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From:
Date: 06 Jun 2006
Time: 12:59:07 +1100
Remote Name: 203.173.145.91
Comments:
to understand the second gas effect you need to understand the CONCENTRATION EFFECT.
- that is increasing the inspired concentration of anaesthetic increases the rate of rise of Fa/Fi
(in theory this is true for any anaesthetic but only clinically significant for N2O or xenon)
- the CONCENTRATION EFFECT is really two phenomena
1. The CONCENTRATING EFFECT (confusing i know)
- this is what you guys have already explained - when anaesthetic is taken up from the alveoli the
alveoli shrinks, increasing the concentration remaining. the concentration remaining is less than
the Fi but more than what you'd think.
- the higher the Fi the higher the concentration after absorption
- for those who want the maths
- say 50% of an anaesthetic agent is absorbed by the pulmonary circulation
- if Fi is 20% ie 20 parts anaes 80 parts other gases; after absorption there remains 10 parts
anaes and 80 parts other gas in a total of 90parts; as a concentration that is 10/90 or 11% anaesthetic
- if Fi is increased to 80% ie 80 parts anaes 20 parts other gas; after absorption there remains
40 parts anaes and 20 parts other gas or 40/60 = 67%
- so a 4x increase in Fi resulted in a 6x increase in Fa
- in the extreme case an Fi of 100% after 50% absorption the Fa would still be 100%
2. The AUGMENTED INFLOW EFFECT
- the decreased alveolar size absorption leads to inflow of fresh gas
- greater Fi leads to greater decrease in alveolar size and therefore greater inflow of gas
- maths
- at Fi of 20% - an additional 10 parts will come in to replace the 10 absorbed. this will contain 2
parts anaestheticand 8 parts other gas. this leaves a total of 10 +2 = 12 parts anaesthetic and 80
+ 8 = 88 parts other gas or a concentration of 12%
- at Fi of 80% - an additional 40 parts will be added containing 32 parts of anaesthetic; leaving
40+32 = 72 parts anaesthetic or Fa of 72%
the SECOND GAS EFFECT is that a high concentration of one gas will increase the rate of rise of
Fi/Fa of another gas by the same mechanisms talked about above.
the second gas effect is greater for more soluble agents due to their slower rates of rise of
Fi/Fa, so it's very clinically significant for halothane, noticable for iso, insignificant for sevo,
and probably virtually nothing for des.