6. Alveolar Gas Equation Flashcards
intro - dont test
The alveolar gas equation allows you to calculate the alveolar partial pressure of oxygen for a given inspired pressure of oxygen and a given alveolar pressure of carbon dioxide.
The examiners may ask about the alveolar gas equation in various guises ranging from a direct question such as, ‘how can the partial pressure of oxygen in alveolar gas be measured?’ to ‘what effect would sudden decompression of a commercial aircraft at an altitude of 35 000 ft have on alveolar oxygen pressure?’
Irrespective of the format of the question it is vital to understand the key role that the alveolar gas equation plays in understanding the causes of hypoxia and ultimately understanding the alveolar–arterial oxygen difference (A–a gradient).
Write the alveolar gas equation.
PiO2 –PACO2
PAO2 = _____________
R
Where:
PAO2 = Alveolar partial pressure of oxygen
PiO2 = Inspired pressure of oxygen
PACO2 =
Alveolar partial pressure of carbon dioxide (approximates with PaCO2 due to rapid diffusion of CO2)
Respiratory Quotient = CO2 production / O2 consumption (N = 0.8).
Explain how PiO2 is calculated
Inspired O2 is different from
atmospheric O2 because it is warmed
and contains added water vapour.
Fractional inspired O2
does not vary with altitude.
However, barometric pressure falls
with increasing altitude;
halving every 18 000 ft.
Partial pressure of water vapour
remains constant at 47 mmHg (6.3 kPa).
Thus:
PiO2 = FiO2 · (Patm − PH2O)
E.g.
At sea level
(barometric pressure 760 mmHg or 101 kPa)
PiO2 = 0.21 · (101 − 6.3) = 19.9 kPa
E.g. At an altitude of 63 000 ft (barometric pressure 47 mmHg or 6.3 kPa)
PiO2 = 0.21 · (6.3 − 6.3) = 0
At 63 000 ft barometric pressure
is equal to the partial pressure of water and
therefore a person’s blood would boil
(as saturated vapour pressure of water
would be equal to barometric pressure).
What factors affect the respiratory quotient (RQ)?
The metabolic substrates used
are the main determinants of the RQ.
> Carbohydrate RQ 1.0
> Protein RQ 0.8–0.9
> Fat RQ 0.7
Why may PAO2 and PaO2 differ?
If PiO2 is held constant
and
PaCO2 increases,
PAO2 and PaO2 will always decrease.
Since PAO2 is a calculation based on known (or assumed) factors, its change is predictable.
PaO2, by contrast, is a measurement whose theoretical maximum value is defined by PAO2 but whose lower limit is determined by:
1 ventilation–perfusion (V /Q ) imbalance,
2 pulmonary diffusing capacity
and
3 oxygen content of blood
entering the pulmonary artery
(mixed venous blood).
In particular, the greater the imbalance
of ventilation–perfusion ratios,
the more PaO2 tends to differ
from the calculated PAO2.
(The difference between
PAO2 and PaO2
is commonly referred to as the
‘A–a gradient’.
However, ‘gradient’ is a misnomer
since the difference is not
due to any diffusion gradient,
but instead to V. /Q. imbalance
and/or right to left shunting of
blood past ventilating alveoli.
Hence ‘A–a O2 difference’
is the more appropriate term
What is the normal A–a gradient?
The A–a gradient varies with age and FiO2. Up to middle age, breathing ambient air, the normal A–a gradient is approximately 1.3 kPa (10 mmHg).
Breathing an FiO2 of 1.0 the normal
A–a gradient ranges up to about 10 kPa.
If the A–a gradient is increased above normal, there is a defect of gas transfer within the lungs; this defect is almost always due to V /Q imbalance.
What are the common causes of an increased A–a gradient?
3x
There are three common causes:
1
> Ventilation–perfusion (V /Q ) mismatching
2
> Diffusion impairment
3
> Anatomical shunt
> Ventilation–perfusion (V /Q ) mismatching
Can it compensate
Blood flowing through high V. /Q. areas
with a higher PO2 cannot compensate
for the blood flowing through
low V. /Q. areas because of the
shape of the oxyhaemoglobin dissociation curve
and
because more of the pulmonary blood usually
flows through low V. /Q. areas.
Diffusion impairment
When does it happen pathologically
Physiologically
may occur in conditions such as
pulmonary fibrosis and pulmonary oedema.
It may also occur if PiO2 is low
(e.g. high altitude)
or
if lung capillary transit time is greatly reduced from its normal 0.75 seconds (e.g. exercise).
> Anatomical shunt
an extreme form of V /Q mismatch
where deoxygenated blood
enters the systemic circulation.
Draw a curve to demonstrate how changes in minute ventilation affect the partial pressures of alveolar oxygen and alveolar carbon dioxide.
As minute ventilation increases,
PaCO2 and hence PACO2 decreases
(PaCO2 approximates PACO2
due to the rapid diffusion of CO2).
This results in a reciprocal increase in PAO2.
Fig. 6.1 The effect of changes in minute ventilation on PAO2 and PACO2
diagram pg 19
