1Departments of Experimental and Clinical Anaesthesiology, Heinrich-Heine-University, Moorenstr. 5, D-40225 Duesseldorf, Germany. 2Department of Anaesthesiology, Merheim Hospital, Clinics of Cologne, Ostmerheimer Str. 200, D-51109 Cologne, Germany
Accepted for publication: February 15, 2000
Abstract
Dry lime causes a loss of volatile anaesthetics by degrading and adsorbing them. Degradation produces toxic substances and heat. Rehydration of lime stops degradation. If humidified breathing gases rehydrate lime, closed anaesthesia-circuits may reduce the loss of anaesthetics. To test this hypothesis we ventilated a reservoir bag with PhysioFlex®-devices using fresh (F) and dried (D) soda lime both in the presence (+H) and absence (H) of halothane. We measured halothane delivery, humidity, temperature, and lime weight. Halothane was lost for 13 min in D+H. Humidity increased steeper with fresh lime, whereas absorbent weight increased more with dried lime; halothane increased both variables (F+H: 99%, 8 g; FH: 93%, 6 g; D+H: 58%, 17 g; DH: 24%, 15 g). Surprisingly, temperature remained constant, probably because of the high gas flow (70 litres min1) generated inside the Physioflex®. These findings indicate rehydration of dried lime by humid gases and a rapid cessation of the loss of halothane in the PhysioFlex®.
Br J Anaesth 2000; 85: 30810
Keywords: equipment, anaesthesia machines, carbon dioxide absorbtion; anaesthetics volatile, halothane
Fresh carbon dioxide absorbents (soda lime and Baralyme®) contain about 1217% of water.1 2 Various situations result in unnoticed drying of lime2 3 which may be harmful since dried lime degrades volatile anaesthetics to toxic products (e.g. carbon monoxide).1 2 Additionally, degradation produces heat that may be deleterious to the airways.2 Rehydration of dried lime abolishes these effects.1 2
Humidification of breathing gases, which is best in closed-circuits may lead to fast rehydration of dried lime. Therefore, we hypothesized that the PhysioFlex® closed-circuit anaesthesia machine reduces degradation of volatile anaesthetics by dried soda lime. This machine generates a high gas flow inside the circuit to homogenize the gas components and to prevent rebreathing of expired gas.4 This gas flow may promote heat transfer from the absorbent to the patient, increasing the risk of airway injury. We also tested if, as assumed elsewhere,1 the degradation of halothane liberates water.
Methods and results
To study how the loss of halothane from the PhysioFlex® is influenced by the limes water content, we used either fresh (F+H, n=4) or dried (D+H, n=4) soda lime (Draegersorb 800, Draeger, Lübeck, Germany). It was dried by passing oxygen (10 litres min1) through an absorbent-packed cylinder until the repeatedly measured weight (scale type 16410; Maul, Bad König, Germany) of the cylinder was constant. Then, an oxygen flow of 1 litres min1 was used to keep the lime dry.
The PhysioFlex® has been described in detail previously.4 It is a closed-circuit anaesthesia machine that delivers the chosen volatile anaesthetic with end-expiratory anaesthetic concentration as the feedback-controlled variable. (With feedback control of the inspiratory concentration the machine works as a semi-closed circuit. This would have been inappropriate for our purpose.) Whenever the anaesthetic concentration (measured by infrared spectroscopy) is lower than set, liquid anaesthetic is injected into the circuit. Thus, the amount of anaesthetic that is necessary to reach and maintain the set concentration can be quantified. Because degradation and adsorption of the volatile anaesthetic will increase anaesthetic delivery by lowering the measured anaesthetic concentrations,3 we expected a difference in anaesthetic delivery between our F+H- and D+H-experiments, i.e. a loss of anaesthetic from the gaseous phase of the system. As the PhysioFlex® removes surplus volatile anaesthetic in a built-in charcoal filter, we removed the charcoal to avoid interference with our measurements.
The circuit was dried to 58% humidity during 23 h with siliceous earth, a strongly hygroscopic agent (Caesar & Lorentz, Hilden, Germany). From the start of the drying procedure until the end of each experiment, the PhysioFlex® ventilated a 3 litre reservoir bag (tidal volume 700 ml, respiratory rate 9 min1, positive end-expiratory pressure 0 cmH2O, 30% oxygen in air).
After drying of the PhysioFlex®, we exchanged the siliceous earth with the absorbent sample to be studied without opening the circuit, took baseline measurements, and started carbon dioxide supply (200 ml min1). (The PhysioFlex® has two absorbent containers, one of which can be selectively bypassed. This allows the introduction of an absorbent sample without opening the circuit.) Carbon dioxide absorption contributes essentially to the rehydration of dried lime by producing water. Thereafter, we set an end-expiratory halothane concentration (Halet) of 2.0 vol% in the respective experiments and maintained this concentration until the end of the experiments (40 min). We measured continuously halothane delivery (PhysioFlex®), humidity, and temperature (Hygrotest 6200, Testotherm, Lenzkirch, Germany) inside the inspiratory limb of the circuit. Absorbent was weighed before and after each experiment.
All data are shown as median (range).
Substantial amounts of halothane were lost in the initial phase of the D+H experiments (stars in Fig. 1A). The median difference in the delivery of liquid halothane between D+H and F+H experiments was 1.96 ml after 6 min, peaked at 2.20 ml after 13 min and then decreased continuously to 1.59 ml. With D+H, the time to attain the set Halet was increased about 4-fold (D+H, 7 (69) min; F+H, 2 (12) min), the corresponding halothane dose was more than doubled (D+H, 3.9 (3.64.1) ml; F+H, 1.7 (1.21.7) ml) compared with F+H. However, to maintain Halet less halothane was delivered in the D+H experiments. Of note, the end of halothane loss coincided with humidity exceeding 20% during the D+H experiments. These findings indicate a substantial loss of halothane by dried lime in the initial minutes. However, about 30% (0.61 ml liquid halothane) of lost anaesthetic reappeared during the later course of the experiment.
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Comments
We have shown for the first time that the loss of halothane caused by dried soda lime ceases rapidly in the PhysioFlex® and that dried lime is rehydrated at the expense of the humidification of the breathing gases. Although degradation of halothane produces heat, temperature inside the inspiratory limb did not increase.
The loss of halothane which resulted from degradation and adsorption by dried soda lime1 2 5 6 decreased by 0.61 ml after the 13th minute of our experiments without reaching a plateau (Fig. 1A). Thus, at least 27% of the lost anaesthetic was not degraded. It was first adsorbed in the molecular sieve of the dried lime and then replaced by water that rehydrated the absorbent.1 3 Rehydration of dried lime by water vapour is generally possible, as proven by the increasing absorbent weight and initially decreasing humidity in our D+H and DH experiments. The loss of halothane ceased more rapidly when humidity exceeded 20% and an initial humidity of 20% prevented any loss of halothane in similar experiments (data not shown). Thus, a relative humidity of 20% inside the circuit seems to prevent degradation of halothane and this threshold is rapidly reached in the PhysioFlex®. Hence, for the PhysioFlex® we can contradict the assumption that water vapour may not suffice to rehydrate dried lime during a short period.1
The decreasing loss of halothane could be explained if halothane concentration was overestimated due to overlapping infrared spectra of halothane and its degradation products. However, with 1.3 vol% of halothane upstream of an absorber filled with dry soda lime, infrared spectroscopy failed to detect halothane downstream of this absorber for more than 10 min,3 excluding overlapping spectra. Moreover, if we had used isoflurane or enflurane, overlapping spectra may have been feigned and anaesthetic delivery would have differed less, because these two anaesthetics are degraded only partially.6 Therefore, halothane was the best choice for our experiments. (None of the PhysioFlex® used in this study contained sevoflurane or desflurane.)
As discussed above, the cessation of the loss of halothane indicates that the degradation and adsorption of halothane are reduced at this time. In a semi-closed anaesthesia circuit carbon dioxide production peaked 1015 min after dried Baralyme® started to degrade desflurane.5 This time point also indicates a reduction in anaesthetic degradation. Therefore, degradation seems to be reduced twice as fast in the PhysioFlex® than in semi-closed circuits. One possible reason for this is the faster humidification of breathing gases in closed compared to semi-closed circuits, resulting in faster rehydration of dried lime.
Temperature remained constant inside the inspiratory limb of the circuit during our experiments. Thus, although degradation of volatile anaesthetics excessively produces heat2 3 and although heat transfer could be promoted by the high internal gas flow inside the PhysioFlex®,4 thermal injury of the patients airway is unlikely to occur with this device. This is likely to occur as a result of the high gas flow cooling the lime and interrupting the vicious circle of a heat-boostered exothermic reaction.1
Baralyme® also degrades volatile anaesthetics, but it was not used because the two limes contain similar amounts of alkali hydroxidesthe crucial components for degradation of volatile anaestheticsand no qualitative differences between the two limes have been reported concerning anaesthetic degradation.1 Thus, our results may predict the situation with Baralyme®.
In conclusion, dried soda lime causes a loss of halothane that ceases rapidly within the PhysioFlex® because of rapid humidification of the dried lime. Furthermore, thermal injury of the patients airway is unlikely to occur with this device.
References
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