physiological dead space and its measurements

1. 25/5/13
Physiological dead space
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Dead space
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Definition "!the!volume!occupied!by!gas!which!does!not!participate!in!gas!exchange!in!lung.!
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A few different types, including:!
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!!!!anatomical!dead!space!
!!!!physiological!dead!space!
!!!!alveolar!dead!space!
!!!!apparatus!dead!space!
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Anatomical dead space!
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Anatomical!dead!space!is!the!volume!of!the!conducting!airways.!
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=>!about 150mL in an average adult
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=>!or 2.2mLs/kg
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Anatomical dead space is constant regardless of circulation.!
Physiological dead space

2. !
Physiological!dead!space!is!the!part!of!the!tidal!volume!which!does!not!participate!in!gas!
exchange.!
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Includes:
!
!!! anatomical dead space
!!!!alveoli with no perfusion (i.e. infinite V/Q) (e.g. West's zone
1)
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The!difference!between!anatomical!dead!space!and!physiological!dead!space!is!alveolar!dead!
space.!
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With increased cardiac output (e.g. during exercise), physiological dead space is
reduced (due to reduction in alveolar dead space).
Alveolar dead space!
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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)!
!
Apparatus dead space
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When!using!mask!or!anaesthetic!circuit!tubing,!this!adds!to!the!conducting!zone.!
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Measurement
Measurement!of!anatomical!dead!space!

3. !
By using Fowler's method.!
Fowler's!method!
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Based!on!rapid!dilution!of!gas!already!in!lung!(N2!or!CO2)!by!inspired!gas!(100%!O2).!
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!!!1.!Single!breath!of!100%!O2!
!!2.!!During!the!following!expiration,![N2]!increases!from!0%!(pure!dead!space!gas)!to!
equilibrium!(pure!alveolar!gas)!(i.e.!plateau)!
!!!!=>!as!per![N2]!vs!time!graph!
!!!!3.Using![N2]!vs!expired!volume!graph,!anatomical!dead!space!is!taken!to!be!at!the!mid"point!
of!the!transition!from!conducting!zone!to!gas!exchange!zon!
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Measurement of physiological dead space
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By!using!Bohr's!equation!and!Bohr's!method!
Bohr's method
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Based!on!"all!expired!CO2!comes!from!alveolar!gas",!and!dead space doesn't
eliminate CO2.
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VT!x!FECO2!=!!VA!x!FACO2!
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Also,!VT!=!VA!+!VD!!
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=>!VT!x!FECO2!=!!(VT!"!VD)!x!FACO2!

4. !
=>!VT!x!(FACO2!"!FECO2)!=!VD!x!FACO2!
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=>!VD/VT!=!(FACO2!"!FECO2)/FACO2!
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!!!!VA!=!ventilated!alveolar!volume!
!!!!VT!=!tidal!volume!
!!!!VD!=!dead!space!volume!
!!!!FECO2!=!fractional!concentration!of!CO2!in!mixed!expired!air!
!!!!FACO2!=!fractional!concentration!of!CO2!in!alveolus!!
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Bohr!equation:!VD/VT = (PACO2 - PECO2)/PACO2
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Normal!value:!0.2~0.35!
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NB:!PECO2!is!the!partial!pressure!in!MIXED!expired!gas,!NOT!end"tidal!gas!
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Enghoff modification "!using!measured!arterial!PaCO2!as!an!estimate!of!the!ideal!
alveolar!PACO2!
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=>!modified:!VD/VT = (PaCO2 - PECO2)/PaCO2!
!
!!
Additional notes
Factors influencing anatomical dead space!

5. !
!!!!Size of subject
!!!!=>!increases!with!body!size!
!!!!Age
!!!!=>!at!infancy,!anatomical!dead!space!is!higher!for!body!weight!(3.3mL/kg)!
Posture
!!!!=>!sitting!147mL,!supine!101mL!
!!!!Position of neck and jaw
!!!!Lung!volume!at!the!end!of!inspiration!
!!!!=>!anatomical dead space increases by 20mL for each L of
lung volume
!!!!Drugs!
!!!!e.g. bronchodilator will increase dead space
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Factors influencing alveolar dead space
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!!!!1.Low!cardiac!output!can!increase!alveolar!dead!space!(increasing!West's!zone!1)!
!!!2.!Pulmonary!embolism!