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Correspondence |
University of California San Diego, San Diego, USA, E-mail: mgreenbe{at}ucsd.edu
To the Editor:
We recently provided general anesthesia to 16 children undergoing patent ductus arteriosus repair at the Angkor Wat Hospital for Children in Siem Reap, Cambodia. Prior to the first surgery, a thorough machine check was conducted, which demonstrated proper functioning of all elements, including the CO2 absorbent, which was uniformly white in colour. Midway through the first case, it was noted that in spite of adequate minute ventilation with a tidal volume of 10 mL·kg–1, that the PETCO2 ranged from 50–55 mmHg, and that during inspiration the PETCO2 decreased to only 12 mmHg. We suspected incomplete CO2 absorption.
Upon investigation, and much to our surprise, we learned that the fresh Spherasorb CO2 absorbent (Intersurgical Ltd., Wokingham, UK) was pink in colour and turned white with exhaustion. After the CO2 canister was filled with fresh soda lime, the patient’s hypercapnia resolved. The confusion originated from our assumption that fresh CO2 absorbent is always white, which is the case at our home institution, in San Diego, California. In fact, Spherasorb has two types of indicators: one that changes from white to violet, and one that changes from pink to white. In contrast, Baralyme (Chemtron Medical Division, Allied Healthcare Products, St. Louis, MO, USA) has a single formulation that turns the crystals from white to violet with exhaustion.
It appears that the two colour schemes (pink to white, and white to violet), developed independently of each other many decades ago, as different pH sensitive dyes were used as markers for exhausted absorbent. While the American and European manufacturers chose the white to violet route (using ethyl violet dye), in the United Kingdom the pink to white option is employed (using titan yellow dye).1
According to Intersurgical Ltd., countries that use soda lime with a pink to white colour change include: the United Kingdom, much of Australia, New Zealand, India, Pakistan, Bangladesh, Hong Kong (which influenced China), Indonesia, Malaysia, and Sri Lanka. (all countries with a British colonial link.) Most other countries use soda lime that turns from white to violet with absorbent depletion. An additional factor to consider is that the pH and the resulting strength of colour depends on the moisture content of the exhausted soda lime. Higher exhaustion moisture content, as seen with low flows and large absorbers, results in a higher pH and weaker colour change.2,3 In addition, if the soda lime remains in the absorber for 10–48 hr, some of the NaOH regenerates and the pH increases, changing the colour of the soda lime back to its "fresh" appearing colour. This is often mistaken for a regeneration of CO2 absorbing potential. Also, desiccated baralyme may turn a yellow colour,4 further complicating the use of colour as a visual reference to the adequacy of absorbent capacity. Thankfully, the use of capnography allowed us to search for, and solve the problem. The experience highlighted the importance for anesthesiologists undertaking work overseas, to recognize international variances in CO2 colour indicators and the need to identify the indicator as part of the routine preoperative machine check.
Footnotes
Accepted for publication December 1, 2006.
References
1 Gootjes P, Lagerweij E. Quality comparison of different CO2 absorbents. Anaesthetist 1981; 30: 261–4.[Medline]
2 Tsuchiya M, Ueda W. Heat generation as an index of exhaustion of soda lime. Anesth Analg 1989; 68: 783– 7.
3 Ohrn M, Gravenstein N, Good ML. Duration of carbon dioxide absorption by soda lime at low rates of fresh gas flow. J Clin Anesth 1991; 3: 104–7.[Medline]
4 Barth CD, Dunning MB 3rd, Bretscher L, Woehlck HJ. Barium hydroxide lime turns yellow after desiccation. Anesth Analg 2005; 101: 748–52.
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