Gaseous homeostasis during low-flow anaesthesia

The object of this clinical study was to investigate the circle system gas homeostasis during low-flow anaesthesia using a technique designed to keep a constant inspired oxygen fraction of 0.30. Denitrogenation was adequately accomplished with mask preoxygenation, 10 l/min, for 1 min and an initial fresh gas flow of 5 l/min for 6 min after intubation. There was no need to wash out accumulated nitrogen at intervals, since the already low nitrogen concentration in the system tended to decrease after 1 h. The fresh gas flow of nitrous oxide to oxygen ratio and the inspiratory to end-expiratory oxygen concentration difference both reflected the uptake of nitrous oxide. The calculated rates of uptake of nitrous oxide, a subject of controversy, were in accordance with those found by Severinghaus and Barton & Nunn.     

A low flow open circle system for anaesthesia Part 2: Clinical evaluation

A prototype valveless ventilator was attached by open deadspace tubing to a circle system and used to ventilate the lungs of 12 patients with low flows of anaesthetic gases for periods between 60 and 120 minutes during intra-abdominal surgery. Anaesthesia was induced with thiopentone and maintained with nitrous oxide 50% in oxygen and enflurane. This was reduced to 2 litres/minute after a 10-minute period of nitrogen wash out and stabilisation of anaesthetic gas concentration, with an initial anaesthetic gas flow of 6 litres/minute. The concentration of oxygen, carbon dioxide, nitrous oxide and enflurane were measured in the outflow from both the anaesthetic machine and the inspiratory limb of the circle system. The measured mean inspired oxygen and nitrous oxide concentrations showed no significant variation throughout the low flow period of the study. This new low flow open circle ventilation system appears to offer some advantages in terms of safety and versatility over other systems which are discussed.     

A minimal-flow system for xenon anesthesia

We described a minimal-flow system for xenon anesthesia during controlled ventilation. A computer maintained oxygen concentration in the anesthesia circle within +/- 2% of the value set by the anesthesiologist. The ventilator and the circle were connected via a large dead space, through which oxygen from the ventilator entered the circle but which prevented xenon from escaping. This arrangement simplified the computer program. The system was tested on a lung model and in six pigs (37-39 kg). The xenon expenditure and the amount of xenon washed out from the pigs after the anesthetic were measured. Additional experiments with nitrous oxide were made in three pigs. The xenon expenditure during 2 h of xenon anesthesia was 7.6 +/- 0.8 l (mean +/- 1 standard deviation). The corresponding expenditure of nitrous oxide was 16.5 +/- 2.7 l. About 75% of the xenon expenditure was in the 1st h of anesthesia; thereafter 20-40 ml.min-1 was needed to maintain oxygen concentration at 30%. Nitrogen concentration in the circle increased to 12-16% during the xenon anesthetic, although it was preceded by a 20 min denitrogenation period. During the washout phase after the xenon anesthesia, mean expired xenon concentration decreased to below 2% within 4 min. Subsequently, washout was slower and the expired concentration remained above 0.1% for more than 90 min. The estimated total amount of xenon washed out from the lungs and body tissues during 4 h of oxygen breathing was about 4 l. We conclude that xenon anesthesia via a fully automated minimal-flow system is feasible.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inspired oxygen concentrations in semiclosed circle absorber circuits with low flows of nitrous oxide and oxygen

Fresh gas flows of 1–5 L/min containing oxygen concentrations of 25–50% in nitrous oxide were led into a semiclosed circle absorber circuit. The resulting inspired oxygen concentrations were measured under conditions of spontaneous respiration. The lower the fresh gas flow, the greater was the discrepancy between fresh gas and inspired oxygen concentration. No association between minute volume and inspired oxygen concentration was seen. Measured value of inspired oxygen concentration agreed with those derived from the formulae of Mushin and Galloon (1960), and Fitton (1963), except at very low fresh gas flow. It appears possible to predict inspired oxygen concentration in a circle absorber circuit with low fresh gas flows of nitrous oxide and oxygen after a steady state of anaesthesia has been established.     

PREDICTION OF INSPIRED OXYGEN CONCENTRATION WITHIN A CIRCLE ANAESTHETIC SYSTEM

Fresh gas flows of nitrous oxide, oxygen and halothane at 6,3,2and 1 litre/min were introduced into a circle absorber system.Spontaneous respiration and IPPV were studied and a regressionof inspired on delivered oxygen concentration % was calculated.The difference between delivered and inspired oxygen concentration% was increased by decreasing the fresh gas flow and by decreasingthe proportion of oxygen in that flow, especially during IPPV.Circuits designed to allow a maximum overflow of alveolar gasprovided a greater inspired oxygen concentration. The patients’height and weight were related to the scatter of inspired valuesobserved at 1 litre/min of fresh gas flow with IPPV.     

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.     

Evaluation of the PhysioFlex closed-circuit anaesthesia machine

The concentrations of nitrous oxide, sevoflurane and oxygen in the circle system of a closed-circuit anaesthesia machine, the PhysioFlex, were measured in seven patients. During anaesthesia, the settings for each gas were changed and their concentrations recorded. At the induction of anaesthesia, it took 80-510s (median 190s) for the end-tidal sevoflurane concentration to reach 2.0%, and 920-2640s (median 1500s) for the oxygen in the breathing circuit to reach 30%. At this time, the nitrous oxide concentration was 60+/-3% (mean+/-SD). During anaesthesia, it took 90-480s (median 140s) for the end-tidal sevoflurane concentration setting to decrease from 3.0 to 1.0%, and 90-400s (median 110s) to return from 1.0 to 3.0%. When the inspired oxygen was increased from 30 to 50%, circuit concentrations reached equilibrium in 40-60s (median 40s), and when decreased from 50% back to 30%, equilibrium took 310-470s (median 450s). During recovery from anaesthesia, inspiratory sevoflurane concentration took 40-70s (median 50s) to decrease to 0.2%. The PhysioFlex provided adequate control of sevoflurane and oxygen concentrations, but not of increasing nitrous oxide concentrations.     

Exposure to sevoflurane and nitrous oxide during four different methods of anesthetic induction

The National Institute for Occupational Safety and Health-recommended exposure levels for nitrous oxide exposure are 25 ppm as a time-weighted average over the time of exposure. The exposure limit for halogenated anesthetics (without concomitant nitrous oxide exposure) is 2 ppm. Inhaled sevoflurane provides an alternative to i.v. induction of anesthesia. However, the inadvertent release of anesthetic gases into the room is likely to be greater than that with induction involving i.v. anesthetics. We therefore evaluated anesthesiologist exposure during four different induction techniques. Eighty patients were assigned to one of the induction groups to receive: 1) sevoflurane and nitrous oxide from a rebreathing bag, 2) sevoflurane and nitrous oxide from a circle circuit, 3) propofol 3 mg/kg, and 4) thiopental sodium 5 mg/kg. Anesthesia was maintained with sevoflurane and nitrous oxide via a laryngeal mask. Trace concentrations were measured directly from the breathing zone of the anesthesiologist. During induction, peak concentrations of sevoflurane and nitrous oxide with the two i.v. methods rarely exceeded 2 ppm sevoflurane and 50 ppm nitrous oxide. Concentrations during the two inhalation methods were generally <20 ppm sevoflurane and 100 ppm nitrous oxide. During maintenance, median values were near 2 ppm sevoflurane and 50 ppm nitrous oxide in all groups. Sevoflurane concentrations during inhaled induction frequently exceeded the National Institute for Occupational Safety and Health-recommended exposure ceiling of 2 ppm but mostly remained <20 ppm. Exposure during the maintenance phase of anesthesia also frequently exceeded the 2-ppm ceiling. We conclude that operating room anesthetic vapor concentrations are increased during inhaled inductions and remain increased with laryngeal mask ventilation. Implications: We compared waste gas concentrations to sevoflurane and nitrous oxide during four different induction methods. During inhaled induction with a rebreathing bag or a circle circuit system, waste gas concentrations frequently exceed National Institute for Occupational Safety and Health limits of 2 ppm sevoflurane and 50 ppm nitrous oxide. Therefore, we recommend that people at risk (e.g., women of child-bearing age) should pay great attention when using this technique.     

Nitrous oxide solubility in halothane and its effect on the output of vaporizers

The absorption of nitrous oxide in halothane was studied by bubbling nitrous oxide and nitrous oxide/oxygen gas mixtures through a halothane bottle, using 100% oxygen as a control. The gas volume emerging from the halothane bottle was measured each minute, over a period of up to 15 minutes. When oxygen was used as a control gas, the averaged flow rate dropped slightly over the experimental period, due to the cooling of the halothane. However, in the presence of nitrous oxide, the initial flow rate of the gas emerging from the halothane bottle was greatly diminished, but then accelerated rapidly to reach that obtained with oxygen. The results suggested that nitrous oxide dissolved in large quantities in halothane, and the data are consistent with an Ostwald coefficient in excess of 4.0.     

GASEOUS HOMEOSTASIS AND THE CIRCLE SYSTEM: Factors Influencing Anaesthetic Gas Exchange

A mathematical model of a subject breathing from a circle systemhas been used to follow the course of anaesthetic uptake duringthe simulated administration of 60% nitrous oxide, 2% halo-thaneand 2% methoxyflurane, under non-re-breathing conditions andwith fresh gas flows to the circle system of between 8 and 0.25litre min^–. Compared with the non-rebreathing state, theuse of a circle system reduced the initial rate of increaseof alveolar towards fresh gas anaesthetic concentration, andthe rate of increase in body anaesthetic content. The degreeof reduction became more marked as fresh gas flow was reduced,and as agents of increasing blood solubility were used. Theseeffects of a circle system were influenced by the volume ofthe circle system and the composition of gas initially presentwithin the system. When the circle system was in use there wereincreases in the magnitude of both the concentration effectand the second gas effect which were related to the magnitudeof fresh gas flow. The use of a circle system augmented theeffects of changes in cardiac output and reduced the effectsof changes in ventilation on the alveolar concentrations ofthe anaesthetic. These influences of a circle system were alsodependent on the magnitude of fresh gas flow. The degree ofaugmentation of the effects of cardiac output decreased withincreasing blood solubility of the agent in use, whilst thelimitation of the effects of ventilation was greatest with theagent of highest blood solubility. Both under non-rebreathingconditions and with the circle system in use, the effects ofcardiac output and ventilation were greater with 2% nitrousoxide than with 60% nitrous oxide, and were also greater whengases were given separately than when administered in combination.     

The sevoflurane-sparing effect of nitrous oxide: a clinical study

BACKGROUND: We studied the sevoflurane-sparing effect of nitrous oxide in a prospective randomised study. METHODS: Forty-two ASA I-II patients scheduled for elective knee arthroscopy under general anaesthesia were randomly assigned to a fresh gas flow consisting of oxygen in air or oxygen in nitrous oxide 1:2. All patients received a standardised anaesthesia consisting of induction with fentanyl and propofol and maintenance with sevoflurane adjusted according to clinical signs. The sevoflurane consumption was studied by means of weighing the vaporiser before and after every anaesthesia. RESULTS: The mean sevoflurane consumption was reduced from 0.62 to 0.25 g/min, a 60% reduction, by the use of oxygen in nitrous oxide 1:2 in the fresh gas flow. The emergence was faster for the patients receiving nitrous oxide. No major differences were observed during recovery. CONCLUSION: Nitrous oxide was found to be cost-effective for use during short ambulatory knee arthroscopy.     

Diffusion of nitrous oxide into the pleural cavity

We postulated that nitrous oxide transfer into the pleural cavitycan occur by diffusion from the alveoli, independent of vasculartransport. Under general anaesthesia, six sheep were studiedin two phases, a control and an experimental phase. The sheepwere anaesthetized, intubated, and received positive pressuremechanical ventilation. A catheter was placed in the right pleuralcavity and 150 ml air injected. The animals were ventilatedwith 100% oxygen. The inspired gas was changed to a mixtureof 50% nitrous oxide and 50% oxygen, and the rate of increaseof nitrous oxide concentration in the pleural space was measured.The animals were then ventilated with 100% oxygen and then killedby exsanguination while ventilation was continued. The inspiredmixture was changed to 50% nitrous oxide and 50% oxygen andthe rate of increase in nitrous oxide concentration was measuredin the pleural space again. During venitilation with nitrousoxide in the living animals, the concentration of nitrous oxidein the pleural cavity increased rapidly and decreased to zeroduring ventilation with 100% oxygen. During ventilation withoutcirculation, the rate of increase in the concentration of nitrousoxide in the pleural cavity was the same as in the control phase.This suggests that nitrous oxide enters the pleural space bydiffusion, rather than by vascular delivery. This mechanismmay explain the rapid increase in the volume of pneumothoraxif nitrous oxide is given in the inspired gas. Br J Anaesth 2001; 87: 894–6     

Comparison of induction time and characteristics between sevoflurane and sevoflurane / nitrous oxide

A previous investigation using nitrous oxide with 5% enflurane (3.8 MAC) for single breath induction produced a stage of excitement which may be related to the difference in blood/gas coefficient solubility of these agents. The closer blood/gas solubility coefficient of sevoflurane and nitrous oxide may eliminate this phenomenon. We therefore evaluated 40 volunteers in a randomized study using 7.5% sevoflurane (3.7 MAC) in oxygen (n=21) or sevoflurane with nitrous oxide (n=19) using a single breath induction technique. Sevoflurane in nitrous oxide and oxygen reduced induction time by 15% compared to sevoflurane in oxygen alone (41 ±16 and 48±16 sec (s.d.), respectively). This was, however, not statistically significant. There were scarcely induction-related complications, such as coughing, laryngospasm, breath-holding, movements of a limb and excessive salivation, in either group. Thus, the addition of nitrous oxide neither increased the number of complications, nor the speed of induction.     

Expansion of gas bubbles by nitrous oxide and xenon

BACKGROUND: Nitrous oxide is well known to expand gas bubbles trapped in enclosed spaces and is contraindicated in situations where this may occur. Xenon, an anesthetic gas with similar physical properties to nitrous oxide, is also likely to expand gas bubbles, and it has been predicted that microbubbles in the circulation may expand dramatically when exposed to xenon. Because of the possibility that xenon will be used during cardiopulmonary bypass surgery, a procedure that is likely to introduce microbubbles into the circulation, the authors reinvestigated the extent to which xenon expands gas bubbles in aqueous solution. METHODS: Gas bubbles of either air or oxygen were formed in an aqueous solution, and their size was monitored using optical microscopy when they were exposed to a rapidly flowing solution of xenon, nitrous oxide, or a xenon-oxygen mixture. RESULTS: Both nitrous oxide and xenon rapidly expanded air bubbles, although nitrous oxide caused a much larger expansion. The observed expansion was not greatly dependent on the initial size of the bubble but was significantly greater at lower temperatures. Under conditions relevant to cardiopulmonary bypass surgery (50% xenon-50% oxygen, 30 degrees C), the increase in diameter was modest (9.7 +/- 0.8%). CONCLUSIONS: Although xenon does expand small air and oxygen bubbles, the extent to which this occurs under clinically relevant conditions of concentration and temperature is modest.     

Sevoflurane and isoflurane reduce oxygen saturation in infants

Volatile anesthetics are generally known to exert several influences on the respiratory system, but their direct effect on oxygen saturation as measured by pulse oximetry (SpO2) in infants remains unknown. In this study, 70 infants under 2 years of age who received general anesthesia were examined to determine the effects of several volatile anesthetics and nitrous oxide on SpO2. After endotracheal intubation, the subjects were ventilated using a Jackson-Rees circuit with oxygen, nitrous oxide, and either sevoflurane, enflurane, or isoflurane adjusted to twice the adult minimum alveolar concentration (MAC) for the agents when used in combination with 67% nitrous oxide. In all cases, the end-tidal carbon dioxide tension (PetCO2) was maintained within the same range (28-35 mm Hg). Significantly lower SpO2 values (paired t test, P < .05) were observed when the subjects were ventilated with oxygen, 67% nitrous oxide, and sevoflurane or isoflurane--but not with oxygen, 67% nitrous oxide, and enflurane--than when they were administered oxygen, 50% nitrous oxide, and the original concentration of each volatile anesthetic. These results suggest that sevoflurane and isoflurane have different effects from enflurane on gas exchange systems.     

The influence of nitrous oxide on oxyhaemoglobin dissociation and measurement of oxygen tension

There are conflicting reports of the effect of nitrous oxide on the oxyhaemoglobin dissociation curve. We have therefore determined P50 of haemoglobin in the presence of either nitrous oxide or nitrogen and studied the upper portion of the curve in greater detail. No significant differences in the oxyhaemoglobin dissociation curve were observed when nitrous oxide was substituted for nitrogen. The oxygen tensions measured in gas mixtures were not significantly different when determined simultaneously with a polarographic oxygen electrode and the mass spectrometer when nitrous oxide was used instead of nitrogen.     

Predictable nitrous oxide uptake enables simple oxygen uptake monitoring during low flow anaesthesia

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 _N20 1000.t^-0.5. The composition of excess gas was predictable and the following formula for oxygen uptake could be derived: O₂=fgO₂ -0.45 (fgN²O -(kg: 70.1000.t^-0.5)) where oxygen uptake rate (O₂, ml.min^-1) equals oxygen fresh gas flow (fgO₂) minus 0.45 times the difference between the fresh gas flow of nitrous oxide (fgN₂O), 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 inspried oxygen concentration.     

Nitrous oxide increases enflurane concentrations delivered by Ethrane vaporizers

Delivered enflurane concentrations from two calibrated Ethrane vaporizers were determined with total gas flows of 3,5 and 8 L/min. Regardless of total gas flow the presence of 60% nitrous oxide increased enflurane concentrations by 20 to 40% above those concentrations present when only oxygen was flowing through the vaporizer. This nitrous oxide effect was present at all dial settings studied except the lowest engraved (0.25) concentration. Enflurane output at the 0.25% setting was 0.38% with or without nitrous oxide. Maximum changes in enflurane concentrations after adding nitrous oxide required about 5 minutes but the rapidity with which enflurane concentrations approached this maximum value were directly related to total gas flow. Similar effects of nitrous oxide on enflurane output from Cyprane Ethrane vaporizers were also measured. The mechanism of increased vaporizer enflurane output in the presence of nitrous oxide is unknown but may reflect increased gas flow through the vaporizing chamber secondary to increase in gas density associated with nitrous oxide. A similar mechanism has been proposed to explain increased halothane concentrations delivered by Fluotec Mark 2 vaporizers in the presence of nitrous oxide. Clinically, central system stimulation and anesthetic overdose may occur from increased enflurane concentrations delivered when nitrous oxide is added to the gases flowing through the Ethrane vaporizer. The ability to deliver low enflurane concentrations is limited since the measured concentration at the lowest dial setting was nearly 0.4%.     

紧闭式氧化亚氮麻醉方法的探讨

２５例选择期手术病人采用紧闭式氧化亚氮麻醉方法，术中持续监测呼气末氧和氧化亚氮浓度，脉搏血氧饱和度和呼吸循环指标，术中观察紧闭式麻醉后呼吸末氧化亚氮，氧浓度变化，结果：紧闭式麻醉１，２，３ｈ后氧化亚氮浓度分别为５２．７％，５６％，６４．９％，氧浓度为４２．１％，３４．４％，３０．８％，随麻醉时间的延长，气道压力先降后回升，约３ｈ恢复至紧闭麻醉前的水平，紧闭式麻醉前后在本组观察时间内动脉血气分析提示     

Effects of Xenon on the Performance of Various Respiratory Flowmeters

Background: The anesthetic gas xenon has distinctly different physical properties compared with air, nitrous oxide, or oxygen. This led us to predict that xenon would affect the performance of commercially available flowmeters.

Methods: Flow was generated by an anesthesia ventilator connected to a lung simulator via a semiclosed breathing circuit. With the system filled with air or with various concentrations of xenon or nitrous oxide in a balance of oxygen, the tidal volume was measured with two rotating vanes, a Pitot tube, a variable-orifice flowmeter, and two constant-temperature hot-wire flowmeters.

Results: Although xenon minimally affected both rotating vane flowmeters, it caused the Pitot tube and the variable-orifice flowmeters to overread in proportion to the square root of the density of the gas mixture used (xenon is 4.6 times more dense than air). In contrast, the hot-wire anemometers underread with xenon; for example, their readings in the presence of 45% and 70% xenon were less than 10% of those displayed when air was used. Nitrous oxide minimally affected all the flowmeters except the variable-orifice device. The Pitot flowmeter was also affected, but only when its gas analyzer port was open to the ambient air so that it no longer corrected its readings for changes in gas composition. In these cases, nitrous oxide produced overreadings in the same manner as did xenon.