Performance of four carbon dioxide absorbents in experimental and clinical settings

Unexpected deposits in the anesthetic circuit: a possible cause of PEEP/Pmax valve malfunction

PEEP is regulated by the internal PEEP/maximum peak inspiratory pressure limit (Pmax) valve. Malfunctioning of the PEEP/Pmax valve can result in the creation of unintentional or unstable PEEP, and a reduction of inspired tidal volume. Some of our Dräger Fabius® anesthesia machines were noted to exhibit changes in expiratory waveforms and unstable PEEP during general anesthesia. We considered that the cause was associated with PEEP/Pmax valve malfunction, and then investigated the problems in collaboration with the manufacturer. Seven of the 22 Dräger Fabius® anesthesia workstations at our department exhibited problems with their PEEP/Pmax valves. We replaced the PEEP membrane and sealing washers in these seven anesthesia machines, and the problems were temporarily resolved. After a short interval, however, one of the seven machines began to show a similar phenomenon. We then asked the manufacturer to overhaul the PEEP/Pmax valve and the entire breathing circuit of the machine. On close investigation, we found that the valve components and the internal surface of the breathing circuit were contaminated with unexpected deposits. The build-up of deposits occurred within a year after the previous regular inspection. Our troubleshooting process determined the issue with the PEEP/Pmax valve, which could go unnoticed because the valve is encased inside the breathing circuit, and requires disassembly for close inspection. Our findings should raise awareness regarding the importance of the preventive maintenance cycle as a safety precaution to keep the anesthetic circuit free of unexpected contamination.

Important role of calcium chloride in preventing carbon monoxide generation during desflurane degradation with alkali hydroxide-free carbon dioxide absorbents

We investigated whether calcium chloride (CaCl₂), a supplementary additive in carbon dioxide (CO₂) absorbents, could affect carbon monoxide (CO) production caused by desflurane degradation, using a Japanese alkali-free CO₂ absorbent Yabashi Lime^®-f (YL-f), its CaCl₂-free and 1% CaCl₂-added derivatives, and other commercially available alkali-free absorbents with or without CaCl₂. The reaction between 1 L of desflurane gas (3-10%) and 20 g of desiccated specimen was performed in an artificial closed-circuit anesthesia system for 3 min at 20 or 40 °C. The CO concentration was measured using a gas chromatograph equipped with a semiconductor sensor detector. The systems were validated by detecting dose-dependent CO production with an alkali hydroxide-containing CO₂ absorbent, Sodasorb^®. Compared with YL-f, the CaCl₂-free derivative caused the production of significantly more CO, while the 1% CaCl₂-added derivative caused the production of a comparable amount of CO. These phenomena were confirmed using commercially available absorbents AMSORB^® PLUS, an alkali-free absorbent with CaCl₂, and LoFloSorb™, an alkali-free absorbent without CaCl₂. These results suggest that CaCl₂ plays an important role in preventing CO generation caused by desflurane degradation with alkali hydroxide-free CO₂ absorbents like YL-f.

Performance of a new carbon dioxide absorbent, Yabashi lime® as compared to conventional carbon dioxide absorbent during sevoflurane anesthesia in dogs

In the present study, we compare a new carbon dioxide (CO2) absorbent, Yabashi lime(®) with a conventional CO2 absorbent, Sodasorb(®) as a control CO2 absorbent for Compound A (CA) and Carbon monoxide (CO) productions. Four dogs were anesthetized with sevoflurane. Each dog was anesthetized with four preparations, Yabashi lime(®) with high or low-flow rate of oxygen and control CO2 absorbent with high or low-flow rate. CA and CO concentrations in the anesthetic circuit, canister temperature and carbooxyhemoglobin (COHb) concentration in the blood were measured. Yabashi lime(®) did not produce CA. Control CO2 absorbent generated CA, and its concentration was significantly higher in low-flow rate than a high-flow rate. CO was generated only in low-flow rate groups, but there was no significance between Yabashi lime(®) groups and control CO2 absorbent groups. However, the CO concentration in the circuit could not be detected (≤5ppm), and no change was found in COHb level. Canister temperature was significantly higher in low-flow rate groups than high-flow rate groups. Furthermore, in low-flow rate groups, the lower layer of canister temperature in control CO2 absorbent group was significantly higher than Yabashi lime(®) group. CA and CO productions are thought to be related to the composition of CO2 absorbent, flow rate and canister temperature. Though CO concentration is equal, it might be safer to use Yabashi lime(®) with sevoflurane anesthesia in dogs than conventional CO2 absorbent at the point of CA production.

Delivery of sevoflurane to dogs using a Stephens anaesthetic machine

OBJECTIVE

To investigate the sevoflurane concentrations produced within the Stephens anaesthetic machine circuit (vaporizer in-circle system) at different fresh gas flow rates (FGFRs), temperatures, vaporizer settings and vaporizer sleeve positions when used to anaesthetize dogs of different body sizes.

STUDY DESIGN

Experimental non-blinded studies.

ANIMALS

Eighteen mixed breed dogs, weights 4-39 kg.

METHODS

Anaesthetic induction with propofol was followed by maintenance with sevoflurane in oxygen via the Stephens anaesthetic machine. In study 1, the vaporizer setting, temperature and circuit FGFRs were altered with the vaporizer sleeve down (n = 3), or in separate experiments, up (n = 3). Delivered (Fi'SEVO) and expired sevoflurane concentrations were recorded. Study 2 determined the vaporizer settings (sleeve up) required to achieve predetermined multiples of minimal alveolar concentration (MAC) of Fi'SEVO when sevoflurane was delivered to dogs (n = 12) of different bodyweights and at different FGFRs.

RESULTS

Delivered concentrations of sevoflurane were sufficient to maintain anaesthesia in all dogs, regardless of bodyweight, FGFR, vaporizer temperature and sleeve position. Fi'SEVO increased with increasing temperature, when the vaporizer sleeve was down, when vaporizer setting was increased and when FGFR was decreased. As the FGFR increased or the dog's bodyweight decreased, higher vaporizer settings were required to produce the same Fi'SEVO. The median Stephens vaporizer settings to achieve an Fi'SEVO of 1.3 MAC ranged from 4.3 to 5.0 for a small dog (1-10 kg), 2.5 to 5.6 for a medium dog (15-25 kg) and 2.5 to 3.5 for a large dog (30-40 kg), depending on the FGFR.

CONCLUSION AND CLINICAL RELEVANCE

The Stephens anaesthetic machine can deliver to dogs, weighing 4 kg and above, concentrations of sevoflurane sufficient or in excess of that required to maintain anaesthesia, at temperatures from 10 to 35 °C, FGFRs of 1 to 5 times the patient's estimated metabolic oxygen requirement and at any vaporizer sleeve position.

Global warming potential of inhaled anesthetics: application to clinical use

BACKGROUND

Inhaled anesthetics are recognized greenhouse gases. Calculating their relative impact during common clinical usage will allow comparison to each other and to carbon dioxide emissions in general.

METHODS

We determined infrared absorption cross-sections for sevoflurane and isoflurane. Twenty-year global warming potential (GWP(20)) values for desflurane, sevoflurane, and isoflurane were then calculated using the present and previously published infrared results, and best estimate atmospheric lifetimes were determined. The total quantity of each anesthetic used in 1 minimal alveolar concentration (MAC)-hour was then multiplied by the calculated GWP(20) for that anesthetic, and expressed as "carbon dioxide equivalent" (CDE(20)) in grams. Common fresh gas flows and carrier gases, both air/oxygen and nitrous oxide (N2O)/oxygen, were considered in the calculations to allow these examples to represent common clinical use of inhaled anesthetics.

RESULTS

GWP(20) values for the inhaled anesthetics were: sevoflurane 349, isoflurane 1401, and desflurane 3714. CDE(20) values for 1 MAC-hour at 2 L fresh gas flow were: sevoflurane 6980 g, isoflurane 15,551 g, and desflurane 187,186 g. Comparison among these anesthetics produced a ratio of sevoflurane 1, isoflurane 2.2, and desflurane 26.8. When 60% N2O/40% oxygen replaced air/oxygen as a carrier gas combination, and inhaled anesthetic delivery was adjusted to deliver 1 MAC-hour of anesthetic, sevoflurane CDE(20) values were 5.9 times higher with N2O than when carried with air/O2, isoflurane values were 2.9 times higher, and desflurane values were 0.4 times lower. On a 100-year time horizon with 60% N2O, the sevoflurane CDE(100) values were 19 times higher than when carried in air/O2, isoflurane values were 9 times higher, and desflurane values were equal with and without N2O.

CONCLUSIONS

Under comparable and common clinical conditions, desflurane has a greater potential impact on global warming than either isoflurane or sevoflurane. N2O alone produces a sizable greenhouse gas contribution relative to sevoflurane or isoflurane. Additionally, 60% N2O combined with potent inhaled anesthetics to deliver 1 MAC of anesthetic substantially increases the environmental impact of sevoflurane and isoflurane, and decreases that of desflurane. N2O is destructive to the ozone layer as well as possessing GWP; it continues to have impact over a longer timeframe, and may not be an environmentally sound tradeoff for desflurane. From our calculations, avoiding N2O and unnecessarily high fresh gas flow rates can reduce the environmental impact of inhaled anesthetics.