Diffusion of nonelectrolytes in the canine trachea: effect of tight junction

We recently demonstrated through theoretical modeling that the exhaled ethanol (EtOH) profile from humans is consistent with a molecular diffusion coefficient (cm2/s) in the bronchial mucosa (Dti) that is only 8% of the diffusion coefficient in water (Dw; J. Appl. Physiol. 75: 2439-2449, 1993). Because of the small oil-water partition coefficient (lambda o:w) of EtOH (lambda o:w = 0.074), the reduced diffusion coefficient may be due, in part, to the epithelial tight junction in the paracellular pathway. We hypothesized that opening the tight junction would open an aqueous pathway and increase the diffusion coefficient of small (mol wt < 100) hydrophilic compounds. We mounted the mucosa from the membranous canine trachea in an Ussing-type diffusion cell and measured the diffusion coefficient of 2-ethoxyethanol (2-Ethx; lambda o:w = 0.042), EtOH, and methyl ethyl ketone (MEK; lambda o:w = 1.04) in the presence and absence of the epithelial tight junction. The tight junction was opened using a phosphate-buffered saline free of Ca2+ and Mg2+ with 0.5 mM ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, and its integrity was assessed by measuring the transepithelial electrical resistance. Dti/Dw in the presence of Ca2+ and Mg2+ was 0.39, 0.34, and 0.39 for 2-Ethx, EtOH, and MEK, respectively, and increased 24.6, 11.7, and 1.11% in the absence of Ca2+ and Mg2+. We conclude that the effect of the tight junction on Dti increases with increasing water solubility but can account for only a small portion of the reduced Dti of EtOH as predicted by exhaled profiles.

Tracheal gas exchange: perfusion-related differences in inert gas elimination

Exchange of inert gases across the conducting airways has been demonstrated by using an isolated dog tracheal preparation and has been characterized by using a mathematical model (E. R. Swenson, H. T. Robertson, N. L. Polissar, M. E. Middaugh, and M. P. Hlastala, J. Appl. Physiol. 72: 1581-1588, 1992). Theory predicts that gas exchange is both diffusion and perfusion dependent, with gases with a higher blood-gas partition coefficient exchanging more efficiently. The present study evaluated the perfusion dependence of airway gas exchange in an in situ canine tracheal preparation. Eight dogs were studied under general anesthesia with the same isolated tracheal preparation. Tracheal perfusion (Q) was altered from control blood flow (Qo) by epinephrine or papaverine instilled into the trachea and was measured with fluorescent microspheres. Six inert gases of differing blood-gas partition coefficients were used to measure inert gas elimination. Gas exchange was quantified as excretion (E), equal to exhaled partial pressure divided by arterial partial pressure. Data were plotted as ln [E/(l-E)] vs. In (Q/Qo), and the slopes were determined by least squares. Excretion was a positive function of Q, and the magnitude of the response of each gas to changes in Q was similar and highly significant (P < or = 0.0002). These results confirm a substantial perfusion dependence of airway gas exchange.

Modeling steady-state inert gas exchange in the canine trachea

The functional dependence between tracheal gas exchange and tracheal blood flow has been previously reported using six inert gases (sulfur hexafluoride, ethane, cyclopropane, halothane, ether, and acetone) in a unidirectionally ventilated (1 ml/s) canine trachea (J. E. Souders, S. C. George, N. L. Polissar, E. R. Swenson, and M. P. Hlastala. J. Appl. Physiol. 79: 918-928, 1995). To understand the relative contribution of perfusion-, diffusion- and ventilation-related resistances to airway gas exchange, a dynamic model of the bronchial circulation has been developed and added to the existing structure of a previously described model (S. C. George, A. L. Babb, and M. P. Hlastala. J. Appl. Physiol. 75: 2439-2449, 1993). The diffusing capacity of the trachea (in ml gas.s-1.atm-1) was used to optimize the fit of the model to the experimental data. The experimental diffusing capacities as predicted by the model in a 10-cm length of trachea are as follows: sulfur hexafluoride, 0.000055; ethane, 0.00070; cyclopropane, 0.0046; halothane, 0.029; ether, 0.10; and acetone, 1.0. The diffusing capacities are reduced relative to an estimated diffusing capacity. The ratio of experimental to estimated diffusing capacity ranges from 4 to 23%. The model predicts that over the ventilation-to-tracheal blood flow range (10-700) attained experimentally, tracheal gas exchange is limited primarily by perfusion- and diffusion-related resistances. However, the contribution of the ventilation-related resistance increases with increasing gas solubility and cannot be neglected in the case of acetone. The increased role of diffusion in tracheal gas exchange contrasts with perfusion-limited alveolar exchange and is due primarily to the increased thickness of the bronchial mucosa.

Modeling the concentration of ethanol in the exhaled breath following pretest breathing maneuvers

A previously developed mathematical model that describes the relationship between blood alcohol (ethanol) concentration and the concentration of alcohol in the exhaled breath at end-exhalation (BrAC) has been used to quantitate the effect of pretest breathing conditions on BrAC. The model was first used to "condition" the airways with different breathing maneuvers prior to simulating a single exhalation maneuver, the maneuver used in standard breath alcohol testing. On inspiration, the alcohol in the air reaches local equilibrium with the alcohol in the bronchial capillary bed prior to entering the alveolar region. On expiration, approximately 50% of the alcohol absorbed on inspiration is desorbed back to the airways. BrAC correlates with the amount of alcohol that is desorbed to the airways. The six pretest breathing conditions and the percent change in BrAC relative to the control maneuver were: hyperventilation (-4.4%), hypoventilation (3.7%), hot-humid air (-2.9%), hot-dry air (0.66%), cold-humid air (0.13%), and cold-dry air (0.53%). The mechanism underlying these responses is not due to changes in breath temperature, but, rather to changes in the axial profile of alcohol content in the mucous lining of the airways.

Dynamics of soluble gas exchange in the airways. III. Single-exhalation breathing maneuver

The exchange characteristics of a highly soluble gas with the pulmonary airways during a single-exhalation maneuver were analyzed using a mathematical model previously described by our group (M. E. Tsu et al. Ann. Biomed. Eng. 16: 547-571, 1988). The model integrates the simultaneous exchange of water, heat, and a soluble gas with the pulmonary airways. The purpose of this paper is to provide experimental data for model validation. Exhaled ethyl alcohol concentration profiles of human subjects were measured with an Intoxilyzer 5000 and were plotted against exhaled volume measured with a wedge spirometer. Each subject performed a series of breathing maneuvers in which exhalation flow rate was the only variable. Phase III has a positive slope (0.047 +/- 0.0089 mol alcohol in air.mol alcohol in alveolus-1.l-1) that is statistically independent (P > 0.05) of flow rate. Reducing the molecular diffusion coefficient of alcohol in the nonperfused tissue layer improves the fit of the model to the experimental data. The optimal diffusion coefficient of alcohol for all subjects was 12 +/- 5.3 (SD) x 10(-7) cm2/s, which is 8% of the diffusion coefficient of alcohol in water (1.6 x 10(-5) cm2/s). We concluded that the experimental data showing a positive slope of the exhaled alcohol profile are consistent with a reduced diffusivity of alcohol in the respiratory mucosa. The reduced diffusion coefficient enhances reabsorption of alcohol by the airways on exhalation and creates a positive phase III slope.

Tumour blood flow following local ultrasound heating computed from thermal clearance curves

Thermal clearance curves following termination of ultrasound-induced hyperthermia in human mammary carcinomas implanted into the flanks of nude rats were studied. They were found to be monoexponential in form, both with and without blood flow. From the difference between the inverse time constants with and without flow, the tumour blood flow rate could be calculated. Blood flow was found to increase with very short exposure times at the therapeutic hyperthermia temperature and subsequently decrease as the exposure time increased. A higher therapeutic hyperthermia temperature augmented this effect.