S. C. M. Luijendijk

The sloping alveolar plateau at synchronous ventilation

We performed model calculations on the slope of the alveolar plateau at single breath washout of helium (He) and sulphurhexafluoride (SF6). Calculations were carried out on an extensible axissymmetric lung model, which was synchronously ventilated either equally or unequally. The calculated slopes of the alveolar plateau were small (<0.3%), which is in agreement with the results of other investigators. However, calculations carried out on a synchronously equally ventilated axis-asymmetric lung model related to the terminal lung unit (acinus), yielded slopes for the alveolar plateau ranging from 0 up to 14% per liter between 1.2 and 1.8 l expired volume. Even with a post inspiratory apnea of 1 s, we still found slopes ranging from 0–4%.Our findings are in contrast to the conclusions of Engel et al. (1979), who ascribe the slope of the alveolar plateau to asynchronous unequal ventilation.

Uptake of highly soluble gases in the epithelium of the conducting airways

Short duration washin and washout experiments were carried out with the gases ether (diethyl ether), ethyl acetate and acetone in order to study the excretion behaviour in the lung of gases highly soluble in blood and tissues. Carbon dioxide (CO2) was analyzed as a reference gas during the washout. The excretion values for acetone were generally the lowest, those for ether the highest, and intermediate ones were obtained for ethyl acetate. These results are in accord with the experimental data of others. Analysis of the washout curves shows that for acetone the volume of gas expired before the beginning of phase II is considerably smaller than found for CO2 and ether. During washout, the slope of the alveolar plateau for acetone is negative for the first few breaths and becomes positive thereafter; however, it remains lower than the slopes for CO2 and ether, which are always positive. These two phenomena occurring during washout clearly demonstrate that the acetone in the expired air must originate from the epithelial tissue lining the conducting airways. We conclude, therefore, that, in terms of gas transport, the conducting airways behave differently for poorly and highly soluble gases and this provides a physiological basis for the deviating excretion behaviour of highly soluble gases.

Time to peak tidal expiratory flow and the neuromuscular control of expiration

The ratio of the time needed to reach peak tidal expiratory flow (tPTEF) and the duration of expiration (tE) is used to detect airflow obstruction in young children. tPTEF is decreased in patients with asthma, but knowledge about the physiological determinants of this parameter is scarce. This study examined the relationship between tPTEF and postinspiratory activities of inspiratory muscles and evaluated the effects of changing sensory information from the lung. Airflow patterns and electromyographic (EMG) activity of inspiratory muscles were recorded in seven spontaneously breathing, anaesthetized cats. The trachea was cannulated and, as a result, the larynx and upper airways were bypassed. Changes in postinspiratory muscle activity were induced by changing afferent sensory nerve information (by cooling the vagus nerves, by administration of histamine and by additional application of continuous positive airway pressure (CPAP)). Durations of postinspiratory activities of the diaphragm and intercostal muscles (characterized by their time constants tau diaphr and tau interc) correlated strongly with tPTEF (r=0.85 and 0.77, respectively). Tau diaphr, tau interc and tPTEF were significantly increased during cooling of the vagus nerves (4-8 degrees C) compared with values at 22 and 37 degrees C (p<0.05). Conversely, administration of histamine and CPAP caused significant decreases in tau diaphr, tau interc and tPTEF, which were absent during cooling of the vagus nerves. In conclusion, the time needed to reach peak tidal expiratory flow is highly influenced by the activities of inspiratory muscles during the early phase of expiration which, in turn, depend on the activities of vagal receptors in the lung.

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 parallel inhomogeneities 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, lambda, for lambda values less than 4.0.

The ratio of the alveolar ventilations of SF6 and He in patients with lung emphysema and in healthy subjects

This study assess the possible impact of changes in the morphometry of acinar airways and air spaces on the efficacy of intrapulmonary gas mixing for sulphur hexafluoride (SF6) relative to that for helium (He). To that end the alveolar ventilations of He and SF6 were determined in patients with macroscopic lung emphysema and in healthy subjects. He-SF6 washout tests were performed in 17 patients (15 emphysema, 2 chronic bronchitis) and 21 healthy subjects. Using a three-compartment model, the data obtained were used to estimate the overall, effective, alveolar ventilations of SF6 and He, and their ratio VAASF6/VAHe. Mean VAASF6/VAHe (+/- S.D.) for patients (0.80 +/- 0.06) was significantly smaller (P < 0.001) than the value for the group of age-matched healthy subjects (0.90 +/- 0.05) which was non-significantly smaller than the result for the group of young, healthy subjects (0.93 +/- 0.03). In our patients, we also determined a score for emphysema using high resolution computed tomography, and this score correlated inversely with VAASF6/VAHe (r = -0.56, P = 0.018). We have interpreted our observations to mean that in patients with lung emphysema, the efficacy of intrapulmonary gas mixing for SF6 as compared with that for He reflected by VAASF6/VAHe is diminished due to increased diffusive path-lengths within the enlarged air spaces of their lungs which impair diffusive gas mixing for SF6 more than for He.

Diffusion-limited gas mixing in the lung and its consequences for transpulmonary gas transport

AbstractThe ratio of alveolar ventilations of He and SF6 (

The influence of training on oxygen isotope fractionation in healthy subjects.

\dot V_{{\text{A,He}}} /\dot V_{{\text{A,SF}}_{\text{6}} }

Modeling of the expiratory flow pattern of spontaneously breathing cats

) was determined in 7 healthy male subjects at rest and at three different levels of exercise on a bicycle ergometer (75, 150 and 225 W). This ratio was calculated from the ratio of the specific ventilations for these gases which were obtained from the decay of the end-tidal partial pressures of He and SF6 during a simultaneous, multiple-breath washout. In all experiments,