Hypoxia Flashcards
What is meant by hypoxia
Describes a specific
environment
Specifically the PO2 in that environment
Hypoxia is a set of conditions and all the conditions of the person
What is meant by hypoxaemia
Describes the blood environment
Specifically the PaO2
What is meant by ischaemia
Describes tissues receiving inadequate oxygen
e.g. forearm ischaemia
What factors can put the body under hypoxic stress
Hypoxic stress: can be brought on by altitude, exercise and disease e.g. COPD
Transient hypoxic state in exercise is well dealt with in healthy individuals, but may be dangerous in unhealthy individuals.
How can the body respond to hypoxic stress
The body can adapt and compensate for hypoxic circumstances to maintain oxygen delivery
How does the PaO2 reaching the tissues change with age
It decreases with every decade
Mean PO2 in the alveolar space and in the arterial blood decreases with age as seen in the chart
Summarise oxygen transport
§ PO2 drops further into the lungs as mixing and humidification occurs.
§ The bronchial drainage (circulation) adds to the return of blood to the heart making the blood reduce from 100% saturation to 97% saturation.
Summarise the oxygen dissociation curve
§ A range of partial pressures at levels of saturation of oxygen in the blood.
§ Blue line represents shifts due to increased metabolism (e.g. exercise).
o Increase acidity, hypercapnia, 2,3-DPG production etc.
§ P50 line indicates how much loading/unloading is occurring in the haemoglobin on different curves.
What does the oxygen cascade describe
The O2 cascade describes the decreasing oxygen tension from inspired air to respiring cells
What does Fick’s law of diffusion state
Fick’s law of diffusion states that flow rate is proportional to the pressure gradient
What can impact Fick’s law in the lungs
Structural disease reduces this area
Fluid in the alveolar sacks increases this thickness
Breathing hypoxic gas reduces this gradient
All reducing the oxygen gradient
Describe the oxygen cascade
You start with 21.3kPa of oxygen at atmospheric pressure and as you go down the levels, the kPa reduces
humidification reduces the partial pressure to 20.0kPa in the upper airways
mixing reduces the partial pressure to 13.5kPa
There is then NO CHANGE between the alveolar air and post-alveolar capillaries.
Partial pressure decreases to 13.3kPa in the arteries due to bronchial drainage
the oxygen is then utilised (a-vO2 difference)
partial pressure decreases to 5.3kPa in tissues and veins
Describe what can alter the oxygen cascade
O2 therapy increases the partial pressure of oxygen in the ambient air, humid air decreases this
Hyperventilation increases the partial pressure in the alveoli, hypoventilation reduces this pressure
Partial pressure in the post-alveolar capillaries can decrease if there is a diffusion defect (Fick’s law)- less efficient diffusion
exercise decreases the partial pressure of oxygen and veins (as it is respired more) and thus the difference between arterial and venous PO2 increases and more oxygen is delivered to the tissues.
Describe the challenges to the oxygen cascade
Alveolar ventilation: Hyperventilation increases partial pressure in alveoli- increasing diffusion. Hypoventilation reduces this
V/Q matching- blockage in the respiratory tree- no air, but blood there, then no diffusion, lower PO2 in blood. If heart is working with the lungs it will send less blood to areas where no ventilation is taking place
Diffusion capacity (see factors affecting Fick’s law)
Cardiac output- adequate Q to move blood to the tissues- if not high enough, less blood delivered to tissues- lower pressure gradient
What is the role of unbound oxygen
The unbound oxygen (dissolved, not bound to iron) doesn’t deliver oxygen but conducts responses to hypoxaemia.
What happens when you breathe hypoxic air
If you are breathing hypoxic air, all the levels simply reduce
harder to maintain homeostasis
Describe consequences of a changing environment
Originally thought to be impossible to reach the summit of Everest without supplemental oxygen.
We now know this is just about our limit
Even breathing unpressurised 100% O2 is inadequate to maintain arterial saturation
Airplanes are pressurised to about 2000 m
‘Oxygen masks will fall in case of depressurisation
Boiling point is inversely proportional to altitude. ‘You can’t make a good cup of tea on Everest’
Eustace and Baumgarter wore pressurised suits to allow them to breathe (provided a viable pressure gradient) and to stop their bodily fluids from boiling
What is the Armstrong limit
Armstrong limit (H2O boils at 37°C) Boiling point is inversely proportional to altitude Around 20000m
Describe the challenges of high altitude
Hypoxia
Much less oxygen in the ambient air
Thermal stress
Freezing cold weather (-7 °C per 1000 m)
High wind-chill factor
Solar radiation
Less atmospheric screening
Reflection off snow
Hydration
Water lost humidifying inspired air
Hypoxia induced diuresis
Dangerous
Windy, unstable terrain, hypoxia-induced confusion and malcoordination