Model analysis on alveolar-capillary O2 equilibration during exercise.

The present article was aimed at determining the alveolar-capillary PO2 difference (deltaP(AcO2)) during exercise. The working hypothesis was that values of the pulmonary NO diffusing capacity can be used to calculate (deltaP(AcO2)) data on the basis of well-known laws of pulmonary gas exchange. For this purpose, we analysed the pertinent data of three studies performed on 35 healthy, non-athletic non-smokers of similar age at seven different exercise intensities. Calculated mean values of alveolar-capillary PO2 difference aggravated from deltaP(AcO2) at rest to (deltaP(AcO2))=18 mmHg at a performance capacity amounting to 90% of the maximum level. Regression analysis revealed (deltaP(AcO2))=0.31* (V O2/V O2 max)2 at a very high significance level (n=7, r=0.999, P<0.0000082). Due to the non-linear increase of (deltaP(AcO2)) with inclining O(2) consumption, our model analysis confirms the opinion that pulmonary diffusion decreasingly determines maximal aerobic power.

Real-time detection of nitric oxide isotopes in lung function tests.

In lung function tests, the determination of the pulmonary diffusing capacity (D) using the single-breath method is a commonly applied technique. The calculation of D is performed on the basis of accurate measurements of indicator gas concentrations. In this chapter, we demonstrate the appropriateness of the stable nitric oxide (NO) isotopes 14NO and 15NO in revealing reliable data of D. We performed studies on animals (14NO) by using respiratory mass spectrometry (M3) and on humans (15NO) by applying laser magnetic resonance spectroscopy (LMRS). The equipment was characterized by sufficient detection limits of 70 parts/billion at [14NO] = 0.001% (M3) and 40 parts/billion at [15NO] = 0.002 % (LMRS), respectively. Lastly, we were able to show that D-values for 14NO indeed reveal the entire diffusive properties of the alveolar-capillary membrane and that 15NO is a useful indicator gas for reflecting disturbances of pulmonary gas exchange.

Determination of alveolar-capillary O2 partial pressure gradient by using 15NO.

We propose an approach for determining the alveolar-mean capillary oxygen (O(2)) partial pressure gradient to evaluate the efficiency of O(2) equilibration between alveolar space and pulmonary capillary blood. For this purpose, measurements of the pulmonary [(15)N]nitric oxide diffusing capacity are to be interpolated into the recording of O(2) consumption. We expect the O(2) partial pressure gradient amounting to 3.3 mmHg for breathing room air at rest, a third of that commonly given. The simplicity of our method allows its application to children or even artificially ventilated patients. Therefore, it will enable a new insight into pulmonary O(2) equilibration.

Pulmonary 15NO uptake in interstitial lung disease.

Because lung nitric oxide (NO) diffusing capacity (DL) represents alveolar-capillary gas diffusion, we queried as to whether disturbances of pulmonary gas exchange in interstitial lung disease (ILD) are appropriately reflected by using NO. In this pilot study, we applied the (15)N-labeled stable isotope (15)NO (relative abundance 0.37% of total NO) in order to ignore the endogenous NO production. In 10 ILD-outpatients, we measured DL (15)NO by performing the single-breath method. Lung function parameters as well as arterial oxygen partial pressure (PaO(2)) were also tested. Values of DL (15)NO ranged within 50-151 ml (15)NO/(mmHg min). Ratios of DL (15)NO/reference were between 43 and 108% of predicted data as taken from our previous work on healthy volunteers [Eur. J. Physiol. 446 (2003) 256]. We found a significant reduction of DL (15)NO/reference in five patients. Additionally, values of PaO(2) were significantly correlated to ratios of DL (15)NO/reference (adjusted R2 +/-SEE=0.407+/-8.051). In conclusion, (15)NO represents an appropriate indicator gas for reflecting an ILD-induced impairment of alveolar-capillary gas exchange.

Pulmonary 15NO uptake in man.

Nitric oxide (NO) is commonly thought to reveal more precise values of pulmonary gas uptake through alveolar-capillary membranes (DL) than the normally used carbon monoxide (CO). Since such measurements are influenced by a significant endogenous NO delivery within human airways, we propose the use of the naturally occurring (15)N-labelled stable nitric oxide isotope (15)NO. It occurs with a relative abundance of 0.37% of the dominating isotope (14)NO. Therefore, the endogenous (15)NO production can be neglected. In the present pilot study we demonstrate the workability of (15)NO in determining DL in healthy individuals. In seven female and 15 male volunteers, averaged values of DL increase with increasing mean alveolar volume as well as individual body height ( P=0.000001). Due to the very high significance level obtained from the multiple regression analysis, we conclude that the application of (15)NO establishes a novel approach to calculate standard values of DL. Such calculations can be employed to predict a reference for patients who suffer from pulmonary diffusion limitation.

NO diffusing capacity of rabbit lungs increases with ventilator-driven lung expansion.

In this animal study we evaluated the dependence of the pulmonary diffusing capacity (DL) of nitric oxide (DL,NO) on ventilator-driven increases in alveolar volume. According to the common hypothesis as to whether DL,NO reflects the diffusive properties of the alveolar-capillary membrane, DL,NO should be improved by lung expansion. However, the influence of a simultaneous raise in intrapulmonary pressure is unknown. DL,NO was determined by applying the single-breath method to four anaesthetised paralysed rabbits (weight 3.2-3.5 kg) at various alveolar volumes (38-69 ml) and intrapulmonary pressures (0.15-12.5 mmHg). The animals were ventilated with room air, using a computerised ventilatory servo-system that was also employed to perform the single-breath manoeuvres. Inflation procedures were always started from residual volume (12-13.7 ml), using 0.05% NO in N2 as the indicator gas mixture. End-tidal PNO was determined by respiratory mass spectrometry. DL,NO increased simultaneously with lung expansion and with increasing intrapulmonary pressure (P<0.001). We conclude that the DL,NO provides a measure of the pulmonary diffusive properties that is not influenced by a pressure-induced impairment of pulmonary capillary blood flow.

How to calculate the single-breath nitric oxide diffusing capacity in rabbits.

Nitric oxide (NO) is a novel indicator gas for investigating alveolar capillary gas exchange conditions. In clinical practice, pulmonary gas uptake is determined by measuring the single-breath diffusing capacity (DL,NO). Different algorithms are employed to calculate DL,NO. To compare the accuracy of those most commonly used, we performed single-breath experiments on 12 artificially ventilated rabbits. In each animal four manoeuvres, executing breath-holds of 2, 4, 6 and 8 s, were carried out. In each case we administered 55 ml of an indicator gas mixture containing 0.05% NO. Alveolar gas was analysed by respiratory mass spectrometry. The two algorithms for calculating DL,NO based on the conventional solution of the breath-holding equation [Ogilvie et al. (1957) J Clin Invest 36:1-17 and Jones and Meade (1961) Q J Exp Physiol 46:131-143], were compared with the three-equation technique [Graham et al. (1980) IEEE Trans Biomed Eng 27:221-227] as the reference. The deviation between DL,NO calculated from the conventional methods and the reference decreased linearly with increasing duration of NO uptake (deltat). The mean deviations declined from 16.6% (Jones and Meade) or 7.7% (Ogilvie) at deltat=4 s to 5.7% (Jones and Meade) or 2.4% (Ogilvie) at deltat=10 s. The larger mean values are due to the conventional solution where three-tenths of the inflation time is subtracted from deltat. These findings qualify the common prediction that the latter method yields DL,NO values of the highest accuracy. We therefore recommend Ogilvie's procedure if the three-equation technique cannot be employed.

An algebraic solution to dead space determination according to Fowler's graphical method.

According to Fowler's method, anatomical dead space (VD) can be determined graphically or computer-aided by iteration procedures by which phase III of a fraction-volume expirogram F(V) is back-extrapolated by a straight line R(V). Whereas Fowler visually partitioned phase II into two equal areas bordered by F(V), R(V), and VD, in the present paper the area between F(V) and R(V) is set equal to the area of a trapezoid, one side of which is the unknown VD to be determined. We obtained two algebraic equations for both possible conditions, nonsloping and sloping alveolar plateau, and, as the main result, an even more general third equation that includes both Bohr's and Fowler's solution. The formulas exactly represent Fowler's graphical method and can be applied to all gases which are applicable in dead space determination. The derived equations were tested in experimental situations, showing equality between values of dead space determined by using the algebraic solution and the graphical method. Their major advantage is facilitating and speeding up computer-aided on-line determinations of VD.

Single-breath CO diffusing capacity influenced by initial alveolar partial pressure of CO.

The purpose of this study was to assess the influence of incorrect determinations of the initial alveolar partial pressure of carbon monoxide (CO) at the beginning of breath holding (PIACO) on the pulmonary CO diffusing capacity of the lung (DLCO). Single-breath maneuvers were performed on 14 anesthetized and artificially ventilated rabbits, using 0.2% CO in nitrogen as the indicator gas mixture. Inflation and deflation procedures were carried out in an identical manner on each animal, with inflation always starting from residual volume. End-tidal partial pressure of CO was determined by respiratory mass spectrometry and was used to calculate DLCO values with the application of the three-equation (method 1), as well as the conventional (method 2), solution. In each rabbit, method 2 caused DLCO values to be overestimated when compared with method 1, and this overestimation decreased with increasing time intervals of CO uptake. Because we were able to recalculate this deviation using PIACO values that were obtained by taking the diffusive removal of CO during inflation into account, we concluded that errors in estimating PIACO by applying method 2 significantly contribute to the discrepancy between both methods.

Decreasing diffusion limitation on exercise in lower-graded interstitial lung disease.

In this study we investigated the contribution of diffusion limitation to the exercise-induced hypoxaemia in interstitial lung disease (ILD). We applied isotopic analysis to the composition of the stable isotopic oxygen molecules 16O2 and 16O18O in expiratory gas mixtures obtained from six ILD patients and six healthy subjects at rest and during ergometer work (60 W). The changes in the 16O18O/16O2 ratios were interpreted by using the overall fractionation factor of respiration (alpha O) which would be increased towards 1.03 on increasing diffusion limitation. In addition, the O2 partial pressures of alveolar gas and arterial blood (PAO2, PaO2) were determined. In the patients, alpha O was significantly reduced from 1.0066 +/- 0.0004 (mean +/- SD) at rest to 1.0035 +/- 0.0004 during exercise and in the healthy subjects from 1.0072 +/- 0.0008 to 1.0044 +/- 0.0004. Furthermore, the exercise-induced reduction of PaO2 (from 77 to 69 mmHg) was due to a drop of alveolar PO2 found in each patient, whereas in each healthy subject PaO2 was increased on exercise. On the basis of a resistance model we conclude that the patients' data were inconsistent with increasing diffusion limitation but showed an increasing impairment of O2 transport by ventilation.

Theta values for C16O18O and C18O2 related to respective pulmonary diffusing capacities.

The single-breath diffusing capacities for singly and doubly 18O-labeled CO2, DLC16O18O and DLC18O2, as well as for NO, were determined in seven anesthetized rabbits to investigate whether the theoretically predicted ratio of specific blood uptake rates of both isotopic CO2 species, theta C18O2/theta C16O18O = 2.0, can be derived from the measured values of DLC16O18O and DLC18O2. Data of DL were obtained by inflating the lungs with gas mixtures containing 0.35% C16O18O or 0.8% C18O2 or 0.05% NO in nitrogen, with breath-holding periods of 0.05-0.5 s and 2-12 s for the CO2 and NO tests, respectively. theta C18O2/theta C16O18O was calculated by applying the double-reciprocal Roughton-Forster equation to DL values obtained in each animal and by assuming that NO diffusing capacity represents the gas conductance of the alveolar-capillary membrane. The measured ratio was theta C18O2/theta C16O18O = 1.9 +/- 0.2 (mean +/- SD), thus comparing reasonably with the predicted one. Therefore, our findings provide evidence that the greater value of DLC18O2 is mainly due to the twofold higher probability (or theta value) for C18O2 than for C16O18O to disappear within red blood cells via isotopic exchange reactions.

Stratification does not limit O2 uptake in rabbit lungs.

This study was performed to assess the role of stratification, i.e. axial gas mixing deficit within alveolar space, in limiting alveolar gas exchange for oxygen. The single-breath method for varying breath-holding time with oxygen-labelled carbon dioxide, C18O2, was applied to 10 anaesthetized, paralysed and artificially ventilated rabbits. Alveolar partial pressure of C18O2 was analysed using respiratory mass spectrometry. Starting from residual volume, the lungs were rapidly inflated using 40 mL of indicator gas mixture (1% C18O2 in nitrogen). After executing breath-holding, the lungs were rapidly deflated. Pulmonary diffusing capacity of carbon monoxide was determined in the same way. On the basis of a serial compartment model, the lower limit of the stratificational conductance of oxygen was estimated, using the rate constant of C18O2 removal from alveolar space (4 s-1) and Graham's law. We found that the stratificational conductance in rabbits amounts to at least 13.5 mL mmHg-1 min-1. The pulmonary diffusing capacity of oxygen was calculated by multiplying the carbon monoxide diffusing capacity of rabbit lungs by a factor of 1.2, yielding a value of 0.77 mL mmHg-1 min-1. These results show that stratificational conductance is at least 17.5 times higher than pulmonary oxygen diffusing capacity, indicating that stratification does not limit oxygen uptake in rabbit lungs.

Pulmonary diffusing capacities for oxygen-labeled CO2 and nitric oxide in rabbits.

We determined the pulmonary diffusing capacity (DL) for 18O-labeled CO2 (C18O2) and nitric oxide (NO) to estimate the membrane component of the respective gas conductances. Six anesthetized paralyzed rabbits were ventilated by a computerized ventilatory servo system. Single-breath maneuvers were automatically performed by inflating the lungs with gas mixtures containing 0.9% C18O2 or 0.05% NO in nitrogen, with breath-holding periods ranging from 0 to 1 s for C18O2 and from 2 to 8 s for NO. The alveolar partial pressures of C18O2 and NO were determined by using respiratory mass spectrometry. DL was calculated from gas exchange during inflation, breath hold, and deflation. We obtained values of 14.0 +/- 1.1 and 2.2 +/- 0.1 (mean value +/- SD) ml.mmHg-1.min-1 for DL(C18O2) and Dl(NO), respectively. The measured DL(C18O2)/DL(NO) ratio was one-half that of the theoretically predicted value according to Graham's law (6.3 +/- 0.5 vs. 12, respectively). Analyses of the several mechanisms influencing the determination of DL(C18)2 and DL(NO) and their ratio are discussed. An underestimation of the membrane diffusing component for CO2 is considered the likely reason for the low DL(C18O2)/DL(NO) ratio obtained.

Single-breath diffusing capacities for NO, CO and C18O2 in rabbits.

Nitric oxide (NO) has been introduced recently for studying alveolar-capillary gas transfer. Due to extremely fast reaction kinetics for the association of NO with haemoglobin, pulmonary NO uptake is expected to depend only on diffusion, whereas in the case of carbon monoxide (CO) or oxygen-labelled carbon dioxide (C18O2) the alveolar-capillary transfer is, in addition, known to depend on a blood uptake component. To provide further data for NO, CO and C18O2, we determined the pulmonary diffusing capacities (DL) for the indicator gases mentioned, performing single-breath manoeuvres on ten rabbits. The inspired gas mixtures contained 0.05% NO and/or 0. 2% CO or 1% C18O2 in nitrogen. Applying respiratory mass spectrometry to the expirates we obtained the following mean +/- SD values: DL,NO/DL,CO = 3.55 +/- 0.4, DL,C18O2/DL,NO = 6.0 +/- 0.6, DL, C18O2/DL,CO = 21.4 +/- 2.5. Graham's law predicts DL ratios of 1.9 for NO/CO, 12 for C18O2/NO, and 23 for C18O2/CO. Thus we equally underestimated the predicted DL ratios for C18O2/NO and CO/NO by a factor of approximately 0.5. From this, and by excluding significant interactions between the indicator gases and lung tissues, we conclude that the closest approximation of the diffusive component of DL is indeed obtained by using NO.

Effect of inhaled nitric oxide on endotoxin-induced hypoxaemia in rabbits.

In five mechanically ventilated rabbits, we studied the property of inhaled nitric oxide in helping to treat hypoxaemia which was induced by intravenous endotoxin (Escherichia coli-derived lipopolysaccharide, serotype 0111: B4). We used measurements of arterial partial pressure of oxygen to check a therapeutic nitric oxide benefit. Pulmonary artery pressure was continuously monitored. Furthermore, we determined the single-breath diffusing capacity for nitric oxide. Measurements of plasma nitrite/nitrate concentration served as an indicator of endogenous nitric oxide output. The first infusion of endotoxin led to a transient pulmonary vasoconstriction, whereas arterial partial pressure of oxygen was permanently reduced by 30 +/- 10 mmHg (mean +/- SD), attaining minimal values of 48 +/- 3.4 mmHg due to additional endotoxin. Single-breath diffusing capacity for nitric oxide declined by 20 +/- 5.5% of baseline values until the experiments were concluded. Endotoxin induced an increase in plasma nitrite/nitrate concentration in the five rabbits as well as in the control animals (four rabbits) without exogenous nitric oxide supply. During the 25 inhalations of nitric oxide (3-50 ppm), arterial oxygenation did not change significantly. Thus endotoxin permanently impaired pulmonary gas exchange without inducing pulmonary hypertension. Inhaled nitric oxide did not improve arterial oxygenation during endotoxaemia.

Convection as one of the limiting factors of human respiration during normoxic exercise.

Each of the pathways within respiration has been suspected of limiting maximal performance, suggesting that O2 transport may be affected by each single pathway. The use of the stable, isotopic O2 molecules 16O2 and 16O18O is presented as a novel method for assessing respiration. Because of their different molecular weights, 16O2 diffuses 3% more rapidly than 16O18O, whereas 16O2 is convectively transported as rapidly as 16O18O. This can be quantified by using the overall fractionation factor alpha O. The more diffusion becomes limiting, the more 16O2 is transported in preference to 16O18O and alpha O is increased to 1.03. By contrast, the more respiration is limited by convection, the closer alpha O comes to 1.0 during the entire O2 transport. Six untrained subjects underwent normoxic exercise on a cycle ergometer. Isotopic analysis was performed at rest and during exercise loads of 50, 100, 150, 200, and 250 W using respiratory mass spectrometry. With increasing workload, a decrease in alpha O from 1.0072 at rest to 1.0033 at 250 W was determined in all subjects. On the basis of a serial resistance model of respiration, we concluded that, in our subjects, O2 transport was increasingly affected by convection but decreasingly limited by diffusion. The relative contribution of convection to the entire resistance to O2 flow ranged from > or = 44.6% at rest to > or = 74.6% at the most strenuous level of exercise, whereas the diffusive pathways decreasingly contributed to resistance to O2 flow by < or = 24% at rest and < or = 11% at 250 W.

Single-breath diffusing capacity of NO independent of inspiratory NO concentration in rabbits.

Pulmonary diffusing capacity of NO (DLNO) was determined by performing single-breath experiments on six anesthetized paralyzed supine rabbits, applying inspiratory concentrations of NO (FINO) within a range of 10 parts per million (ppm) < or = FINO < or = 800 ppm. Starting from residual volume, the rabbit lungs were inflated by 50 ml of a NO-nitrogen-containing indicator gas mixture. Breath-holding time was set at 0.1, 1, 3, 5, and 7 s. Alveolar partial pressure of NO was determined by analyzing the end-tidal portion from expirates, with the use of respiratory mass spectrometry. In the six animals, pulmonary diffusing capacity of NO averaged DLNO = 1.92 +/- 0.21 ml.mmHg-1.min-1 (mean +/- SD value). Despite extreme variations in FINO, we found very similar DLNO values, and in three rabbits we found identical values even at such different FINO levels of 80 ppm or 500, 20, or 200 ppm as well as 10 or 800 ppm. There was also no dependence of DLNO on the respective duration of the single-breath maneuvers. In addition, the time course of NO removal from alveolar space was independent of applied FINO levels. These results suggest that DLNO determinations are neither affected by chemical reactions of NO in alveolar gas phase as well as in lung tissue nor biased by endogenous release of NO from pulmonary tissue. It is our conclusion that the single-breath diffusing capacity of NO is able to provide a measure of alveolar-capillary gas conductance that is not influenced by the biochemical reactions of NO.