用全自动电动麻醉机进行低流量麻醉的临床观察

目的 采用高精密度监测设备和手段对全自动电动麻醉机物理性能测定后进行低落充量麻醉观察。方法 将４８例胸外科 手术病人随机分成两组,分别给予４Ｌ．ｍｉｎ＾－１和０．８Ｌ．ｍｉｎ＾－１流量麻醉,采用Ｃａｐｎｏｍａｃ Ｄａｔｅｘ ＡＳ／３多功能麻醉气体监测仪与麻醉机自身监测系统同时监测环路内各气体浓度（Ｎ２０、０２、ＣＯ２、Ｉｓｏ）,并记录各气体（Ｎ２Ｏ、Ｏ２、Ｉｓｏ）及钠石灰消耗量,将两监测系统监测值     

Low flow anesthesia at a fresh gas flow of 10 ml.kg-1.min-1 for hours using time-cycled ventilator]

Low flow anesthesia (LFA) at a fresh gas flow (FGF) level of 10 ml.kg-1.min-1 with oxygen flow set at 0.5 ml.kg-1.min-1: 0.5 ml.kg-1.min-1 nitrous oxide and 3% isoflurane was performed using time-cycled ventilator on 10 patients of ASA class I or II, with age of 55 +/- 13 (mean +/- SD) years and body weight of 55 +/- 10 kg for 5 h. Excessive anesthetic gases from the anesthesia gas monitor were led to an expiratory breathing tube. After rapid induction and tracheal intubation, denitrogenation was performed for about 5 min using a 100% oxygen flow of 6 l.min-1 before LFA. The inspired/expired oxygen concentration decreased gradually from 96 +/- 2%/90 +/- 2% at beginning of LFA to 42 +/- 3%/37 +/- 4% at 5 h. The operation was started after 29 +/- 10 min of beginning of LFA. The nitrous oxide concentration reached 37 +/- 4%/35 +/- 4% at the beginning of operation and further increased to 55 +/- 3%/53 +/- 3% at 5 h. The isoflurane concentration reached 1.0 +/- 0.1%/0.8 +/- 0.1% at the beginning of operation and further increased to 1.2 +/- 0.1%/1.0 +/- 0.1% at 5 h. The anesthetic potency was 1.2 +/- 0.1 MAC/1.0 +/- 0.2 MAC at the beginning of operation. The isoflurane vaporizer setting was changed only once in two cases from 3% to 2% exceeding 1.5% in inspired concentration. There was no need to change the flow of oxygen and nitrous oxide for 5 hrs. No SpO2 lower than 95% was observed during this study. This method is a clinically safe, easily applicable anesthesia method and used the smallest FGF reported in LFA without occurrence of low FIO2.     

Clinical experience with minimal flow xenon anesthesia

Xenon is a more potent anesthetic than nitrous oxide, and gives more profound analgesia. This investigation was performed to assess the potential of xenon for becoming an anesthetic inspite of its high manufacturing cost. Seven ASA I—-II patients undergoing cholecystectomy (n = 4), hernia repair (n = 2), or mammoplasty (n=l) were studied. Denitrogenation by 15–20 min of oxygen breathing under propofol anesthesia was followed by fentanyl–supplemented xenon anesthesia administered via an automatic minimal flow system which held the oxygen concentration at 30%. Xenon anesthesia lasted 76–228 min and 8–14 1 of xenon (ATPD) was used, of which 5.6–8.1 1 was expended during the first 15 min. Anesthesia appeared to be satisfactory, and the patients woke up rapidly after xenon was discontinued. The automatic system made minimal flow xenon anesthesia easy to administer, but nitrogen accumulation is still a problem. Assuming a xenon price of 10 US $ per litre, the average cost for xenon was about 65 US $ for the first 15 min and then about 25 USS for each subsequent hour of anesthesia.     

Lack of Degradation of Sevoflurane by a New Carbon Dioxide Absorbent in Humans

Background: Potent inhaled anesthetics degrade in the presence of the strong bases (sodium hydroxide or potassium hydroxide) in carbon dioxide (CO2) absorbents. A new absorbent, Amsorb (Armstrong Medical Ltd., Coleraine, Northern Ireland), does not employ these strong bases. This study compared the scavenging efficacy and compound A production of two commercially available absorbents (soda lime and barium hydroxide lime) with Amsorb in humans undergoing general anesthesia.

Methods: Four healthy volunteers were anesthetized on different days with desflurane, sevoflurane, enflurane, and isoflurane. End-tidal carbon dioxide (ETco2) and anesthetic concentrations were measured with infrared spectroscopy; blood pressure and arterial blood gases were obtained from a radial artery catheter. Each anesthetic exposure lasted 3 h, during which the three fresh (normally hydrated) CO2 absorbents were used for a period of 1 h each. Anesthesia was administered with a fresh gas flow rate of 2 l/min of air:oxygen (50:50). Tidal volume was 10 ml/kg; respiratory rate was 8 breaths/min. Arterial blood gases were obtained at baseline and after each hour. Inspired concentrations of compound A were measured after 15, 30, and 60 min of anesthetic administration for each CO2 absorbent.

Results: Arterial blood gases and ETco2 were not different among three CO2 absorbents. During sevoflurane, compound A formed with barium hydroxide lime and soda lime, but not with Amsorb.     

Lack of degradation of sevoflurane by a new carbon dioxide absorbent in humans

BACKGROUND: Potent inhaled anesthetics degrade in the presence of the strong bases (sodium hydroxide or potassium hydroxide) in carbon dioxide (CO2) absorbents. A new absorbent, Amsorb (Armstrong Medical Ltd., Coleraine, Northern Ireland), does not employ these strong bases. This study compared the scavenging efficacy and compound A production of two commercially available absorbents (soda lime and barium hydroxide lime) with Amsorb in humans undergoing general anesthesia. METHODS: Four healthy volunteers were anesthetized on different days with desflurane, sevoflurane, enflurane, and isoflurane. End-tidal carbon dioxide (ETCO2) and anesthetic concentrations were measured with infrared spectroscopy; blood pressure and arterial blood gases were obtained from a radial artery catheter. Each anesthetic exposure lasted 3 h, during which the three fresh (normally hydrated) CO2 absorbents were used for a period of 1 h each. Anesthesia was administered with a fresh gas flow rate of 2 l/min of air:oxygen (50:50). Tidal volume was 10 ml/kg; respiratory rate was 8 breaths/min. Arterial blood gases were obtained at baseline and after each hour. Inspired concentrations of compound A were measured after 15, 30, and 60 min of anesthetic administration for each CO2 absorbent. RESULTS: Arterial blood gases and ETCO2 were not different among three CO2 absorbents. During sevoflurane, compound A formed with barium hydroxide lime and soda lime, but not with Amsorb. CONCLUSIONS: This new CO2 absorbent effectively scavenged CO2 and was not associated with compound A production.     

Managing fresh gas flow to reduce environmental contamination

Anesthetic drugs have the potential to contribute to global warming. There is some debate about the overall impact of anesthetic drugs relative to carbon dioxide, but there is no question that practice patterns can limit the degree of environmental contamination. In particular, careful attention to managing fresh gas flow can use anesthetic drugs more efficiently--reducing waste while achieving the same effect on the patient. The environmental impact of a single case may be minimal, but when compounded over an entire career, the manner in which fresh gas flow is managed by each individual practitioner can make a significant difference in the volume of anesthetic gases released into the atmosphere. The maintenance phase of anesthesia is the best opportunity to reduce fresh gas flow because circuit gas concentrations are relatively stable and it is often the longest phase of the procedure. There are, however, methods for managing fresh gas flow during induction and emergence that can reduce the amount of wasted anesthetic vapor. This article provides background information and discusses strategies for managing fresh gas flow during each phase of anesthesia with the goal of reducing waste when using a circle anesthesia system. Monitoring oxygen and anesthetic gas concentrations is essential to implementing these strategies safely and effectively. Future technological advances in anesthetic delivery systems are needed to make it less challenging to manage fresh gas flow.     

Technical communication: An initial evaluation of a novel anesthetic scavenging interface

Waste anesthetic gas scavenging technology has not changed appreciably in the past 30 years. Open reservoir systems entrain high volumes of room air and dilute waste gases before emission into the atmosphere. This process requires a large vacuum pump, which is both costly to install and, although efficient, operates continuously and at near-full capacity. In an era of increasing energy costs and environmental awareness, carbon footprint reduction is a priority and a more efficient system of safely scavenging waste anesthetic gases is desirable. We tested a low-flow scavenger interface to evaluate the potential for cost and energy savings. The use of this interface in a suite of 4 operating rooms reduced scavenging flow from a constant 37 L/min to a value equal to the fresh gas flow (usually 2 L/min) for each anesthesia machine. Using the ventilator increased this flow by approximately 6 L/min because of the exhaust of ventilator drive gas into the scavenging circuit. Daytime workload of the central vacuum pump decreased from 92% to 12% (expressed as duty cycle). The new system produces energy savings and may increase vacuum pump lifespan.     

Effect of N2O on Sevoflurane Vaporizer Settings during Minimal- and Low-flow Anesthesia

Background: Uptake of a second gas of a delivered gas mixture decreases the amount of carrier gas and potent inhaled anesthetic leaving the circle system through the pop-off valve. The authors hypothesized that the vaporizer settings required to maintain constant end-expired sevoflurane concentration (Etsevo) during minimal-flow anesthesia (MFA, fresh gas flow of 0.5 l/min) or low-flow anesthesia (LFA, fresh gas flow of 1 l/min) would be lower when sevoflurane is used in oxygen-nitrous oxide than in oxygen.

Methods: Fifty-six patients receiving general anesthesia were randomly assigned to one of four groups (n = 14 each), depending on the carrier gas and fresh gas flow used: group Ox.5 l (oxygen, MFA), group NOx.5 l (oxygen-nitrous oxide, MFA after 10 min high fresh gas flow), group Ox1 l (oxygen, LFA), and group NOx1 l (oxygen-nitrous oxide, LFA after 10 min high fresh gas flow). The vaporizer dial settings required to maintain Etsevo at 1.3% were compared between groups.

Results: Vaporizer settings were higher in group Ox.5 l than in groups NOx.5 l, Ox1 l, and NOx1 l; vaporizer settings were higher in group NOx.5 l than in group NOx1 l between 23 and 47 min, and vaporizer settings did not differ between groups Ox1 l and NOx1 l.     

Respiratory flow measurements and anesthetic drugs: some in vitro observations.

STUDY OBJECTIVE: To illustrate the influence of anesthetic gases on respiratory flow measurements and to correct this influence. DESIGN: In vitro evaluation. SETTING: Laboratory. MEASUREMENTS AND MAIN RESULTS: An in vitro method using a 120-L Tissot water-seal spirometer was used along with a Bicore CP-100, designed for use in intensive care units, and a Datex Ultima, designed for use in the operating room. The flow transducer of one of the instruments being tested was placed in the gas inlet of the Tissot so that simultaneous measurements could be made. Timed flows (2 to 60 L/min) of various gases (O2 and air) and gas mixtures (halothane-O2, isoflurane-O2, N2O-O2, and N2O-O2-isoflurane) were used and the measurements made by the Tissot and the test instrument compared. The Datex Ultima, which includes software corrections for anesthetics, was able to accurately measure gas flows (2 to 60 L/min) of air, 100% oxygen, and anesthetic gas mixtures to within +/- 10% of measurements made by the Tissot. The Bicore CP-100, intended for use with mechanically ventilated patients, accurately measured air and 100% oxygen flow rate to within +/- 8% of the measurements made by the Tissot, but there were large errors (up to 40%) when anesthetics were used. CONCLUSIONS: This study illustrates the effects of anesthetic gases on measurements of ventilatory flow and the need to ascertain whether corrections are needed to improve the accuracy of flow transducers.     

Preparation of the Siemens KION anesthetic machine for patients susceptible to malignant hyperthermia

BACKGROUND: Preparation of anesthetic machines for use with malignant hyperthermia-susceptible (MHS) patients requires that the machines be flushed with clean fresh gas. We investigated the washout of inhalational anesthetics from the KION anesthetic machine. METHODS: In part 1, halothane was circulated through KION anesthetic machines for either 2 or 12 h using a test lung. The times to washout halothane (to 10 parts per million [ppm]) first, from the internal circuitry and second, from the ventilator-patient cassette (without the carbon dioxide absorber) were determined at 5 and 10 l/min fresh gas flow (FGF). In part 2, the rates of washout of halothane or isoflurane from either the KION or Ohmeda Excel 210 machines were compared. The effluent gases were analyzed using calibrated Datex Capnomac Ultima (Helsinki, Finland) and a Miran LB2 Portable Ambient Air Analyzer (Foxboro, Norwalk, CT). RESULTS: Halothane was washed out of the internal circuitry of the KION within 5 min at 10 l/min FGF. Halothane was eliminated from the ventilator-patient cassette in 22 min at the same FGF. The times to reach 10 ppm concentration of halothane and isoflurane in the KION at 10 l/min FGF, 23 to 25 min, was four-fold greater than those in the Ohmeda Excel 210, 6 min. CONCLUSIONS: To prepare the KION anesthetic machine for MHS patients, the machine without the carbon dioxide absorber must be flushed with 10 l/min FGF for at least 25 min to achieve 10 ppm anesthetic concentration. This FGF should be maintained throughout the anesthetic to avoid increases in anesthetic concentration in the FGF.     

Humidification of anesthetic systems for prolonged procedures.

The desirability of humidification of anesthesia systems for prolonged surgical procedures has been documented previously. Dry anesthetic gases damage the ciliated epithelium and cause respiratory heat loss. Chalon suggests that from 12 to 16 mg. of water/L. of gas is necessary to prevent damage to the tracheal epithelium. This study describes a method of obtaining values of from 21.5 plus or minus 0.4 to 39.3 plus or minus 0.1 mg. of water/L. by cycling the fresh gas flow through the carbon dioxide (CO2) absorber before exposure to the patient.     

Preparation of anesthesia machines for patients susceptible to malignant hyperthermia

Malignant hyperthermia is a potentially lethal syndrome that can be triggered by inhaled anesthetics. Thus, it may be appropriate to utilize equipment that minimizes exposure of susceptible patients to inhaled anesthetics. The rate of release of anesthetic stored in anesthesia delivery systems is unknown. To determine residual anesthetic concentrations, the washout rates of halothane and isoflurane were compared, and the effects of a 1-l/min and a 10-l/min fresh gas flow were evaluated. Halothane concentrations were also measured in samples taken from the fresh gas outlet and the Y-piece of the circle system during four separate studies in which various components of the anesthesia system were replaced. In each study an Ohio Modulus anesthesia machine equipped with an Air-Shields ventilator was exposed to 2% halothane for 18 h. Anesthetic concentrations were determined by a gas chromatograph having a sensitivity of 0.1 ppm. Isoflurane washed out 3-4 times faster than halothane. Residual halothane concentration was approximately equal to tenfold greater when the fresh gas flow was 1 l/min rather than 10 l/min: 194 versus 19 ppm after 1 h of washout. Using a 10-l/min fresh gas flow, halothane concentrations in samples obtained from the Y-piece were similar with original or fresh soda lime but were more than tenfold lower after the fresh gas outlet hose and circle system were replaced (approximately equal to 50 ppm vs. approximately equal to 5 ppm after 5 min of washout).(ABSTRACT TRUNCATED AT 250 WORDS)     

The comparative effects of propofol versus thiopental on middle cerebral artery blood flow velocity during electroconvulsive therapy

Electroconvulsive therapy provokes abrupt changes in both systemic and cerebral hemodynamics. An anesthetic that has a minor effect on cerebral hemodynamics might be more suitable for patients with intracranial complications, such as cerebral aneurysm. The purpose of our present study was to compare the effects of thiopental and propofol on cerebral blood flow velocity. We continuously compared cerebral blood flow velocity at the middle cerebral artery (MCA) during electroconvulsive therapy, using propofol (1 mg/kg, n = 20) versus thiopental (2 mg/kg, n = 20) anesthesia. Systemic hemodynamic variables and flow velocity at the MCA were measured until 10 min after the electrical shock. Heart rate and arterial blood pressure increased in the thiopental group until 5 min after the electrical shock. In the propofol group, an increase in mean blood pressure was observed to 1 min after the electrical shock. Mean flow velocity at the MCA decreased after anesthesia in both groups, and increased at 0.5-3 min after the electrical shock in the thiopental group and at 0.5 and 1 min after the shock in the propofol group. The flow velocities at 0.5-5 min after the electrical shock were significantly more rapid in the thiopental group compared with the propofol group. ?abs? Implications: Cerebral blood flow velocity change, measured by transcranial Doppler sonography during electroconvulsive therapy, was minor using propofol anesthesia compared with barbiturate anesthesia. Propofol anesthesia may be suitable for patients who cannot tolerate abrupt cerebral hemodynamic change.     

D-glucaric acid. A new method proposed to investigate damage from exposure to anesthetic gas and vapors

Acute exposure and hepatotoxicity following full anesthetic doses is controversial, despite various experimental trials. On the other hand, microsomial enzyme induction following the chronic use of anesthetic gases and vapours, even with minimal metabolism, has been established. Therefore, there is increasing interest in the field of prevention to develop techniques and instruments to minimize pollution of anesthetic vapours and screening methods to detect early liver-damage. We have evaluated the reliability of D-Glucaric Acid test in monitoring microsomial enzyme induction after anesthesia. We evaluated urinary excretion of D-Glucaric Acid before and after exposure to anesthetic gases in 53 subjects, including medical personnel and patients. Statistical analysis of these data confirm the usefulness of this technique to assess acute liver damage in patients undergoing general anesthesia (postoperative increase of 10.5 microns/l as average value of urinary acid excretion, chi 2 = 9.8; p less than 0.01). This technique is also valuable in demonstrating damage in operating room personnel due to chronic exposure (base values greater than 12 microns/l in anesthesiologists with a 9 microns/l increase at the end of surgical intervention, chi 2 = 8.1; p less than 0.01). Data of patients submitted to local anesthesia are more difficult to interprete. They presented a decrease in acid urinary excretion (less than 10 microns/l; chi 2 = 1.93; p less than 0.2) probably due to hemodynamic changes which occurred during spinal block or due to degree of sedation related to de-afferentation itself.     

Contributions of liver perfusion flow rate and enzyme inhibition to altered verapamil clearance with halothane: a study in the isolated perfused rat liver

Verapamil clearance is reduced during halothane administration. This study evaluated relative contributions of reduced hepatic flow rate and hepatic metabolizing enzyme inhibition by halothane as a cause of reduced verapamil clearance. An isolated perfused rat liver model was utilized in which flow rate could be fixed during halothane administration. Perfusions were performed on five to six livers under each of the following conditions: (a) control--40 mL/min flow rate with no anesthetic exposure; (b) 1.5% halothane--40 mL/min; (c) 2.25% halothane--40 mL/min; (d) reduced flow--20 mL/min with no anesthetic exposure; and (e) reduced flow with 1.5% halothane--20 mL/min. Halothane caused dose-dependent decreases in both total hepatic and intrinsic clearance rates (P less than 0.05). With no anesthetic exposure, a flow reduction of 50% (20 mL/min) also gave a large reduction (P less than 0.05) in hepatic clearance of verapamil compared with the control condition (40 mL/min). The addition of 1.5% halothane to the reduced flow condition was not associated with further reduction in hepatic clearance rate. Results of this study suggest that although both reduced hepatic perfusion and hepatic enzymatic inhibition by halothane administration are associated with decreased verapamil clearance, a greater proportion of this decrease appears to be due to reductions in hepatic flow. The present results may apply to other drugs used in anesthesia that have high hepatic extraction ratios; thus, clearance of these drugs may be more dependent on hepatic blood flow than on hepatic enzyme activity.     

Preparation of the Siemens KION Anesthetic Machine for Patients Susceptible to Malignant Hyperthermia

Background: Preparation of anesthetic machines for use with malignant hyperthermia-susceptible (MHS) patients requires that the machines be flushed with clean fresh gas. We investigated the washout of inhalational anesthetics from the KION anesthetic machine.

Methods: In part 1, halothane was circulated through KION anesthetic machines for either 2 or 12 h using a test lung. The times to washout halothane (to 10 parts per million [ppm]) first, from the internal circuitry and second, from the ventilator-patient cassette (without the carbon dioxide absorber) were determined at 5 and 10 l/min fresh gas flow (FGF). In part 2, the rates of washout of halothane or isoflurane from either the KION or Ohmeda Excel 210 machines were compared. The effluent gases were analyzed using calibrated Datex Capnomac Ultima (Helsinki, Finland) and a Miran LB2 Portable Ambient Air Analyzer (Foxboro, Norwalk, CT).

Results: Halothane was washed out of the internal circuitry of the KION within 5 min at 10 l/min FGF. Halothane was eliminated from the ventilator-patient cassette in 22 min at the same FGF. The times to reach 10 ppm concentration of halothane and isoflurane in the KION at 10 l/min FGF, 23 to 25 min, was four-fold greater than those in the Ohmeda Excel 210, 6 min.     

Hemodynamic effects of anesthetic agents and bypass flow during orthotopic liver transplantation in pigs

Although the hemodynamics during orthotopic liver transplantation is unstable, it is very important for new liver to get well-controlled hemodynamics. Thus hemodynamic changes were studied, especially in relation to the influence of anesthetic agents and bypass flow in orthotopic liver transplantation in pig. Hemodynamic changes associated with NLA, GOS (sevoflurane) and GOF anesthesia were evaluated. It was difficult to maintain arterial pressure and to recover cardiac output with NLA after the bypass was removed. GOS, with which the hemodynamic condition was best maintained and no hepatotoxicity was manifest, proved the most useful of the three anesthetic agents. Hemodynamic studies based on bypass flow were made by comparing two groups, a high flow (31 +/- 4ml/kg/min) and a low flow (19 +/- 2ml/kg/min) groups, following the bypass model study conducted by 20, 30 and 40ml/kg/min. flow rates with the fixed infusion speed. In the high flow group, cardiac output and pulmonary arterial pressure were better maintained during the anhepatic phase and at the removal of the bypass. It is estimated that the low flow group was within the limits permitted, but beyond safety limits, also from the bypass model study. It is suggested that approximately 30ml/kg/min was the appropriate flow rate in pig.     

Success rate of unilateral spinal anesthesia is dependent on injection flow

BACKGROUND AND OBJECTIVES: The dependence of unilateral spinal anesthesia on injection flow is controversial. We hypothesized that it is possible to achieve strictly unilateral sympathetic block (as assessed by temperature measurements of the limbs) and unilateral sensory and motor block, respectively, during spinal anesthesia by a slow and steady injection of a hyperbaric local anesthetic solution. METHODS: Forty-four patients (American Society of Anesthesiologists [ASA] physical status I-III) undergoing surgery of one lower extremity were randomly assigned to one of two groups. Dependent on the patients' height, 1.4 to 1.7 mL hyperbaric bupivacaine 0.5% was injected manually with the patient in the lateral decubitus position, which was maintained for 30 minutes after injection. Injection flow was approximately 0.5 mL/min in group I ("air-buffered" injections performed by 4 mL air between the local anesthetic and the syringe's plunger, n = 25) and approximately 7.5 mL/min in group II ("conventional" injections, n = 19). Sympathetic block was defined as a temperature increase of more than 0.5 degrees C at the foot. Any reduction in the ability to move the hip, knee, or ankle as well as loss of temperature discrimination and/or pinprick even in one dermatome on the nondependent side was considered as a bilateral block. RESULTS: Before surgery, significant differences (P < .05) were observed for unilateral motor paralysis (92% in group I v 68.4% in group II), unilateral sensory block (48.0% v 10.5%), and unilateral sympathetic block (72% v 42.1%). Strictly unilateral spinal anesthesia was found to be significantly more frequent in group I (40% v 5.3%). Significant hemodynamic differences between the groups were not detected. CONCLUSIONS: For hyperbaric spinal anesthesia, the injection flow is an important factor in achieving unilateral sympathetic block. A slow injection proves useful to restrict spinal anesthesia to the side of surgery.     

High Carboxyhemoglobin Concentrations Occur in Swine during Desflurane Anesthesia in the Presence of Partially Dried Carbon Dioxide Absorbents

Background: Increased carboxyhemoglobin concentrations in patients receiving inhalation anesthetics (desflurane, enflurane, and isoflurane) have been reported. Recent in vitro studies suggest that dry carbon dioxide absorbents may allow the production of carbon monoxide.

Methods: The authors used high fresh oxygen flow (5 or 10 l/min) through a conventional circle breathing system of an anesthesia machine for 24 or 48 h to produce absorbent drying. Initial studies used 10 l/min oxygen flow with the reservoir bag removed or with the reservoir bag left in place during absorbent drying (this increases resistance to gas flow through the canister). A third investigation evaluated a lower flow rate (5 l/min) for absorbent drying. Water content of the absorbent and temperature were measured. Pigs received a 1.0 (human) minimum alveolar concentration desflurane anesthetic (7.5%) for 240 min using a 1 l/min oxygen flow rate with dried absorbent. Carbon monoxide concentrations in the circuit and carboxyhemoglobin concentrations in the pigs were measured.

Results: Pigs anesthetized with desflurane using Baralyme exposed to 48 h of 10 l/min oxygen flow (reservoir bag removed) had extremely high carboxyhemoglobin concentrations (more than 80%). Circuit carbon monoxide concentrations during desflurane anesthesia using absorbents exposed to 10 l/min oxygen flow (reservoir bag, 24 h) reached peak values of 8,800 to 13,600 ppm, depending on the absorbent used. Carboxyhemoglobin concentrations reached peak values of 73% (Baralyme) and 53% (soda lime). The water content of Baralyme decreased from 12.1 +/- 0.3% (mean +/- SEM) to as low as 1.9 +/- 0.4% at the bottom of the lower canister (oxygen flow direction during drying was from bottom to top). Absorbent temperatures in the bottom canister increased to temperatures as high as 50 [degree sign] Celsius. With the reservoir bag in place during drying (10 l/min oxygen flow), water removal from Baralyme was insufficient to produce carbon monoxide (lowest water content = 5.5%). Use of 5 l/min oxygen flow (reservoir bag removed) for 24 h did not reduce water content sufficiently to produce carbon dioxide with desflurane.     

Effect of N2O on sevoflurane vaporizer settings during minimal- and low-flow anesthesia

BACKGROUND: Uptake of a second gas of a delivered gas mixture decreases the amount of carrier gas and potent inhaled anesthetic leaving the circle system through the pop-off valve. The authors hypothesized that the vaporizer settings required to maintain constant end-expired sevoflurane concentration (Etsevo) during minimal-flow anesthesia (MFA, fresh gas flow of 0.5 l/min) or low-flow anesthesia (LFA, fresh gas flow of 1 l/min) would be lower when sevoflurane is used in oxygen-nitrous oxide than in oxygen. METHODS: Fifty-six patients receiving general anesthesia were randomly assigned to one of four groups (n = 14 each), depending on the carrier gas and fresh gas flow used: group Ox.5 l (oxygen, MFA), group NOx.5 l (oxygen-nitrous oxide, MFA after 10 min high fresh gas flow), group Ox1 l (oxygen, LFA), and group NOx1 l (oxygen-nitrous oxide, LFA after 10 min high fresh gas flow). The vaporizer dial settings required to maintain Etsevo at 1.3% were compared between groups. RESULTS: Vaporizer settings were higher in group Ox.5 l than in groups NOx.5 l, Ox1 l, and NOx1 l; vaporizer settings were higher in group NOx.5 l than in group NOx1 l between 23 and 47 min, and vaporizer settings did not differ between groups Ox1 l and NOx1 l. CONCLUSIONS: When using oxygen-nitrous oxide as the carrier gas, less gas and vapor are wasted through the pop-off valve than when 100% oxygen is used. During MFA with an oxygen-nitrous oxide mixture, when almost all of the delivered oxygen and nitrous oxide is taken up by the patient, the vaporizer dial setting required to maintain a constant Etsevo is lower than when 100% oxygen is used. With higher fresh gas flows (LFA), this effect of nitrous oxide becomes insignificant, presumably because the proportion of excess gas leaving the pop-off valve relative to the amount taken up by the patient increases. However, other unexplored factors affecting gas kinetics in a circle system may contribute to our observations.