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.

Gas exchange in the airways.

The primary function of the lungs is to exchange the respiratory gases, O2 and CO2, between the atmosphere and the blood. Our overall understanding of the lungs as a gas-exchanging organ has improved considerably over the past four decades. We now know that the dynamics of gas exchange depend on the blood solubility (beta b, ml gas ml blood-1 atm-1) of the gas. While the major focus of research has rightly been on the respiratory gases, the lungs exchange a wide spectrum of gases ranging from very low solubility gases such as SF6 or helium (beta b = 0.01) to water vapor (beta b = 20,000). O2 (beta b = 0.7) and CO2 (beta b = 3.0) exchange primarily in the alveolar region of the lung and their exchange is limited by the rate of ventilation and perfusion. In contrast, highly soluble gases (beta b > 100) are likely to exchange primarily in the airways of the lung. We have used exhaled ethanol (beta b = 1756) profiles for humans, steady-state exchange of six inert gases (0.01 < beta b < 300) in an in situ dog trachea, and a mathematical model to analyze the dynamics of airway gas exchange. We make the following conclusion: (1) ethanol exchanges entirely within the airways, and (2) the magnitude of perfusion- and diffusion-related resistance to airway gas exchange is the same.

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.

Dynamics of soluble gas exchange in the airways: II. Effects of breathing conditions.

A mathematical model of the airways is developed which focuses on the dynamic exchange characteristics of heat, water and soluble gas. A typical airway segment is divided radially into three regions: the airway lumen, a thin mucous layer of variable thickness coating the airway wall, and an underlying nonperfused tissue layer. A bronchial circulation capillary bed lies beyond the nonperfused tissue layer. The simultaneous exchange of water, heat and soluble gas is dealt with using the model of Tsu et al. (Ann. Biomed. Eng. 16:547-571, 1988). In the case of excretion of ingested ethyl alcohol from the bronchial and pulmonary circulations, the model predicts that during inspiration, because of the alcohol flux from the airway mucosa, a concentration of alcohol in equilibrium with mucus is achieved in the inspired air before the respiratory bronchioles are reached. During exhalation, much of this alcohol redeposits on the airway surface. The net flux of alcohol from the airway surface exceeds the flux of alcohol from the mouth in the exhaled gas indicating that the exhaled alcohol comes from the airways and bronchial circulation rather than from the alveoli and the pulmonary circulation. Alcohol flux moves farther into the airways with oral breathing compared to nasal breathing. Increased ventilation shifts the alcohol flux more alveolarward. Changes in inspired air temperature and humidity have almost no effect on the distribution of alcohol flux in the airways.

Accurate measurement of blood alcohol concentration with isothermal rebreathing.

The importance of interaction of exhaled air with the airway surface was evaluated by comparing the effects of different breathing maneuvers and inhaled air temperature on the relationship between breath alcohol concentration (BRAC) and blood alcohol concentration (BAC). Breath alcohol was measured with an infrared absorption unit. Blood and simulator liquid alcohol concentrations were measured by gas chromatography. Breath samples were measured after both low and high exhaled volumes and after rebreathing. Breathing maneuvers were performed after either hyperventilation, breathhold or normal breathing. Inspired air temperature was varied between 0 degree C and 40 degrees C. The rebreathing method for sampling alveolar alcohol samples was evaluated with a new isothermal rebreather that was designed to provide a substantial amount of heat to the rebreathed air in order to heat the airway surfaces. Using a single breath test, the indicated BAC values vary from 14% above the actual BAC to as low as 55% below the actual BAC. Hyperventilation caused a significant decrease in BRAC and breathhold caused a significant increase in BRAC. When isothermal rebreathing is applied to such tests, the breath test results were always within +/- 10% of the true BAC, even with an altered breathing pattern. Isothermal rebreathing provided an accurate sample of alveolar air that was not affected by altered breathing pattern or air temperature.

Influence of gas physical properties on pulmonary gas exchange.

Overall, the exchange of gas by the lung is strongly dependent on the blood-gas partition coefficient of that gas and weakly dependent on the molecular weight of the gas. The exchange of very soluble inert gases is dependent on interaction with the airway surface during inspiration and expiration.

Dynamics of heat, water, and soluble gas exchange in the human airways: 1. A model study.

In order to provide a means for analysis of heat, water, and soluble gas exchange with the airways during tidal ventilation, a one dimensional theoretical model describing heat and water exchange in the respiratory airways has been extended to include soluble gas exchange with the airway mucosa and water exchange with the mucous layer lining the airways. Not only do heat, water, and gas exchange occur simultaneously, but they also interact. Heating and cooling of the airway surface and mucous lining affects both evaporative water and soluble gas exchange. Water evaporation provides a major source of heat exchange. The model-predicted mean airway temperature profiles agree well with literature data for both oral and nasal breathing validating that part of the model. With model parameters giving the best fit to experimental data, the model shows: (a) substantial heat recovery in the upper airways, (b) minimal respiratory heat and water loss, and (c) low average mucous temperatures and maximal increases in mucous thickness. For resting breathing of room air, heat and water conservation appear to be more important than conditioning efficiency. End-tidal expired partial pressures of very soluble gases eliminated by the lungs are predicted to be lower than the alveolar partial pressures due to the absorption of the expired gases by the airway mucosa. The model may be usable for design of experiments to examine mechanisms associated with the local hydration and dehydration dynamics of the mucosal surface, control of bronchial perfusion, triggering of asthma, mucociliary clearance and deposition of inhaled pollutant gases.

Presumptive nickel dermatitis from hemodialysis.

Presumptive cutaneous hypersensitivity to nickel administered intravenously during hemodialysis was observed in a patient with nickel sensitivity. The source of the nickel was a 316 stainless-steel fitting that came in contact with 6N hydrochloride during dialysis. An in vitro experiment was performed to demonstrate the ability of nickel to be dialyzed into blood through a standard hemodialysis system. A plasma nickel level 89% higher than that of the original dialysate was achieved with a single cycle of dialysis. This is consistent with the plasma protein binding of nickel.

Clinical studies of a continuous extracorporeal cyanate treatment system for patients with sickle cell disease.

An extracorporeal carbamylation system was evaluated in two patients with sickle cell disease. Access was achieved with existing veins in one patient and an AV fistula in the second. Modifications in the treatment procedure were made as experience indicated. Levels of carbamylation of greater than 1 mol/mol of hemoglobin tetramer were achieved with 4 to 6 hr of treatment every 2 weeks. Cyanate returned to the patient averaged 68.5 mg per treatment. As expected, P50 values decreased and hemoglobin levels increased as the treatment progressed. Patterns of cyanate distribution among the red cell population were those predicted by computer calculations. These observations document the safety and reliability of the treatment system but do not permit conclusions as to efficacy.

Effect of incubation temperature on human red cell survival.

The effect of in vitro response of normal human erythrocytes to temperatures between 41 degrees C and 43 degrees C for two hours was determined. In vivo survival was measured employing 51Cr and 14C-cyanate as red cell tags. No significant difference was observed between cells exposed to these temperatures and erythrocytes incubated at 37 degrees C. The results of this study are of value in various types of extracorporeal blood treatment where an elevated temperature is desired.

The middle molecule hypothesis in perspective.

The middle molecule (MM) hypothesis states that molecules in the molecular weight range of 500 to 2000 daltons/molecule accumulate in uremia and are one cause of peripheral neuropathy. Evidence for and against this hypothesis is reviewed and evaluated. The course of events in the core of early hemodialysis patients who developed neuropathy provides important support ofr the hypothesis. Failure of early peritoneal dialysis patients to develop neuropathy suggested that the peritoneum is more permeable to MM's than early hemodialysis membranes, a fact that was later hemodialysis membranes, a fact that was later confirmed by appropriate investigations. Several investigators have demonstrated that MM's actually do accumulate in uremia and disappear rapidly following a renal transplant, which suggests a high clearance by the human kidney. Retrospective and prospective studies have demonstrated a protective effect of residual renal function against MM intoxication. Our prospective studies to produce MM intoxication are reviewed and their strengths and weaknesses delineated. Similarly, the investigations of others that are cited as evidence against the MM hypothesis are reviewed. It is the opinion of hte authors that the weight of evidence supports the validity of the MM hypothesis; but that final proof still is lacking. A protocol for more definitive studies is discussed.

Creatinine degradation. II: Mathematical model including the effect of extra-renal removal rates.

Previous models of the patient-artificial kidney system have neglected the contribution of creatinine degradation as a pathway for creatinine removal. Creatinine degradation can remove significant amounts of creatinine from dialysis patients, and models which neglect this mechanism are susceptible to error in predicting creatinine concentrations. In this study three models of the patient-artificial kidney system have been developed. The single pool model is the easiest to use since it requires a minimum of patient information, but it is the least accurate in predicting creatinine plasma concentrations. A two-pool model predicts experimental data within 5 percent, and appears to be the most useful model from the consideration of ease of use and accuracy. A three-pool model was derived that is suitable for use on a mini-computer or programmable calculator providing the device is capable of inverting a 3 x 3 matrix. The three-pool model predicts the measured plasma concentrations with 0.5 percent. These models provide a means to account for creatinine degradation and accurately predict creatinine concentrations in dialysis patients.

Creatinine degradation I: the kinetics of creatinine removal in patients with chronic kidney disease.

The existence of an alternative route of excretion for creatinine in subjects with chronic renal failure has been demonstrated. The data presented in this study confirm the hypothesis that creatinine is converted into other metabolites, probably by action of the gut flora. Creatinine degradation was quantitated in a group of subjects that spanned a wide range of kidney functions from normal to no renal function. Five patients were analyzed who were on maintenance dialysis, five were predialysis and two subjects were normal with respect to kidney function. Creatinine degradation expressed as a percentage of production varied from 13.9 to 27.7% in the dialysis patients, 0 to 42.3% in the pre-dialysis patients and was 0% in the controls. Creatinine degradation was correlated with plasma creatinine degradation was correlated with plasma creatinine levels in predialysis (r = 0.73, p less than 0.01), but not in dialysis patients. No correlation was found between creatinine degradation and production in either group. It is concluded that significant amounts of creatinine are degraded in dialysis patients, and this removal mechanism must be accounted for in models of the patient-artificial kidney system.

Creatinine production rate measured by whole body counting of 40K.

The creatinine production rate in dialysis patients was measured by potassium-40 whole body counting and carbon-14 creatinine injection. The correlation coefficient for the two methods was 0.96, p less than 0.005. An equation predicting whole body potassium (WBK) for normal subjects was found to accurately predict the WBK of dialysis patients as well. Two equations predicting creatinine production from WBK were compared with measured production rates and were found to agree within experimental error. It is thus possible to use the predictive equations to accurately estimate creatinine production without resorting to experimental measurements. These findings should simplify the use of computer models of the patient-artificial kidney system where accurate estimations of creatinine production rates are essential.

Measurement of the carbamylation kinetics and antisickling mechanism in hemoglobin S blood.

The kinetics of HbS carbamylation in whole blood have been investigated under conditions anticipated in extracorporeal treatment systems. The reaction was well represented by a bimolecular, irreversible, second-order mechanism, and the overall carbamylation rate was enhanced by increasing the temperature and decreasing the pH and PO2. An expression was developed to predict the carbamylation rate for a range of experimental conditions. The relative amount of beta chain carbamylation was increased for those conditions under which the overall carbamylation rate was lowered, i.e., lower temperature, higher PO2, and higher pH. Morphological examination of cells with predominantly beta chain carbamylation showed that the antisickling effect, as measured by this technique, could be accounted for entirely by an increase in the oxygen affinity. Although this observation does not exclude an effect independent of change in the oxygen affinity of carbamylated hemoglobin, such an effect, if it occurs, is not detectable by this method. The results of this study were used to design a reaction vessel for an extracorporeal treatment system for sickle cell anemia patients.