CO2 Partial Pressure Differences Research Articles

Remotely sensed retrieval of air-sea carbon flux and acidification risk in Chinese Bohai Sea based on a semi-analytical mechanism model with hour-level GOCI image and ERA5 reanalysis data

Marine carbon sinks act as a buffer against global warming, but raise the risk of acidification, especially in the marginal shelf seas which are rich in terrigenous carbon input. The Chinese Bohai Sea performs generally as a weak carbon source while carbon fluxes (fCO2), pH, and aragonite saturation states (Ωarag) vary in time and space under intensive land-sea interaction. However, there are still 1) insufficient spatiotemporal resolution in existing remotely sensed retrieval of carbon flux, 2) inadequate analytic mechanism for existing empirical models, which are not suitable for case II waters in Bohai Sea, and 3) limited research on remotely sensed retrieval of acidification risk expressed as pH and Ωarag. Thus, we proposed a semi-analytical mechanism algorithm (MeSAA) to gain seawater partial pressure of CO2 (pCO2sw) with hour-level GOCI imagery and seawater carbonate equilibrium equation (CO2SYS). With the assistance of ERA5 reanalysis data, the gained pCO2sw was then used to obtain fCO2 by using the sea-air CO2 partial pressure difference (ΔpCO2) method. Similarly, with the assistance of remotely sensed retrieval, two indices, pH and Ωarag were also gained from CO2SYS to identify the acidification risk. The results showed that Bohai Sea was a weak carbon source with the positive values of fCO2 (0.5–6 mmol C·m−2·day−1) and total emission (0.956 Tg C·yr−1). It suffered from a high acidification risk, especially in three bays with low values of Ωarag (1.15–1.3) from May to September 2011. Although photosynthetic carbon sequestration was intensive near shore, it could not consume the large amount of the rich carbon input, and resulting a monthly increase for pCO2sw and fCO2 and a monthly decrease for pH and Ωarag from May to September. The distribution of pCO2sw was in accord with a former study, but the values were not. The local parameter adjustment of MeSAA in the Bohai Sea was analyzed for this issue, so did the effect of uncertainty analysis of mapping at different time scales (hourly and daily). Moreover, the contrast of absolute deviation and relative deviation on different time scales verified the advantage of hour-level GOCI imagery in improving assessment accuracies. This study gained a more precise change trend of pCO2sw, fCO2, pH, and Ωarag in the Bohai Sea from May to September 2011, which would be beneficial to the study of the carbon cycle in the marginal sea of the shelf under the conditions of climate change.

Venous Minus Arterial Carbon Dioxide Gradients in the Monitoring of Tissue Perfusion and Oxygenation: A Narrative Review.

According to Fick's principle, the total uptake of (or release of) a substance by tissues is the product of blood flow and the difference between the arterial and the venous concentration of the substance. Therefore, the mixed or central venous minus arterial CO2 content difference depends on cardiac output (CO). Assuming a linear relationship between CO2 content and partial pressure, central or mixed venous minus arterial PCO2 differences (Pcv-aCO2 and Pmv-aCO2) are directly related to CO. Nevertheless, this relationship is affected by alterations in the CO2Hb dissociation curve induced by metabolic acidosis, hemodilution, the Haldane effect, and changes in CO2 production (VCO2). In addition, Pcv-aCO2 and Pmv-aCO2 are not interchangeable. Despite these confounders, CO is a main determinant of Pcv-aCO2. Since in a study performed in septic shock patients, Pmv-aCO2 was correlated with changes in sublingual microcirculation but not with those in CO, it has been proposed as a monitor for microcirculation. The respiratory quotient (RQ)-RQ = VCO2/O2 consumption-sharply increases in anaerobic situations induced by exercise or critical reductions in O2 transport. This results from anaerobic VCO2 secondary to bicarbonate buffering of anaerobically generated protons. The measurement of RQ requires expired gas analysis by a metabolic cart, which is not usually available. Thus, some studies have suggested that the ratio of Pcv-aCO2 to arterial minus central venous O2 content (Pcv-aCO2/Ca-cvO2) might be a surrogate for RQ and tissue oxygenation. In this review, we analyze the physiologic determinants of Pcv-aCO2 and Pcv-aCO2/Ca-cvO2 and their potential usefulness and limitations for the monitoring of critically ill patients. We discuss compelling evidence showing that they are misleading surrogates for tissue perfusion and oxygenation, mainly because they are systemic variables that fail to track regional changes. In addition, they are strongly dependent on changes in the CO2Hb dissociation curve, regardless of changes in systemic and microvascular perfusion and oxygenation.

Diagnosing CO2 emission-induced feedbacks between the Southern Ocean carbon cycle and the climate system: A multiple Earth System Model analysis using a water mass tracking approach

AbstractAnthropogenic CO2emission-induced feedbacks between the carbon cycle and the climate system perturb the efficiency of atmospheric CO2uptake by land and ocean carbon reservoirs. The Southern Ocean is a region where these feedbacks can be largest and differ most among Earth System Model projections of 21stcentury climate change. To improve our mechanistic understanding of these feedbacks, we develop an automated procedure that tracks changes in the positions of Southern Ocean water masses and their carbon uptake. In an idealised ensemble of climate change projections, we diagnose two carbon–concentration feedbacks driven by atmospheric CO2(due to increasing air-sea CO2partial pressure difference,dpCO2, and reducing carbonate buffering capacity) and two carbon–climate feedbacks driven by climate change (due to changes in the water mass surface outcrop areas and local climate impacts). Collectively these feedbacks increase the CO2uptake by the Southern Ocean and account for one-fifth of the global uptake of CO2emissions. The increase in CO2uptake is primarily dpCO2-driven, with Antarctic intermediate waters making the largest contribution; the remaining three feedbacks partially offset this increase (by ~25%), with maximum reductions in Subantarctic mode waters. The process dominating the decrease in CO2uptake is water mass-dependent: reduction in carbonate buffering capacity in Subtropical and Subantarctic mode waters, local climate impacts in Antarctic intermediate waters, and reduction in outcrop areas in circumpolar deep waters and Antarctic bottom waters. Intermodel variability in the feedbacks is predominatelydpCO2–driven and should be a focus of efforts to constrain projection uncertainty.

P0345 / #1784: EVALUATION OF THE VENOUS TO ARTERIAL CO2 PARTIAL PRESSURE DIFFERENCE AS AN INDIRECT MEASURE OF ADEQUACY OF TISSUE BLOOD FLOW AFTER CONGENITAL HEART DISEASE SURGERY

Aims & Objectives: In the postoperative period of cardiac surgery with extracorporeal circulation, the occurrence of low cardiac output is common, especially in neonates and infants under one year old. The venous to arterial CO2 partial pressure difference (VACO2) may be an indirect measure of cardiac output, and, in normal conditions, varies between 2 and 6 mmHg. Higher values of VACO2 may indicate insufficient cardiac output or inadequate microcirculatory blood flow. To evaluate the value of VACO2 as an indirect measure of adequacy of tissue blood flow and cardiac output in children after cardiac surgery and verify its association with outcome. Methods: Prospective observational cohort study involving patients between 0 and 18 years old admitted to the pediatric intensive care unit (PICU) after cardiac surgery with extracorporeal circulation at HCFMRP-USP. In the first 48 postoperative hours, laboratory tests, clinical data, illness severity scores and vasoactive inotropic score were registered. VACO2 and lactate clearance was calculated every 6h. Clinical outcome and the length of stay in the hospital and in PICU were evaluated. Patients were divided in two groups according to the values of VACO2 at 24h after PICU admission (≥ 6 and < 6 mmHg) and were compared by Mann-Whitney U test. Results: Values of VACO2 ≥ 6mmHg were more frequent in younger patients, with lower weight. They were also associated with greater length of stay in hospital and in PICU. There was no correlation with lactate clearance. Conclusions: High VACO2 values at 24h following cardiac surgery for congenital heart disease were associated with worse postoperative outcomes.

Variability in a four-network composite of atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; differences between three primary baseline sites

Abstract. Spatial differences in the monthly baseline CO2 since 1992 from Mauna Loa (mlo, 19.5∘ N, 155.6∘ W, 3379 m), Cape Grim (cgo, 40.7∘ S, 144.7∘ E, 94 m), and South Pole (spo, 90∘ S, 2810 m) are examined for consistency between four monitoring networks. For each site pair, a composite based on the average of NOAA, CSIRO, and two independent Scripps Institution of Oceanography (SIO) analysis methods is presented. Averages of the monthly standard deviations are 0.25, 0.23, and 0.16 ppm for mlo–cgo, mlo–spo, and cgo–spo respectively. This high degree of consistency and near-monthly temporal differentiation (compared to CO2 growth rates) provide an opportunity to use the composite differences for verification of global carbon cycle model simulations. Interhemispheric CO2 variation is predominantly imparted by the mlo data. The peaks and dips of the seasonal variation in interhemispheric difference act largely independently. The peaks mainly occur in May, near the peak of Northern Hemisphere (NH) terrestrial photosynthesis/respiration cycle. February–April is when interhemispheric exchange via eddy processes dominates, with increasing contributions from mean transport via the Hadley circulation into boreal summer (May–July). The dips occur in September, when the CO2 partial pressure difference is near zero. The cross-equatorial flux variation is large and sufficient to significantly influence short-term Northern Hemisphere growth rate variations. However, surface–air terrestrial flux anomalies would need to be up to an order of magnitude larger than found to explain the peak and dip CO2 difference variations. Features throughout the composite CO2 difference records are inconsistent in timing and amplitude with air–surface fluxes but are largely consistent with interhemispheric transport variations. These include greater variability prior to 2010 compared to the remarkable stability in annual CO2 interhemispheric difference in the 5-year relatively El Niño-quiet period 2010–2014 (despite a strong La Niña in 2011), and the 2017 recovery in the CO2 interhemispheric gradient from the unprecedented El Niño event in 2015–2016.

Measuring turbulent CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes with a closed-path gas analyzer in a marine environment

Abstract. In this study, we introduce new observations of sea–air fluxes of carbon dioxide using the eddy covariance method. The measurements took place at the Utö Atmospheric and Marine Research Station on the island of Utö in the Baltic Sea in July–October 2017. The flux measurement system is based on a closed-path infrared gas analyzer (LI-7000, LI-COR) requiring only occasional maintenance, making the station capable of continuous monitoring. However, such infrared gas analyzers are prone to significant water vapor interference in a marine environment, where CO2 fluxes are small. Two LI-7000 analyzers were run in parallel to test the effect of a sample air drier which dampens water vapor fluctuations and a virtual impactor, included to remove liquid sea spray, both of which were attached to the sample air tubing of one of the analyzers. The systems showed closely similar (R2=0.99) sea–air CO2 fluxes when the latent heat flux was low, which proved that neither the drier nor the virtual impactor perturbed the CO2 flux measurement. However, the undried measurement had a positive bias that increased with increasing latent heat flux, suggesting water vapor interference. For both systems, cospectral densities between vertical wind speed and CO2 molar fraction were distributed within the expected frequency range, with a moderate attenuation of high-frequency fluctuations. While the setup equipped with a drier and a virtual impactor generated a slightly higher flux loss, we opt for this alternative for its reduced water vapor cross-sensitivity and better protection against sea spray. The integral turbulence characteristics were found to agree with the universal stability dependence observed over land. Nonstationary conditions caused unphysical results, which resulted in a high percentage (65 %) of discarded measurements. After removing the nonstationary cases, the direction of the sea–air CO2 fluxes was in good accordance with independently measured CO2 partial pressure difference between the sea and the atmosphere. Atmospheric CO2 concentration changes larger than 2 ppm during a 30 min averaging period were found to be associated with the nonstationarity of CO2 fluxes.

CO2 and CH4 gas permeation study via zeolitic imidazolate framework (ZIF)-8 membrane

In the present work, the effects of CO2 feed composition (15%–90%), pressure difference (100 kPa–700 kPa) and temperature (303 K–343 K) on the separation of CO2 from CH4 via ZIF-8 membrane were investigated. The membrane was synthesized through a simple, feasible and reproducible solvent evaporation seeding method coupled with microwave-assisted secondary solvothermal growth. Subsequently, the membrane was characterized by using scanning electron microscopy. The results showed that, for single gas permeation, CO2 permeances remained constant while CH4 permeances increased slightly with the increasing pressure differences at temperature of 303 K. Besides, CO2 permeances decreased while CH4 permeances increased slightly with the increase in temperature at pressure difference of 100 kPa. For binary gas separation, maximum CO2/CH4 separation factor of 5.07 and CO2 permeance of 6.595 × 10−8 mol m−2 s−1 Pa−1 were obtained at CO2 feed composition of 15%, pressure difference of 100 kPa and temperature of 303 K. Besides, CO2 fluxes approached constant value with the increasing CO2 partial pressure differences at CO2 feed composition of 15%, which was mainly attributed to the competitive adsorption between CO2 and CH4. On the other hand, CH4 fluxes increased with increasing CH4 partial pressure differences even at higher CO2 composition in the feed, owing to its adsorption capacity and gas transport pathway via membrane intercrystalline pores.

Selective removal of carbon dioxide contained in the effluent from ion chromatography suppressors using a new non-vacuum device

In this paper, a new CO2 gas removal device optimized to selectively remove CO2 gas contained in the effluent from suppressors used in ion chromatography (IC) under non-vacuum conditions is described. This device consists of a closed vessel equipped with gas permeable tubing (GPT) and a CO2 adsorbent. During operation, the CO2 adsorbent adsorbs CO2 gas in the vessel, creating CO2 partial pressure difference between the inside of the GPT and the vessel. The CO2 gas contained in the effluent being pumped into the GPT is selectively removed from the effluent based on the diffusion of the CO2 associated with the CO2 partial pressure difference. The purpose of this study is to optimize the IC operating conditions with the aim of selectively removing HCO3− (CO32−) contained in the effluent and reducing the electrical conductivity of the effluent under non-vacuum conditions. The electrical conductivity of the effluent and the signal intensity of the water dip is decreased by approximately 25μS/cm (from 30 to 5μS/cm) and by approximately twentieth, respectively, using the optimized CO2 remover. In addition, the anion detection limit achieved in IC instruments with a CO2 remover is on the order of a few ppb.

Global air-sea surface carbon dioxide transfer velocity and flux estimated using 17 a altimeter data and a new algorithm

The global distributions of the air-seaCO(2) transfer velocity and flux are retrieved from TOPEX/Poseidon and Jason altimeter data from October 1992 to December 2009 using a combined algorithm. The 17 a average global, area-weighted, Schmidt number-corrected mean gas transfer velocity is 21.26 cm/h, and the full exploration of the uncertainty of this estimate awaits further data. The average total CO2 flux (calculated by carbon) from atmosphere to ocean during the 17 a was 2.58 Pg/a. The highest transfer velocity is in the circumpolar current area, because of constant high wind speeds and currents there. This results in strongCO(2) fluxes. CO2 fluxes are strong but opposite direction in the equatorial east Pacific Ocean, because the air-sea CO2 partial pressure difference is the largest in the global oceans. The results differ from the previous studies calculated using the wind speed. It is demonstrated that the air-sea transfer velocity is very important for estimating air-sea CO2 flux. It is critical to have an accurate estimation for improving calculation of CO2 flux within climate change studies.

Investigating the influence of sweep gas on CO2/N2 membranes for post-combustion capture

CO2 partial pressure difference is the driving force for CO2-selective membranes and is emerging as the critical issue, when applying CO2-selective membranes for post-combustion capture. Using sweep gas can be an option to increase the driving force for CO2 permeation without changing the total pressure. Hence, in contrast to the application of compressors and vacuum pumps, the usage of sweep gas achieves separation without extra energy consumption. Nevertheless, it has the inherent feature of diluting the permeate. In this paper, two membrane concepts using air as sweep gas for post-combustion capture were investigated. The influence of the sweep gas on membrane area and the degree of CO2 separation was explored. An energetic and economic analysis was carried out comparing this concept with the cascade membrane concept without sweep gas. The technical route using sweep gas makes it possible to achieve an efficiency loss of 3.8%-points with a 70% degree of CO2 separation for a 600MW reference power plant.

Carbon sources and sinks from an Ensemble Kalman Filter ocean data assimilation

[1] We quantify contemporary and preindustrial net air-sea CO2 fluxes by an Ensemble Kalman Filter assimilation of interior ocean observations and compare results with published estimates in the light of sensitivity to model transport and input data reconstructions. Four different published reconstructions of anthropogenic carbon and the ΔCgasex tracer are assimilated into different versions of the Bern3D ocean model. The two tracers represent the components of dissolved inorganic carbon due to the anthropogenic perturbation and the air-sea gas exchange of preindustrial CO2. Contemporary air-sea fluxes for broad latitudinal bands are consistent with those from earlier ocean inversions and the observed air-sea CO2 partial pressure differences. We infer modest meridional transport rates of up to 0.5 GtC yr−1 for the preindustrial and the contemporary ocean and a small carbon transport across the equator. The anthropogenic perturbation offsets the preindustrial net sea-to-air flux yielding a weak contemporary carbon sink in the Southern Ocean (south of 44°S) of 0.15 ± 0.25 GtC yr−1. Preindustrial Southern Ocean outgassing varies by almost a factor of 2 among the four ΔCgasex reconstructions. Large differences in regional fluxes are found between an earlier ocean inversion using Green's function and this study for the same model and input data calculation. Systematic differences in assimilated and optimized ΔCgasex fields are large in both inversions, and the contemporary, anthropogenic, and preindustrial air-sea CO2 flux in the high-latitude and midlatitude Southern Hemisphere remains uncertain.

Growth, carbon‐isotope discrimination, and drought‐associated mortality across a Pinus ponderosa elevational transect

AbstractDrought‐ and insect‐associated tree mortality at low‐elevation ecotones is a widespread phenomenon but the underlying mechanisms are uncertain. Enhanced growth sensitivity to climate is widely observed among trees that die, indicating that a predisposing physiological mechanism(s) underlies tree mortality. We tested three, linked hypotheses regarding mortality using a ponderosa pine (Pinus ponderosa) elevation transect that experienced low‐elevation mortality following prolonged drought. The hypotheses were: (1) mortality was associated with greater growth sensitivity to climate, (2) mortality was associated with greater sensitivity of gas exchange to climate, and (3) growth and gas exchange were correlated. Support for all three hypotheses would indicate that mortality results at least in part from gas exchange constraints. We assessed growth using basal area increment normalized by tree basal area [basal area increment (BAI)/basal area (BA)] to account for differences in tree size. Whole‐crown gas exchange was indexed via estimates of the CO2 partial pressure difference between leaf and atmosphere (pa−pc) derived from tree ring carbon isotope ratios (δ13C), corrected for temporal trends in atmospheric CO2 and δ13C and elevation trends in pressure. Trees that survived the drought exhibited strong correlations among and between BAI, BAI/BA, pa−pc, and climate. In contrast, trees that died exhibited greater growth sensitivity to climate than trees that survived, no sensitivity of pa−pc to climate, and a steep relationship between pa−pc and BAI/BA. The pa−pc results are consistent with predictions from a theoretical hydraulic model, suggesting trees that died had a limited buffer between mean water availability during their lifespan and water availability during drought – i.e., chronic water stress. It appears that chronic water stress predisposed low‐elevation trees to mortality during drought via constrained gas exchange. Continued intensification of drought in mid‐latitude regions may drive increased mortality and ecotone shifts in temperate forests and woodlands.

Evidence from the Baltic Sea for an enhanced CO2 air—sea transfer velocity

Surface water total CO2 concentrations (CT) and the CO2 partial pressure of the surface water and in the atmosphere were measured in the eastern Gotland Sea at approximately monthly intervals during five cruises in the winter of 1999/2000. Taking into account vertical/lateral exchange processes and the decomposition of organic matter, the monthly changes in CT were used to determine CO2evasion fluxes. In addition, the CO2 fluxes were calculated on the basis of the CO2 partial pressure differences using local wind speed (u) records and different currently applied parametrizations of the gas exchange transfer velocity (k). The latter resulted in substantially lower monthly fluxes that indicated a considerable underestimation of k from the k(u) functions used. To achieve an optimal agreement between the flux calculations and the balance-derived CO2 fluxes, the coefficients of both a simple quadratic and cubic function k(u) were iterated using a least-squares fitting procedure. The resulting equations, which refer to short-term wind data and to the CO2 exchange at 20 ¼C, were k=(0.45 ± 0.10)u2 and k=(0.037 ± 0.008)u3 (k, cm h−1; u, m s−1). These yielded higher k values than most of the previously proposed parametrizations. Unfortunately, our data did not allow us to decide whether the quadratic or cubic function is more appropriate to describe the gas exchange dynamics.

Nitric oxide inhalation increases alveolar gas exchange by decreasing deadspace volume.

To test the hypothesis that nitric oxide inhalation facilitates CO2 elimination by decreasing alveolar deadspace in an ovine model of acute lung injury. Prospective, placebo-controlled, randomized, crossover model. University research laboratory. Eleven mixed-breed adult sheep. To induce acute lung injury, hydrochloric acid was instilled into the tracheas of paralyzed sheep receiving controlled mechanical ventilation. Each sheep breathed 0 ppm, 5 ppm, and 20 ppm nitric oxide in random order. Estimates of alveolar deadspace volumes and arterial-to-end tidal CO2 partial pressure differences were used as indicators of CO2 elimination efficiency. At a constant minute ventilation, nitric oxide inhalation caused dose-independent decreases in Paco2 (p <.05), alveolar deadspace (p <.01), and arterial-to-end tidal CO2 partial pressure differences (p <.01). We found that estimates of arterial-to-end tidal CO2 partial pressure differences may be used to predict alveolar deadspace volume (r2 =.86, p <.05). Estimates of arterial-to-end tidal CO2 partial pressure differences are reliable indicators of alveolar deadspace. Both values decreased during nitric oxide inhalation in our model of acutely injured lungs. This finding supports the idea that nitric oxide inhalation facilitates CO2 elimination in acutely injured lungs. Future studies are needed to determine whether nitric oxide therapy can be used to reduce the work of breathing in selected patients with cardiopulmonary disorders.

Changes in respiration and ethylene production of apples in response to internal and external oxygen partial pressures

Variations in respiration rate and ethylene production of ‘Cox's Orange Pippin’ and ‘Granny Smith’ apples in response to reduced oxygen partial pressures were characterised by studying O2, CO2 and C2H4 partial pressure differences between the internal and external atmospheres (ΔpO2, ΔpCO2 and ΔpC2H4, Pa) of individual fruit maintained in different O2 atmospheres at 20 ± 1°C. ΔpO2 decreased at low O2 levels, reflecting the decreased rate of O2 uptake in low O2 partial pressures. Oxygen uptake relative to that in air (relrO2) approximately followed Michaelis-Menten kinetics, with a half-maximal rate at 3.3 kPa O2 for internal oxygen (piO2) and 10.4 kPa O2 for external oxygen (peO2). An equation was developed to describe aerobic and anaerobic components of CO2 production relative to that in air (relrCO2) as a function of peO2 or piO2. The relationship between rate of C2H4 production relative to that in air (relrC2H4) and piO2 was reasonably described by a Michaelis-Menten type hyperbolic curve, confirming the expectation that small changes in O2 partial pressure would have a much greater effect on relrC2H4 at low piO2 than they do at high piO2. In contrast, presence of the skin as a diffusion barrier resulted in development of an apparent ‘lag phase’ in the relationship between relrC2H4 and peO2 such that it became essentially sigmoidal. These differences are attributed to differences in O2 partial pressure between internal and external atmospheres.

CO2 transport at the air‐sea interface: Effect of coupling of heat and matter fluxes

Existing data on CO2 fluxes between air and sea have been calculated by assuming, incorrectly, that the thermodynamic driving force for the CO2 flux is provided solely by the CO2 partial pressure difference between the two phases. Because both heat and matter transfer occur under non‐equilibrium conditions, with finite temperature and composition differences across the interface, a proper description of transport at the air‐water interface requires the use of irreversible thermodynamics, according to which the heat and matter fluxes are coupled through the heat of solution of the gas. The present treatment shows that the driving force for CO2 transport due to the temperature difference across the interface is comparable with that arising from the partial pressure difference.

Arterial and Mixed Venous Blood Gas Status during Apnoea of Intubation — Proof of the Christiansen-Douglas-Haldane Effect in Vivo

The Christiansen-Douglas-Haldane effect, in short the Haldane effect, describes the dependence of the CO2 binding of blood on the degree of oxygenation of haemoglobin. Under the physiological conditions of an 'open' system between blood and alveoli the partial pressure of arterial CO2 (PaCO2), must be less than that of mixed venous blood (PvCO2). During the unphysiological conditions of a 'closed' system, e.g. hyperoxic apnoea, i.e. continuous oxygen uptake without CO2 delivery by the lungs, the PaCO2 will not only approximate the PvCO2 but will even exceed it. Without the Haldane effect, rapid adjustment of PaCO2 to PvCO2 would be expected during apnoea due to the lack of CO2 excretion. If, however, as undertaken in this study, ongoing oxygenation (high alveolar PO2 (PACO2) with concomitant lack of CO2 delivery (apnoea, i.e. the CO2 concentration remains constant) lead to a continuing sufficient oxygenation of blood during its passage through the lung capillaries, then this leads to a rightwards shift of the CO2 binding curve--the Haldane effect. The resulting increase in PCO2 as shown here actually leads to an arterial-mixed venous CO2 partial pressure difference (avDPCO2) of 2.8 +/- 1.8 mmHg. The results described substantiate for the first time the existence of the Haldane effect under clinical conditions.

Blood-gas equilibrium of carbon dioxide in lungs: a continuing controversy.

This review presents the experimental evidence that has been published in recent years both against and in support of the occurrence of negative blood-gas CO2 partial pressure differences (delta PCO2) in lungs in rebreathing equilibrium and during steady-state gas exchange in hypercapnia. Although some sources of potential experimental error can be pointed out, the reasons for the remarkably pronounced disagreement between the experimental data of the different studies cannot be definitely identified. Since a consistent and reproducible occurrence of negative blood-gas delta PCO2 in lungs in gas-blood equilibrium is not convincingly proved, it appears to be justified to continue accepting the validity of the conventional concept of equal PCO2 in blood and gas in equilibrium. Because the issue is of considerable importance in the analysis and understanding of alveolar gas exchange, pertinent evidence is expected from future work.

Analysis of the factors influencing peritubular PCO2 in the rat.

We have developed a mathematical model to assess the relative contributions of several factors to the high CO2 partial pressures observed in rat peritubular capillaries. This model is based on a single nephron and focuses specifically on the CO2 partial pressure differences (delta PCO2) between peritubular capillaries and the afferent arteriole. The model is formulated by writing steady-state mass balances for the glomerulus, proximal tubule, and peritubular capillaries in addition to equilibrium relationships for CO2, HCO3-, blood protein buffers, and hemoglobin carbamino compounds. Principal input parameters include glomerular blood flow rate, rates of HCO3- and water reabsorption, and the rate of metabolic CO2 production. Under conditions representative of normal Munich-Wistar rats, the model predicts delta PCO2 to be 4.1 mmHg, in approximate agreement with experimental observations reported elsewhere. Metabolic CO2 production is responsible for roughly half of this predicted delta PCO2, the remainder being attributable to reabsorption processes. In examining the sensitivity of delta PCO2 to changes in physiological conditions, we consistently found it to be inversely related to glomerular blood flow rate. The influence of changes in HCO3- reabsorption on delta PCO2 is variable and highly dependent on the arterial acid-base status and the ratio of HCO3- reabsorption to water reabsorption.

Reversal of arterial-to-expired CO2 partial pressure differences during rebreathing in goats.

Whether CO2 partial pressure (PCO2) in expired gas may exceed that in arterial blood has been controversial. We measured arterial PCO2 (Paco2) and end-tidal PCO2 (PETco2) in four awake goats during air breathing and during hyperoxic CO2 rebreathing in various conditions of acid-base balance. During air breathing, Paco2 was slightly higher than PETco2; i.e., the mean (+/- SE) difference, Paco2 - PETco2, was positive by + 2.36 +/- 0.53 Torr (P less than 0.001). In contrast, during CO2 rebreathing with the same techniques of measurement, this difference was always negative (mean +/- SE = -11.63 +/- 0.22 Torr, P less than 0.001), and it widened as Paco2 increased with rebreathing. Magnitude of the negative difference during rebreathing was too great to be accounted for by incorrect assumptions or measurement error, even if reasonable contributions from all known sources of error were concurrently invoked. We conclude that during hyperoxic CO2 rebreathing in goats, PETco2 exceeds Paco2.