Concentration and second gas effects: can the accepted explanation be improved?

Effects of N2 O elimination on the elimination of second gases in a two-step mathematical model of heterogeneous gas exchange

We have investigated the elimination of inert gases in the lung during the elimination of nitrous oxide (N2 O) using a two-step mathematical model that allows the contribution from net gas volume expansion, which occurs in Step 2, to be separated from other factors. When a second inert gas is used in addition to N2 O, the effect on that gas appears as an extra volume of the gas eliminated in association with the dilution produced by N2 O washout in Step 2. We first considered the effect of elimination in a single gas-exchanging unit under steady-state conditions and then extended our analysis to a lung having a log-normal distribution of ventilation and perfusion. A further increase in inert gas elimination was demonstrated with gases of low solubility in the presence of the increased ventilation-perfusion mismatch that is known to occur during anesthesia. These effects are transient because N2 O elimination depletes the input of that gas from mixed venous blood to the lung, thereby rapidly reducing the magnitude of the diluting action.

Elucidating the roles of solubility and ventilation-perfusion mismatch in the second gas effect using a two-step model of gas exchange

Gas exchange in the lung can always be represented as the sum of two components: gas exchange at constant volume followed by gas exchange on volume correction. Using this sequence to study the second gas effect, low gas solubility and increased ventilation-perfusion mismatch are shown to act together to enhance second gas uptake. While appearing to contravene classical concepts of gas exchange, a detailed theoretical analysis shows it is fully consistent with these concepts.

1-1-8 one-step sevoflurane wash-in scheme for low-flow anesthesia: simple, rapid, and predictable induction

Background

Sevoflurane is suitable for low-flow anesthesia (LFA). LFA needs a wash-in phase. The reported sevoflurane wash-in schemes lack simplicity, target coverage, and applicability. We proposed a one-step 1-1-8 wash-in scheme for sevoflurane LFA to be used with both N₂O and Air. The objective of our study was to identify time for achieving each level of alveolar concentration of sevoflurane (F_AS) from 1 to 3.5% in both contexts.

Methods

We recruited 199 adults requiring general anesthesia with endotracheal intubation and controlled ventilation—102 in group N₂O and 97 in group Air. After induction and intubation, a wash-in was started using a fresh gas flow of O₂:N₂O or O₂:Air at 1:1 L·min^− 1 plus sevoflurane 8%. The ventilation was controlled to maintain end-tidal CO₂ of 30–35 mmHg.

Results

The rising patterns of F_AS and inspired concentration of sevoflurane (F_IS) are similar, running parallel between the groups. The F_AS/F_IS ratio increased from 0.46 to 0.72 within 260 s in group N₂O and from 0.42 to 0.69 within 286 s in group Air. The respective time to achieve an F_AS of 1, 1.5, 2, 2.5, 3, and 3.5% was 1, 1.5, 2, 3, 3.5, and 4.5 min in group N₂O and 1, 1.5, 2, 3, 4, and 5 min in group Air. The heart rate and blood pressure of both groups significantly increased initially then gradually decreased as F_AS increased.

Conclusions

The 1-1-8 wash-in scheme for sevoflurane LFA has many advantages, including simplicity, coverage, swiftness, safety, economy, and that it can be used with both N₂O and Air. A respective F_AS of 1, 1.5, 2, 2.5, 3, and 3.5% when used with N₂O and Air can be expected at 1, 1.5, 2, 3, 3.5, and 4.5 min and 1, 1.5, 2, 3, 4, and 5 min.

Trial registration

This study was retrospectively registered with ClinicalTrials.gov (NCT03510013) on June 8, 2018.

Effect of net gas volume changes on alveolar and arterial gas partial pressures in the presence of ventilation-perfusion mismatch

The second gas effect (SGE) occurs when nitrous oxide enhances the uptake of volatile anesthetics administered simultaneously. Recent work shows that the SGE is greater in blood than in the gas phase, that this is due to ventilation-perfusion mismatch, that as mismatch increases, the SGE increases in blood but is diminished in the gas phase, and that these effects persist well into the period of nitrous oxide maintenance anesthesia. These modifications of the SGE are most pronounced with the low soluble agents in current use. We investigate further the effect of net gas volume loss during nitrous oxide uptake on low concentrations of other gases present using partial pressure-solubility diagrams. The steady-state equations of gas exchange were solved assuming a log-normal distribution of ventilation-perfusion ratios using Lebesgue-Stieltjes integration. It was shown that under these conditions the classical partial pressure-solubility diagram must be modified, that for currently used volatile anesthetic agents the alveolar-arterial partial pressure difference is less than that predicted in the past, and that the alveolar-arterial partial pressure difference may even be reversed during uptake in the case of highly insoluble gases such as sulfur hexafluoride. Comparing this with the situation described previously for nitrogen in steady-state air breathing, we show that for nitrogen, the direction of the alveolar-arterial gradient is opposite to the direction of net gas volume movement. Although gas uptake with ventilation-perfusion inequality exceeding that when matching is optimal is shown to be possible, it is less likely than alveolar-arterial partial pressure reversal. NEW & NOTEWORTHY Net uptake of gases administered with nitrous oxide may proceed against an alveolar-arterial partial pressure gradient. The alveolar-arterial gradient for nitrogen in the steady-state breathing air depends not only on the existence of a distribution of ventilation-perfusion ratios in the lung but also on the presence of a net change in gas volume and is opposite in direction to the direction of net gas volume uptake.

Can Mathematical Modeling Explain the Measured Magnitude of the Second Gas Effect?

BACKGROUND

Recent clinical studies suggest that the magnitude of the second gas effect is considerably greater on arterial blood partial pressures of volatile agents than on end-expired partial pressures, and a significant second gas effect on blood partial pressures of oxygen and volatile agents occurs even at relatively low rates of nitrous oxide uptake. We set out to further investigate the mechanism of this phenomenon with the help of mathematical modeling.

METHODS

Log-normal distributions of ventilation and blood flow were generated representing the range of ventilation-perfusion scatter seen in patients during general anesthesia. Mixtures of nominal delivered concentrations of volatile agents (desflurane, isoflurane and diethyl ether) with and without 70% nitrous oxide were mathematically modeled using steady state mass-balance principles, and the magnitude of the second gas effect calculated as an augmentation ratio for the volatile agent, defined as the partial pressure in the presence to that in the absence of nitrous oxide.

RESULTS

Increasing the degree of mismatch increased the second gas effect in blood. Simultaneously, the second gas effect decreased in the gas phase. The increase in blood was greatest for the least soluble gas, desflurane, and least for the most soluble gas, diethyl ether, while opposite results applied in the gas phase.

CONCLUSIONS

Modeling of ventilation-perfusion inhomogeneity confirms that the second gas effect is greater in blood than in expired gas. Gas-based minimum alveolar concentration readings may therefore underestimate the depth of anesthesia during nitrous oxide anesthesia with volatile agents. The effect on minimum alveolar concentration is likely to be most pronounced for the less soluble volatile agents in current use.

Pretreatment with nitrous oxide enhances induction of anesthesia with sevoflurane: A randomized controlled trial

Background and Aims:

Inhalation anesthesia with sevoflurane may be enhanced by several drugs or techniques. The aim of the present study was to investigate the effect of nitrous oxide (N₂O) pretreatment on the speed of anesthesia induction with sevoflurane.

Material and Methods:

Eighty patients scheduled for hysteroscopy under general anesthesia were randomly assigned to inhale for 10 min before induction 50% N₂O in oxygen or air via a facemask. Anesthesia was induced with 7-8% sevoflurane in oxygen via a facemask. Bispectral index (BIS), end-tidal carbon dioxide (EtCO₂) tidal volume, respiratory rate, oxygen saturation (SpO₂), and heart rate were recorded every minute during the 10 min pretreatment periods and every 30 s during the first 300 s of induction with sevoflurane. During induction of anesthesia inspired and end-tidal sevoflurane concentrations were also recorded.

Results:

During the 10 min of inspired 50% N₂O or air BIS, EtCO₂, tidal volume, respiratory rate and heart rate values did not differ between the two groups except for the SpO₂, which was higher in the N₂O group (P < 0.001). During induction of anesthesia the N₂O group exhibited lower BIS values (P = 0.001), being significant at 60-150 s (P < 0.001, P < 0.001, P = 0.002, P = 0.014) as well as at 270 s (P = 0.004). EtCO₂ and tidal volume were consistently lower in the N₂O group (P = 0.001, P = 0.041 respectively) and respiratory rate was higher (P = 0.007).

Conclusion:

Our results show that pretreatment of the patients with 50% N₂O for 10 min enhances the speed of induction with sevoflurane as assessed by the BIS monitoring.

Combining nitrous oxide with carbon dioxide decreases the time to loss of consciousness during euthanasia in mice--refinement of animal welfare?

Carbon dioxide (CO(2)) is the most commonly used euthanasia agent for rodents despite potentially causing pain and distress. Nitrous oxide is used in man to speed induction of anaesthesia with volatile anaesthetics, via a mechanism referred to as the "second gas" effect. We therefore evaluated the addition of Nitrous Oxide (N(2)O) to a rising CO(2) concentration could be used as a welfare refinement of the euthanasia process in mice, by shortening the duration of conscious exposure to CO2. Firstly, to assess the effect of N(2)O on the induction of anaesthesia in mice, 12 female C57Bl/6 mice were anaesthetized in a crossover protocol with the following combinations: Isoflurane (5%)+O(2) (95%); Isoflurane (5%)+N(2)O (75%)+O(2) (25%) and N(2)O (75%)+O(2) (25%) with a total flow rate of 3 l/min (into a 7 l induction chamber). The addition of N(2)O to isoflurane reduced the time to loss of the righting reflex by 17.6%. Secondly, 18 C57Bl/6 and 18 CD1 mice were individually euthanized by gradually filling the induction chamber with either: CO(2) (20% of the chamber volume.min-1); CO(2)+N(2)O (20 and 60% of the chamber volume.min(-1) respectively); or CO(2)+Nitrogen (N(2)) (20 and 60% of the chamber volume.min-1). Arterial partial pressure (P(a)) of O(2) and CO(2) were measured as well as blood pH and lactate. When compared to the gradually rising CO(2) euthanasia, addition of a high concentration of N(2)O to CO(2) lowered the time to loss of righting reflex by 10.3% (P<0.001), lead to a lower P(a)O(2) (12.55 ± 3.67 mmHg, P<0.001), a higher lactataemia (4.64 ± 1.04 mmol.l(-1), P = 0.026), without any behaviour indicative of distress. Nitrous oxide reduces the time of conscious exposure to gradually rising CO(2) during euthanasia and hence may reduce the duration of any stress or distress to which mice are exposed during euthanasia.

Persisting concentrating and second gas effects on oxygenation during N2O anaesthesia

Theoretical modelling predicts that the concentrating effect of nitrous oxide (N2O) uptake on alveolar oxygenation is a persisting phenomenon at typical levels of ventilation - perfusion (V/Q) inhomogeneity under anaesthesia. We sought clinical confirmation of this in 20 anaesthetised patients. Arterial oxygen pressure (P(aO2)) was measured after a minimum of 30 min of relaxant general anaesthesia with an inspired oxygen (F(I O2)) of 30%. Patients were randomly allocated to two groups. The intervention group had N2O introduced following baseline blood gas measurements, and the control group continued breathing an identical F(I O2) in nitrogen (N2). The primary outcome variable was change in P(aO2). Mean (SD) in P(aO2) was increased by 1.80 (1.80) kPa after receiving a mean of 47.5 min of N2O compared with baseline conditions breathing O2/N2 (p = 0.01). This change was significantly greater (p = 0.03) than that in the control group: + 0.09 (1.37) kPa, p = 0.83 and confirms the presence of significant persisting concentrating and second gas effects.

Large volume N 2 O uptake alone does not explain the second gas effect of N 2 O on sevoflurane during constant inspired ventilation †

BACKGROUND

The second gas effect (SGE) is considered to be significant only during periods of large volume N(2)O uptake (VN(2)O); however, the SGE of small VN(2)O has not been studied. We hypothesized that the SGE of N(2)O on sevoflurane would become less pronounced when sevoflurane administration is started 60 min after the start of N(2)O administration when VN(2)O has decreased to approximately 125 ml min(-1), and that the kinetics of sevoflurane under these circumstances would become indistinguishable from those when sevoflurane is administered in O(2).

METHODS

Seventy-two physical status ASA I-II patients were randomly assigned to one of six groups (n=12 each). In the first four groups, sevoflurane (1.8% vaporizer setting) administration was started 0, 2, 5 and 60 min after starting 2 litre min(-1) O(2) and 4 litre min(-1) N(2)O, respectively. In the last two groups, sevoflurane (1.8 or 3.6% vaporizer setting) was administered in 6 litre min(-1) O(2). The ratios of the alveolar fraction of sevoflurane (Fa) over the inspired fraction (Fi), or Fa/Fi, were compared between the groups.

RESULTS

Sevoflurane Fa/Fi was larger in the N(2)O groups than in the O(2) groups, and it was identical in all four N(2)O groups.

CONCLUSIONS

We confirmed the existence of a SGE of N(2)O. Surprisingly, when using an Fa of 65% N(2)O, the magnitude of the SGE was the same with large or small VN(2)O. The classical model and the graphical representation of the SGE alone should not be used to explain the magnitude of the SGE. We speculate that changes in ventilation/perfusion inhomogeneity in the lungs during general anaesthesia result in a SGE at levels of VN(2)O previously considered by most to be too small to exert a SGE.

Anesthetic uptake of sevoflurane and nitrous oxide during an inhaled induction in children

The uptake of sevoflurane and nitrous oxide (N(2)O) was characterized during the mask induction of anesthesia in healthy children. We assessed concentration and second gas effects by determining the influence of two different inspiratory N(2)O concentrations on the rate at which the estimated alveolar concentration (FA) increased to the inspired gas concentration (FI). Eighteen children aged 4-12 yr old were randomly assigned to receive a 6% sevoflurane mixture with either a large or a small N(2)O concentration with balance O(2). End-tidal and inspiratory concentrations of respiratory and anesthetic gases were continuously assessed during the induction. The FA/FI for the small N(2)O was 0.87 +/- 0.09 (mean +/- SD) and increased to 0.92 +/- 0.08 for the large N(2)O (P < 0.01). Both groups differed significantly at 3, 4, and 5 min. The FA/FI for sevoflurane increased but more slowly than for N(2)O. The mean only differed significantly at 3 min. Equilibration between FA and FI for N(2)O and sevoflurane was attained rapidly. Consistent with their respective blood/gas partition coefficients, the FA/FI for N(2)O increased more rapidly than that for sevoflurane. Increasing FI-N(2)O produced a leftward shift in gas equilibration curves. A concentration effect was confirmed with N(2)O and a brief second gas effect, probably explained by the higher solubility of sevoflurane.

Effect of ventilation-perfusion inhomogeneity and N(2)O on oxygenation: physiological modeling of gas exchange

Ventilation-perfusion (VA/Q) inhomogeneity was modeled to measure its effect on arterial oxygenation during maintenance-phase anesthesia involving an inspired mixture of 30% O(2) and either N(2)O or N(2). A multialveolar compartment computer model was constructed based on a log normal distribution of VA/Q inhomogeneity. Increasing the log SD of the distribution of blood flow from 0 to 1.75 produced a progressive fall in arterial PO(2) (Pa(O(2))). The fall was less steep in the presence of N(2)O than when N(2) was present instead. This was due mainly to the concentrating effect of N(2)O uptake on alveolar PO(2) in moderately low VA/Q compartments. The improvement in Pa(O(2)) when N(2)O was present instead of N(2) was greatest when the degree of VA/Q inhomogeneity was in the range typically seen in anesthetized patients. Models based on distributions of expired and inspired alveolar ventilation give quantitatively different results for Pa(O(2)). In the presence of VA/Q inhomogeneity, second-gas and concentrating effects may have clinically significant effects on arterial oxygenation even at "steady-state" levels of N(2)O uptake.

Clinical evaluation of the Mapleson theoretical ideal fresh gas flow sequence at the start of low-flow anaesthesia with isoflurane, sevoflurane and desflurane

Mapleson used a computer spreadsheet model to predict the theoretical ideal fresh gas flow sequence at the start of low-flow anaesthesia. The aim was to increase the end-expired partial pressure of inhalational agent (PE'an) to one minimum alveolar concentration (MAC) as quickly as practicable and then to keep it constant. Ninety adult patients undergoing elective tonsillectomy under general anaesthesia were randomly allocated to one of three groups (n = 30) to receive isoflurane, sevoflurane or desflurane in oxygen. Fresh gas flow and vaporiser settings as specified by Mapleson were followed in all cases except that the maximum setting for desflurane was 18% (2.7 MAC instead of 3 MAC). Recordings of PE'an were made at 1, 2, 3, 4, 5, 7, 10, 15 and 20 min. Mean values of PE'an exceeded 1 MAC by 2 min in all three groups and remained above this value throughout. Each group's PE'an measurements were divided by their respective 1-MAC value. A simple two-level model (with patients at level 2 and time at level 1), with measurements at 1 min excluded, showed that the fitted value at 2 min and the time-weighted mean for 2-20 min for PE'iso (1.042 [95% CI 0.980-1.104] and 1.044 [0.984-1.104], respectively) were not significantly different from its 1-MAC value, whereas those of the PE'sevo (1.169 [1.119-1.219] and 1.143 [1.119-1.219]) and PE'des (1.305 [1.261-1.349] and 1.140 [1.098-1.182]) were significantly higher than their respective 1-MAC values. The Mapleson concept of an initial high fresh gas flow and high vaporiser settings, followed first by reduced high fresh gas flow, as followed in this clinical study, results in PE'an values close to or slightly higher than predicted in the spreadsheet model.

Model-based administration of inhalation anaesthesia. 2. Exploring the system model

We explored our model by displaying its new capabilities, testing its sensitivity to variations in input data and illustrating its use. Its multiple-gas character allows simulation of the mechanisms governing concentration and second gas effects. Simulating the volume of a standing bellows makes it possible to test algorithms for automated closed-circuit anaesthesia. Using desflurane, the model's sensitivity to changes in blood/gas partition coefficient (range 0.42-0.576), cardiac output and minute ventilation was analysed. The model was very sensitive to changes in blood solubility; other results agreed with those reported previously. An alveolar isoflurane tension of 1% atm was rapidly attained and maintained, even using 0.5 litres min(-1) of fresh gas, when isoflurane was 'co-administered' through a vaporizer set to 3.5 vol% and a single aliquot (1.25 ml liquid) injected into the expiratory limb. As a result of its credibility and capabilities, the model is to be tested in the clinical setting.

Second gas effect of N2O on oxygen uptake

PURPOSE

The concept of the second gas effect is well known, however, there have been no studies that showed the relationship between alveolar oxygen concentration and arterial oxygen tension (PaO2) after the inhalation of nitrous oxide (N2O) in humans. The purpose of this study was to examine the changes in both end-tidal oxygen fraction (F(ET)O2) and PaO2 after N2O inhalation in patients under general anesthesia.

METHODS

Fifteen patients scheduled for elective orthopedic surgery were enrolled in this study. Anesthesia was maintained with the continuous infusion of propofol and with nitrogen (N2) and oxygen (O2) (6 L x min(-1), F1O2, 0.33). In all patients, the lungs were ventilated with a Servo 900C ventilator equipped with a gas mixer for O2, N2O, and N2. After obtaining baseline data, N2 was replaced with N2O maintaining FIO2 constant at 0.33. The changes in fractional concentration of O2, N2O, and N2 were continuously measured using mass spectrometer in a breath-by-breath basis. PaO2 and hemodynamic data were obtained at 1, 5, 10, 30 and 60 min after the start of N2O inhalation.

RESULTS

Five minutes after N2O inhalation, F(ET)O2 increased from 0.27+/-0.01 to 0.31+/-0.02 (P<0.01) and PaO2 increased from 172.0+/-22.5 mm Hg to 201.0+/-10.3 mm Hg (P<0.01). These effects produced by N2O were observed for 30 min.

CONCLUSIONS

These results confirm the concept of second gas effect of N2O on oxygen uptake in humans and provide evidence that the PaO2 increase correlated with the increase in F(ET)O2 after N2O inhalation.

A demonstration of the concentration and second gas effects in humans anesthetized with nitrous oxide and desflurane

In the present study, we explored both the existence of and the basis for the concentration and second gas effects. Groups of six normocapnic patients were given one of three gas mixtures via a nonrebreathing system: 65% nitrous oxide (N2O) plus 4% desflurane; 5% N2O plus 4% desflurane; or 65% N2O plus 0.5% desflurane plus 2% xenon (Xe). End-tidal carbon dioxide (CO2) was held constant by adjustments in ventilation. Confirming the existence of the concentration effect, the end-tidal (F(A)) concentration of N2O increased toward the inspired (F(I)) concentration more rapidly (i.e., F(A)/F(I) increased more rapidly) when the inspired concentration was 65% than when it was 5%. The F(A)/F(I) for desflurane also increased more rapidly when desflurane was given with 65% rather than 5% N2O, confirming the existence of the second gas effect. The small uptake of the second gas (desflurane) did not influence its own F(A)/F(I) or that of N2O; that is, the administration of 4%, rather than 0.5%, desflurane did not increase the rate of rise of F(A)/F(I) of either N2O or desflurane. One of the bases of the concentration and second gas effects, a concentrating of residual gases, was confirmed: administration of Xe with 65% N2O produced an F(A)/F(I) for Xe that exceeded 1.0. Patient sex did not seem to influence the rate of rise of F(A)/F(I) of either N2O or desflurane. Finally, we unexpectedly found that, despite an equal solubility in blood, the rise in F(A)/F(I) for N2O exceeded that for desflurane, perhaps because of differences in tissue solubilities and intertissue diffusion.

IMPLICATIONS

As predicted by the concentration and second gas effects, increasing the inspired concentration of nitrous oxide accelerated its rate of rise and the rate of rise of concurrently administered desflurane in humans.