End-tidal carbon dioxide tension reflects arterial carbon dioxide tension in the heat-stressed human with and without simulated hemorrhage

End-tidal carbon dioxide tension (Pet(CO(2))) is reduced during an orthostatic challenge, during heat stress, and during a combination of these two conditions. The importance of these changes is dependent on Pet(CO(2)) being an accurate surrogate for arterial carbon dioxide tension (Pa(CO(2))), the latter being the physiologically relevant variable. This study tested the hypothesis that Pet(CO(2)) provides an accurate assessment of Pa(CO(2)) during the aforementioned conditions. Comparisons between these measures were made: 1) after two levels of heat stress (N = 11); 2) during combined heat stress and simulated hemorrhage [via lower-body negative pressure (LBNP), N = 8]; and 3) during an end-tidal clamping protocol to attenuate heat stress-induced reductions in Pet(CO(2)) (N = 7). Pet(CO(2)) and Pa(CO(2)) decreased during heat stress (P < 0.001); however, there was no group difference between Pa(CO(2)) and Pet(CO(2)) (P = 0.36) nor was there a significant interaction between thermal condition and measurement technique (P = 0.06). To verify that this nonsignificant trend for the interaction was not due to a type II error, Pet(CO(2)) and Pa(CO(2)) at three distinct thermal conditions were also compared using paired t-tests, revealing no difference between Pa(CO(2)) and Pet(CO(2)) while normothermic (P = 0.14) and following a 1.0 ± 0.2°C (P = 0.21) and 1.4 ± 0.2°C (P = 0.28) increase in internal temperature. During LBNP while heat stressed, measures of Pet(CO(2)) and Pa(CO(2)) were similar (P = 0.61). Likewise, during the end-tidal carbon dioxide clamping protocol, the increases in Pet(CO(2)) (7.5 ± 2.8 mmHg) and Pa(CO(2)) (6.6 ± 3.4 mmHg) were similar (P = 0.31). These data indicate that mean Pet(CO(2)) reflects mean Pa(CO(2)) during the evaluated conditions.     

The influence of oxygen and carbon dioxide on diving behaviour of tufted ducks, Aythya fuligula

While optimal diving models focus on the diver's oxygen (O(2)) stores as the predominant factor influencing diving behaviour, many vertebrate species surface from a dive before these stores are exhausted and may commence another dive well after their O(2) stores have been resaturated. This study investigates the influence of hypoxia and also hypercapnia on the dive cycle of tufted ducks, Aythya fuligula, in terms of surface duration and dive duration. The birds were trained to surface into a respirometer box after each dive to a feeding tray so that rates of O(2) uptake (VO2) and carbon dioxide output (VCO2) at the surface could be measured. Although Vco2 initially lagged behind Vo2, both respiratory gas stores were close to full adjustment after the average surface duration, indicating that they probably had a similar degree of influence on surface duration. Chemoreceptors, which are known to influence diving behaviour, detect changes in O(2) and CO(2) partial pressures in the arterial blood. Thus, the need to restore blood gas levels appears to be a strong stimulus to continue ventilation. Mean surface duration coincided with peak instantaneous respiratory exchange ratio due to predive anticipatory hyperventilation causing hypocapnia. For comparison, the relationship between surface duration and O(2) uptake in reanalysed data for two grey seals indicated that one animal tended to dive well after fully restocking its O(2) stores, while the other dived at the point of full restocking. More CO(2) is exchanged than O(2) in tufted ducks during the last few breaths before the first dive of a bout, serving to reduce CO(2) stores and suggesting that hypercapnia rather than hypoxia is more often the limiting factor on asphyxia tolerance during dives. Indeed, according to calculations of O(2) stores and O(2) consumption rates over modal diving durations, a lack of O(2) does not seem to be associated with the termination of a dive in tufted ducks. However, factors other than CO(2) are also likely to be important, and perhaps more so, such as food density and rate of food ingestion. Because some predictive success has been demonstrated for optimal diving models, they should continue to incorporate O(2) stores as a variable, but their validity is likely to be improved by also focusing on CO(2) stores.     

Biofeedback of alveolar carbon dioxide tension and levels of arousal

This is a preliminary study designed to investigate the potential usefulness of alveolar (lung) CO2 feedback training in promoting sleep onset in primary insomniacs. The present study was undertaken to determine if normal subjects could, without obvious manipulation of breathing, bring alveolar (lung) CO2 tension under voluntary control using biofeedback techniques and, if so, whether this control would be accompanied by shifts in level of wakefulness. Subjects participated in five baseline and five training sessions in which EEG, alveolar CO2 tension, and thoracic/abdominal respiratory movement were monitored. The feedback consisted of a pitch-modulated tone plus visual scores. We found that CO2 tension in awake portions of "up" trials was significantly higher than for awake portions of "down" trials (p less than .01), indicating that learning had occurred. In the initial trials, when subjects raised CO2 tension they became drowsy and often fell asleep, and when they lowered CO2 tension they aroused themselves. However, when subjects were awakened immediately upon falling asleep, there developed a decoupling of EEG and CO2 changes. The presence of such a decoupling phenomenon makes it unclear whether CO2 feedback will be useful in promoting sleep onset in primary insomniacs.     

Biofeedback of alveolar carbon dioxide tension and levels of arousal

This is a preliminary study designed to investigate the potential usefulness of alveolar (lung) CO₂ feedback training in promoting sleep onset in primary insomniacs. The present study was undertaken to determine if normal subjects could, without obvious manipulation of breathing, bring alveolar (lung) CO₂ tension under voluntary control using biofeedback techniques and, if so, whether this control would be accompanied by shifts in level of wakefulness. Subjects participated in five baseline and five training sessions in which EEG, alveolar CO₂ tension, and thoracic/abdominal respiratory movement were monitored. The feedback consisted of a pitch-modulated tone plus visual scores. We found that CO₂ tension in awake portions of up trials was significantly higher than for awake portions of down trials (p<.01), indicating that learning had occurred. In the initial trials, when subjects raised CO₂ tension they became drowsy and often fell asleep, and when they lowered CO₂ tension they aroused themselves. However, when subjects were awakened immediately upon falling asleep, there developed a decoupling of EEG and CO₂ changes. The presence of such a decoupling phenomenon makes it unclear whether CO₂ feedback will be useful in promoting sleep onset in primary insomniacs.This study was supported by National Institute of Mental Health Research Fellowship MH05151, Research Grant MH29369, Research Scientist Development Award MH38897, and by a Biomedical Research Support Grant to Langley Porter Institute, 507RR05755.     

Effects of sodium chloride and carbon dioxide on oxygen binding byArtemia hemoglobin

Summary The effects of both sodium chloride and CO₂ on oxygen binding by component II of the hemoglobin fromArtemia franciscana have been determined. Sodium chloride decreases both the oxygen affinity and cooperativity: the Hill coefficient decreases from 2.4 to 1.7 in the presence of 0.5 M NaCl at pH 7.5, 20°C. In contrast, CO₂ increases both the oxygen affinity and cooperativity. The effects of both agents are small and increase with the degree of oxygenation.     

Effect of oxygen concentration on leaf photosynthesis and resistances to carbon dioxide diffusion

Summary Net photosynthesis of tropical legume leaves increased by 44% and that of tropical grass leaves was unaffected when oxygen concentration was reduced from 21 to 0.2%. Stomatal resistance to carbon dioxide diffusion was unaltered in both cases but mesophyll resistance of legume leaves decreased with oxygen concentration. It is proposed that the decrease in mesophyll resistance is accompanied by decreases in excitation and carboxylation resistances.     

Linkage between carbon dioxide binding and four-step oxygen binding to hemoglobin

In order to elucidate the molecular mechanism of the effect of carbon dioxide on the four-step oxygenation equilibria of hemoglobin, accurate oxygen equilibrium curves of human adult hemoglobin were determined at different concentrations of CO₂ and in the presence and absence of chloride (Cl^?), 2,3-diphosphoglycerate (P₂G), and/or inositol hexaphosphate (IHP) and were analyzed according to Adair's stepwise oxygenation scheme to evaluate the four Adair constants, k_i (i = 1 to 4). The effects of CO₂ on oxygen affinity and co-operativity are influenced by H⁺, Cl^?, P₂G and IHP. The shape of the oxygen equilibrium curve varies with changes of CO₂ concentration; the four Adair constants are affected by CO₂ non-uniformly. Hence, the number of CO₂ molecules released upon oxygenation is not the same in the individual oxygenation steps. In the absence of added Cl^?, CO₂ lowers the overall oxygen affinity expressed by median oxygen pressure (p_m) and increases the co-operativity expressed by Hill's coefficient (n_max) by reducing k₁, k₂ and k₃ without changing k₄. significantly. The effect of CO₂ on oxygen affinity becomes smaller with decrease in pH, disappearing below pH 6.5. The alkaline Bohr effect is reduced by CO₂. The first oxygenation step contributes to the reduction of the Bohr effect more than the fourth step. When log p_m is plotted against log [CO₂] at several constant Cl^? concentrations, the plots converge to a common point that is named “iso-effective point”. When log p_p is plotted against log [Cl^?] at several constant CO₂ concentrations, the plots also converge to an iso-effective point. This phenomenon can be explained in terms of linkage relations in oxygen-linked competitive binding of CO₂ and Cl^?. It was found to be useful to consider in this analysis that the bicarbonate ion introduced by added CO₂ exerts a heterotropic effect equivalent to that of Cl^?. The combined effects of Cl^?, CO₂ and IHP were not explained satisfactorily by the present analysis using linkage relations.     

Plant cell growth under different levels of oxygen and carbon dioxide

Summary An experimental system, in which gases of known composition were passed through flasks, was used to systematically study the effects of oxygen and carbon dioxide on plant cell growth. As expected, oxygen limiting conditions resulted in suppressed growth of Catharanthus roseus cultures. Oxygen limitations did not alter the amount of cell mass produced per gram of sugar consumed which suggests that the production of fermentative metabolites was limited. Varying levels of carbon dioxide were observed to have no effect on the growth rates of either C. roseus or Daucus carota cultures. The amount of C. roseus cell mass generated per gram of sugar consumed appeared to be slightly increased at higher carbon dioxide levels.