Lecture 5.2: Altitude and Exercise Flashcards
What is Hypercapnia?
• It is when the concentration of carbon dioxide (CO2) in the bloodstream rises
above a certain level
• Partial pressure of carbon dioxide rises, partial pressure of oxygen falls
What is Hypoxia?
A state in which oxygen is not available in sufficient amounts at the tissue level to maintain adequate homeostasis
What is Persisting Hypercapnia?
• As disease progresses CO2 retained because of inadequate ventilation
• pCO2 now chronically elevated
• Now changes in CSF pH is compensated by the choroid plexus cells retaining
HCO3
• Central chemoreceptors therefore ‘reset’
•Ventilation no longer stimulated
What can Persisting Hypercapnia lead to?
• Normally rises in pCO2 lead to drive to increased ventilation
• Produces breathlessness
• Which, initially may drive ventilation enough to keep pCO2 down and pO2 up
• May even produce hypocapnia
What is Hypocapnia?
Decrease in alveolar and blood carbon dioxide (CO2) levels below the normal reference range of 35 mmHg
Persisting Hypercapnia (from Cerebrospinal Fluid [CSF] pH perspective)
• As disease progresses CO2 retained because of inadequate ventilation
• pCO2 now chronically elevated
• Now changes in CSF pH is compensated by the choroid plexus cells retaining
HCO3-
• Central chemoreceptors therefore ‘reset’
• Ventilation no longer stimulated
What organ compensates systemic acid base balance and how?
• Kidneys
• By retaining HCO3-
Why can excess oxygen lead to Hypercapnia?
• High O2 causes respiratory depression
• Thus decrease in breathing rate
• Thus very high CO2 as hypoventilation
What are the Symptoms of Acute Hypoxia? (5)
• Initially Pleasant Euphoria
• Nausea
• Lack of Coordination
• Unconsciousness
• Death
Acute Exposure to High Altitude: Effects (20,000 and 29,000ft)
• Above 20,000ft acute exposure leads rapidly to
unconsciousness
• Exposure at 29,000ft leads to unconsciousness in 40 sec
What is Acute Hypoxia detected by?
• Acute hypoxia detected by peripheral chemoreceptors
What happens to respiratory control in acute exposure?
• Try to increase breathing
• As ventilation increases pCO2 falls
• CSF becomes alkaline
• Alkaline CSF inhibits central chemoreceptors
• Counteracts hypoxic drive
What happens when hypoxic drive is counteracted?
• System Trapped!
• Breathe More = Die from Alkalosis
• Don’t Breathe More = Die from Hypoxia
• System Gives Up = You Die
Initial Effects of Gradual Exposure
• Mild hypoxia stimulates ventilation enough to raise CSF pH slightly
• Choroid plexus cells respond by exporting HCO3- from CSF
• CSF pH corrected, so hypoxic drive now expressed
• Subject breathes more
Effects of Longer Term Acclimatisation
• Oxygen carrying capacity of blood increased (polycythaemia, 2,3 DPG)
• Cardiac output increased
• Cardiac output directed to vital organs
• Systemic acid/base balance corrected
