Anesthesia Pharmacology Chapter 4:  Physics and Anesthesiology

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  • The carbon dioxide absorber is located on the exhalation side of the circle. Exhaled gas goes through the canister which contains soda lime and the carbon dioxide is removed. Soda lime mainly consists of calcium hydroxide with smaller quantities of the important activators, sodium and potassium hydroxides. The chemical reactions responsible for carbon dioxide removal is exothermic, producing water as a byproduct.  

  • The removal of carbon dioxide involves chemical reactions. Soda lime is consumed by the reaction must be replenished periodically.

  • Indication of the need to replenish includes a change in the color of soda lime from white and blue.  The basis of this change is that as the chemical reaction proceeds, the pH decreases and a pH indicator changes color to blue.

  • 21Klide, A  &Teichner,  J, Rebreathing Systems arts of the Circle System: Carbon Dioxide Absorber University of Pennsylvania School of Veterinary Medicine


  • 22Rebreathing Circuits

    • In rebreathing systems, expired CO2 is absorbed by soda lime or Baralyme which is contained in a canister as noted above.  The rationale is that expired gas, with its CO2 content now removed, could be used as part of inspired gas without promoting CO2 accumulation

    • As noted above soda lime is a mixture which contains about 94% calcium hydroxide, 5% sodium hydroxide, and 1% potassium hydroxide.  Furthermore, soda lime must contain water since CO2 must be dissolved in water prior to reaction.  Silica is included for granularity and the pH-sensitive dying is used to assess when to change the canister due to loss of CO2  absorption capacity.

    • The chemical reaction that results in absorption of CO2  is described below:

    • It is quite important to know when to change the soda lime in the canister.  Depending on the system loss of capacity may be noted when the color changes from white to bloom (purple) or another system from pink to white.  At all events, this color change which indicates the need for fresh absorbent may disappear when soda lime is allowed to stand.  Therefore, immediate change is required.

    • The relationship between the CO2   canister and other elements of the anesthesia circuit are noted below:

    • 22This figure is a representation of a "Circle Absorber" in which the carbon dioxide absorber canister is labeled (C); the breathing bag (B), inspiratory valve (Vi); expiratory valve (Ve) and pressure relief valve (V).  Fresh gas supply occurs by means of tubing labeled F.  Elements not represented include hoses leading to the patient and the "Y" piece.

    • 22The function of the system is described below in the animation.

  • 23Compound A, a metabolite of sevoflurane (Sevorane, Ultane), has been suggested to be a renal toxin.

    • Endpoints of renal toxicity might include abnormal albumin excretion, urinary glucose, and alpha-glutathione-S-transferase. 

    • Using these endpoints, compound A at doses < 240 ppm/hour did not induce abnormal levels of the aforementionned compounds. 

    • By contrast, patients receiving doses > 240 ppm/hour exhibited increased, abnormal excretion of urinary albumin, glucose, and glutathione-S-transferase. 

    • In terms of relating these results to sevoflurane (Sevorane, Ultane) administration, Compound A levels exceeding 240 ppm//hour might be expected following prolonged sevoflurane (Sevorane, Ultane) anesthesia, for example eight hours at 2L/min flowrate. 

    • This administration corresponds to about 30 ppm Compound A exposure .which would then be expected or could produce a transient albuminuria and enzymuria in otherwise healthy patients


Dessicated Absorbents will degrade these Anesthetics

Enflurane (Ethrane)

Desflurane (Suprane)

Isoflurane (Forane)


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