eLFH - Apnoeic Oxygenation and Differential Equations Flashcards
Rate of rise of PaCO2 in apnoea
0.3 - 0.7 kPa/min
Mechanical forces acting to maintain functional residual capacity
Thoracic cage tending to spring outwards
Lung tending to collapse inwards with its own weight and fluid surface tension in alveoli
How much CO2 passes across the alveolar membrane per minute in apnoea
Assumption of PACO2 = PaCO2
Rate if rise of PaCO2 is 0.5 kPa/min
Thus PACO2 rises by 0.5 kPa/min
Total Pi = 100 kPa so content of CO2 rises by 0.5% as per Dalton’s Law
Therefore, taking FRC as ~2L (for ease of maths), 10 ml CO2 crosses alveolar membrane each minute into FRC volume
Apnoeic Mass Transfer Oxygenation (AMTO)
Passive ventilation into open airway in apnoea
FRC volume remains constant by mechanical forces acting upon it
O2 uptake is 250 ml/min
CO2 crossing alveolar membrane in 10 ml/min
Therefore deficit of 240 ml/min volume being lost
Provided airway is open, 240 ml will be drawn into alveolar space to maintain FRC volume
Combined gas law PV = nRT
V, R and T are constant
Therefore Pressure is proportional to quantity of gas
More moles of gas removed than are added so pressure falls to below atmospheric pressure causing mass transfer of gas down pressure gradient (note mass transfer is not simple diffusion)
What is rate of fall of PAO2 during apnoea
Provided there is:
- Adequate preoxygenation
- Open airway
- 100% O2 in airway for AMTO being passively ventilated
Then PAO2 falls at same rate as PACO2 rises
Therefore PAO2 falls at rate of 0.5 kPa/min
Compare fall in PAO2 if only 50% O2 in airway
PAO2 falls much quicker than would expect
Volume deficit remains the same, but now the 240 ml is replaced with 120 ml O2 only, so now losing 130 ml O2 per minute
(130 ml / 2000 ml) x 100 = 6.5%
6.5% of 100 kPa = 6.5 kPa/min
Now fall in PAO2 is 6.5 kPa/min
Over 10x faster fall than with 100% O2 in airway
How does preoxygenation affect time to reach hypoxic threshold (PAO2 ~ 10 kPa)
Impacts the starting point for PAO2 and for any particular FRC the store of O2 that can be used to compensate for the excess of O2 uptake to delivery
How does FRC size affect time to reach hypoxic threshold (PAO2 ~ 10 kPa)
Determines the store of O2 for any given FAO2
How does FIO2 affect time to reach hypoxic threshold (PAO2 ~ 10 kPa)
Determines proportion of the indrawn gas that is O2 and hence delivery of O2 into alveolar space by AMTO
How does oxygen consumption affect time to reach hypoxic threshold (PAO2 ~ 10 kPa)
Uptake determines the demand upon the stores of O2 in the alveolar space and hence rate of fall of PAO2
How does CO2 production affect time to reach hypoxic threshold (PAO2 ~ 10 kPa)
Determines rate of rise of PaCO2 which in turn determines rate of transfer of CO2 across alveolar membrane
