Altitude Lecture 4

Question 1

Describe hypoxia due to altitude in humans

Answer 1

Tolerant until 60mmHg or 8kPa of oxygen in blood- when desaturation starts

Symptoms start to appear when blood is less than 90% saturated eg. Loss of visual acuity then postural stability and recall and reaction time decreases under 80%

Question 2

What are the acute responses to altitude?

Answer 2

Low PO2 stimulates peripheral chemoreceptors leading to ventilation (which is opposed to varying degrees by hypoxia depressing respiratory centres)

Hyperventilation causes a fall in PaCO2➡ hypocapnia and rise in pH (respiratory alkalosis)➡ inhibition of peripheral and central chemoreceptors➡ slight fall in total ventilation- reduces the response to hypoxia

Fall in CO2 also means that oxygen delivery to tissues is not as efficient as Hb affinity for oxygen increases

Loss of CO2 can also cause Cheyne-Stokes respiration while asleep

However hyperventilation is necessary to reduce the risk of severe hypoxia by decreasing the space taken up by CO2 in the alveoli and keeps the partial pressure of the gases as high as possible for effective diffusion

Question 3

Describe the cardiovascular response to altitude

Answer 3

• Suppression of vagal activity by increased respiratory activity
• This increases HR and CO
• Reduced peripheral resistance raises tissue perfusion

Cerebral blood flow- vasoconstriction due to the altitude induced hyperventilation- very responsive to CO2 but not O2- severe hypoxia leads to vssodilation due to Adenosine, K+ and NO

Pulmonary circulation- hypoxic vasoconstriction helps V/Q- promotes blood flow to the best alveoli

Question 4

What are cerebral and pulmonary responses to hypoxia?

Answer 4

Cerebral blood flow- vasoconstriction due to the altitude induced hyperventilation- very responsive to CO2 but not O2- severe hypoxia leads to vssodilation due to Adenosine, K+ and NO

Pulmonary circulation- hypoxic vasoconstriction helps V/Q- promotes blood flow to the best alveoli

Question 5

What is mountain sickness?

Answer 5

Headache above 2500m plus: dizziness, irritability, vomiting, nausea, sweating, breathlessness, insomnia, fatigue

Caused by:

Thought to be caused by cerebral oedema- hypoxia dilation increases cerebral capillary filtration pressure and hypoxia induced increased permeability

Pulmonary oedema- uneven hypoxia vasoconstriction of pulmonary vessels leading to pulmonary hypertension, increased capillary permeability- protein leakage

Treated by supplementary oxygen and reducing altitude

Question 6

How is pH restored to normal in high altitude?

Answer 6

Adaptive and alcclimatization - Body compensates for low PO2 and improve O2 delivery to tissues

Hyperventilation increases again as the altered pH is restored and peripheral chemoreceptor sensitivity increases

1. Cerebral pH is restored

pH of CSF decreases

2. Decreased reabsorption of HCO3- and decreased H+ secretion in urine restores plasma pH

Metabolic compensation for respiratory alkalosis takes 2-3 weeks

Question 7

What does chronic hypoxia lead to?

Answer 7

• Leads to 2,3-DPG production -> causes right shift leading to decreased O2 affinity
• Increased conc. of cyt. oxidase so mitochondria utilise O2 more effectively
• Increased myoglobin in skeletal muscle raises O2 reserve store
• Chronic hypoxia activates hypoxia-inducible factor

Causes:

• Angiogenesis
• Erythropoiesis (HCT increases to 55% - this leads to increased viscosity)
• Carotid body sensitivity adjustments

Ability to acclimatise is genetically determined

Also the O2 carrying capacity in blood is increased by increased EPO, increased RBC count and Hb conc
More pulmonary capillaries open

HR increases but SV decreases so CO is restored
Angiogenesis, increased cytochrome oxidase, increased myoglobin content in sketal muscle in tissues