Respiratory gas exchange. Anesthesia with enflurane or isoflurane in nitrous oxide during spontaneous and controlled ventilation

The estimation of oxygen consumption and carbon dioxide elimination is essential for predicting the metabolic activity and needs of any patient having anaesthesia. During anaesthesia oxygen consumption can be measured and compared to a predicted value. However, oxygen uptake is affected by anaesthetic agents, which complicates the interpretation of measured oxygen uptake rate. The purpose of this study was to investigate whether there are any differences in respiratory gas exchange during anaesthesia with enflurane and isoflurane and also to assess the effects of spontaneous versus controlled ventilation. METHODS. Forty orthopedic patients were randomized to enflurane or isoflurane anaesthesia in nitrous oxide with either spontaneous or controlled ventilation. A fresh low-gas-flow technique was used. Inspiratory oxygen and end-tidal carbon dioxide concentrations and expiratory minute ventilation were measured in a circle absorber system between the y-piece and the endotracheal tube with a sampling analyser. Between the mixing box and the absorption canister, carbon dioxide concentration was continuously measured. The carbon dioxide elimination was calculated from mixed expired concentration and expiratory minute ventilation. Excess gas was collected every 10 min in a non-permeable mylar plastic bag connected to the excess valve. The excess gas flow was calculated and the oxygen uptake rate was assumed to be the difference between the oxygen fresh gas flow and the oxygen excess gas flow. RESULTS. The grand mean oxygen uptake rate was 2.5 ml.kg-1 x min-1 or 100 ml.min-1 x m-2. There were no statistically significant differences in oxygen uptake between enflurane and isoflurane anaesthesia or between spontaneous and controlled ventilation. The mean oxygen uptake rate at 10 min was between 2.0 and 2.2 ml.kg-1 x min-1 in all groups. At 30 min the mean oxygen uptake rates were 2.6 to 2.8 ml.kg-1 x min-1. Carbon dioxide elimination was closely associated with expired minute ventilation, with a carbon dioxide excretion of about 30 ml per litre gas exhaled, irrespective of ventilatory mode employed.     

Three isoflurane preparations from various manufacturers. A comparison of isoflurane concentrations in fresh gas after vaporization with a Drager vaporizer type 19.3

OBJECTIVES: The purpose of this study was to check the precision of the Dr?ger vaporizer model 19.3 when filled with three different preparations of isoflurane. METHODS: Six Dr?ger vaporizers model 19.3 calibrated with forene were filled with forene (Abbott), isoflurane (Lilly) and isoflurane (Pharmacia); gas output was measured by infrared absorption (Irina, Dr?ger) at vaporizer settings of 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 and 5.0 vol%, starting with a fresh gas flow of 2 1/min followed by 4, 6 and 12 1/min. Thus each of the three isoflurane preparations was checked in six different vaporizers and with four different fresh gas flows. RESULTS: Within the concentration range used in clinical practice there was no significant difference in the delivery of the three isoflurane preparations. Each of the six vaporizers produced a controllable and predictable concentration of the three preparations. CONCLUSION: Vaporizers of Dr?ger type 19.n calibrated with forene deliver the same predictable concentration of the volatile anaesthetic when filled with isoflurane from Lilly or isoflurane from Pharmacia instead of forene and may be used without impairment in patient safety. In addition, no specific calibration with one of the new isoflurane preparations is required.     

Changing from isoflurane to desflurane toward the end of anesthesia does not accelerate recovery in humans

BACKGROUND: In an attempt to combine the advantage of the lower solubilities of new inhaled anesthetics with the lesser cost of older anesthetics, some clinicians substitute the former for the latter toward the end of anesthesia. The authors tried to determine whether substituting desflurane for isoflurane in the last 30 min of a 120-min anesthetic would accelerate recovery. METHODS: Five volunteers were anesthetized three times for 2 h using a fresh gas inflow of 2 l/min: 1.25 minimum alveolar concentration (MAC) desflurane, 1.25 MAC isoflurane, and 1.25 MAC isoflurane for 90 min followed by 30 min of desflurane concentrations sufficient to achieve a total of 1.25 MAC equivalent ("crossover"). Recovery from anesthesia was assessed by the time to respond to commands, by orientation, and by tests of cognitive function. RESULTS: Compared with isoflurane, the crossover technique did not accelerate early or late recovery (P > 0.05). Recovery from isoflurane or the crossover anesthetic was significantly longer than after desflurane (P < 0.05). Times to response to commands for isoflurane, the crossover anesthetic, and desflurane were 23 +/- 5 min (mean +/- SD), 21 +/- 5 min, and 11 +/- 1 min, respectively, and to orientation the times were 27 +/- 7 min, 25 +/- 5 min, and 13 +/- 2 min, respectively. Cognitive test performance returned to reference values 15-30 min sooner after desflurane than after isoflurane or the crossover anesthetic. Isoflurane cognitive test performance did not differ from that with the crossover anesthetic at any time. CONCLUSIONS: Substituting desflurane for isoflurane during the latter part of anesthesia does not improve recovery, in part because partial rebreathing through a semiclosed circuit limits elimination of isoflurane during the crossover period. Although higher fresh gas flow during the crossover period would speed isoflurane elimination, the amount of desflurane used and, therefore, the cost would increase.     

Steady-state PVECP is superior to transient PVECP in the diagnosis of optic neuritis

The isoflurane-saving and CO2-retaining effects of a charcoal filter were compared with a Siemens standard heat and moisture (HME) exchanger and an emptied specimen (dummy). Isoflurane was delivered during the inspiratory phase and consumption investigated at 10, 15 and 25 cycles min-1. The investigation was performed by ventilation with humidified air with a constant end-tidal CO2 and temperature. For a comparison, isoflurane was delivered in a conventional manner via the ventilator. The arrangement with a charcoal filter reduced the isoflurane consumption by a factor of 2.0-2.6, depending on ventilatory rate. Most of the saving was a result of the method of isoflurane delivery (factor 1.4-2.0), while adding the reflector gave a further reduction (factor 1.3-1.5). One circumstance that reduced the net efficiency of the charcoal filter was that it also reflected CO2; consequently, total minute ventilation had to be increased to maintain constant end-tidal CO2.     

Anaesthetic machine and breathing system contamination and the efficacy of bacterial/viral filters

Contamination of the anaesthetic machine and breathing system by the environment and by patient exposure has been shown to occur. Outside the intensive care setting, however, it is difficult to demonstrate that the anaesthetic machine and breathing system are a vector for patient cross-infection. Bacterial and viral filters for use within the breathing system have been shown to be very effective for filtration, yet their use has not been demonstrated to be of benefit in the prevention of cross-infection between patients. Several instances of patient morbidity are a direct consequence of filter use. The use of bacterial/viral filters may represent another step towards defensive medical practice.     

Dependence of surgical trauma in aortic interventions on the approach chosen. A prospective study

A study was undertaken to assess the performance of the Komesaroff vaporizer, placed within the circuit, in ventilated patients during maintenance of closed circuit anaesthesia with halothane or isoflurane. Following intravenous induction, anaesthesia was maintained by inhalation. This was achieved using a conventional vaporizer outside the circle for the first 10 minutes to manage the fast uptake phase. The fresh gas flow was then reduced to the basal oxygen requirement with the Komesaroff vaporizer within the circle maintaining inhalational anaesthesia. Complete isolation of the circuit was achieved by returning all anaesthetic gases to the circuit following analysis and using a bag-in-bottle ventilator. The Komesaroff vaporizer dial was positioned at between the first and second division and end-tidal volatile anaesthetic agent levels were measured. This study demonstrated that at dial positions 1 or 1.5 with either agent, the end-tidal volatile concentration plateaued at clinically acceptable levels. The Komesaroff vaporizer can therefore be used safely in ventilated patients to maintain closed circuit anaesthesia provided clinical observation and monitoring are meticulous.     

Comparison of disposable circle and 'to-and-fro' breathing systems during anaesthesia in dogs

Low-flow anaesthesia is beneficial in terms of reducing atmospheric pollution with waste anaesthetics and improving economy. This study compared a disposable circle and a 'to-and-fro' breathing system at low fresh gas flows (10 ml/kg/minute) in 19 dogs undergoing ovariohysterectomy. Ten dogs were assigned to the circle and nine to the to-and-fro breathing system. Fractional inspired halothane, end-tidal carbon dioxide and halothane were higher and mean blood pressure was lower in dogs using the to-and-fro system, possibly indicating an increased anaesthetic depth in this group. Use of both systems resulted in an elevated inspired carbon dioxide level, although this was significantly lower in the circle system. Further work will be required to determine the clinical relevance of this difference and whether rebreathing can be eliminated by higher fresh gas flows. The disposable circle studied may be used safely in dogs.     

Toxic effect of systemic administration of low doses of the plasticizer di-(2-ethyl hexyl) phthalate [DEHP] in rats

A spreadsheet model of a circle breathing system and a 70-kg anaesthetised 'standard man' has been used to simulate the first 20 min of low-flow anaesthesia with halothane, enflurane, isoflurane, sevoflurane and desflurane in oxygen. It is shown that, with the fresh-gas flow set initially equal to the total ventilation and the fresh-gas partial pressure to 3 MAC, the end-expired partial pressure can be raised to 1 MAC in 1 min with desflurane and sevoflurane, 1.5 min with isoflurane, 2.5 min with enflurane and 4 min with halothane. Sequences of lower fresh-gas flow and partial pressure settings are given for then maintaining 1 MAC end-expired partial pressure, with a minimum usage of anaesthetic, e.g. 13 ml of liquid desflurane in 20 min (of which only 33% is taken up by the patient) if the minimum acceptable flow is 11.min-1, or 8 ml (with 57% in the patient) if the minimum is 250 ml.min-1.     

Occupational exposure to inhalation anesthetics at the work-place of veterinary surgery

In a prospective study we evaluated the work-place pollution by isoflurane and nitrous oxide during various anaesthetic procedures in animal surgery. The study was conducted during one working week at an University Animal Department. Trace concentrations of isoflurane and nitrous oxide were directly measured every minute in the breathing zone by means of a photoacoustic infrared spectrometer in two different operating rooms (OR) with an air turnover of 17 changes per hour. In one OR the 8-hour time-weighted average (mean +/- SEM) was calculated to be 12.3 +/- 9.9 ppm nitrous oxide and 1.9 +/- 2.5 ppm isoflurane. The other OR, where only isoflurane was used, was contaminated with 5.3 +/- 8.1 ppm isoflurane. In the first OR, the trace gas concentrations were low and comparable to values obtained under human anaesthesia in adults and children. The higher contamination in the second OR resulted from performing inhalational anaesthesia with an open mask system in birds and small animals. Although the mean values were below the recommended occupational exposure standards, some high peak values (> 300 ppm isoflurane) violated these threshold limits. We recommend the use of a local scavenging device, if other alternatives such as total intravenous anaesthesia are not possible.     

Accumulation of acetone in blood during long-term anaesthesia with closed systems

During closed system anaesthesia with isoflurane, patients with a preoperative increase in blood concentration of acetone (> 10 mg litre-1) had a significantly greater concentration of acetone than patients with an initial normal blood concentration of acetone (P < 0.01). Flushing the closed system with a high flow of fresh gas had no effect on the blood concentration of acetone. Using a large fresh gas flow, there was no increase in blood acetone concentration. Acetone concentrations of about 50 mg litre-1 cause problems such as nausea and vomiting in the postoperative period. These symptoms occurred more frequently after closed system anaesthesia.     

Residual dipolar couplings as new conformational restraints in isotropically 13C-enriched oligosaccharides

Closed-circuit anesthesia (CCA) has certain advantages such as decreased cost, decreased anesthetic gas pollution, improved inhalational gas humidity and temperature in comparison to conventional inhalational anesthesia using a high fresh gas flow, i.e. more than 2 L x min(-1), with a semi-closed breathing circuit. The main disadvantage of CCA is the possibility of hypoxic anesthetic gas delivery. This potentially lethal situation is caused by an insufficient oxygen flow rate for the body metabolism or by the accumulation of inactive gas, usually nitrogen, within the breathing circuit in spite of a sufficient oxygen concentration in the fresh gas supply to the breathing circuit. In the latter case, the accumulation of inactive gas may also lead an increased risk of awareness because of its dilution effect on the concentrations of inhalational anesthetics. We herein present a case of air contamination of the breathing circuit through a sampling line of an anesthetic gas monitor. The air caused a decrease in the oxygen concentration during closed circuit anesthesia.     

Prevention of contamination with a heat-and-moisture-exchanger (HME) and bacterial filter during clinical anesthesia

Although the use of HME and bacterial filter is a common practice to protect the anesthesia machine as well as the patients from bacterial contaminants, there is no report demonstrating the effectiveness of this filter in clinical anesthesia setting. We evaluated the actual effectiveness of the filter during clinical use. While the anesthesia breathing circuit, two bacterial filters (BB 50 T, Nihon PALL) and anesthesia bag, which were sterilized with ethylen oxide gas (EOG), were connected to the anesthesia machine and used continuously for one week, EOG sterilized HME and bacterial filter (BB 25 A, Nihon PALL) were changed before each anesthesia. Culture samples were taken from the BB 25 A, the breathing circuit and the machine side of the BB 50 T. Of the 117 BB 25 A samples taken, 6 were positive for Micrococcus, alpha-Streptococcus, Bacillus, and Staphylococcus epidermidis. From 21 breathing circuit "internal" samples, one was positive for Bacillus, Staphylococcus epidermidis. But the contamination from outside sources was suspected, since all the BB 25 as used with this circuit were negative. Use of BB 25 A prevents contamination of the breathing circuit for a period of one week. If we use BB 25 A in every anesthesia case, the changing of the breathing circuit is unnecessary, reducing the cost and simplifying procedures during clinical practice.     

Behavior of Field-Scale Biotrickling Filter under Nonsteady State Conditions

The performance of a field-scale biotrickling filter was investigated for the treatment of styrene vapors released from a bathtub manufacturing process. The two-stage biotrickling filter was operated in series with an average gas flow rate of 350 m3?h?1 corresponding to an overall empty bed gas contact time of 84 s. Daily average values of styrene removal efficiency varied from 40 to 90% with inlet concentrations ranging between 0.4 and 1.7 g?m?3. System performance was not significantly affected by changes in temperature and was moderately susceptible to 3-day starvation or complete system shutdown. After 7 months of styrene treatment, toluene contaminated air was fed to the system and experiments were performed in which styrene and toluene were fed alternately at 3-h intervals. While styrene elimination remained unchanged over the cycles, the elimination capacity of toluene increased with the number of cycles, indicating some adaptation of the process culture to the new contaminant. Overall, the results suggest that biotrickling filters for air pollution control can be successful even under greatly varying operating conditions.     

Arthroscopy in the elderly

The uptake rate of oxygen and nitrous oxide were studied during low flow anaesthesia with enflurane or isoflurane in nitrous oxide with either spontaneous or controlled ventilation. The excess gas flow and composition were analysed. The nitrous oxide uptake rate was in agreement with Severinghaus' formula VN20 1000.t-0.5. The composition of excess gas was predictable and the following formula for oxygen uptake could be derived: VO2 = VfgO2-0.45 (VfgN2(0)-(kg: 70.1000.t-0.5)) where oxygen uptake rate (VO2, ml.min-1) equals oxygen fresh gas flow (VfgO2) minus 0.45 times the difference between the fresh gas flow of nitrous oxide (VfgN2O), ml.min-1 and estimated uptake of nitrous oxide. The equation assumes constant inspired gas concentrations of 30% oxygen and 65-70% nitrous oxide. The oxygen uptake rates calculated from this formula were in good agreement with measured uptake rates. Thus, continuous monitoring of oxygen uptake rates is possible by using only reliable flowmeters and analysis of inspired oxygen concentration.     

Application of a new method for analysis of exhaled gas in critically ill patients

OBJECTIVE: Application of a new method for analysis of exhaled gas in critically ill patients. DESIGN: Open study. SETTING: Surgical intensive care unit of an university hospital. PATIENTS: Thirty-seven consecutive, critically ill, mechanically ventilated patients. INTERVENTIONS: None. MEASUREMENTS AND RESULTS: Chemical analysis of the patient's exhaled gas was based upon substance adsorption and concentration onto activated charcoal, microwave desorption and gas chromatographic separation. Patients with acute respiratory distress syndrome (ARDS) exhaled less isoprene than those without ARDS [9.8 (8.2-21.6) vs 21.8 (13.9-41.4) nmol/m2 per min [median (95% confidence interval)], p = 0.04]. In patients who developed pulmonary infection, pentane elimination increased from 0.4 (0.0-5.4) to 2.7 (0.6-6.1, p = 0.05) nmol/m2 per min and isoprene elimination decreased from 5.2 (0-33) to 5.0 (0-17, p = 0.05) nmol/m2 per min, resulting in a significant increase in pentane/isoprene ratio from 0.1 (0-0.3) to 0.4 (0-15, p = 0.007) when compared to patients without pulmonary infection. CONCLUSIONS: The new method allows quantitative analysis of human gas samples with low substance concentrations and is well suited for clinical studies which involve the investigation of metabolic processes in the lung and the body.     

The mechanism of increased renal clearance of amylase in acute pancreatitis

Penicillium urticae (NRRL 2159A) was grown in culture broth containing 1 muCi of [1-14C-A1acetate to produce [14C]patulin. [14C]patulin was purified from the broth and added to apple cider. After the patulin concentration of the cider was adjusted to 30 mug/ml with unlabeled patulin, the cider was subjected to various charcoal treatments. [14C]patulin was completely removed by shaking the cider with 20 mg of activated charcoal per ml and by eluting the cider through a 40- to 60-mesh charcoal column. Activated charcola at 5 mg/ml reduced patulin in naturally contaminated cider to nondetectable levels.     

Economic considerations of the use of new anesthetics: a comparison of propofol, sevoflurane, desflurane, and isoflurane

Cost control in anesthesia is no longer an option; it is a necessity. New anesthetics have entered the market, but economic differences in comparison to standard anesthetic regimens are not exactly known. Eighty patients undergoing either subtotal thyroidectomy or laparoscopic cholecystectomy were randomly divided into four groups, with 20 patients in each group. Group 1 received propofol 1%/sufentanil, Group 2 received desflurane/sufentanil, Group 3 received sevoflurane/sufentanil, and Group 4 received isoflurane/sufentanil (standard anesthesia) for anesthesia. A fresh gas flow of 1.5-2 L/min and 60% N2O in oxygen was used for maintenance of anesthesia, and atracurium was given for muscle relaxation. Concentrations of volatile anesthetics, propofol, and sufentanil were varied according to the patient's perceived need. Isoflurane, desflurane, and sevoflurane consumption was measured by weighing the vaporizers with a precision weighing machine. Biometric data, time of surgery, and time of anesthesia were similar in the four groups. Times for extubation and stay in the postanesthesia care unit (PACU) were significantly longer in the isoflurane group. Use of sufentanil and atracurium did not differ among the groups. Propofol patients required fewer additional drugs in the PACU (e.g., antiemetics), and thus showed the lowest additional costs in the PACU. Total (intra- and postoperative) costs were significantly higher in the propofol group ($30.73 per patient; $0.24 per minute of anesthesia). The costs among the inhalational groups did not differ significantly (approximately $0.15 per minute of anesthesia). We conclude that in today's climate of cost savings, a comprehensive pharmacoeconomic approach is needed. Although propofol-based anesthesia was associated with the highest cost, it is doubtful whether the choice of anesthetic regimen will lower the costs of an anesthesia department. IMPLICATIONS: Cost analysis of anesthetic techniques is necessary in today's economic climate. Consumption of the new inhaled drugs sevoflurane and desflurane was measured in comparison to a standard anesthetic regimen using isoflurane and an IV technique using propofol. Propofol-based anesthesia was associated with the highest costs, whereas the costs of the new inhaled anesthetics sevoflurane and desflurane did not differ from those of a standard, isoflurane-based anesthesia regimen.     

The parallel Lack anaesthetic breathing system

The parallel Lack system is a new modification of the Mapleson A system comprising separate inspiratory and expiratory tubes. To determine that the function of the system was that anticipated of a Mapleson A, the fresh gas flow requirements to prevent rebreathing during spontaneous ventilation were assessed in three situations: (1) a lung model (2) conscious volunteers and (3) anaesthetised patients. Two sets of criteria to define rebreathing were used; (A) those based on changes in ventilation or end-expired carbon dioxide tension and (B) minimum inspired carbon dioxide tension. Using A, rebreathing occurred at a fresh gas flow to minute ventilation ratio (VF/VE) of 0.75 for the lung model, and 0.73 for conscious volunteers. These results were comparable to those obtained for a Magill attachment. They were also close to the point at which mechanical dead space began to increase in the lung model. Criteria B gave much lower values for the onset of rebreathing. Rebreathing was present by criteria A in five of the six anaesthetised patients at a fresh gas flow of 60 ml.kg-1.min-1 (VF/VF of 0.78). The results confirm that the parallel Lack behaves as a Mapleson A system. The resistance to breathing posed by the parallel Lack was also comparable to the Magill system.     

Normocapnic ventilation using the circle system

A formula is derived for maintaining normocapnia during controlled ventilation using a circle system without carbon dioxide absorption. In a series of 70 patients, unselected in terms of age, sex, obesity, ASA status, body position during operation, type of anaesthetic administered or type of circle system used, it was found that a total fresh gas flow of 50 ml/kg body weight/min and a minute ventilation of 120 to 150 ml/kg body weight at a rate of 10 to 12/min achieved normocapnia. For moderate hypocapnia a total fresh gas flow of 60 ml/kg body weight/min and a minute ventilation of 120 ml/kg at a rate of 10-12/min is suggested.     

Ondansetron for prevention of postoperative nausea and vomiting following minor oral surgery: a double-blind randomized study

The efficacy and safety of ondansetron in preventing postoperative nausea and vomiting following minor oral surgery was evaluated in a prospective randomized double-blind study. Of a total of seventy-seven patients, randomly 38 had 4 mg of ondansetron and 39 had normal saline as placebo intravenously immediately prior to induction of anaesthesia. A standard general anaesthetic with thiopentone, suxamethonium, fentanyl, nitrous oxide and isoflurane was employed. Postoperatively nausea was assessed verbally and on a visual analog scale at 1, 4 and 24 hours from the time of awakening. Episodes of vomiting were recorded. Eight patients (21.1%) in the ondansetron group compared to 19 (48.7%) in the placebo group had nausea (P < 0.05) and 1 (2.6%) in the ondansetron group compared with 9 (23.1%) in the placebo group vomited (P < 0.05). Patients who vomited twice or more and the number who required a rescue antiemetic were significantly fewer in the ondansetron group (P < 0.05). Cardiovascular parameters were stable and showed no significant difference in the two groups. There were no significant adverse effects that could be directly attributable to ondansetron.