Gas Exchange Model Research Articles

Adaptive Inert Gas Exchange Model for Improved Hypobaric Decompression Sickness Risk Estimation

INTRODUCTION: Future high-altitude military operations and spaceflight will require new procedures to protect crews from decompression sickness while limiting the operational impact. It is hypothesized that the current prediction models do not accurately reflect actual inert gas dynamics, making them unsuitable for the risk estimation of new hypobaric exposure profiles. METHODS: A biophysical gas exchange model was created, allowing modification of various physiological parameters. Predicted nitrogen (N2) volume flows were compared with an experimental study by the Swedish Aerospace Physiology Centre. Bubble growth predictions, made using the Tissue Bubble Dynamics Model, were compared with measured venous gas emboli (VGE). RESULTS: While the simulated washout curves captured the general trends, some important discrepancies were observed when using the nominal model parameters. The new biophysical gas exchange model, incorporating changes in cardiac output and individual anthropometric variations, improved the predictions and approximated the experimentally observed N2 washout. The standard bubble growth predictions did not match measured VGE. Using weighing factors based on the N2 gas flow components predicted by the new biophysical model, the bubble growth pattern agrees much better with the measured VGE scores. DISCUSSION: Traditional decompression models do not account for variations in physiological and environmental factors, leading to incorrect estimates of N2 washout and bubble growth predictions. Using an adaptive biophysical gas exchange model significantly improves the predictions for various altitude exposure profiles. We therefore strongly recommend incorporating adaptive physiological parameters in any model to be used for estimating decompression sickness risk and designing mitigation procedures. De Ridder S, Neyt X, Germonpré P. Adaptive inert gas exchange model for improved hypobaric decompression sickness risk estimation. Aerosp Med Hum Perform. 2025; 96(2):85–92.

Accuracy of two-compartment modelling of gas exchange with ventilation-perfusion mismatch in inhalational anesthesia.

Multi-compartment computer models of heterogeneity in alveolar ventilation-perfusion ratios (VA/Q scatter) across the lung explain the significant alveolar-arterial (A-a) partial pressure gradients and associated alveolar dead-space fractions (VDA/VA) seen in anesthetized patients for both carbon dioxide and for anesthetic gases of different blood solubilities. However, the accuracy of a simpler two-compartment model of VA/Q scatter to do this has not been tested or compared to calculations from the traditional Riley model with "ideal", unventilated (shunt) and unperfused (deadspace) compartments. Measurements of gas partial pressures in inspired and expired gas and arterial and mixed venous blood from 29 patients undergoing inhalational general anesthesia for cardiac surgery was used to compare the accuracy of two simple models of VA/Q scatter and lung gas exchange in predicting measured alveolar and arterial partial pressure differences, and associated alveolar dead-space calculations for the modern anesthetic gases isoflurane, sevoflurane and desflurane. These models were the Riley model, and a two-compartment model with reciprocal proportions of allocation of VA and Q, with and without additional true-shunt. A multi-compartment "log-normal" model was also tested. Mean (95% confidence interval) of the measured alveolar dead-space fraction for the three anesthetic gases G combined (VDA/VAG) was 0.557 (0.523, 0.592). Mean VDA/VAG from the two-compartment model incorporating an additional true-shunt lung compartment (0.539 (0.498, 0.580) was similar to the measured value (p = 0.347), and was 0.501 (0.457, 0.546) without a true-shunt compartment. The log-normal model outputs were 0.491 (0.453, 0.529)). The Riley model outputs for VDA/VAG severely underestimated this (0.327 (0.294, 0.361)). Satisfactory prediction of the A-a partial pressure gradients and alveolar dead-space for the modern volatile anesthetic gases measured in vivo requires a model with more than one gas-exchanging lung compartments, which the traditional Riley model lacks. A simple "reciprocal" two-compartment model achieves this.

Evaluation of the Effect of Aeration Tubes and Holes on Soil Gas Exchange Using a Simulation Model

ABSTRACTA prevalent strategy to mitigate the negative impacts of soil compaction or sealing on urban trees involves installing aeration tubes, aeration holes or near surface aeration horizons to enhance gas exchange between the soil air and the atmosphere. Despite their widespread use, there is currently no scientific evidence confirming their effectiveness. In this study, gas exchange between the atmosphere and soils that were aerated using different methods was modelled and evaluated in the laboratory and in the field. Both the laboratory and field measurements could be modelled well with the simulation model. The research showed that the effectiveness of aeration tubes and holes is highly dependent on the air‐filled porosity of the soil. The more gas exchange can take place via the soil pores, the lesser the influence of the aeration equipment. Thus, the use of aeration tubes is not necessary when using tree substrates in unsealed tree pits but could mitigate disturbances in soil aeration in compacted and fine‐grained soils with low air capacity. However, modelling shows that the effect of aeration tubes and holes is less than expected and that near‐surface aeration layers are generally more effective than vertical aeration systems. With the help of the modelling of gas exchange as presented here, it is possible to optimise the level of aeration of the soil depending on the existing degree of compaction and/or the planned surface sealing.

The sensitivity of reconstructed carbon dioxide concentrations to stomatal preparation methods using a leaf gas exchange model.

Mechanistic models using stomatal traits and leaf carbon isotope ratios to reconstruct atmospheric carbon dioxide (CO2) concentrations (c a ) are important to understand the Phanerozoic paleoclimate. However, methods for preparing leaf cuticles to measure stomatal traits have not been standardized. Three people measured the stomatal density and index, guard cell length, guard cell pair width, and pore length of leaves from the same Ginkgo biloba, Quercus alba, and Zingiber mioga leaves growing at known CO2 levels using four preparation methods: fluorescence on cleared leaves, nail polish, dental putty on fresh leaves, and dental putty on dried leaves. There are significant differences between trait measurements from each method. Modeled c a calculations are less sensitive to method than individual traits; however, the choice of assumed carbon isotope fractionation also impacted the accuracy of the results. We show that there is not a significant difference between c a estimates made using any of the four methods. Further study is needed on the fractionation due to carboxylation of ribulose bisphosphate (RuBP) in individual plant species before use as a paleo-CO2 barometer and to refine estimates based upon widely applied taxa (e.g., Ginkgo). Finally, we recommend that morphological measurements be made by multiple observers to reduce the effect of individual observational biases.

The influence of applying skin temperature corrections to gas exchange models on air-sea oxygen flux estimates

The skin of the ocean is often slightly cooler than the surface mixed layer due to net surface heat loss (cool skin effect), and sometimes slightly warmer in areas with extreme solar radiation (warm layer effect). In previous work (Yang et al., 2022), with the skin temperature correction term (ΔT) derived from the fifth generation European Center for Medium-Range Weather Forecasts Reanalysis (ERA5) and oxygen (O2) data from three Argo profiling floats, we showed that skin temperature correction is critical for air-sea O2 flux calculation. In this work, we applied the same method to the World Ocean Atlas 2018 (WOA2018) dataset with two widely used air-sea gas exchange models (an empirically derived quadratic bulk flux model W14, and a mechanistic bubble-mediated model E19), to evaluate the influence of skin temperature correction on large-scale air-sea O2 flux estimate. To avoid the influence of sea ice on air-sea gas exchange (and possibly on the ERA5 reanalysis), we limited our analysis between 50°S and 50°N. The result revealed that for both W14 and E19 models the skin temperature correction lowered annual sea-to-air O2 flux between 50°S and 50°N by 25 % for the E19 model and by 22 % for the W14 model. Larger ΔT (further from zero), higher temperature, higher wind speed, and larger O2 concentration difference across the air-sea interface led to larger difference in O2 fluxes calculated with and without the skin temperature correction. With the E19 model, using the ERA5-based ΔT for areas between 50°S and 50°N and a fixed ΔT of −0.17 K for high latitude areas (50°N-90°N and 50°S-90°S), we made an estimate of O2-based global air-to-sea carbon flux of 3.84 Pg C yr−1 (using O2 to C ratio of 1.45 from Hedges et al., 2002), which was comparable to other latest estimates.

Modeling pulmonary perfusion and gas exchange in alveolar microstructures

Pulmonary capillary perfusion and gas exchange are physiological processes that take place at the alveolar level and that are fundamental to sustaining life. Present-day computational simulations of these phenomena are based on low-dimensional mathematical models solved in idealized alveolar geometries, where the chemical reactions between O2-CO2 and hemoglobin are simplified. While providing general insights, current modeling efforts fail to capture the complex chemical reactions that take place in pulmonary capillary blood flow on arbitrary geometries and ignore the crucial impact of microstructural morphology on pulmonary function. Here, we propose a coupled continuum perfusion and gas exchange model that captures complex gas and hemoglobin dynamics in realistic geometries of alveolar tissue. To this end, we derive appropriate governing equations incorporating a two-way Hill-like relationship between gas partial pressures and hemoglobin saturations. We numerically solve the resulting boundary-value problem using a non-linear finite-element approach to simulate and validate velocity, partial pressure, and hemoglobin saturation fields in simple geometries. We further perform sensitivity studies to understand the impact of blood speed and acidity variability on key physiological fields. Notably, we simulate perfusion and gas exchange on anatomical alveolar domains constructed from 3D μ-computed-tomography images of murine lungs. Based on these models, we show that morphological variations decrease O2 and CO2 diffusing capacity, predicting trends and values that are consistent with current medical knowledge. We envision that our model will provide an effective in silico framework to study how exercise and pathological conditions affect perfusion dynamics and the overall gas exchange function of the respiratory system. Source code is available at https://github.com/comp-medicine-uc/alveolar-perfusion-transport-modeling.

Sensitivity analysis of models of gas exchange for lung hyperpolarised 129Xe MR.

Sensitivity analysis enables the identification of influential parameters and the optimisation of model composition. Such methods have not previously been applied systematically to models describing hyperpolarised 129Xe gas exchange in the lung. Here, we evaluate the current 129Xe gas exchange models to assess their precision for identifying alterations in pulmonary vascular function and lung microstructure. We assess sensitivity using established univariate methods and scatter plots for parameter interactions. We apply them to the model described by Patz et al and the Model of Xenon Exchange (MOXE), examining their ability to measure: i) importance (rank), ii) temporal dependence and iii) interaction effects of each parameter across healthy and diseased ranges. The univariate methods and scatter plot analyses demonstrate consistently similar results for the importance of parameters common to both models evaluated. Alveolar surface area to volume ratio is identified as the parameter to which model signals are most sensitive. The alveolar-capillary barrier thickness is identified as a low-sensitivity parameter for the MOXE model. An acquisition window of at least 200 ms effectively demonstrates model sensitivity to most parameters. Scatter plots reveal interaction effects in both models, impacting output variability and sensitivity. Our sensitivity analysis ranks the parameters within the model described by Patz et al and within the MOXE model. The MOXE model shows low sensitivity to alveolar-capillary barrier thickness, highlighting the need for designing acquisition protocols optimised for the measurement of this parameter. The presence of parameter interaction effects highlights the requirement for care in interpreting model outputs.

Establishment and Solution of a Finite Element Gas Exchange Model in Greenhouse-Grown Tomatoes for Two-Dimensional Porous Media with Light Quantity and Light Direction

An accurate gas utilization model is essential for precisely detecting plant photosynthetic capacity. Existing equipment for measuring the plant photosynthetic rate typically considers the key parameters of mesophyll cell conductance and a photosynthetic model based on the carbon reaction process under direct light conditions. However, the light environment signals received by the plant canopy not only vary significantly in incidence angles, but the effective light intensity also differs greatly from the measured values under vertical incidence conditions. To reduce the deviation between existing photosynthetic models and the actual photosynthetic efficiency of leaves, this study employs the gas diffusion method from engineering, using the finite element approach. Based on elastic mechanics and seepage mechanics, the internal stress field control equation of tomato leaves and the two-phase flow equation under a CO2 porous medium were derived. A mathematical model of porous gas–liquid two-phase fluid-solid coupling was established, solved, and analyzed. Preliminary verification was conducted through tests. The results show that in the initial stage of CO2 entering the leaf, the gas flow velocity is higher because of the larger pressure gradient between the pore and the leaf. In this stage, the gas diffusion rate is higher. As the intake time increases, the pressure gradient gradually decreases, and the inlet velocity slows down. Consequently, the diffusion rate gradually reduces. Because of the coupling of light quantity and light direction, the gas diffusion rate significantly increases compared with the uncoupled model. Additionally, a diffusion model that does not consider fluid–solid coupling will overestimate the gas flow rate as the depth of gas entry increases. Therefore, the internal gas diffusion model must account for the effect of coupling on the diffusion rate.

Air-surface exchange of halomethoxybenzenes in a Swedish subarctic catchment

Halomethoxybenzenes (HMBs) and related halomethoxyphenols are produced naturally in the marine and terrestrial environment and some also have anthropogenic origins. They are relatively volatile and water soluble and undergo atmospheric exchange with water bodies and soil. Here we report air-surface exchange of HMB compounds brominated anisoles and chlorinated dimethoxybenzenes in a Subarctic lake and catchment in Sweden during September 2022. HMBs were isolated from water on solid-phase extraction cartridges and from ground litter/soil by solvent extraction and determined by capillary gas chromatography - quadrupole mass spectrometry. Identified compounds in lake and stream water in the 10–100 pg L−1 range were 1,2,4,5-tetrachloro-3,6-dimethoxybenzene (DAME) > 2,4-dibromoanisole (DiBA) ≥ 2,4,6-tribromoanisole (TriBA) > 1,2,3,4-tetrachloro-5,6-dimethoxybenzene (tetrachloroveratrole, TeCV). DAME and the related compound 2,3,5,6-tetrachloro-4-methoxyphenol (DA) are reported in Subarctic litter/soil in the range 0.005–1.1 mg kg−1 dry weight (dw), whereas DiBA and TriBA were not detected in any litter/soil sample and TeCV in only one. Exchanges were assessed from concentrations in water and soil, air concentrations from a monitoring station at Pallas, Finland, and the physicochemical properties of the HMBs. Fluxes to and from the lake were estimated using the two-film gas exchange model. Net loadings (deposition minus volatilization) for the month of September were − 23, −15 and − 68 g for DiBA, TriBA and DAME, respectively, which amounted to about 4–7 % of the estimated lake inventory. An exchange assessment for DAME from litter/soil showed significant net volatilization at five sites, net deposition at one site and near-equilibrium at one site. The Torneträsk catchment appeared close to steady state with respect to HMB exchange during September 2022. The situation could be different during the warmer and colder seasons, and extending the study to cover these periods is a suggested next step.

Vertical gradient of needle ozone uptake within the canopy of Cryptomeria japonica

Leaf ozone uptake through the stomata is an important index for the ozone risk assessments on trees. Stomatal conductance (gs) and ozone concentration ([O3]), determinants of the leaf ozone uptake, are known to show vertical gradients within a tree canopy. However, less is known about the within-canopy vertical gradient of leaf ozone uptake. This study was aimed to elucidate how the vertical gradient of [O3] and gs affect needle ozone uptake within a canopy of mature Cryptomeria japonica trees in a suburban forest at Tokyo, Japan. For this purpose, a multilayer gas exchange model was applied to estimate the vertical gradient of needle gs and the accumulated ozone uptake during the study period (POD1, Phytotoxic Ozone Dose above a threshold of 1 nmol m−2 s−1). In addition, we also tested several scenarios of vertical gradient of [O3] within the canopy for sensitivity analysis. The POD1 was declined from the top to the bottom of the canopy. This tendency strongly depended on the vertical gradient of gs and was hardly affected by the changes in simulated vertical reductions of the [O3]. We further assessed the photosynthesis of sunlit needles (needles absorbing both direct and diffuse light) and shaded needles (needles only absorbing diffuse light). The photosynthesis of shaded needles in the upper half of the canopy made a great contribution to the entire canopy photosynthesis. In addition, given that their POD1 was lower than that of sunlit needles, ozone may affect sunlit and shaded needles differently. We concluded that these considerations should be incorporated into modeling of the calculation of ozone uptake for mature trees to make accurate predictions of the ozone effects on trees at the canopy scale.

Mixing and transport of CO2 across a monolayer-covered surface in an open cylinder driven by a rotating knife edge

The dissolution of CO2 into water has attracted much attention for the modeling of air-sea gas exchange, CO2 sequestration into porous aquifers, and gas transport in bioreactors. In nearly all circumstances, the surface of water is covered by surfactants that make the gas-liquid interface behave elastically and exhibit surface (excess) shear viscosity. Here, transport and mixing of CO2 across the interface is studied in an axisymmetric system with inertia. A rotating knife edge at the interface with negligible contact surface area takes advantage of surface shear viscosity to drive a strong overturning flow in the bulk via viscous coupling. When CO2 is dissolved into water, this system has a forced convection nature that enhances the transport of CO2 into water. This transport leads to intense radial gradients in the dissolved CO2 resulting in baroclinic production of azimuthal vorticity which alters the underlying meridional flow and further enhances mixing by breaking down transport barriers. Flow visualization experiments reveal baroclinic instability and numerical simulations detail the hydrodynamics, including the effects of varying the CO2 partial pressure above the water.

ROBUST CONTROL OF OXYGEN SATURATION DURING MECHANICAL VENTILATION

Acute respiratory distress syndrome (ARDS) is a disease that has a high reported mortality rate. The treatment for ARDS typically involves mechanical ventilation that is tailored to each patient's needs. A crucial aspect of this treatment is maintaining adequate oxygen saturation of haemoglobin by setting the fraction of inspired oxygen. This paper proposes a design method of robust proportional-integral-derivative (PID) controllers using a gas exchange model during ARDS. Several PID controllers were synthesized for different sub-operational ranges defined by measurable quantities of the mechanical ventilator and the patient using a mixed sensitivity H∞ approach. In simulations, the controller demonstrated high robustness to external changes and changes in the patient's condition, with saturation always above 88%. Although further validation of the controller is required, the results indicate that the presented robust control method has the potential to be clinically relevant.

Combined leaf gas-exchange system for model assessment.

Leaf gas-exchange measurements are useful in assessing plant environmental responses. However, uncertainties in the leaf gas-exchange model potentially limit its application. The main challenge in the model-dependent calculations is to detect violations of assumptions. Here, we developed a system that integrates into one instrument the direct measurement of leaf intercellular CO2 concentration and the standard open-flow (OF) and novel open-diffusion (OD) systems for flux measurement. In the OD system, a gas-permeable membrane between the leaf ambient air and outside air creates CO2 and H2O differentials, rather than the air flow in the OF chamber. We measured hypostomatous and amphistomatous leaves of several species with different photosynthetic capacities [sunflower (Helianthus annuus), grape (Vitis vinifera), lemon (Citrus limon), and cherry (Prunus avium)]. The CO2 and H2O differentials in the OD system strictly depend on the flux measured by the OF system. The lower permeability of the membrane resulted in a larger differential per flux, indicating that the OD system can increase the resolution for a small flux. An analysis of the conductance model along with observations suggested that cuticle and leaf intercellular conductances and the unsaturation of leaf humidity contributed to discrepancies between the direct measurement and standard calculation. The combined system developed here provides an opportunity to address these overlooked concepts in leaf gas exchange.

Coupled effect of injection position variation on gas exchange stability of a free-piston engine

Gas exchange fluctuations may lead to operation instability of the free-piston engine (FPE), because the engine has a coupling effect between the gas exchange and the piston motion, which is different from a conventional engine. It is, therefore, important to study the gas exchange instability to ensure the stable operation of the engine. This study focuses the effect of variable piston position when injecting on the gas exchange stability of a looped scavenging FPE, and a systematic full-cycle gas exchange model is presented by developing a dynamic model, a combustion model, and a multidimensional scavenging model. Meanwhile, an iterative method between motion and scavenging is used to solve the model numerically for gas exchange simulation, and the effects of variable piston position when injecting on piston motion, thermodynamic in cylinder, and gas exchange process are studied. The results indicate that retarding injection induces instability of gas exchange duration which first increases and then decreases, and there is an injection position which is most conducive to the increase of capture efficiency of gas exchange, although the scavenging efficiency is lowest under this condition. This result also suggests that more complete exhaust may be achieved by a different injection position.

Comparing Methodologies for Stomatal Analyses in the Context of Elevated Modern CO2.

Leaf stomata facilitate the exchange of water and CO2 during photosynthetic gas exchange. The shape, size, and density of leaf pores have not been constant over geologic time, and each morphological trait has potentially been impacted by changing environmental and climatic conditions, especially by changes in the concentration of atmospheric carbon dioxide. As such, stomatal parameters have been used in simple regressions to reconstruct ancient carbon dioxide, as well as incorporated into more complex gas-exchange models that also leverage plant carbon isotope ecology. Most of these proxy relationships are measured on chemically cleared leaves, although newer techniques such as creating stomatal impressions are being increasingly employed. Additionally, many of the proxy relationships use angiosperms with broad leaves, which have been increasingly abundant in the last 130 million years but are absent from the fossil record before this. We focus on the methodology to define stomatal parameters for paleo-CO2 studies using two separate methodologies (one corrosive, one non-destructive) to prepare leaves on both scale- and broad-leaves collected from herbaria with known global atmospheric CO2 levels. We find that the corrosive and non-corrosive methodologies give similar values for stomatal density, but that measurements of stomatal sizes, particularly guard cell width (GCW), for the two methodologies are not comparable. Using those measurements to reconstruct CO2 via the gas exchange model, we found that reconstructed CO2 based on stomatal impressions (due to inaccurate measurements in GCW) far exceeded measured CO2 for modern plants. This bias was observed in both coniferous (scale-shaped) and angiosperm (broad) leaves. Thus, we advise that applications of gas exchange models use cleared leaves rather than impressions.

Simulation of human breathing gas exchange for the ventilation regulation study

The chemoreflex control of breathing plays a major role in human lung ventilation adjustment in response to metabolic demands and CO2, O2 partial pressure changes in the inhaled air. The extreme conditions, e.g. emergency work in mines or deep-sea diving, space flights, can change the respiratory system reaction to CO2 and O2. The study of the relevant respiratory system characteristics is an important fundamental and practical task. One of the convenient ways of research is mathematical simulation, which allows to reduce the number of experiments in extreme conditions or experiments for the personal protective equipment testing, as well as to forecast the estimated time of effective human work in such conditions. The model describes the dynamics of the gas content in the 3 compartments of the biological system and the external environment represented by the 4th compartment. The external environment can be limited by the volume of the device to which a person is connected, or by the volume of a closed hermetic object, or represented by a sufficiently large volume, conditionally being an atmosphere with appropriate parameters. A mathematical model of the breathing gas exchange with the external environment (the atmosphere or any other limited space) is presented. It is the first time, the simulation results of breathing at rest, during hyperventilation and rebreathing tests are presented, including gas dynamics in the pulmonary and tissue compartments, as well as in the brain compartment. Hypercapnia ventilation reaction during rebreathing tests with a hyperoxic - hypercapnic gas mixtures and different rebreathing bags in comparison with stationary methods of breathing control study. Verification of the model by simulation results with the literature data comparison showed the simulation model’s adequacy. A sensitivity table of the simulation model behavior in response to parameters changes is presented.

Assessment of the variation of the pulmonary gas exchange model in the critical care setting

Background and Aims: In intensive care units (ICUs), clinicians use various mathematical models to assess and monitor respiratory illness. However, evidence suggests that some pulmonary internal and external physiolog¬ical factors are not integrated in the calculation of these models. This research aimed to demonstrate the variation in these models within a short period in critical care settings. Methods: A retrospective cohort study was conducted from 2017 to 2019 with adults admitted to an ICU and mechanically ventilated for more than 24 hours. Two different arterial blood gas samples were selected within a mean of 120 minutes. The coefficient of variation percent [(CV%)=SD/Mean] was calculated. Results: In total, 211 intubated ICU patients were included. The mean age was 57 (SD +/- 19.62) years, and 33% were female. The degree of variation of the average CV% for all the mathematical models was 14%-30%. The mean CV% for the arterial oxygen tension to inspired oxygen fraction (PaO2/FiO2) and arterial/alveolar oxygen tension ratio (PaO2/ PAO2) was 14%, which is a low variation compared to the respiratory index (RI) for which the mean CV% was 30%. Conclusions: The findings of this study indicate that most of the existing oxygen mathematical models used in clinical settings lack physiological realism, necessitating a new mathematical model that integrate both assess illness severity and enhance other models’ accuracy.

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.

A modeling approach to investigate drivers, variability and uncertainties in O2 fluxes and O2 : CO2 exchange ratios in a temperate forest

Abstract. The O2 : CO2 exchange ratio (ER) between terrestrial ecosystems and the atmosphere is a key parameter for partitioning global ocean and land carbon fluxes. The long-term terrestrial ER is considered to be close to 1.10 mol of O2 consumed per mole of CO2 produced. Due to the technical challenge in measuring directly the ER of entire terrestrial ecosystems (EReco), little is known about variations in ER at hourly and seasonal scales, as well as how different components contribute to EReco. In this modeling study, we explored the variability in and drivers of EReco and evaluated the hypothetical uncertainty in determining ecosystem O2 fluxes based on current instrument precision. We adapted the one-dimensional, multilayer atmosphere–biosphere gas exchange model “CANVEG” to simulate hourly EReco from modeled O2 and CO2 fluxes in a temperate beech forest in Germany. We found that the modeled annual mean EReco ranged from 1.06 to 1.12 mol mol−1 within the 5-year study period. Hourly EReco showed strong variations over diel and seasonal cycles and within the vertical canopy profile. The determination of ER from O2 and CO2 mole fractions in air above and within the canopy (ERconc) varied between 1.115 and 1.15 mol mol−1. CANVEG simulations also indicated that ecosystem O2 fluxes could be derived with the flux-gradient method using measured vertical gradients in scalar properties, as well as fluxes of CO2, sensible heat and latent energy derived from eddy covariance measurements. Owing to measurement uncertainties, however, the uncertainty in estimated O2 fluxes derived with the flux-gradient approach could be as high as 15 µmol m−2 s−1, which represented the 90 % quantile of the uncertainty in hourly data with a high-accuracy instrument. We also demonstrated that O2 fluxes can be used to partition net CO2 exchange fluxes into their component fluxes of photosynthesis and respiration if EReco is known. The uncertainty of the partitioned gross assimilation ranged from 1.43 to 4.88 µmol m−2 s−1 assuming a measurement uncertainty of 0.1 or 2.5 µmol m−2 s−1 for net ecosystem CO2 exchange and from 0.1 to 15 µmol m−2 s−1 for net ecosystem O2 exchange, respectively. Our analysis suggests that O2 measurements at ecosystem scale have the potential to partition net CO2 fluxes into their component fluxes, but further improvement in instrument precision is needed.

Therapeutic Hypothermia for Refractory Hypoxemia on Venovenous Extracorporeal Membrane Oxygenation: An In Silico Study.

Patients with respiratory failure may remain hypoxemic despite treatment with venovenous extracorporeal membrane oxygenation (VV-ECMO). Therapeutic hypothermia is a potential treatment for such hypoxia as it reduces cardiac output () and oxygen consumption. We modified a previously published mathematical model of gas exchange to investigate the effects of hypothermia during VV-ECMO. Partial pressures were expressed as measured at 37°C (α-stat). The effect of hypothermia on gas exchange was examined in four clinical scenarios of hypoxemia on VV-ECMO, each with different physiological derangements. All scenarios had arterial partial pressure of oxygen (PaO2) ≤ 46 mm Hg and arterial oxygen saturation of hemoglobin (SaO2) ≤ 81%. Three had high with low extracorporeal blood flow to ratio (). The problem in the fourth scenario was recirculation, with normal . Cooling to 33°C increased SaO2 to > 89% and PaO2 to > 50 mm Hg in all scenarios. Mixed venous oxygen saturation of hemoglobin as % () increased to > 70% and mixed venous partial pressure of oxygen in mm Hg () increased to > 34 mm Hg in scenarios with low . In the scenario with high recirculation, and increased, but to < 50% and < 27 mm Hg, respectively. This in silico study predicted cooling to 33°C will improve oxygenation in refractory hypoxemia on VV-ECMO, but the improvement will be less when the problem is recirculation.