A new type of constitutive equation for nonlinear elastic bodies. Fitting with experimental data for rubber-like materials

In this article, we develop a new implicit constitutive relation, which is based on a thermodynamic foundation that relates the Hencky strain to the Cauchy stress, by assuming a structure for the Gibbs potential based on the Cauchy stress. We study the tension/compression of a cylinder, biaxial stretching of a thin plate and simple shear within the context of our constitutive relation. We then compare the predictions of the constitutive relation that we develop and that of Ogden’s constitutive relation with the experiments of Treloar concerning tension/compression of a cylinder, and we show that the predictions of our constitutive relation provide a better description than Ogden’s model, with fewer material moduli.

Inflation of residually stressed Fung-type membrane models of arteries

Elastic arteries are idealised as being incompressible orthotropic nonlinearly elastic cylinders. They are further idealised as being membranes in order to analyse the effect of the experimentally observed pre-stressing of arterial tissue on inflation. The pre-stress is modelled here using the opening-angle method. It is shown that there can be multiple unloaded states of arterial segments of Fung materials, suggesting the corresponding set of material parameters will not yield reliable predictions of arterial stress in three dimensions as there is no experimental evidence to support this non-uniqueness. It is also shown that the circumferential pre-strain has a simple magnifying scaling effect on the pressure-radial strain relation and on the axial force needed to maintain the membrane length during inflation; the effect of the axial pre-strain is more nuanced.

The residually stressed unloaded state of arteries: Membrane and thin cylinder approximations

A solution is obtained for incompressible non-linearly elastic membranes that describes the bending of a cylindrical sector to form a perfect cylinder for a wide class of materials that includes isotropic materials and orthotropic materials reinforced by two families of mechanically equivalent fibres that are wound helically about the axial direction. Despite the relative simplicity of the physical problem, the solution of the corresponding boundary value problem for thick cylinders requires a numerical solution for even the simplest models. It is shown, however, that the thin shell solution provides an excellent approximation to the solution of the problem for cylindrical sectors whose thicknesses are an order of magnitude greater than that assumed for membranes. The approximate stress distribution in such thin shells is obtained. In these residually stressed cylinders, the radial stress is approximately zero but the hoop and axial stresses are finite. Estimates of the residual stresses in the unloaded state are obtained. A closed-form solution for the bending moment necessary to effect the deformation is also obtained.

On an Implicit Model Linear in Both Stress and Strain to Describe the Response of Porous Solids

We study some mathematical properties of a novel implicit constitutive relation wherein the stress and the linearized strain appear linearly that has been recently put into place to describe elastic response of porous metals as well as materials such as rocks and concrete. In the corresponding mixed variational formulation the displacement, the deviatoric and spherical stress are three independent fields. To treat well-posedness of the quasi-linear elliptic problem, we rely on the one-parameter dependence, regularization of the linear-fractional singularity by thresholding, and applying the Browder-Minty existence theorem for the regularized problem. An analytical solution to the nonlinear problem under constant compression/extension is presented.

Modeling of the Aorta: Complexities and Inadequacies

The aorta is a very complex organ comprising three layers, consisting of four kinds of tissues. It is an anisotropic, inhomogeneous, multiconstituent, and living organ that presents both a formidable challenge and a tremendous opportunity to a modeler to mathematically characterize its structure. Unfortunately, even the most sophisticated models in vogue do not faithfully take into consideration its various complexities, falling very short of putting into place a reasonable model, as they ignore many of the quintessential features that need to be taken into account. In this article, we address the various features that need to be taken into account to develop a meaningful model of the aorta.

Segmental Variations in the Peel Characteristics of the Porcine Thoracic Aorta

Aortic dissection occurs predominantly in the thoracic aorta and the mechanisms for the initiation and propagation of the tear in aortic dissection are not well understood. We study the tearing characteristics of the porcine thoracic aorta using a peeling test and we estimate the peeling energy per unit area in the ascending and the descending segments. The stretch and the peel force per unit width undergone by the peeled halves of a rectangular specimen are measured. We find that there can be significant variation in the stretch within the specimen and the stretch between the markers in the specimen varies with the dynamics of peeling. We found that in our experiment the stretch achieved in the peeled halves was such that it was in the range of the stretch at which the stress-stretch curve for the uniaxial experiment starts deviating from linearity. Higher peeling energy per unit area is required in the ascending aorta compared to the descending aorta. Longitudinal specimens required higher peeling energy per unit area when compared to the circumferential specimens.

Flow of a new class of non-Newtonian fluids in tubes of non-circular cross-sections

Fluids described by constitutive relations wherein the symmetric part of the velocity gradient is a function of the stress can be used to describe the flows of colloids and suspensions. In this paper, we consider the flow of a fluid obeying such a constitutive relation in a tube of elliptic and other non-circular cross-sections with the view towards determining the velocity field and the stresses that are generated at the boundary of the tube. As tubes are rarely perfectly circular, it is worthwhile to study the structure of the velocity field and the stresses in tubes of non-circular cross-section. After first proving that purely axial flows are possible, that is, there are no secondary flows as in the case of many viscoelastic fluids, we determine the velocity profile and the shear stresses at the boundaries. We find that the maximum shear stress is attained at the co-vertex of the ellipse. In general tubes of non-circular cross-section, the maximum shear stress occurs at the point on the boundary that is closest to the centroid of the cross-section. This article is part of the theme issue 'Rivlin's legacy in continuum mechanics and applied mathematics'.

On the states of stress and strain adjacent to a crack in a strain-limiting viscoelastic body

The viscoelastic Kelvin-Voigt model is considered within the context of quasi-static deformations and generalized with respect to a nonlinear constitutive response within the framework of limiting small strain. We consider a solid possessing a crack subject to stress-free faces. The corresponding class of problems for strain-limiting nonlinear viscoelastic bodies with cracks is considered within a generalized formulation stated as variational equations and inequalities. Its generalized solution, relying on the space of bounded measures, is proved rigorously with the help of an elliptic regularization and a fixed-point argument.

Nonlinear elasticity with limiting small strain for cracks subject to non-penetration

A major drawback of the study of cracks within the context of the linearized theory of elasticity is the inconsistency that one obtains with regard to the strain at a crack tip, namely it becoming infinite. In this paper we consider the problem within the context of an elastic body that exhibits limiting small strain wherein we are not faced with such an inconsistency. We introduce the concept of a non-smooth viscosity solution which is described by generalized variational inequalities and coincides with the weak solution in the smooth case. The well-posedness is proved by the construction of an approximation problem using elliptic regularization and penalization techniques.

Implicit constitutive relations for nonlinear magnetoelastic bodies

Implicit constitutive relations that characterize the response of elastic bodies have greatly enhanced the arsenal available at the disposal of the analyst working in the field of elasticity. This class of models were recently extended to describe electroelastic bodies by the present authors. In this paper, we extend the development of implicit constitutive relations to describe the behaviour of elastic bodies that respond to magnetic stimuli. The models that are developed provide a rational way to describe phenomena that have hitherto not been adequately described by the classical models that are in place. After developing implicit constitutive relations for magnetoelastic bodies undergoing large deformations, we consider the linearization of the models within the context of small displacement gradients. We then use the linearized model to describe experimentally observed phenomena which the classical linearized magnetoelastic models are incapable of doing. We also solve several boundary value problems within the context of the models that are developed: extension and shear of a slab, and radial inflation and extension of a cylinder.

Fidelity of the estimation of the deformation gradient from data deduced from the motion of markers placed on a body that is subject to an inhomogeneous deformation field

Practically all experimental measurements related to the response of nonlinear bodies that are made within a purely mechanical context are concerned with inhomogeneous deformations, though, in many experiments, much effort is taken to engender homogeneous deformation fields. However, in experiments that are carried out in vivo, one cannot control the nature of the deformation. The quantity of interest is the deformation gradient and/or its invariants. The deformation gradient is estimated by tracking positions of a finite number of markers placed in the body. Any experimental data-reduction procedure based on tracking a finite number of markers will, for a general inhomogeneous deformation, introduce an error in the determination of the deformation gradient, even in the idealized case, when the positions of the markers are measured with no error. In our study, we are interested in a quantitative description of the difference between the true gradient and its estimate obtained by tracking the markers, that is, in the quantitative description of the induced error due to the data reduction. We derive a rigorous upper bound on the error, and we discuss what factors influence the error bound and the actual error itself. Finally, we illustrate the results by studying a practically interesting model problem. We show that different choices of the tracked markers can lead to substantially different estimates of the deformation gradient and its invariants. It is alarming that even qualitative features of the material under consideration, such as the incompressibility of the body, can be evaluated differently with different choices of the tracked markers. We also demonstrate that the derived error estimate can be used as a tool for choosing the appropriate marker set that leads to the deformation gradient estimate with the least guaranteed error.

Modeling of Deformation-Accelerated Breakdown of Polylactic Acid Biodegradable Stents

The use of biodegradable polymers in biomedical applications has been successful in nonload bearing applications, such as biodegradable implants for local drug delivery, and in simple load bearing situations such as surgical sutures and orthopedic fixation screws. The desire to incorporate these materials in more complex load bearing situations, such as tissue engineering scaffolds and endovascular or urethral stents, is strong, but the lack of constitutive models describing the evolution of biodegradable polymers over the course of degradation has severely hampered the rational design process for these more complex biodegradable medical applications. With the objective of predicting biodegradable stent behavior, we incorporated constitutive models of biodegradable polymeric materials in a computational setting and the mechanical response of three different stent designs were analyzed as degradation progressed. A thermodynamically consistent constitutive model for materials undergoing deformation-induced degradation was applied to a commonly employed biodegradable polymer system, poly(L-lactic acid), and its specific form was determined by corroboration against experimental data. Depreciation of mechanical properties due to degradation confers time-dependent characteristics to the response of the biodegradable material: the deformation imparted by a constant load increases over time, i.e. the body creeps, and the stress necessary to keep a fixed deformation decreases, i.e. the body relaxes. Biodegradable stents, when subjected to constant pressure in its exterior, deflect inwards and ultimately fail as the structure loses its mechanical integrity. The complex geometry of endovascular stents and their physiological loading conditions lead to inhomogeneous deformations, and consequently, inhomogeneous degradation ensues. Degradation is mostly confined to the bends of the stent rings and junction points, which are the locations that carry most of the deformation, whereas mostly undeformed connector bars remain less degraded. If failure occurs, it will occur most likely at those sensitive locations and large, nondegraded pieces can provoke severe embolic problems. Highly nonuniform degradation indicates that some stent designs are at higher risk for complications. Deformation patterns of stents made of a material that loses its integrity are different than those of permanent stents. Blind adaptation of permanent stent design concepts is ill-suited for biodegradable stent design. The time-dependent aspect of the implant not only must be taken into account but should also be used to interact with the body’s reaction and to enhance healing.

A model for the formation, growth, and lysis of clots in quiescent plasma. A comparison between the effects of antithrombin III deficiency and protein C deficiency

A mathematical model comprised of 23 reaction-diffusion equations is used to simulate the biochemical changes and transport of various reactants involved in coagulation and fibrinolysis in quiescent plasma. The growth and lysis of a thrombus, as portrayed by the model equations, is governed by boundary conditions that include the surface concentration of TF-VIIa, the generation of XIa by contact activation (in vitro), and the secretion of tPA due to endothelial activation. We apply the model to two clinically relevant hypercoagulable states, caused by deficiency of either antithrombin III or protein C. These predictions are compared with published experimental data which validate the utility of the developed model under the special case of static conditions. The incorporation of varying hemodynamic conditions in to the current fluid static model remains to be performed.

Towards an understanding of the mechanics underlying aortic dissection

Acute aortic dissection and associated aortic catastrophes are among the most devastating forms of cardiovascular disease, with a remarkably high morbidity and mortality despite current medical and surgical treatment. The mechanics underlying aortic dissection are incompletely understood, and a further understanding of the relevant fluid and solid mechanics may yield not only a better appreciation of its pathogenesis, but also the development of improved diagnostic and therapeutic strategies. After illustrating some of the inadequacies with respect to the extant work on the mechanics of aortic dissection, we alternatively postulate that the clinical hemodynamic disturbances that render the aorta susceptible to the initiation of dissection are principally elevated maximum systolic and mean aortic blood pressure, whereas the hemodynamic disturbances that facilitate propagation of dissection are principally elevated pulse pressure and heart rate. Furthermore, abnormal aortic mechanical properties and/or geometry are requisite for dissection to occur. Specifically, we propose that the degree of anisotropy will directly influence the probability of future aortic dissection. Imaging of the aorta may provide information regarding aortic anisotropy and geometry, and in combination with a hemodynamic risk assessment, has the potential to be able to prospectively identify patients at high risk for future aortic dissection thereby facilitating prophylactic intervention. The aim of the paper is to identify the main mechanical issues that have a bearing on aortic dissection, and to suggest an appropriate mathematical model for describing the problem.

A theoretical model of enlarging intracranial fusiform aneurysms

The mechanisms by which intracranial aneurysms develop, enlarge, and rupture are unknown, and it remains difficult to collect the longitudinal patient-based information needed to improve our understanding. We submit, therefore, that mathematical models hold promise by allowing us to propose and test competing hypotheses on potential mechanisms of aneurysmal enlargement and to compare predicted outcomes with limited clinical information--in this way, we may begin to narrow the possible mechanisms and thereby focus experimental studies. In this paper, we present a constrained mixture model of evolving thin-walled, fusiform aneurysms and compare multiple competing hypotheses with regard to the production, removal, and alignment of the collagen that provides the structural integrity of the wall. The results show that this type of approach has the capability to infer potential means by which lesions enlarge and whether such changes are likely to produce a stable or unstable process. Such information can better direct the requisite histopathological examinations, particularly on the need to quantify collagen orientations as a function of lesion geometry.

A Model for the Formation and Lysis of Blood Clots

Both biochemical and mechanical factors have to be taken into account if a meaningful model for the formation, growth, and lysis of clots in flowing blood is to be developed. Most models that are currently in use neglect one or the other of these factors. We have previously reported a model [J Theoret Med 2003;5:183-218] that we believe is a step in this direction, incorporating many of the crucial biochemical and rheological factors that play a role in the formation, growth, and lysis of clots. While this model takes into account the extrinsic pathway of coagulation, it largely ignores the intrinsic pathway. Here, we discuss some of the general issues with respect to mathematical modeling of thrombus formation and lysis, as well as specific aspects of the model that we have developed.

Heat-Induced Changes in the Finite Strain Viscoelastic Behavior of a Collaagenous Tissue

Supra-physiological temperatures are increasingly being used to treat many different soft tissue diseases and injuries. To identify improved clinical treatments, however, there is a need for better information on the effect of the mechanics on the thermal damage process as well as the effect of the incurred damage on the subsequent mechanical properties. In this paper, we report the first biaxial data on the stress relaxation behavior of a collagenous tissue before and after thermal damage. Based on a two-dimensional finite strain viscoelastic model, which incorporates an exponential elastic response, it is shown that the thermal damage can significantly decrease the characteristic time for stress relaxation and the stress residual.

A constrained mixture model for arterial adaptations to a sustained step change in blood flow

A sustained change in blood flow results in an arterial adaptation that can be thought to consist of two general steps: an immediate vasoactive response that seeks to return the wall shear stress to its homeostatic value, and a long-term growth and remodeling process that seeks to restore the intramural stresses and, if needed, the wall shear stress toward their homeostatic values. Few papers present mathematical models of arterial growth and remodeling in general, and fewer yet address flow-induced changes. Of these, most prior models build upon the concept of "kinematic growth" proposed by Skalak in the early 1980s (Skalak R (1981) In: Proceedings of the IUTAM Symposium on finite elasticity. Martinus Nijhoff, The Hague, pp 347-355). Such approaches address important consequences of growth and remodeling, but not the fundamental means by which such changes occur. In this paper, therefore, we present a new approach for mathematically modeling arterial adaptations and, in particular, flow-induced alterations. The model is motivated by observations reported in the literature and is based on a locally homogenized, constrained mixture theory. Specifically, we develop a 3-D constitutive relation for stress in terms of the responses of the three primary load-bearing constituents and their time-varying mass fractions, with the latter accounting for the kinetics of the turnover of cells and extracellular matrix in changing, stressed configurations. Of particular importance is the concept that the natural configurations of the individual constituents can evolve separately and that this leads to changes in the overall material properties and empirically inferred residual stress field of the vessel. Potential applications are discussed, but there is a pressing need for new, theoretically motivated data to allow the prescription of specific functional forms of the requisite constitutive relations and the values of the associated material parameters.

Gas mixing for achieving suitable conditions for single point aerosol sampling in a straight tube: experimental and numerical results

Experimental measurements of velocity and tracer gas concentration are taken in a straight tube to evaluate the effectiveness of mixing in achieving conditions as required by ANSI N13.1-1999 for single point extractive sampling from stacks and ducts of nuclear facilities. Mixing is evaluated for inlet turbulent intensities of 1.5%, 10%, and 20%, achieved by introducing various bi-plane grids, and for conditions generated by a commercial static gas mixer. The data obtained (at Reynolds number = 15,000) highlight the importance of inlet turbulence intensity in the process of turbulent dispersion of a dilute gas. The gas mixer does not introduce significant pressure losses and unlike bi-plane grids, the turbulence downstream of the mixer is not homogenous. A judicious choice of the release location that uses the large scale eddies and inhomogeneity of the turbulence ensures that the specified ANSI N13.1-1999 criteria are attained within 7 diameters downstream of the duct inlet. This is significantly more effective than a bi-plane grid where even with 20% inlet intensity the criteria are met only at 21 diameters downstream. The predictions of a proposed semi-empirical correlation match favorably with data. For example, at 18 diameters downstream with inlet intensities of 1.5% and 10%, the predicted coefficients of variation (COVs) of 150% and 65% are close to the actual values of 154% and 50%; where the COV of a set of measurements is the ratio of the standard deviation of the set to its mean value. The corresponding results obtained using commercially available software are 141% and 12%. Results from a particle-tracking model show good qualitative trends, but they should not be used to determine compliance with the requirements of the ANSI standard.

Lung function during moderate hypobaric hypoxia in normal subjects and patients with chronic obstructive pulmonary disease

BACKGROUND

We sought to describe changes in spirometric variables and lung volume subdivisions in healthy subjects and patients with chronic obstructive pulmonary disease (COPD) during moderate acute hypobaric hypoxia as occurs during air travel. We further questioned whether changes in lung function may associate with reduced maximum ventilation or worsened arterial blood gases.

METHODS

Ambulatory patients with COPD and healthy adults comprised the study populations (n = 27). We obtained baseline measurements of spirometry, lung volumes and arterial blood gases from each subject at sea level and repeated measurements during altitude exposure to 8000 ft (2438 m) above sea level in a man-rated hypobaric chamber.

RESULTS

Six COPD patients and three healthy subjects had declines in FVC during altitude exposure greater than the 95% confidence interval (CI) for expected within day variability (p < 0.05). Average forced vital capacity (FVC) declined by 0.123 +/- 0.254 L (mean +/- SD; 95% CI = -0.255, -0.020; p < 0.05) for all subjects combined. The magnitude of decline in FVC did not differ between groups (p > 0.05) and correlated with increasing residual volume (r = -0.455; <0.05). Change in maximum voluntary ventilation (MVV) in the COPD patients equaled -1.244 +/- 4.797 L x min(-1) (95% CI = -3.71, 1.22; p = 0.301). Decline in maximum voluntary ventilation (MVV) in the COPD patients correlated with decreased FVC (r = 0.630) and increased RV (r = -0.546; p < 0.05). Changes in spirometric variables for patients and controls did not explain significant variability in the arterial blood gas variables PaO2, PaCO2 or pH at altitude.

CONCLUSIONS

We observed a decline in forced vital capacity in some COPD patients and normal subjects greater than expected for within day variability. Spirometric changes correlated with changes in reduced maximum voluntary ventilation in the patients but not with changes in resting arterial blood gases.

A single integral finite strain viscoelastic model of ligaments and tendons

A general continuum model for the nonlinear viscoelastic behavior of soft biological tissues was formulated. This single integral finite strain (SIFS) model describes finite deformation of a nonlinearly viscoelastic material within the context of a three-dimensional model. The specific form describing uniaxial extension was obtained, and the idea of conversion from one material to another (at a microscopic level) was then introduced to model the nonlinear behavior of ligaments and tendons. Conversion allowed different constitutive equations to be used for describing a single ligament or tendon at different strain levels. The model was applied to data from uniaxial extension of younger and older human patellar tendons and canine medial collateral ligaments. Model parameters were determined from curve-fitting stress-strain and stress-relaxation data and used to predict the time-dependent stress generated by cyclic extensions.

A mathematical model for shear-induced hemolysis

The time-varying history of stress exposure within a rotary blood pump makes it difficult to arrive at a quantifiable design criterion for predicting cell traumatization. Constant stress experiments have revealed that there is a threshold stress level above which damage to blood cells occurs depending upon the time of exposure. The shear stress history experienced by cells within a rotary blood pump, however, is highly unsteady. In order to better predict cell trauma under these realistic conditions, a mathematical damage model based on a concept of "damage accumulation" has been developed. This model is evaluated within the context of red cell trauma. Experimental results support the hypothesis that the rate of damage accumulation increases nonlinearly with the stress level as well as the age of the cell.

Identification of elastic properties of homogeneous, orthotropic vascular segments in distension

Characterization of the constitutive behavior of normal and pathological blood vessel segments could provide the clinician with a means to predict the onset and assess the severity of certain vascular maladies. Many of the constitutive models that have been proposed to date either fail to properly consider certain features of the anatomic structure and function of vascular tissue or are so mathematically complex that their utilization is intractable. We have developed a material identification technique that first required the adaptation and validation of a constitutive law describing the nonlinear, three-dimensional behavior of orthotropic, compressible, hyperelastic vascular segments. By coupling a nonlinear finite element program and experimental data with a robust nonlinear least-squares regression algorithm, a set of elastic parameters (moduli) is obtained. Regressions on data for a canine carotid artery and rabbit infrarenal aorta yielded coefficients of variation of 0.21 and 0.08, respectively. The estimated moduli demonstrated certain trends found by other investigators: both the canine carotid artery and rabbit aorta were found to be stiffer radially than circumferentially, and the former was found to be stiffer circumferentially than longitudinally. Using these material constants and measured arterial pressures, the stress distribution was computed for each specimen. The predicted radial stress was consistent with a transmural variation of approximately--p (applied luminal pressure) to approximately zero in both specimens, while the circumferential stresses ranged from 2.2p to 0.7p for the canine carotid, and from 6.4p to 3.7p for the rabbit aorta. The stress distributions qualitatively agreed with those reported in previous investigations, as well as with certain physiologic observations. Based on the results of our two sample cases, we believe that our technique could be beneficial to the assessment of the three-dimensional, anisotropic behavior of vascular tissue.

Hemodynamic effects of altitude exposure and oxygen administration in chronic obstructive pulmonary disease

PURPOSE

Cardiovascular events are the leading cause of death during air travel. Because patients with chronic obstructive pulmonary disease (COPD) develop severe hypoxemia at altitude, we sought to determine whether changes in systemic hemodynamics may contribute to health risks during hypobaric hypoxia.

PATIENTS AND METHODS

We recorded radial artery catheter blood pressure, cardiac frequency, and cardiac ectopy in 18 men (aged 68 +/- 6 years, mean +/- SD) with severe COPD (forced expiratory volume in 1 second 0.97 L +/- 0.32 L) at sea level, after 45 minutes of steady-state hypobaric hypoxia at 2,438 m in a hypobaric chamber, and after oxygen supplementation at 2,438 m.

RESULTS

Mean arterial pressure (mm Hg), systolic blood pressure (SBP), diastolic blood pressure, and pulsus paradoxus during acute hypobaric exposure did not differ from baseline. During oxygen supplementation, SBP declined (p = 0.028). Decreases in pulsus paradoxus and pulse pressure were noted on oxygen (p < 0.05). We found no changes in cardiac frequency. Cardiac ectopy was uncommon; for one subject, ectopy increased with hypobaric hypoxia and decreased with oxygen administration.

CONCLUSION

Vasopressor responses to hypoxia do not add to the risk of air travel in patients with severe COPD. Supplemental oxygen may cause beneficial hemodynamic changes in patients with COPD during acute hypobaric exposure.

Sensitivity and specificity of bronchial provocation testing. An evaluation of four techniques in exercise-induced bronchospasm

The thresholds used to define a positive result for bronchial provocation challenges (BPC) are arbitrary. Requiring smaller decrements in expired flow to define a positive study would capture more cases of reactive airways (increased sensitivity) but would include some "normal" responses (decreased specificity). To examine the relationship between threshold definition and the ability to correctly classify subjects as either normal or as having airways hyperresponsiveness (AHR), four different BPC tests were administered on different days to 20 patients with a clinical diagnosis of exercise-induced bronchospasm (EIB) and 20 control subjects. The four BPC tests were indoor exercise on a cycle ergometer, methacholine inhalation challenge (MIC), eucapnic voluntary hyperventilation (EVH) with dry gas, and EVH with cold gas. Our results indicate that the thresholds which best separate the two groups are different for each of the four BPC techniques. For methacholine inhalation (MIC), a fall in FEV1 (d%FEV1) of 15 percent or greater at 188 cumulative breath units was 100 percent specific for AHR but had a sensitivity of only 55 percent. Eucapnic voluntary hyperventilation (EVH) with room temperature dry gas was 100 percent specific at a d%FEV1 of 11 percent, but, at that threshold, sensitivity was only 50 percent. EVH with cold air was 100 percent specific at a d%FEV1 of 12 percent but sensitivity was only 35 percent. The bicycle ergometer challenge was far too insensitive to be of value in evaluating AHR. Based on their respective receiver operating characteristic curves, the best separation of the two subject groups occurred at a d%FEV1 of 5 percent and 12 percent for the two EVH techniques and MIC, respectively. An individual's response to one test was highly correlated with the response to either of the other two (r = 0.66, p less than 0.001 for dry vs cold gas EVH; r = 0.56, p less than 0.001 for dry gas EVH vs methacholine; and r = 0.69, p less than 0.001 for cold gas EVH vs methacholine). Thus, MIC and EVH techniques are equally useful in defining AHR and each has its optimal threshold for a positive test result.

Exercise responses prior to pregnancy and in the postpartum state

Increased women in the work force and requirements for maximal employee productivity have necessitated examination of the optimal time for parturients to resume normal activities. This prospective study was designed to determine whether prepregnancy measures of aerobic capacity are regained by 4-8 wk postpartum. Weight, percent body fat, recall energy expenditure, and exercise responses via a stage 1, graded cycle ergometer exercise test were determined in 11 subjects (mean age = 27.56 +/- 2.2) in a postabsorptive state prior to pregnancy and 4-8 wk postpartum. Subject characteristics were compared by the Student's t-test and differences across workloads and time by analysis of variance with repeated measures. Prepregnant weight (mean = 58.80 +/- 7.26 kg) was significantly less (P less than 0.05) than postpartum weight (mean = 62.81 +/- 9.12 kg), and prepregnant energy expenditure (1352 +/- 453 kJ) per day was significantly higher (P less than 0.05) than in the postpartum period (274 +/- 333 kJ). Maximal oxygen uptake was significantly higher (35.2 +/- 0.7 vs 30.5 +/- 2.0 ml.kg-1min-1) in the prepregnant as compared with the postpartum period. Further, heart rate at 125 and 150 W was significantly lower prepregnancy as compared with postpregnancy. Results support a detraining effect in the early postpartum period. Whether this detraining is an inevitable factor associated with pregnancy or whether exercising throughout pregnancy can ameliorate the decline in aerobic capacity postpartum is uncertain.

Oxygen supplementation during air travel in patients with chronic obstructive lung disease

The objective of this study was to quantitate the effects of O2 supplementation by nasal cannula (NC) and Venturi mask (VM) on PaO2 in patients with chronic obstructive pulmonary disease (COPD) during acute hypobaric exposure, simulating a commercial jet aircraft cabin. We conducted a crossover intervention trial in which subjects served as their own controls in an ambulatory outpatient pulmonary disease service of a tertiary care military medical center and a hypobaric research facility. The subjects were a volunteer sample of 18 men with stable severe COPD, not requiring long-term O2 therapy, and uncomplicated by hypercapnea or cardiac disease. Mean age was 68 years, and mean FEV1 was 0.97 L (31.3 percent predicted). We exposed patients to conditions equivalent to 8,000 feet in a hypobaric chamber. Radial artery catheters provided blood samples at ground level and 8,000 feet. O2 was sequentially administered at 8,000 feet by NC at 4 L/min and 24 percent or 28 percent VM. We describe changes in blood gas data from baseline values and between interventions. O2 at 4 L/min NC flow at 8,000 feet caused PaO2 to increase from 47.4 +/- 6.3 mm Hg to 82.3 +/- 14 mm Hg (n = 18), an increase of 34.9 +/- 14.8 mm Hg. Supplementation of O2 by 24 percent VM caused PaO2 at 8,000 feet to increase by 12.7 +/- 3.8 mm Hg. Twenty-eight percent VM caused PaO2 at 8,000 feet to increase by 19.7 +/- 8.2 mm Hg. Changes in PaO2 with 4 L/min NC were greater than those with either VM. The increase with 28 percent VM was greater than that caused by 24 percent VM (p less than 0.05). Compared with ground level, 4 L/min NC increased mean PaO2 by 9.9 +/- 12.6 mm Hg; 24 percent and 28 percent VM did not cause mean PaO2 to increase above ground level values. We describe a range of capability of familiar O2 therapy devices to increase PaO2 to levels that will maintain tissue oxygenation of patients during acute altitude exposure.

Urinary desmosine excretion as a marker of lung injury in the adult respiratory distress syndrome

Desmosine, the intermolecular and intramolecular cross link between the chains of elastin polypeptide, may be useful as a marker of a lung injury in adult respiratory distress syndrome (ARDS). A radioimmunoassay for rabbit antibody developed against desmosine, conjugated to bovine serum albumin, can detect as little as 100 pg of desmosine in plasma or urine. Desmosine is not metabolically absorbed, reused, or catabolized by the body, but rather eliminated unchanged in the urine as low molecular weight peptides. The lung is relatively rich in elastin, and we reasoned that a timed collection could be used as an index of elastin degradation in vivo. A 2-h collection of urine for desmosine assay was obtained at the time of Swan-Ganz catheter insertion in 41 consecutive patients. On the basis of clinical and initial Swan-Ganz catheter data, the patients were assigned to one of three groups: an ARDS group (n = 12); a cardiogenic pulmonary edema (CPE) group (n = 12); and a critically ill, nonpulmonary edema group (NPE, n = 17). The mean urine desmosine concentration (mg/L) for the ARDS group (0.728 +/- 0.22 SE) differed from the CPE group (0.149 +/- 0.07; p less than 0.001). The total excretion (microgram/2 h) was 64.95 +/- 24.7 in the ARDS group and 24.71 +/- 11.7 in the CPE group (p less than 0.05). Urine desmosine concentration/serum creatinine index for the ARDS group (0.78 +/- 0.28) was greater than in the CPE group (0.07 +/- 0.04; p = 0.019). Desmosine excretion was increased in the NPE group compared with CPE and ARDS groups, possibly reflecting heterogeneity in this group. In the differentiation of ARDS from CPE, we conclude that substantial increases in urinary desmosine excretion favor a diagnosis of ARDS.

Expiratory muscle recruitment during inspiratory flow-resistive loading and exercise

Both exercise and inspiratory flow-resistive loading may cause recruitment of expiratory muscles. To evaluate the extent of recruitment in combined exercise and flow-resistive loading, and to estimate the effect on inspiratory muscle work, we studied five men, 26 to 39 yr of age, during mild exercise with different degrees of inspiratory flow-resistive loading. Each subject performed four 1-h exercise runs at 30% of their maximal oxygen consumption on different days while inspiring through an external resistor of either 1.4, 14.5, 19.9, or 30.6 cm H2O/s/L. Mouth and esophageal pressure, inspiratory flow rate, and abdominal and rib cage motion were recorded continuously. Abdominal expansion tended to lead and rib cage expansion tended to lag the start of inspiration as judged from the beginning of negative pressure development at the mouth. These time differences increased as resistive load increased. Plots of abdominal versus rib cage motion also showed increase in phase shift, with the abdomen leading the rib cage on inspiration. For all subjects, the esophageal pressure at the end of expiration became less negative as the resistive load increased, indicating that the end-expiratory volume decreased with increasing resistive load. We conclude that there was increasing use of expiratory muscles as the resistive load increased, and that the initial expansion of the abdomen at high resistive loads represented elastic recoil of structures that had been compressed below the volume at FRC by the expiratory muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Accuracy of oxyhemoglobin saturation monitors during simulated altitude exposure of men with chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary disease (COPD) are at risk for hypoxemia during air travel. We assessed the comparative performance of oxyhemoglobin saturation (%O2Hb) monitors on these patients during hypobaric exposure. We measured %O2Hb by arterial catheter blood co-oximetry (COOX) and compared these values to those from a transmittance ear oximeter and a reusable digital pulse oximeter. Additionally, we examined the effect of oxygen supplementation (4 L/min) on %O2Hb. A total of 18 ambulatory males with severe COPD were exposed to 8,000 ft (565 mm Hg) in a hypobaric chamber. Multiple measures of %O2Hb were made with each monitor at sea level and at 8,000 ft, with and without supplemental oxygen. By COOX, %O2Hb fell at altitude to clinically significant levels, and was subsequently corrected with supplemental oxygen. Saturations measured by the transmittance ear oximeter were very close to the COOX, underestimating the true value by only 0.6% at altitude (p less than 0.05), while the reusable digital pulse oximeter over-estimated %O2Hb alinearly by a mean of 3.8% at altitude.

Respiratory failure in rapidly progressing pulmonary lymphoma. Role of immunophenotyping in diagnosis

An asymptomatic man was found to have bilateral small pulmonary infiltrates on a preoperative chest roentgenogram. Over a 4-wk period there was rapid progression of the infiltrates with a clinical picture suggestive of adult respiratory distress syndrome. Open lung biopsy showed a high grade lymphoma filling and distorting the pulmonary parenchyma. Flow cytometry of pleural fluid showed an aberrant phenotype consistent with T cell lymphoma. The patient died of progressive respiratory failure 6 wk after the first radiographic abnormalities were detected. With further experience, immunophenotyping of pleural fluid may secure definitive diagnoses in certain clinical situations, obviating more invasive procedures.

Hypoxemia during air travel in patients with chronic obstructive pulmonary disease

STUDY OBJECTIVE

To quantitate and identify determinants of the severity of hypoxemia during air travel in patients with chronic obstructive pulmonary disease.

DESIGN

Prospective study of physiologic variables before and during intervention.

SETTING

Referral-based pulmonary disease clinic at a U.S. Army medical center.

PATIENTS

Eighteen ambulatory retired servicemen (age 68 +/- 6 [SD] years) with severe chronic obstructive pulmonary disease (forced expiratory volume in the first second [FEV1] 31% +/- 10% of predicted).

INTERVENTION

Altitude simulation equivalent to 2438 meters (8000 feet) above sea level in a hypobaric chamber.

MEASUREMENTS AND MAIN RESULTS

Radial artery catheter blood oxygen tension in the patients declined from a ground value (PaO2G) at sea level of 72.4 +/- 9 mm Hg to an altitude value (PaO2Alt) of 47.4 +/- 6 mm Hg after 45 minutes of steady state hypobaric exposure. The PaO2G correlated with PaO2Alt (r = 0.587; P less than 0.01). Multiple regression analysis revealed that the preflight FEV1 reduced the variability in PaO2Alt not explained by PaO2G in the equation: PaO2Alt = 0.453 [PaO2G] + 0.386 [FEV1% predicted] + 2.440 (r = 0.847; P less than 0.001). Residuals from two previously published formulas using PaO2G also correlated with FEV1 (r greater than or equal to 0.765; P less than 0.001).

CONCLUSIONS

Arterial blood oxygen tension declined to clinically significant levels in most patients during hypobaric exposure. When combined with the preflight arterial PO2 at ground level (PaO2G), the measurement of the preflight FEV1 improved prediction of PaO2 at altitude (PaO2Alt) in patients with severe chronic obstructive pulmonary disease.

Determinants of maximum exercise capacity in patients with chronic airflow obstruction

Patients with chronic airflow obstruction (CAO) often develop impairment of respiratory muscle function. We hypothesized that inspiratory muscle strength, as assessed by resting, peak inspiratory pressure (PIP) may be an important determinant of maximum exercise capacity in patients with CAO. Twenty ambulatory male patients (mean age, 56 +/- 3 years [+/- SE]) with CAO (FEV1, 1.72 +/- 0.21 L) comprised the studied population. Oxygen consumption at incremental cycle ergometry to tolerance (VO2max, 1.80 +/- 0.20 L/min) served as the dependent variable for regression vs measures of resting pulmonary function. Significant correlations with VO2max included power output in watts (r = 0.951), VEmax (r = 0.858), Dsb (r = 0.841), PIP (r = 0.816), age (r = -0.809), FEV1 (r = 0.763), and FVC (r = 0.663). The FEV1, Dsb, and PIP each entered into a multiple linear regression relationship describing VO2max. Also, when paired with VEmax as independent variables in multiple regression, PIP and Dsb each improved description of VO2max over VEmax alone (p less than 0.05), whereas FEV1 and FVC did not (p greater than 0.05). We conclude that factors other than ventilatory capacity also have a quantitative effect on VO2max and that PIP constitutes a determinant of maximum exercise capacity in patients with CAO.

Thyroid hormone resistance in a large kindred: physiologic, biochemical, pharmacologic, and neuropsychologic studies

PURPOSE

Thyroid hormone resistance affects the pituitary gland and a variety of other tissues. We studied a large kindred with this disorder and measured a number of clinical markers of tissue metabolism to determine if these markers were useful in elucidating the sites and degree of resistance.

PATIENTS

A kindred of 89 persons in four generations was identified; 44 had thyroid function tests, and 14 (five to 67 years old) were found to have thyroid hormone resistance.

RESULTS

The inheritance pattern was autosomal dominant, with no common HLA haplotype. Physiologic measurements in five affected members showed marked heterogeneity. Four patients had normal baseline cardiac contractility, but only two experienced a shortening of their QKd interval into the hyperthyroid range with triiodothyronine (T3) therapy. Intrathyroidal 127I content was increased in two patients and was normal in two. Bone mineral content was normal in two men, but two women had marked osteopenia. The propositus, hypothyroid after inappropriate 131I therapy, had a hypothyroid ventilatory response to hypercapnea. This response became low normal during T3 (100 micrograms/day) administration but not during long-term thyroxine (T4) (300 micrograms/day) administration. Three other patients had values within normal limits and one had a hyperthyroid ventilatory response. Peripheral biochemical markers of thyroid hormone action were measured in 13 affected and 19 unaffected family members. Sex hormone-binding globulin was increased in zero of 13 affected patients (versus 19 of 20 hyperthyroid, chi 2:p less than 0.001); ferritin was elevated in two of 13 patients (versus 11 of 20 hyperthyroid, p less than 0.02); angiotensin converting enzyme activity was increased in one of 13 patients (versus 12 of 20 hyperthyroid, p less than 0.025). The eldest patient had marked cardiac sensitivity despite normal biochemical markers. We attempted to suppress the integrated thyroid-stimulating hormone (TSH) response to thyrotropin-releasing hormone (TRH) using T3 (72 and 100 percent suppression in two patients), dopamine (40 percent suppression in one), 3,5,3'-triiodothyroacetic acid (TRIAC) (94 percent suppression in one), and verapamil (10 percent and 40 percent suppression in two). Neuropsychologic function was studied in 14 individuals (11 affected, three unaffected). Although mild impairments were detected, they were not specific for thyroid hormone resistance.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropsychologic symptoms in obstructive sleep apnea improve after treatment with nasal continuous positive airway pressure

To describe the affective changes associated with sleep restoration we assessed psychologic symptoms using the Profile of Mood States questionnaire before and two months after treatment with nasal continuous positive airway pressure (NCPAP) in seven men with obstructive sleep apnea (OSA). The results were compared with those of a control group of patients with OSA who did not receive NCPAP. Two of six mood factors, depression and fatigue, improved significantly following treatment with NCPAP. Total Mood Disturbance (TMD) score was used to assess global mood differences. The mean TMD score for the patients before treatment was 1.7 and during treatment decreased to -7.6 (p less than 0.05). This mean decrease of 9.3 in the TMD score implies generalized improvement in mood. These findings support the opinion that sleep fragmentation and abnormalities of respiration during sleep are at least partially responsible for affective changes seen in sleep apnea. These psychologic disturbances improve after treatment with NCPAP.

The effect of participation in a regular exercise program upon aerobic capacity during pregnancy

This study examined the questions of whether pregnancy decreases physical fitness, as measured by maximal oxygen consumption, between the second and third trimesters, and whether maintaining a regular exercise program during the second half of pregnancy affects fitness. At the beginning of their second trimester, pregnant women were randomly assigned to either a nonexercising control group or an exercising group. They completed a maximal progressive exercise test on a cycle ergometer at 20 and 30 weeks, during which pulmonary parameters of aerobic capacity were measured. The exercising group demonstrated greater improvement in aerobic capacity than did the control group, manifested by increases in tidal volume and oxygen consumption and a stable ventilatory equivalent for oxygen. Pregnancy did not reduce maximal oxygen consumption between the second and third trimesters of pregnancy.

Respiratory control in presenile dementia

To evaluate the role of the cerebral cortex in the response to externally added inspiratory flow-resistive load, we studied 7 patients manifesting clinical presenile dementia of the Alzheimer's type. All subjects exhibited diffuse cerebral cortical atrophy on computerized tomography of the brain. The mean age of the group was 45.6 yr. The rebreathing technique was used to assess minute ventilation (VE) and occlusion pressure (P100) responses to progressive hypercapnia. Rebreathing runs were performed before and during the addition of an inspiratory flow-resistive load of 18 cm H2O.L-1.s. The respiratory control data of these patients were compared with data obtained by similar techniques in a matched normal volunteer control group. In the patient group, with the addition of load, the VE/PCO2 response slope decreased (p less than 0.005), whereas the P100/PCO2 response slope did not significantly change. In the control group, P100/PCO2 response slope increased with load to maintain ventilation. These results suggest that in presenile dementia, during added inspiratory load, the drop in VE is associated with an inadequate increase in respiratory neuromuscular output. This lack of load compensation in patients with presenile dementia suggests a role for the cerebral cortex in the response to externally added load.

Orthodeoxia: a new finding in interstitial fibrosis

Orthodeoxia--arterial desaturation accentuated by the upright position and improved by recumbency--has been described earlier with true pulmonary vascular shunts and intracardiac shunts. This phenomenon has been described in some parenchymal lung diseases, but has not been reported with interstitial fibrosis. We describe 2 patients with predominantly basal interstitial fibrosis, disabling dyspnea, and severe hypoxemia who demonstrated this positional oxygenation change. Large or surgically correctable arteriovenous malformations (AVM), or intracardiac shunts, were not demonstrated in either patient. This finding has important implications for oxygen prescription and the explanation of positional dyspnea.

Obstructive sleep apnea in adults with tonsillar hypertrophy

Tonsillar enlargement is a more common cause of obstructive sleep apnea (OSA) in the pediatric population than it is in adults. The small number of reported cases suggests that tonsillectomy for adult patients with this association may be as a successful as it is known to be for children. We compared polysomnographic findings and/or symptomatology both before and after tonsillectomy in six patients aged 22 to 52 years, who had adenotonsillar hypertrophy and OSA. Tonsillectomy provided lasting relief of OSA in four patients. We review the literature for available polygraphic data from similar patients before and after tonsillectomy. Our combined experience indicates that many adults with OSA can clearly benefit from removal of hypertrophied tonsils. Such success may, for some adults, be transient or may not occur. This should be considered during initial patient counseling and when planning postoperative follow-up.

Pulmonary function in sickle cell trait

Pulmonary function abnormalities, which have been reported to occur in persons with sickle cell trait (hemoglobin AS), could intensify the hypoxic stimulus for the systemic formation of sickle cells at high altitude. We sought to determine whether pulmonary function abnormalities occur as a result of exposure to high altitudes in persons with hemoglobin AS. In a prospective study, 13 men with hemoglobin AS ("cases") and 13 controls (hemoglobin AA) matched by age, sex, and race were exposed to five to seven altitude simulations (ranging from 1524 to 7620 m [5000 to 25,000 ft]) in a hypobaric chamber. Measurements of diffusing capacity for carbon monoxide (DLco), forced vital capacity, forced expiratory volume in one second, and forced midexpiratory flow were obtained before and after each exposure. Data before exposures did not differ statistically between cases and controls. Altitude had no systematic effect on DLco or spirometric values in cases compared with values in controls (p greater than 0.05). Individual declines in forced vital capacity or DLco of more than 10% occurred with similar frequency in both groups. Measurements made after the series of exposures showed no change from those made before. We conclude that short serial exposures to hypoxia at high altitudes does not acutely or cumulatively alter DLco or spirometric values in healthy, nonexercising persons with sickle cell trait.

Effects of medroxyprogesterone acetate in obstructive sleep apnea

We studied the effects of medroxyprogesterone acetate, a respiratory stimulant, on the incidence and duration of episodes of apnea and disordered breathing in 13 nonhypercapnic men with obstructive sleep apnea. Nocturnal polysomnography was done before and after four weeks of treatment with medroxyprogesterone acetate (60 mg/day) and one week after cessation of treatment. There were no significant (p less than 0.05) differences in the mean frequency of apneic episodes per hour of sleep before (31.3 +/- 5.7 [+/- SE]), during (26.8 +/- 6.6), or after (23.6 +/- 7.0) treatment, or in the mean number of disordered breathing episodes per hour of sleep before (19.4 +/- 5.6), during (21.4 +/- 5.8), or after (23.1 +/- 6.3) the period of treatment. Medroxyprogesterone did not alter significantly the total time of apnea or the total time for disordered breathing, expressed as percentages of total sleep time. Arterial oxygen desaturation during apnea and disordered breathing did not change with treatment. Medroxyprogesterone increased the minute ventilation and occlusion pressure responses to hypercapnia measured in the awake state; however, the results of this study demonstrate that medroxyprogesterone does not improve the breathing disorders during sleep in the nonhypercapnic patient with obstructive sleep apnea.

Overnight nasal CPAP improves hypersomnolence in sleep apnea

Eleven adult men with sleep apnea underwent nocturnal polysomnography on two successive nights. The first study, done without NCPAP, served as the control. The second (treatment) was done with the application of 7.5 to 15 cm H2O nasal continuous positive airway pressure (NCPAP). A subjective sleepiness index (SSI) was noted upon awakening from each night of polygraphic recording. During the control night, the mean frequency of apnea episodes/sleep hr was 35.95 +/- 4.5 SE, and the mean duration was 28.68 +/- 2.7 sec. Mean frequency of disorder of breathing (DOB) episodes/sleep hr was 19.25 +/- 6.2 and mean duration of DOB episodes was 23.1 +/- 2.8 sec. During the treatment night, all obstructive apnea episodes were abolished. During the control night, the mean decrease in arterial oxygen saturation during obstructive apnea episodes was 11.2 +/- 1.9 percent and the mean lowest saturation was 67.6 +/- 4.0 percent. NCPAP eliminated arterial oxygen desaturation. While 44.5 +/- 5.7 percent of total sleep time was spent in either apnea or disordered breathing during the control night, NCPAP decreased this to 0.73 +/- 0.3 percent. In addition to the improvement in respiration during sleep, SSI decreased from a mean of 3.73 +/- 0.49 after the control night to 1.64 +/- 0.24 after treatment, reflecting an improvement in daytime hypersomnolence. We conclude that nasal CPAP is effective in eliminating obstructive apnea episodes, and results in a marked decrease in daytime hypersomnolence after one treatment night.

Prediction of ventilation at maximal exercise in chronic air-flow obstruction

In chronic air-flow obstruction (CAO), inspiratory mechanics constitute a potential mechanism of compensation for limitation of expiratory air flow. We sought to determine whether assessment of inspiratory function could improve our ability to predict ventilation at maximal exercise (Ve max) in patients with CAO. Resting inspiratory and expiratory pulmonary function studies from 20 patients with ventilatory limitation of exercise due to CAO provided data for development of a new regression model for Ve max. Maximal exercise was quantitated from breath-by-breath analysis of exercise responses at cycle ergometry with work increments of 25 watts each min to tolerance. Multiple regression analysis by 3 methods gave identical results. A 2-variable formula containing peak inspiratory flow rate (PIFR) and the forced expiratory volume in one second (FEV1) correlated strongly with Ve max (r = 0.967) in the formula Ve max (L/min) = 21.34 FEV1 (L) + 6.28 PIFR (L/s) +3.94 (95% Cl = +/- 18 L/min). This model was significantly different from published models containing FEV1 alone (p = 0.0002) and was not improved by additional variables. Similar formulas derived for emphysematous and bronchitic clinical types of CAO did not exhibit significantly different slope and intercept coefficients. Both PIFR and FEV1 correlated strongly with tidal volume at maximal exercise. The PIFR also correlated well with resting peak inspiratory airway pressure (r = 0.775). We conclude that consideration of PIFR in addition to FEV1 can improve our clinical ability to predict Ve max in patients with CAO.