Influence of gas composition on convective and diffusive intrapulmonary gas transport

The influence of gas composition on convective and diffusive gas transport in the lungs was assessed by studying the dispersion of combined particle and argon (Ar) boluses induced by the passage through the lungs filled with three different gas mixtures. Particles, as a "nondiffusing gas," served as a tracer for convective gas transport, while the significance of diffusive gas transport was inferred from the difference in the behavior of Ar and particles. The lungs of six anesthetized and mechanically ventilated beagle dogs were equilibrated with air or either of the test atmospheres, He-O2 or SF6-O2, where nitrogen was replaced by helium (He) or sulfur hexafluoride (SF6). Due to differences in gas density and gas viscosity Reynolds numbers varied by a factor of twelve and Ar diffusivity by a factor of four between He-O2 and SF6-O2, suggesting that both kinds of intrapulmonary gas transport, convection and diffusion, should be affected. Combined particle and Ar boluses were inhaled into various lung depths and the extent of gas transport was inferred from changes in bolus shape induced by the passage through the lungs. In air, convective bulk gas transport generally followed the symmetric first-in, last-out principle and acted at all tested lung depths. Within the conducting airways, gas transport to the lung periphery was primarily due to convection but beyond these airways diffusion became rapidly significant. Breathing test atmospheres affects intrapulmonary gas transport only slightly. The extent of convective mixing was increased by 4% in SF6-O2 (p < .01) and reduced by 5% in He-O2 (p < .01) as compared to air. The symmetry of convective lung filling and emptying was slightly disturbed. In SF6-O2 the mean of the exhaled bolus was shifted by 8% toward the lung periphery. In He-O2 it was shifted by 4% toward the airway opening. In both test atmospheres exhaled Ar boluses were similar, suggesting that diffusive gas transport overwhelms the small changes in convective gas transport. Hence, factors other than gas composition-related flow characteristics, e.g., nonreversibility of in- and expiratory flow profiles or features of lung geometry, are the major determinants of gas transport in the lungs.

Clinical non-invasive measurement of effective pulmonary capillary blood flow

Since traditional pulmonary function testing is centered on measurements of air flow and lung volume, a method to assess the pulmonary circulation might improve our ability to evaluate diseases that impact upon pulmonary hemodynamics. We have developed a PC based application that rapidly calculates pulmonary blood flow. Subjects rebreath a mixture of 10% argon and 3.5% freon for 20 seconds. Gas concentrations at the mouth are monitored by a clinical mass spectrometer and signals are acquired and processed with off-the-shelf hardware. To test the accuracy and reproducibility of this technique, patients with pulmonary artery catheters were assessed by standard thermodilution methods and the rebreathing test. Measurements using this non-invasive technology closely corelate with invasive thermodilution methods (r = 0.980) and show equivalent reproducibility (average standard error = 2.5%). This application of signal processing technology can extend the role of pulmonary function testing to include routine evaluation of the pulmonary circulation.

Vapour transfer in two-layer clothing due to diffusion and ventilation

An experiment was carried out to measure the vapour resistance of two-layer clothing ensembles as a function of air permeability of the outer layer, open or closed apertures, wind, and walking, both for the total ensemble and for the outer garment alone. Six subjects walked on a treadmill (0.0, 2.5, and 5 km.h-1) which was placed in a wind tunnel (0.2, 0.7, and 3.0 m.s-1). They wore long underwear and an outer garment made of impermeable (imp), microporous (mpo), low air permeable (loa), or high air permeable (hia) fabric. Vapour resistances were determined by a trace gas method, calibrated against water vapour resistance. The vapour resistances of the underclothing and the outer garment were calculated from the measured data, as was the ventilation through the apertures. The vapour resistance of the underclothing was almost constant at 5 mm air equivalent. The ventilation was strongly dependent on wind and motion but still so low (54 l.min-1) that only the impermeable garment could benefit from it noticeably. The vapour resistance of the garments also varied strongly (imp 55-200 mm, mpo 12-20 mm, loa and hia 1-14 mm air equivalent). For the imp garment, this is due to leakage of air, whereas the vapour permeable garments were dominated by the diffusion and air penetration through the fabric. It is concluded that ventilation with vents cannot match the effect of vapour permeability, and that real low vapour resistances are only possible with air permeable fabrics.

Construction and uses of a concentric catheter for gas sampling in lung airways

A catheter for intra-airway sampling of gas concentrations was constructed from concentric polyethylene tubes. The internal tube (0.58 mm ID, 0.91 mm OD) was connected to a gas analyzer while the external tube (1.20 mm ID, 1.75 mm OD) was constantly flushed by air or a calibration gas, except during sampling. Injection and sampling dead spaces were 0.35 and 0.28 ml, respectively. Delay at 4-ml/min sampling rate was 4.0 +/- 0.2 s. The 0-90% step response to a sudden change in gas composition was 0.24 s when connected to a mass spectrometer. This catheter was used to assess tracer gas dispersion during oscillatory flow (1-20 Hz) in a straight long tube. Local concentrations measured through the catheter, after a small bolus of tracer gas was injected through the external tube, compared favorably with direct measurements through needles inserted via the tube wall and with theoretical predictions. The catheter was also used to measure intra-airway gas concentrations in dog airways during spontaneous breathing, conventional mechanical ventilation, high-frequency ventilation, high-frequency vibration ventilation, and constant-flow ventilation. It ws placed by a fiber-optic bronchoscope and used to measure local quasi-steady concentrations of CO2 and local dispersion with the bolus method. The occurrence of catheter clogging with secretions was substantially reduced with flow through the external tube. Transmitting a calibration gas through the external tube facilitated in situ recalibration of the gas analyzer without removing the catheter. The use of this catheter improved the efficiency and accuracy of measurements of gas concentrations inside lung airways.

Transport of inert gases in mammalian myocardium: comparison with a convection-diffusion model

Because tracer techniques are gaining an increasing importance for imaging flow (and metabolism) in the heart, experimental evidence is needed on the role of convection and diffusion in the transcoronary transport of solutes. In the present work, the transport of four different inert gases through the coronary system is studied in five closed-chest dog experiments and is compared with a digital multicapillary convection-diffusion model. Transport may be defined as flow dependent, as judged by the gross similarity of shape of the time-normalized dilution curves. However, the results show that the transcoronary transport of helium and xenon is more dispersed than that of argon and krypton, probably because of differences in diffusibility and solubility. A comparison of the animal and model experiments emphasizes the importance of diffusive transport of the gases. It is suggested that there is a diffusion shunt that is mainly located within the capillary network itself rather than between conduit vessels. Only for helium (which has the highest diffusivity) was a small arteriovenous shunt fraction seen that is thought to bypass the capillary exchange region. The conclusion is that although there is evidence of diffusional shunting at a capillary level, the inert gas kinetics in the heart are compatible with a basically flow-limited transport.