Effect of Controlled Oxygen Therapy on Arterial Blood Gases in Acute Respiratory Failure

Seven patients in acute exacerbation of chronic respiratory failure were given 24·5% and later 28% oxygen through Ventimasks. The mean increases in arterial PO₂ were 11 and 21 mm. Hg while breathing 24·5% and 28% oxygen respectively compared with control values while breathing air. Associated increases in arterial PCO₂ were 4 and 8 mm. Hg, respectively. In five of the patients these increases in inspired oxygen concentration resulted in useful increases in tissue oxygen supply without significant deterioration in ventilation, but in two patients arterial PCO₂ rose excessively and artificial ventilation was required.     

Effects of inhalational anesthesia on blood gases and pH in California sea lions

Despite the widespread use of inhalational anesthesia with spontaneous ventilation in many studies of otariid pinnipeds, the effects and risks of anesthetic‐induced respiratory depression on blood gas and pH regulation are unknown in these animals. During such anesthesia in California sea lions (Zalophus californianus), blood gas and pH analyses of opportunistic blood samples revealed routine hypercarbia (highest P_CO2 = 128 mm Hg [17.1 kPa]), but adequate arterial oxygenation (P_O2 > 100 mm Hg [13.3 kPa] on 100% inspiratory oxygen). Respiratory acidosis (lowest pH = 7.05) was limited by the increased buffering capacity of sea lion blood. A markedly widened alveolar‐to‐arterial P_O2 difference was indicative of atelectasis and ventilation‐perfusion mismatch in the lung secondary to hypoventilation during anesthesia. Despite the generally safe track record of this anesthetic regimen in the past, these findings demonstrate the value of high inspiratory O₂ concentrations and the necessity of constant vigilance and caution. In order to avoid hypoxemia, we emphasize the importance of late extubation or at least maintenance of mask ventilation on O₂ until anesthetic‐induced respiratory depression is resolved. In this regard, whether for planned or emergency application, we also describe a simple, easily employed intubation technique with the Casper “zalophoscope” for sea lions.     

A comparison of biologically variable ventilation to recruitment manoeuvres in a porcine model of acute lung injury

BackgroundBiologically variable ventilation (return of physiological variability in rate and tidal volume using a computer-controller) was compared to control mode ventilation with and without a recruitment manoeuvre – 40 cm H₂O for 40 sec performed hourly; in a porcine oleic acid acute lung injury model.MethodsWe compared gas exchange, respiratory mechanics, and measured bronchoalveolar fluid for inflammatory cytokines, cell counts and surfactant function. Lung injury was scored by light microscopy. Pigs received mechanical ventilation (F_IO₂ = 0.3; PEEP 5 cm H₂O) in control mode until PaO₂ decreased to 60 mm Hg with oleic acid infusion (PaO₂/F_IO₂ <200 mm Hg). Additional PEEP to 10 cm H₂O was added after injury. Animals were randomized to one of the 3 modes of ventilation and followed for 5 hr after injury.ResultsPaO₂ and respiratory system compliance was significantly greater with biologically variable ventilation compared to the other 2 groups. Mean and mean peak airway pressures were also lower. There were no differences in cell counts in bronchoalveolar fluid by flow cytometry, or interleukin-8 and -10 levels between groups. Lung injury scoring revealed no difference between groups in the regions examined. No differences in surfactant function were seen between groups by capillary surfactometry.ConclusionsIn this porcine model of acute lung injury, various indices to measure injury or inflammation did not differ between the 3 approaches to ventilation. However, when using a low tidal volume strategy with moderate levels of PEEP, sustained improvements in arterial oxygen tension and respiratory system compliance were only seen with BVV when compared to CMV or CMV with a recruitment manoeuvre.     

Analysis of relative contributions to the alveolar-arterial oxygen gradient

A fundamental analysis has been undertaken of O₂ transmission across a static inert gas with simultaneous diffusion of CO₂ in the reverse direction. The overall alveolar-arterial, (A-a) DO₂, gradient has then been derived as the simple sum of four terms representing shunt, ventilation/perfusion inequalities, membrane diffusion and airway diffusion with due allowance for any variation of each process throughout the lung. The expression provides a mathematical framework from which to isolate the net contribution of each process to the overall gradient—each value being a mean weighted according to the ventilation distribution of the gas exchange units. Airway diffusion resistance can be isolated with change of pressure and, from such data, has been estimated as about 6–7% of total (A-a) DO₂ in a healthy man breathing air at normal pressure. The same O₂ data has enabled the effective diameter of the functional gas exchange unit to be estimated as 14.2 mm.—well within the morphological limits for the secondary lobule.     

Metabolic and ventilatory responses to hypoxia in two altitudinal populations of the toad, Bufo bankorensis

Effects of hypoxia on resting oxygen consumption ( ), lung ventilation, and heart rate at different ambient P_O2 were compared between lowland and high altitude populations of the toad, Bufo bankorensis. Resting decreased significantly in mild hypoxia (P_O2=120 mm Hg) at 10°C and in moderate hypoxia (P_O2=80 mm Hg) at 25°C in both altitudinal populations; however, resting did not differ significantly between the two populations. Numbers of lung ventilation periods (VP) and total inspired volume (V_L) did not change with P_O2 at 10°C, but did increase at moderate and severe hypoxia (40 mm Hg), respectively, at 25°C. Resting heart rates did not change during hypoxia and did not differ between altitude populations. The results suggest (1) the effect of P_O2 change on should be considered in future studies involving transfer of anurans to a different altitude; and (2) the metabolic and ventilatory physiology in B. bankorensis does not compensate for the low temperature and P_O2 at high altitude.     

Gill ventilation and O2 extraction during graded hypoxia in two ecologically distinct species of flatfish,the flounder (Platichthys flesus) and the plaice (Pleuronectes platessa)

Synopsis Gill ventilation, breathing frequency, breath volume, oxygen extraction from the ventilatory water current and oxygen uptake through the gills were measured in flounder, Platichthys flesus, and plaice, Pleuronectes platessa, at water O₂ tensions ranging from 35 to 155 mm Hg at 10° C. Ventilation volumes were similar in the two species at high water O₂ tension. Exposure to hypoxic water elicited a larger increase in ventilation in the flounder. The per cent extraction of O₂ from water decreased slightly in both species as water O₂ tension was lowered. At comparable levels of ventilation O₂ extraction was higher in flounder. At the higher levels of water O₂ tension, O₂ uptake across the gills of flounder was stable, the critical O₂ tension being between 60 and 100 mm Hg. The plaice behaved as an oxygen conformer over the entire range of O₂ tensions investigated. The superior ability of the flounder in maintaining OZ uptake across the gills during a reduction in water O₂ tension may in part explain why the species, unlike plaice, inhabits very shallow waters with large fluctuations in dissolved oxygen.     

The influence of gas exchange on lung gas concentrations during air breathing

A model study is made of the contribution that continuing respiratory gas exchange makes to the alveolar plateau slope for O₂ during air breathing. Calculations in the model of the O₂ concentration appearing at the mouth during expiration, are performed for single breaths of air at constant flow rates 18 litres/min and 120 litres/min. At 18 litres/min the breathing period is 5 sec, the initial lung volume is 2300 ml, and the O₂ uptake rate is 300 ml STPD/min; whereas at 120 litres/min these parameters are 4 sec, 1200 ml, and 1800 ml STPD/min respectively. In each case the initial lung O₂ tension is taken to be 98 mm Hg. It is found that at 18 litres/min, the O₂ concentration difference on the alveolar plateau over the last second of expiration is 0.4 mm Hg when gas exchange is omitted and 1.2 mm Hg when gas exchange is included in the model. At 120 litres/min, this difference is zero and 5.0 mm Hg respectively. The gas exchange component predicted from a corresponding well-mixed compartment model is the same at 18 litres/min (0.8 mm Hg) but is 6.0 mm Hg at 120 litres/min.     

Effect of Nitrazepam in Chronic Obstructive Bronchitis

The effect of nitrazepam on ventilatory capacity and arterial blood gas tensions was studied in a double-blind controlled crossover trial in six patients with chronic obstructive bronchitis in ventilatory failure (raised Paco₂). Nitrazepam produced a fall in ventilatory capacity and worsening of the ventilatory failure. In the sixth patient, while on the active drug, the Pao₂ fell from 48 to 35 mm Hg while the Paco₂ tension rose from 59·5 to 68 mm Hg. It is suggested that nitrazepam is contraindicated in patients with severe chronic obstructive bronchitis.     

Differentiation-arrested rat fetal lung in primary monolayer cell culture

Summary Differentiation-arrested lung cell cultures were developed from fetal rats of various gestational ages. In contrast to previously published observations with cultures in a pO₂ of ∼142 mm Hg, cultures developed in a pO₂ of ∼30 mm Hg, close to the normal fetal arterial pO₂, have improved plating efficiency and a slightly increased growth rate. They did not, however, show gestation-dependent increases of choline incorporation into phospholipids, nor did immature lung cell cultures respond to dexamethasone or triiodothyronine, singly or in combination, by increased choline incorporation into saturated lecithin. The incorporation of choline and glycerol into lipids suggested a mature rate of lipid synthesis by immature cultures at a pO₂ ∼30 mm Hg, despite preservation of an immature morphology. Electron microscope observations revealed no gross differences between immature cultures developed at either pO₂. The cellular mechanisms underlying these differences are unclear but suggest that oxygen tension may significantly influence results obtained with in vitro studies of lipid synthesis by immature lung. This work was supported by grants from the Medical Research Council of Canada, the Ontario Thoracic Society, and the Physicians' Services Incorporated Foundation.     

The influence of dissolved oxygen concentration on phosphofructokinase and the glucose metabolism ofEscherichia coli K-12

The glucose metabolism and the response of phosphofructokinase activity to oxygen were investigated using glucose-limited chemostat cultures ofE. coli K-12. With a dilution rate of 0.2 hr^–1 and a glucose input concentration of 0.83 g/litre, 10 steady states were obtained ranging from 320 to 0 mm HgO₂. Dissolved oxygen reached zero level at a pO₂ of 25.8 mm Hg. The specific phosphofructokinase activity was constant above 28 mm Hg O₂ and increased linearly at lower pO₂ levels until it reached highest activity at 0 mm Hg O₂. Cell dry weight also started to decrease linearly from 28 to 5.9 mm Hg O₂, and fell sharply thereafter. Acid production rate did not start before pO₂ reached 25.6 mm Hg, increased progressively with an additional sharp increase below 5.9 mm Hg O₂. The main endproducts formed were acetic acid and ethanol with lactic acid appearing below 5.9 mm Hg O₂. The results suggest an effect of oxygen on phosphofructokinase synthesis rather than an ATP inhibition of the enzyme.This work was supported by a grant from the Australian Research Grant Commission.     

Modification of alveolar hyperoxia induced pulmonary vasodilatation by indomethacin

Decrease in pulmonary vascular resistance was observed in neonatal minature pigs breathing 100% O₂ or 95% O₂:5% CO₂. The pulmonary vasodilator response to hyperoxia ventilation was reduced by indomethacin in the intact animal and in the isolated perfused lung preparation. In the isolated perfused lung preparation, it was also shown that lung alveolar pO₂ rather than pulmonary arterial pO₂ was responsible for the pulmonary vasodilation. The study suggests that alveolar hyperoxia induced decrease in pulmonary vascular resistance may be mediated in part by release of prostaglandins. The relevance of this study with oxygen therapy in newborn infants is also discussed.     

Negative Pressure Artificial Respiration: Use in Treatment of Respiratory Distress Syndrome of the Newborn

Forty-five newborn infants in respiratory failure with respiratory distress syndrome were treated with intermittent negative pressure ventilation (INPV). There was a survival rate of 38% (17/45).All infants were initially treated without nasotracheal intubation. However, 24 of these developed a Paco₂ greater than 70 mm. Hg and were subsequently intubated. Intubation was followed by a decrease in the degree of hypercarbia in each instance and simultaneous increase in Pao₂.Complications encountered during ventilation were: emphysema (one patient), aspiration pneumonia (two patients), septicemia (two patients), misplaced nasotracheal tube (one patient).Follow-up of the 17 surviving patients for periods of four to 36 months disclosed two patients with post-intubation hoarseness. One infant initially had spastic quadriplegia with EEG abnormalities, both of which cleared by 5 months of age. In the remaining 14 infants, the results of physical, neurological and psychological examinations have remained within normal limits.     

Hypoxemia, atelectasis, and the elevation of arterial pressure and heart rate in paralyzed artificially ventilated rat.

Rats prepared while anesthetized with halothane, ether or pentobarbital, subsequently paralyzed with curare, and maintained with or without anesthetic, by artificial ventilation with room air are hypoxemic in association with elevated arterial pressures and heart rates. The hypoxemia can occur with normal Pa_CO2, is associated with a marked increase in the alveolar-arterial P_O2 difference, and is not reversed by hyperventilation or hyperinflation. The lungs, visualized directly through a thoracotomy during ing artificial ventilation, are segmentally collapsed and at postmortem demonstrate focal and diffuse signs of atelectasis. Hypoxemia and an elevation of the alveolar-arterial P_O2 difference occur within 20 minutes after the onset of anesthesia, prior to paralysis. We conclude that anesthetized rats develop atelectasis soon after the onset of anesthesia. The atelectasis, and resultant hypoxemia persist during subsequent paralysis despite an adequate minute volume and absence of anesthesia. Despite atelectasis, blood gases, arterial pressures and heart rates may be maintained near normal values by ventilation of paralyzed rats with 50% O₂ and 50% N₂.     

Ventilatory responses to temperature variation in the fresh water turtle,Mauremys caspica leprosa

A study of lung gas exchange in the fresh water turtle Mauremys caspica leprosa at normal physiological body temperatures (15, 25 and 35 °C) was extended to extreme temperatures (5 and 40 °C) to determine whether the direct relationship between body temperature and ventilatory response found in many lung-breathing ectotherms including other chelonian species was maintained. From 5 to 35 °C the lung ventilation per unit of O₂ uptake and CO₂ removed declined with temperature. Consequently, lung CO₂ partial pressure increased with temperature. Its value was maintained within narrow limits at each thermal constant, suggesting a suitable control throughout the complete ventilatory cycle. At 40 °C the ventilatory response showed the opposite trend. The ratios of ventilation to lung gas exchange increased compared to their values at 35 °C. The impact of this increased breathing-lowering the estimated mean alveolar CO₂ partial pressure-was nevertheless less than expected due to an increase in calculated physiological dead space. This suggests that the relative hyperventilation in response to hyperthermia found in Mauremys caspica leprosa is related to evaporative heat loss.Abbreviations BTPS body temperature, ambient pressure, saturated with water vapour - CTM critical thermal maximum - F_N2 fractional concentration of nitrogen - PA _CO2or PL _CO2 alveolar or lung CO₂ pressure - PA_O2or PL_O2 alveolar or lung O₂ pressure - PI_O2 inspired O₂ pressure - R respiratory exchange ratio - STPD standard temperature, standard pressure, dry - T _a ambient temperature - T _b body temperature - VA alveolar ventilation - VA/VCO₂ relative alveolar ventilation (alveolar ventilation per unit of CO₂ removed) - VO₂ O₂ uptake - VCO₂ CO₂ output - V _D anatomical dead space volume - V _D physiological dead space volume - VE/VO₂ ventilatory equivalent for O₂ - VE pulmonary ventilation or expiratory minute volume - VE/VCO₂ ventilatory equivalent for CO₂ - V _T tidal volume     

Negative Pressure Artificial Respiration: Use in Treatment of Respiratory Failure of the Newborn

Ninety-one infants with respiratory failure secondary to primary pulmonary disease and with a birth weight of 1000 g. or over have been managed in a negative-pressure respirator (Air-Shields) over a three-year period. Of these the failure in 87 was due to respiratory distress syndrome (RDS) and in four it resulted from massive meconium aspiration. Respiratory failure was indicated initially by arterial blood gas tensions (while breathing 100% O₂) of Po₂ <40 mm. Hg, pH <7.10 and Pco₂ >75 mm. Hg in the initial 47 cases; these levels were subsequently raised to Po₂ < 50 mm. Hg, pH <7.20 and Pco₂ >70 mm. Hg for the remainder. Fifty-four (59.3%) of the infants survived the use of the respirator and 47 of these (51.6%) were subsequently discharged alive and well. Mean time in hours to normalization of blood gas values while on the respirator were as follows: for Po₂, 10.5; for pH, 11.6; and for Pco₂, 22.6. These values indicate that the respirator is more efficient in promoting oxygenation (raising Po₂) than ventilation (lowering Pco₂). They also suggest that the observed acidosis is in large part secondary to the hypoxia rather than the result of co₂ retention. For the survivors the average time of total respirator dependency before commencement of weaning was 53.7 hours. All the infants were managed without the use of endotracheal tubes although the use of the respirator and/or administration of 100% oxygen were either continuous or intermittent for periods of up to two weeks. There have been no instances of so-called respirator lung disease in the survivors or in those who died, which suggests that the use of high oxygen concentration by itself is not the major factor in the pathogenesis of this complication.     

THE INFLUENCE OF HYPOCAPNIA UPON INTRACELLULAR pH AND UPON SOME CARBOHYDRATE SUBSTRATES, AMINO ACIDS AND ORGANIC PHOSPHATES IN THE BRAIN

Abstract— In order to evaluate the influence of hypocapnia upon the energy metabolism of the brain, lightly anaesthetized rats were hyperventilated to arterial CO₂ tensions of 26, 15 and 10 mm Hg respectively, with subsequent measurements of intracellular pH and of tissue concentrations of carbohydrate substrates, amino acids and organic phosphates. At P_co1= 26 there was a moderate increase in the intracellular pH but when the P_co2 was reduced further to 10 mm Hg the intracellular pH returned to normal, or slightly subnormal, values. The reduction in P_Co2 was accompanied by increased cerebral cortical concentrations of lactate, pyruvate, citrate, α-ketoglutarate, malate and glutamate and by decreased aspartate concentrations. It is concluded that the accumulation of metabolic acids explains the normal value for intracellular pH at very low CO₂ tensions. Previous results obtained in man indicate that there is an increased anaerobic production of lactic acid in the brain in extreme hypocapnia. At comparable CO₂ tensions the present results showed a small fall in phosphocreatine and a small rise in ADP. However, since the ammonia concentrations were normal or decreased and since there was an increase in citrate, the results give no direct support to the hypothesis of an activation of phosphofructokinase. Since the cerebral venous P_o2 was reduced to 20 mm Hg at an arterial CO₂ tension of 10 mm Hg the accumulation of acids was probably secondary to tissue hypoxia. However, since there was no, or only a very small, increase in the calculated cytoplasmic NADH/NAD⁺ ratio, it appears less likely that acids accumulated due to lack of NAD⁺.     

The influence of chronic anaemia on catecholamine secretion in the rainbow trout (Oncorhynchus mykiss)

The effect of long-term (7 day) anaemia on catecholamine release was examined in rainbow trout (Oncorhynchus mykiss) in vivo during acute exposure to hypoxia and in situ using a perfused post-cardinal vein preparation. The first goal was to distinguish among reductions in blood O₂ partial pressure, O₂ concentration and haemoglobin percentage saturation as potential stimuli for, or correlates of, catecholamine secretion during hypoxia. The second goal was to elucidate the role of these factors in promoting enhanced chromaffin cell responsiveness in trout subjected to chronic hypoxia (Montpetit and Perry 1998). Anaemic fish (haematocrit lowered from 28.4±2.4% to 11.9±1.6%) displayed a marked reduction in haemoglobin-O₂ binding affinity [P ₅₀ (P _aO₂ at 50% Hb-O₂ saturation) was increased from 14.7 mm Hg to 24.3 mm Hg]. Upon exposure to hypoxia, the anaemic fish released catecholamines into their circulation at higher values of arterial O₂ partial pressure (∼52 mm Hg versus ∼18 mm Hg) and haemoglobin O₂ saturation (<70% versus <55%) than did control fish. In addition, anaemic fish achieved significantly greater circulating levels of total catecholamines (noradrenaline plus adrenaline) during acute hypoxia (294.8±67.3 versus 107.0±35.6 nmol l⁻¹). These results do not support the view that catecholamine release is triggered by a reduction in haemoglobin O₂ saturation or arterial PO₂, per se. Nor are they consistent with the idea that catecholamine release occurs at a threshold value of arterial PO₂ corresponding to a critical reduction in blood O₂ concentration. The effects of the non-selective cholinergic receptor carbachol on catecholamine secretion from chromaffin tissue were assessed using perfused posterior cardinal vein preparations derived from control or anaemic fish. For adrenaline secretion, there was no statistically significant change in the ED₅₀ (dose eliciting 50% response). For noradrenaline secretion however, preparations originating from anaemic fish displayed an enhanced responsiveness to carbachol as indicated by a significant 4.5-fold reduction in the carbachol ED₅₀ value from 2.53 × 10⁻⁶ mol kg⁻¹ to 5.67 × 10⁻⁷ mol kg⁻¹. These results demonstrate that anaemia-induced hypoxaemia, in the absence of any lowering of PO₂, is able to modulate the responsiveness of chromaffin cells to cholinergic stimulation. Accepted: 21 April 1999     

Quantitative aspects of oxygen and carbon dioxide exchange through the lungs in Ocypode ceratophthalmus (Crustacea: Decapoda) during rest and exercise in water and air

Ghost crabs Ocypode ceratophthalmus were exercised in air and water to measure CO₂ and O₂ exchange rates using the method of instantaneous measurements of oxygen consumption rate (MO₂) where applicable. Average heart rate increased from 100 to nearly 400 pulses per minute after five minutes of exercise on a treadmill at a run rate of 0.133 m s^?1. It took less than a minute for oxygen taken up through the lung epithelium from the air inside the branchial cavity to reach the maximal oxygen consumption rate of 26.1 mmol O₂ kg^?1 h^?1. Resting MO₂ was 4.06 mmol O₂ kg^?1 h^?1 in air, but decreased to 3.37 mmol O₂ kg^?1 h^?1 in seawater. Radioactive CO₂ from injected l-lactate is released linearly by the lung. The percent accumulated 14-CO₂ in exhaled air, plotted against time, intersects zero time on the x -axis, indicating rapid gas exchange at the lung surface. The P ₅₀ values for native haemocyanin of 4.89 mm Hg before exercise, and 8.99 mm Hg after exercise, are typical of a high-affinity haemocyanin usually associated with terrestrial crabs. The current notion that Ocypode ceratophthalmus drown when submerged in seawater was not substantiated by our experiments. MO₂ in seawater increased from 3.37 mmol O₂ kg^?1 h^?1 for resting crabs to 5.72 mmol O₂ kg^?1 h^?1 during exercise. When submerged by wave-seawater in the natural environment and during exercise in respirometer-seawater O. ceratophthalmus do not swim but, having a specific density of 1.044, float nearly weightless with a minimum of body movements.     

Very low tidal volume ventilation with associated hypercapnia--effects on lung injury in a model for acute respiratory distress syndrome

Background

Ventilation using low tidal volumes with permission of hypercapnia is recommended to protect the lung in acute respiratory distress syndrome. However, the most lung protective tidal volume in association with hypercapnia is unknown. The aim of this study was to assess the effects of different tidal volumes with associated hypercapnia on lung injury and gas exchange in a model for acute respiratory distress syndrome.

Methodology/Principal Findings

In this randomized controlled experiment sixty-four surfactant-depleted rabbits were exposed to 6 hours of mechanical ventilation with the following targets: Group 1: tidal volume = 8–10 ml/kg/PaCO₂ = 40 mm Hg; Group 2: tidal volume = 4–5 ml/kg/PaCO₂ = 80 mm Hg; Group 3: tidal volume = 3–4 ml/kg/PaCO₂ = 120 mm Hg; Group 4: tidal volume = 2–3 ml/kg/PaCO₂ = 160 mm Hg. Decreased wet-dry weight ratios of the lungs, lower histological lung injury scores and higher PaO₂ were found in all low tidal volume/hypercapnia groups (group 2, 3, 4) as compared to the group with conventional tidal volume/normocapnia (group 1). The reduction of the tidal volume below 4–5 ml/kg did not enhance lung protection. However, oxygenation and lung protection were maintained at extremely low tidal volumes in association with very severe hypercapnia and no adverse hemodynamic effects were observed with this strategy.

Conclusion

Ventilation with low tidal volumes and associated hypercapnia was lung protective. A tidal volume below 4–5 ml/kg/PaCO₂ 80 mm Hg with concomitant more severe hypercapnic acidosis did not increase lung protection in this surfactant deficiency model. However, even at extremely low tidal volumes in association with severe hypercapnia lung protection and oxygenation were maintained.     

Respiratory rate,haemolymph oxygen tension and haemocyanin level in the shrimp Palaemon adspersus: Rathke

The marine shrimp Palaemon adspersus Rathke, frequenting Zostera meadows, shows an oxygen consumption rate (MO₂) that is independent of water oxygen tension (P_wO₂) as PO₂ decreases to a critical point (P_cr) near 70 mm Hg. This respiratory independence is associated with maintenance of a relatively constant “arterial” (post-branchial) haemolymph tension (P_aO₂) at 70–80 mm Hg. At lower P_wO₂ values, both MO₂ and P_aO₂ fall, reflecting, in contrast to the above independence, a direct dependence of MO₂ on internal PO₂.Haemolymph copper measurements demonstrate relatively high haemocyanin concentrations and oxygen-carrying capacities, but MO₂ levels reflect an insignificant rôle for the pigment as an oxygen store.The data are discussed as regards adaptations for aerobic metabolism.