A vacuum chamber for simplifying the collection of expired air samples

Summary Metalized bags for collection and storage of expired air for the study of oxygen consumption in man have been described by Johnsonet al. [J. appl. Physiol.22, 377–379 (1967)]. We have developed a vacuum chamber to permit rapid, multiple sampling with these metalized bags. The reproducibility and reliability of oxygen and carbon dioxide measurements of samples collected with the vacuum chamber compare favorably with accepted standard techniques, such as that of Campney and Pleasants [Res. Quart. Amer. Ass. Hlth phys. Educ.36, 207–210 (1965)].     

A simple method for measuring calorie expenditures during sleep

Summary A simple method for the continuous determination of O₂ consumption and CO₂ production in sleeping subjects is described. Face masks and moth pieces have been eliminated and the method has been successful with babies and adults. The apparatus consists of a ventilated hood connected to a vacuum cleaner. The subject sleeps with his head on a pillow inside the hood and fresh air is sucked over his face. Diluted expired air leaves the hood through an outlet in the rear wall and its volume is measured with a gas meter. A side-arm fitted to the outlet enables continuous samples of diluted expired air to be diverted by an aquarium pump into a sampling bag. Safety circuits are incorporated into the hood.     

The relation of ventilation to metabolic rate during moderate exercise in man

Summary To characterize more precisely the relationship between ventilation (V _E) and CO₂ output (VCO₂) during incremental exercise, 35 healthy males were studied at rest and during upright cycle ergometry, with the work rate incremented every 4 min up to each subject's anaerobic threshold ( _an). Twenty-one subjects had arterial blood sampled at rest and in the steady state at each work rate to determine the relationship between physiological dead space ventilation (V _D) and VCO₂. At these work rates arterial PCO₂ was regulated at the resting, control value. V _E (BTPS) was linearly related to VCO₂ from rest to _an with a slope of 24.6. However, the regression had a significant positive intercept of 3.2 L·min^–1. This causes the ventilatory equivalent for CO₂ (i.e., V _E/VCO₂) to decrease with increasing work rates. V _D also increased linearly with increasing VCO₂. However, this was consequent to increased breathing frequency as V _D remained constant. Thus, the observed fall in V _E/VCO₂ with increasing work rates is due to the positive intercept but the inherent relationship between V _E and VCO₂, reflected by the linear regression slope, remains unchanged from rest through moderate exercise.This investigation was supported by National Institutes of Health Grant HL-11907     

Phospholipase A2 activity determines rate of mitochondrial respiration in hibernating animals

Institute of Biological Physics, Academy of Sciences of the USSR, Pushchino. (Presented by Academician of the Academy of Medical Sciences of the USSR S. V. Severin.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 108, No. 10, pp. 488–489, October, 1989.     

In vitro effects of platinum compounds on renal cellular respiration in mice

Background: Cisplatin, carboplatin and oxaliplatin are structurally-related compounds, which are commonly used in cancer therapy. Cisplatin (Platinol^®) has Boxed Warning stating: “Cumulative renal toxicity associated with PLATINOL is severe”, while carboplatin and oxaliplatin are less nephrotoxic. These drugs form platinum adducts with cellular DNA. Their bindings to cellular thiols (e.g., glutathione and metallothionein) are known to contribute to drug resistance while thiol depletion augments platinum toxicity. Methods: Using phosphorescence oxygen analyzer, this study investigated the effects of platinum drugs on renal cellular respiration (mitochondrial O₂ consumption) in the presence and absence of the thiol blocking agent N-ethylmaleimide (used here as a model for thiol depletion). Renal cellular ATP was also determined. Kidney fragments from C57BL/6 mice were incubated at 37°C in Krebs-Henseleit buffer (gassed with 95% O₂:5% CO₂) with and without 100 μM platinum drug in the presence and absence of 100 μM N-ethylmaleimide for ≤ 6 h. Results: Platinum drugs alone had no effects on cellular respiration (P ≥ 0.143) or ATP (P ≥ 0.161). N-ethylmaleimide lowered cellular respiration (P ≤ 0.114) and ATP (P = 0.008). The combination of platinum drug and N-ethylmaleimide significantly lowered both cellular respiration (P ≤ 0.006) and ATP (P ≤ 0.003). Incubations with N-ethylmaleimide alone were associated with moderate-to-severe tubular necrosis. Incubations with cisplatin+N-ethylmaleimide vs. cisplatin alone produced similar severities of tubular necrosis. Tubular derangements were more prominent in carboplatin+N-ethylmaleimide vs. carboplatin alone and in oxaliplatin+N-ethylmaleimide vs. oxaliplatin alone. Conclusions: These results demonstrate the adverse events of thiol depletion on platinum-induced nephrotoxicities. The results suggest cellular bioenergetics is a useful surrogate biomarker for assessing drug-induced nephrotoxicities.     

A Mathematical Model to Predict CO2 Removal in Hollow Fiber Membrane Oxygenators

A mathematical model has been developed to predict CO₂ removal in hollow fiber membrane oxygenators. The model is analogous to one developed previously for predicting O₂ transfer. A mass transfer correlation was determined in water for O₂ and CO₂ exchange and collapsed onto one universal curve. The correlation was used to predict CO₂ removal in blood by incorporating a ‘facilitated diffusivity’ to account for the transport of CO₂ present as bicarbonate. The diffusion of bicarbonate greatly increased the ability of the oxygenator to remove CO₂ in blood compared to water. A fiber bundle module was fabricated to test the model predictions. The fiber bundle had a length of 13 cm and a bundle thickness of 0.2 cm. The module was tested in bovine blood at flowrates of 0.75, 1.5, and 2.2 L/min and CO₂ removal rate predictions were within 9% of experimental measurements at all flowrates. The O₂ transfer rate predictions were within 10% of experimental measurements. A second module was manufactured with a bundle of length 4 cm and thickness of 1 cm. The CO₂ removal predictions were within the standard deviation of the experimental measurements.     

A flow cytometric approach to assess phytoplankton respiration

Microbial respiration in the ocean is considered as the major process representative of the organic matter biological oxidation.The corresponding metabolic CO₂ production was estimated to be about 22 Pg C y^–1 [24]. However, the in situ respiration rate is generally too low (by several orders of magnitude) to be accessible to the available direct measurement methods. Some fluorescent probes, such as DiOC₆(3) (Molecular Probes, USA) have been shown to be very sensitive to changes in the proton electrochemical potential difference (~_H+), characterising mitochondrial and plasmic membranes bearing the cell respiratory system in eukaryotic and prokaryotic cells respectively [22, 37]. In mitochondria, ~_H+ is linked to the flux of oxygen uptake by a linear relationship [32]. To our knowledge, no such relationship has been established in the case of whole marine cells. In the present work, we addressed the dark respiration rate of the Chlorophyceae Dunaliella tertiolecta (Butcher) in axenic cultures, both directly by using a highly sensitive oxygraph (Oroboros) and by staining cells with DiOC₆(3). We found and standardized a linear relationship between oxygen uptake by D. tertiolecta and its green fluorescence inducedby DiOC₆(3), enabling the determination by flow cytometry of the respiration rate of D. tertiolecta.     

A method for estimating bicarbonate buffering of lactic acid during constant work rate exercise

A method to estimate the CO₂ derived from buffering lactic acid by HCO₃ ^– during constant work rate exercise is described. It utilizes the simultaneous continuous measurement of O₂ uptake ( O₂) and CO₂ output ( CO₂), and the muscle respiratory quotient (RQ_m). The CO₂ generated from aerobic metabolism of the contracting skeletal muscles was estimated from the product of the exercise-induced increase in O₂ and RQ_m calculated from gas exchange. By starting exercise from unloaded cycling, the increase in CO₂ stores, not accompanied by a simultaneous decrease in O₂ stores, was minimized. The total CO₂ and aerobic CO₂ outputs and, by difference, the millimoles (mmol) of lactate buffered by HCO₃ ^– (corrected for hyperventilation) were estimated. To test this method, ten normal subjects performed cycling exercise at each of two work rates for 6 min, one below the lactic acidosis threshold (LAT) (50 W for all subjects), and the other above the LAT, midway between LAT and peak O₂ [mean (SD), 144 (48) W]. Hyperventilation had a small effect on the calculation of mmol lactate buffered by HCO₃ ^– [6.5 (2.3)% at 6 min in four subjects who hyperventilated]. The mmol of buffer CO₂ at 6 min of exercise was highly correlated (r = 0.925, P < 0.001) with the increase in venous blood lactate sampled 2 min into recovery (coefficient of variation = ±0.9 mmol·l^–1). The reproducibility between tests done on different days was good. We conclude that the rate of release of CO₂2 from HCO₃ ^– can be estimated from the continuous analysis of simultaneously measured CO₂, O₂, and an estimate of muscle substrate.     

Pump for accurate mixing of three gas components with predetermined fractions

A mixing pump that creates an accurate mixture of three gases at predetermined fractional ratios that can be set in steps of 10 ppm is described. A nearly continuous flow of each of the three component gases is produced by pistons driven by stepping motors; the gas mixture is forwarded by a fourth piston. The flow of each component gas is adjusted by the stepping frequency of the motor and a microcomputer system is used to adjust the three frequencies according to the desired fractional concentrations. The total flow of the gas mixture is adjustable between 0.1–500 ml/min and is nearly independent of the after-load. The accuracy of the pump was tested by mixing the respiratory gases, O₂ and CO₂, with various carrier gases (N, He or Ar) at various fractional ratios and total flow rates. The fractions of O₂ and CO₂ in the mixture were analysed with the Scholander technique. In the physiological range, the mixing error in the gas fractions was less than 4%. The pump is, thus, suited for producing calibration mixtures.     

The effectiveness of the control of pH in the extracellular fluid of the brain by the respiratory control system

Summary The effectiveness of the respiratory control system as a regulator of the pH in the extracellular fluid of the brain is defined by pH_{ECF op}/pH_{ECF cl} where pH_{ECF op} means the primary or open loop shift and pH_{ECF cl} the final or closed loop shift of brain extracellular fluid pH. The analysis of a steady state model described in a preceding paper (Loeschcke, 1973) allows, in the limits of the suppositions and simplifications, to calculate the effectiveness of the feedback regulator in the cases of increased metabolism, metabolic acidosis-alkalosis and inhalation of CO₂. The effectiveness is diminished if CO₂ production is increased, it drops in metabolic acidosis and rises in metabolic alkalosis and it drops steeply if CO₂ is inhaled. The effectiveness of this control system depends on the controlling action of the controlling element (the ventilation) rather than on varying sensitivity of the sensing element. The controlling effect is defined asC=–d P _CO2/d V _A orC=d pH_ECF/d V _A.Im Rahmen des Programms des Sonderforschungsbereiches 114 (Bionach) der Deutschen Forschungsgemeinschaft.     

Nonlinearity identified by neural network models inP CO 2 control system in humans

The nonlinearity included in the P_{CO 2} control system in humans is evaluated using the degree of nonlinearity based on a difference of residuals. An autoregressive moving average (ARMA) model and neural networks (linear and nonlinear) are employed to model the system, and three types of network (Jordan, Elman and fully interconnected) are compared. As the Jordan-type linear network cannot approximate respiratory data accurately, the other two types and the ARMA model are used for the evaluation of the nonlinearity. The results of the evaluation indicate that the linear assumption for the P_{CO 2} control system is invalid for three subjects out of seven. In particular, strong nonlinearity was observed for two subjects.     

New acquisitions in the assessment of breath-by-breath alveolar gas transfer in humans

We summarise recent results obtained in testing some of the algorithms utilised for estimating breath-by-breath (BB) alveolar O₂ transfer (VO_2A) in humans. VO_2A is the difference of the O₂ volume transferred at the mouth minus the alveolar O₂ stores changes. These are given by the alveolar volume change at constant O₂ fraction (F _AiO₂ V _Ai) plus the O₂ alveolar fraction change at constant volume [V _Ai–1(F _Ai–F _Ai–1)O₂], where V _Ai–1 is the alveolar volume at the beginning of the breath i. All these quantities can be measured BB, with the exception of V _Ai–1, which is usually set equal to the subjects functional residual capacity (FRC) (Auchincloss algorithm, AU). Alternatively, the respiratory cycle can be defined as the time elapsing between two equal O₂ fractions in two subsequent breaths (Grønlund algorithm, GR). In this case, F _AiO₂=F _Ai–1O₂ and the term V _Ai–1(F _Ai–F _Ai–1)O₂ disappears. BB alveolar gas transfer was first determined at rest and during exercise at steady-state. AU and GR showed the same accuracy in estimating alveolar gas transfer; however GR turned out to be significantly more precise than AU. Secondly, the effects of using different V _Ai–1 values in estimating the time constant of alveolar O₂ uptake (O_2A) kinetics at the onset of 120 W step exercise were evaluated. O_2A was calculated by using GR and by using (in AU) V _Ai–1 values ranging from 0 to FRC +0.5 l. The time constant of the phase II kinetics (₂) of O_2A increased linearly, with V _Ai–1 ranging from 36.6 s for V _Ai–1=0 to 46.8 s for V _Ai–1=FRC+0.5 l, whereas ₂ amounted to 34.3 s with GR. We concluded that, when using AU in estimating O_2A during step exercise transitions, the ₂ value obtained depends on the assumed value of V _Ai–1.     

Analysis of alveolar PCO 2 control during the menstrual cycle

We attempted to analyze how is regulated during progesterone-induced hyperventilation in the luteal phase. A model for the CO₂ control loop was constructed, in which the function of the CO₂ exchange system was described as and that of the CO₂ sensing system as . Using this model, we estimated (1) the primary increase in produced by progesterone stimulation and (2) the effectiveness (E) of the loop to regulateP _{A CO 2}, defined as P _{A CO 2} (op)/P _{A CO 2} (cl) in which op signifies open-loop and cl, closed-loop. These respiratory variables were investigated throughout the menstrual cycle in 8 healthy women. During the luteal phase, on average, increased by 9.4% andP _{A CO 2},B andH decreased by 0.33 kPa (2.5 mm Hg), 0.47 kPa (3.5 mm Hg) and 13.6%, respectively, whileS and did not change significantly. (op) increased progressively on successive days of the luteal phase whileE remained unchanged at a value of 7.9, thus there was a progressive decrease inP _{A CO 2}. The decrease inH was considered to lessen P _{A CO 2} (op) and so reduce the final deviation ofP _{A CO 2} (P _{A CO 2} (cl)) during the luteal phase. The decrease inB was found to be dependent on (op).     

Kinetics of oxygen uptake and recovery for supramaximal work of short duration

In order to follow the time pattern of oxygen uptake and recovery for supramaximal work of short duration, 35 male subjects (mean age 21.4 years, mean body weight 71.9 kg) pedalled a bicycle ergometer at maximal speed for 1 min. A constant frictional resistance of 5.5 kg was used, resulting in a total work output of 2890 kpm (85 revolutions, SD = 7.5). The total percent decrement in work output from the initial rate on this test was 59.7 %. The total oxygen uptake during the work averaged 2.35 l, the net oxygen recovery was 4.891, while the net work efficiency was 19.3 %. One and two component exponential curves fit the observed oxygen uptake and recovery measures with a high degree of accuracy. Comparison of the curve parameters with published data showed large differences for the post exercise oxygen recovery and the slow component of the recovery curve. The magnitude of the fast component of recovery was similar to other data. The total oxygen uptake during the test was found to be 10% lower than the maximal oxygen uptake determined on a seperate progressive step-increment test. It was shown, by curve analysis, that the maximal oxygen uptake would have been reached in approximately 2 min.     

Sensation and control of breathing: A dynamic model

A dynamic model of the CO₂ respiratory control system is proposed, which can provide a qualitative basis for predicting breathing sensations. The discomfort index, which represents breathing sensations, is assumed to be composed of two sources: the arterial CO₂ level and the respiratory motor command. The respiratory controller receives inhibitory neuromechanical and excitatory CO₂ signals from the plant. The CO₂ signal is enhanced by exercise stimuli. This dynamic multiplicative-type controller is used in simulations of key experiments: exercise and CO₂ rebreathing with and without resistive loading. The dynamics of the discomfor index, the respiratory motor command, ventilation, and arterial CO₂ concentration conform to the experimental data. The perceptual sensitivity to CO₂ relative to respiratory effort is significantly correlated with the slope of hypercapnic ventilatory response. This result shows a clear linkage between ventilatory response and breathing sensations. Although it is shown that the automatic controller effectively minimizes the discomfort index for perturbations about an operating point under certain conditions, the discomfort index itself does not seem to be an underlying control principle of the proposed automatic controller model. Rather, breathing sensations may influence ventilatory responses by modifying the output of the automatic controller.     

CO2 control of the respiratory system: Plant dynamics and stability analysis

A stability analysis of respiratory chemical control is developed using a mathematical model of CO₂ mass transport dynamics. Starting with a 3-compartment model of CO₂ stores that distinguishes alveolar, muscle, and other tissue, model reduction techniques are applied to obtain a first-order representation of the respiratory plant. This model contains an effective tissue volume for CO₂, whose derived value is much smaller than previously predicted. To investigate oscillatory instabilities, a controller which incorporates only peripheral chemoreceptor responses was added to the first-order plant model. An explicit stability index (SI) is obtained analytically from a linearized version of this model. SI varies directly with the controller gain and circulation delay time and inversely with the effective tissue volume and inspired CO₂ concentration. Numerical simulations using the first-order nonlinear model show that SI is a good predictor of system stability. According to the linearized model, the system is stable for SI<1; from the nonlinear model, the system is stable for SI<1.1. For typical normal adults, the SI value is well within the stable region. Ahmed ElHefnawy is supported in part by a fellowship from the Egyptian Ministry of Higher Education; he is currently on leave of absence from Cairo University.