SODA LIME TEMPERATURE MONITORING IS A RELIABLE WARNING METHOD OF CARBON MONOXIDE PRODUCTION DURING DESFLURANE AND ISOFLURANE ANESTHESIA IN SWINE

Abstract

S174 INTRODUCTION: Carbon monoxide (CO) production from desflurane and isoflurane breakdown in dry soda lime in anesthetic circuits can result in intraoperative CO toxicity. Nowadays, only mass spectrometers can indirectly warn of CO production by displaying a wrong agent or an agent mixture from detection of anesthetic breakdown byproducts. However they may not be sensitive enough to warn of isoflurane breakdown. Knowing that anesthetic breakdown is an exothermic reaction we determined if soda lime temperature monitoring could provide a clinical useful warning of CO production during isoflurane and desflurane anesthesia with circle breathing systems. METHODS: Eight pigs (weight 38 +/- 4 Kg) were anesthetized with intraperitoneal metomidate and azaperon. Mechanical ventilation was instituted at 15 breaths/min and a tidal volume of 12-14 ml/Kg adjusted to maintain a PaCO2 of 37-43 mmHg. After standard monitoring was performed, we started the administration of the halogenated agent, setting the vaporizer at the desired alveolar concentration. A high FGF (8 L/min) was adjusted during the first 15 min of anesthesia and reduced thereafter to one tenth of the minute volume until the end of the study. Fresh soda lime with 16% water content was used in four pigs (2 with 7% des and 2 with 1.5% iso) and with 1% water content in another four (2 with 7% des and 2 with 1.5% iso). Dry soda lime was obtained by exposing fresh soda lime (900 gr) to 8 L/min of continuous O2 flow during 65 hours. Soda lime temperature was continuously monitored by a thermometer probe located in the middle of the canister. Temperatures at 0, 5, 15, 30, 60, 90, 120 minute intervals and time of peak temperature were recorded. Inspired gas was sampled with gas-tight syringes at the same time intervals and CO was measured thereafter by chromatography. Data are presented as mean +/- SD. RESULTS: Soda lime temperature and CO circuit concentrations are shown in Figure 1 and Figure 2 respectively. During fresh soda lime studies inspired CO was undetectable and soda lime temperature did not vary until FGF was reduced, increasing slowly thereafter and peaking at 120 min-time with both agents (36 +/- 4[degree sign]C w/iso and 33 +/- 5[degree sign]C w/des). In contrast, with dried soda lime, temperature peaked to 63 +/- 6[degree sign]C with isoflurane at 12 +/- 2 min-time and to 52 +/- 1[degree sign]C with desflurane at 46 +/- 14 min-time from the beginning of the anesthesia, and decreased thereafter. Peak temperatures coincided in time with peak inspired CO concentrations for both agents although temperatures were not proportional to CO levels.Figure 1Figure 2DISCUSSION: Fresh soda lime prevented CO production and consequently, temperature changes were small, progressive and related solely to CO2 absorption during the rebreathing period. In dry soda lime studies, temperatures were higher at any time; with isoflurane we observed a more rapid and higher soda lime peak temperature but lower circuit CO concentrations compared to desflurane. This way, temperature changes may reflect anesthetic breakdown more than CO production derived from this reaction. However, soda lime temperature monitoring seems a useful and inexpensive tool to warn of anesthetic breakdown and CO production in circle circuits, easy to be applied in clinical anesthesia.