypoxia on cardiac output?

Answer 19

Early increased cardiac output at rest at any given exercise workload

Following acclimitisation, cardiac output at rest and at submaximal exercise returns to sea level values

HR increased and SV reduced

Myocardial function maintained at sea level values

Question 20

What are the effects of hypoxia on systemic capillary vessels?

Answer 20

Increased capillary density in brain, retina, liver skeletal muscle

Potential advantage of capillary angiogenesis
> increased surface area of capillary endothelium
> reduced diffusing distance
> slowed blood velocity at constant total flow rate

Question 21

Why is muscle wasting at high altitude an advantage?

Answer 21

Decreased body weight at altitude decreases the distance between capillaries and therefore shorter distance for oxygen to travel

Question 22

Tissue changes at moderate altitudes

Answer 22

Increased myoglobin which increases O2 solubility within cytosol and increases diffusion to mitochondria

Increased enzymes of glycolytic pathway, TCA cycle and ETC

Increased mitochondrial density

Question 23

Tissue changes at extreme altitudes (> 5000m)

Answer 23

Progressive wasting and decreased muscle mass (advantageous)

Decreased O2 needed for function

No increase in enzymes of TCA cycle or ETC

Question 24

Ways to reduce oxygen requirement

Answer 24

Reduce activity
Reduce workrate when active
Reduce weight - altitude is potent anorexigen