Concentration and second-gas effects in the water analogue

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.

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.

Simulation in anesthesia: the merits of large simulators versus small simulators

Anesthesia simulation is generally perceived as involving large simulators that provide a limited number of operating room scenarios, especially crisis management. The scope of both anesthesia and flight simulation is much wider, and this review summarizes the range of the former. The areas where simulation has been used include training, education and science. The diversity of its uses may surprise the reader. The models that are used in simulations are important, and these are discussed in part of the discussion. As a result of the current imbalance in perception, I emphasize the merits of small simulators at the expense of large simulators.

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.