Acclimitisation Part 2 Flashcards
What are the early responses of the pulmonary circulation to hypoxia?
Diffuse hypoxic vasoconstriction causes direct action on pulmonary vessels independent of hormonal or neural mechanisms
How does diffuse hypoxic vasoconstriction effect the circulation?
Increased pulmonary arterial pressure and right ventricle workload
How does V/Q change at high altitude?
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
Increased PAP and pulmonary vascular resistance is due to what two mechanisms?
Diffuse hypoxic vasoconstriction & pulmonary arteriolar remodelling
What changes occur to the pulmonary vasculature during hypoxia?
Increased SM media, thicker adventitia, inward remodelling
What is obliterative vascular remodelling?
Affects 1-5% population, aggresive remodelling of vessels causing increased SM, adventitia and resistance leading to narrowed lumen and right heart failure
What are the responses to sustained hypoxia?
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
What is acute mountain sickness?
In susceptible individuals, progressive increase in resistance and PAP leads to right hear failure and death
What is polycythaemia and what causes it?
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
What contributes to increased haematocrit?
Early reduction in plasma volume causes early increase in Hct as well as the later increase in erythropoesis
Why is haematocrit important for exercise performance?
Decreased Hct reduces exercise performance whereas too high Hct decreases performance
Optimum Hct needed to balance viscosity
Optimum - where viscosity outweighs oxygen carriage
Does blood transfusion before ascent increase performance?
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)
What are the affects of reducing Hb on exercise capacity?
Dilution of Hb in acclimitised lowlanders did not reduce maximal oxygen consumption (exercise capacity)
What is the effect of respiratory alkalosis on oxygen saturation?
Respiratory alkalosis causes a leftward shift leading to increased affinity of Hb for O2 and higher oxygen content at any given PO2
What does acclimitisation do to the Hb-O2 curve?
Left shift means higher affinity of O2 for Hb making it harder to unload O2 to the tissues (disadvantage)
How does renal compensation affect Hb-O2 curve?
Leftward shift due to respiratory alkalosis, increased BPG and renal pH correction return curve to normal position
How is the cardiovascular system affected by hypoxia?
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
What are the effects of sustained hypoxia on TPR?
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)
What are the effects of sustained hypoxia on cardiac output?
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
What are the effects of hypoxia on systemic capillary vessels?
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
Why is muscle wasting at high altitude an advantage?
Decreased body weight at altitude decreases the distance between capillaries and therefore shorter distance for oxygen to travel
Tissue changes at moderate altitudes
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
Tissue changes at extreme altitudes (> 5000m)
Progressive wasting and decreased muscle mass (advantageous)
Decreased O2 needed for function
No increase in enzymes of TCA cycle or ETC
Ways to reduce oxygen requirement
Reduce activity
Reduce workrate when active
Reduce weight - altitude is potent anorexigen
