Quote from: jafo1964 on December 09, 2009, 05:25:42 PM
My understanding of the subject

All materials absorb gases in varying concentrations

So Nitrous Oxide has been found to permeate across the thin PVC sheet that forms the cuff. That explains that N2O can move around
In fact various rubber and plastic tubings can absorb varying quantities of inhalational agents as gases
This uptake by circuit needs to be factored in when practising low flow anaesthesia techniques
These taken up gases by circuit may be relaesed later and can theorotically delay recovery
SO N2O can move across the thin PVC cuff

If there is a semipermeable membrane gas molecules move across it based on concentration gradient from high to low concentration. That simply explains how CO2 is given into lungs and O2 taken up at lungs. Lungs have less CO2 and more O2 compared to venous blood. The vice versa happens at the tissue level
Obviously the cuff is filled with air ( no N2O) while the inspired gas has 66% of N2O so obviously N2O will move in according to the concentration gradient. and it will continue to move until equilibrium is acheived across the membrane . Meaning inside the ETT cuff too there must be 66% N2O to prevent further movement.
There is another additinal problem. N2O is almost 30 times more diffusible than other gases. So in the time taken for 1 air molecule to move out nearly 30 N2O molecules can move in . So N2O fills in faster than air moves out( SIMILAR OCCURENCE DURING FINK EFFECT / DIFFUSION HYPOXIA). If this change occurs in a compliant space like air in intestines, then the intestines bloat as during laprotomy.
IF N2O moves into non-compliant spaes like middle ear or ETT cuff the volume cannot expand much so the pressure increases producing the damages

In ETT cuff N2O moves in faster than air moves out leads to increase in presure above the safe  allowed 18 mm Hg and can compromise tracheal mucosal blood flow
So you need to check pressure if you use N2O and deflate and make adjustments in volume as necessary

ALTERNATIVES
Fill cuff with saline or any liquid
Use Inspiratory gas mixture( 66% N2O and 33% O2) to fill cuff
Use manometer to measure and adjust pressure
Use self regulating ETT cuff systems like Lanz system or Mcginnis cuff balloon system

Hope this explanation is decipherable and
Hope to God this is the correct explanation for this oft ignored phenomenon

regs