Comparison of three devices for measuring the second derivative of a langmuir probe curve

We have compared three devices, employing different principles, which plot the second derivative (SD) of the current voltage curve of a Langmuir probe. The probe measurements were performed in the positive column of a helium discharge. Two devices, using low modulation frequencies, produced within 1% the same shape of the SD curve. The third (200 kHz) yielded 10 to 25% lower values in the maximum of the SD curve; the discrepancy depended upon the pressure and diminished towards more negative probe potentials. The influence on the SD curves of the magnitude of the modulation voltages (up to 1 V peak to peak) is negligible in the case of helium. The determination of the space potential using the SD curves is discussed

Histamine induced bronchoconstriction and end tidal inspiratory acitivty in man

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Breathing pattern during bronchial challenge in humans.

Contains fulltext : 25385___.PDF (publisher's version ) (Open Access

Inert tracer gas washout from mixed venous blood: the sloping alveolar plateau

The aim of this model study was to investigate the mechanisms underlying the sloping alveolar plateau for inert tracer gases supplied to the lung by mixed venous blood. Transpulmonary gas exchange was simulated in an asymmetric lung model for conditions at rest and in exercise. For highly soluble gases, the calculations show that the varying amount of tracer gas dissolved in superficial parenchymal tissue and capillary blood causes a sustained stratification in the acinus during expiration and that this is mainly responsible for the slope. For this type of tracer gas, the slope is almost independent of variations in the molecular diffusion coefficient (D) of the gases. In contrast, for poorly soluble gases, the contributions of local parallelinhomogenetics of gas concentrations in the acinus and the continued gas exchange across the alveolo-capillary membrane are mainly responsible for the slope. The first factor, which depends on the asymmetric branching pattern of intra-acinar airways, increases with decreasing D values. The contribution of continued gas exchange to the slope is most pronounced under exercise conditions. This contribution is almost independent of the blood/gas partition coefficient, γ, for γ values less than 4.0

The sloping alveolar plateau of tracer gases washed out from mixed venous blood in man

We have investigated the slope of the alveolar plateau for inert tracer gases that were washed out from mixed venous blood. Two pairs of tracer gases were used (He, SFe) and (C2H2, Freon 22). The gases of each pair share almost the same blood-gas partition coefïicient but they have different diffusive properties in the gas phase. The experiments were performed in healthy subjects at rest and at three levels of exercise (75, 150,225 W). Each experiment started with the alveolar washin of the tracer gases by adding these gases to inspired air. This washin was continued for several minutes in order to dissolve suffìcient amounts of the tracer gases in the body tissues. Subsequently, the tracer gases were washed out. In this paper, the slopes of the alveolar plateaus are defined as the relative increase of the concentration per second. Steeper slopes were found for the heavier gases (SF6 and Freon 22) in comparison with those for the lighter gases of the two pairs (He and C2H2). This finding ma¡' be ascribed to the contribution of diffusionlimited gas mixing in the lung to the slope of the alveolar plateau. For each gas, the slope for the first expiration during washout (alveolar washout) was considerably smaller than that for the later part of washout (mixed venous u'ashout), and the difference amounts to about 560/o and 760/o of the slope during mixed venous washout at rest and at the highest level of exercise, respectively. As a ñrst approximation, this difference may be considered to represent the contribution to the slope of the continued gas exchange across the alveolo-capillary membrane during expiration. ln theory. this contribution can be computed from the ratio of the alveolar ventilation and alveolar volume (VA/VA), and this is confirmed by our results

Validation of bioelectrical-impedance measurements as a method to estimate body-water compartments

Validation of bioelectrical-impedance measurements as a method to estimate body-water compartments. van Marken Lichtenbelt WD, Westerterp KR, Wouters L, Luijendijk SC. Department of Human Biology, University of Limburg, Maastricht, Netherlands. Single-frequency bioelectrical-impedance analyses (BIA) and bioelectrical spectroscopy (modeling multifrequency measurements; BIS) were validated as methods to predict body water (BW) compartments in 29 healthy adults. Total body water (TBW) and extracellular water (ECW) were determined by deuterium- and bromide-dilution techniques. Contribution of BIA to anthropometry and influence of the position of electrodes were examined. Stepwise-regression analysis revealed that inclusion of BIA to anthropometric data greatly improved the fit with BW compartments. Highly significant correlations were observed between BIS and BW compartments (TBW: r2 = 0.98, SEE = 1.2; ECW: r2 = 0.95, SEE = 0.6). Cross-correlation (14 males, and 15 females) indicated predictive potential of the method. Results from BIS and BIA were comparable, but it is argued that BIS has the potential of improved standardization of the method

Intrapulmonary gas mixing and the sloping alveolar plateau in COPD patients with macroscopic emphysema.

Dept of Pulmonology, University Hospital Maastricht, The Netherlands. Chronic obstructive pulmonary disease patients, especially those with emphysema, show steep slopes of the alveolar plateau (S). This study tested the hypothesis that continued gas exchange between poorly and well-ventilated lung units by means of collateral ventilation would contribute to S in these patients. Nine young volunteers, nine older volunteers and 11 patients with macroscopic emphysema performed wash-out tests with helium (He) and sulphur hexafluoride (SF6). S was determined for breaths 1-5 (range 1), and for breaths between 95% and 98% of complete wash-out (range 2). An unequal ventilation index (UVI) was defined as the ratio between the estimated mean alveolar pressure and the end tidal pressure (PET) of each tracer gas, calculated over range 2. Over the same range, a phase III ratio was calculated by dividing PET by the estimated pressure at Fowler dead space. In all groups of subjects, the S for He and SF6 were greater for range 2 than for range 1 (p< or =0.012). In the emphysema patients, the correlations between S and UVI were 0.72 for He (p=0.012) and 0.81 for SF6 (p=0.002), while the mean phase III ratios were 1.7 for He and 2.4 for SF6, much less than their theoretical maxima. It was concluded that in patients collateral ventilation may account for only a small part of the increase in the alveolar plateau slope between ranges 1 and 2, and that this increase was mainly caused by unequal ventilation in combination with sequential emptying of lung units. The degree of sequential emptying, however, was modest compared with its full potential