Write short notes on physiological dead space (1990)
Definition - the volume occupied by gas which does not participate in gas exchange in lung.
A few different types, including:
Anatomical dead space is the volume of the conducting airways.
=> about 150mL in an average adult
=> or 2.2mLs/kg
Anatomical dead space is constant regardless of circulation.
Physiological dead space is the part of the tidal volume which does not participate in gas exchange.
Includes:
The difference between anatomical dead space and physiological dead space is alveolar dead space.
With increased cardiac output (e.g. during exercise), physiological dead space is reduced (due to reduction in alveolar dead space).
Alveolar dead space is the part of the inspired gas which passes through the anatomical dead space to mix with gas at the alveolar level, but does not participate in gas exchange. (i.e. infinite V/Q)
When using mask or anaesthetic circuit tubing, this adds to the conducting zone.
By using Fowler's method.
Based on rapid dilution of gas already in lung (N2 or CO2) by inspired gas (100% O2).
[Diagram 20050228(1) - Fowler's method]
By using Bohr's equation and Bohr's method
Based on "all expired CO2 comes from alveolar gas", and dead space doesn't eliminate CO2.
VT x FECO2 = VA x FACO2
Also, VT = VA + VD
=> VT x FECO2 = (VT - VD) x FACO2
=> VT x (FACO2 - FECO2) = VD x FACO2
=> VD/VT = (FACO2 - FECO2)/FACO2
Bohr equation: VD/VT = (PACO2 - PECO2)/PACO2
Normal value: 0.2~0.35
NB: PECO2 is the partial pressure in MIXED expired gas, NOT end-tidal gas
Enghoff modification - using measured arterial PaCO2 as an estimate of the ideal alveolar PACO2
=> modified: VD/VT = (PaCO2 - PECO2)/PaCO2
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Created | 20050228 |
Updated | 20050228 |