Lecture 23: Hypoxia Flashcards
What is hypoxia?
Low oxygen content
What is hypoxaemia?
Low PaO2
___________ produces ________ but the converse is not true
Hypoxaemia produces Hypoxia
What is cyanosis?
Blue colour from deoxygenated Hb that occurs in tissue hypoxia and hypoxaemic hypoxia
What are 5 causes of hypoxaemia?
- Low PiO2 (inspired)
- Hypoventilation
- Diffusion limitation
- Right-Left Shunt
- V’/Q’ mismatch
___% of people who drown don’t get water in the lungs. Instead they die because of ___________.
60
Laryngeospasm
What happens when you get fresh water in the lungs?
- Fresh water is hypotonic and so rapidly diffuses and causes epithelial cell rupture
- However the fluid gets absorbed by the blood
- Eventually leads to haemolysis
- Dilution of the blood
What happens when you get salt water in the lungs?
- Salt water is Hypertonic and thus draw water out of the cells causing oedema
- Causes electrolyte issues due to electrolyte absorption
- Slows or halts diffusion of gases
- reduces ventilation because it’s filled with fluid
What causes hypoventilation?
- Respiratory muscle failure
- Restrictive disease (↓ compliance)
- Obstructive disease (↑ Resistance)
Hypoventilation makes you _________ and _________.
Hypoxic and Hypercapnic
Alveolar CO2 is proportional to _________
PACO2 ∝ V’CO2/V’A
During hypoventilation PAO2 always ______ and is _______ to PaO2
Falls
PAO2 equal to PaO2
During hypoventilation PACO2 always ________ and is ______ to PaCO2
Increases
PACO2 equal to PaCO2
What is the Alveolar Gas Equation
PAO2 = PiO2 - PACO2/RQ
How is RQ calculated?
V’CO2/V’O2
What does the alveolar gas equation predict?
The change in PaCO2 from PaCO2
What can we do to the alveolar gas equation if gas exchange is normal?
We can substitute PAO2 = PaO2 and substitute PACO2 = PaCO2
How do we calculate the Alveolar-Arterial O2 difference?
AaDO2 = PAO2 (calculated) - PaO2 (measured)
What does an AaDO2>15mmHg suggest?
Abnormality with gas exchange
How do we calculate Pressure of inspired O2 (PiO2)?
Fraction of O2 x (Barometric pressure - water pressure)
What is a shunt?
Blood is bypassing the lung or alveoli
What are the causes of an AaDO2>15mmHg?
Shunt
V’/Q’ mismatch
Diffusion (D[small L])
If AaDO2 is normal then the cause of the problem is ______________
Hypoventilation
