Bulk elastic properties of excised lungs and the effect of a transpulmonary pressure gradient

Simulation of airway closure during forced vital capacity

Airway closure, which disconnects peripheral respiratory units from the trachea, has been observed during expiration to residual volume. It is attributed to dynamic compression that may cause unstable collapse and closure of small airways. During forced vital maneuvers, airway closure is expected to be more significant owing to the maximum expiratory effort. In the present study we have added a simulation of airway closure to the model developed by Elad and associates which simulated flow limitation during forced expiration. Progressive closure is simulated by variation in the number of branches and their cross-sectional areas rather than by change in tube law. The results demonstrate that peripheral airway closure may explain the reduction in maximal flow rate at small lung volumes. It can reproduce either the abrupt fall in maximal expiratory flow-volume curves as observed in dog lungs or the gradual decrease that has been observed in humans.

Is airway closure caused by a liquid film instability?

A physical model for small airway closure is developed, based on the assumption that closure occurs as a result of a surface tension-induced instability of the thin liquid film lining the airways. To distinguish this mechanism from others involving airway compliance, experiments were performed in rigid tubes, 1 mm in diameter, with length-to-diameter ratios between one and ten. Oil was added to the film in small increments and photographed at each stage. For total liquid volumes (V) less than some critical value (Vc) surface tension draws the oil into an axi-symmetric film on the tube walls leaving the lumen relatively unobstructed. When V exceeds Vc, the film becomes unstable and collapses, bridging the lumen and causing obstruction. The ratio of Vc to the tube diameter cubed was found to be approximately 0.7 for the entire range of tube lengths studied. These experimental findings were then used to predict airway closure in a morphometric model of the bronchial tree. Assuming that the liquid film at TLC is 10 microns and that the volume of each airway varies in direct proportion to lung volume, the model predicts that airway closure will first occur in the terminal bronchioles at a lung volume of 23% TLC, in approximate agreement with observed values of residual volume.

In vivo hysteresis of airspace dimensions measured by aerosol recovery

In anesthetized mongrel dogs, we made measurements of single breath aerosol recovery (RC) at equal volume points on the inflation and deflation limb of the quasi-static pressure-volume (P-V) curve of the lungs. Using a 1.2 micron monodisperse aerosol, a large aerosol tidal volume (Vt), and a breathing period of 5 sec, we found that losses of particles were primarily due to sedimentation in pulmonary airspaces distal to anatomic dead space. Thus, the RC measurements could be related to a mean radius (R) of airspaces filled with aerosol over the course of the breath. Furthermore, at a given volume, differences between inflation and deflation limb RC could be attributed to differences in R for the two measurements (i.e., RI vs RD). We found that at isovolume, RC as measured from the inflation limb was larger than that measured from the deflation limb for low lung volumes (less than 0.75 TLC). However, the recoveries were similar as lung volume approached TLC (greater than 0.75 TLC). These results implied that at the same volume, RI greater than RD expect at volumes approaching TLC, i.e. a larger mean airspace dimension on the inflation limb than on the deflation limb at equal volume. The findings of this study support a model of nonuniform changes in airspace dimensions associated with in vivo inflation and deflation of the lungs.

Analysis of elastic and surface tension effects in the lung alveolus using finite element methods

Displacement method finite element theory is used to examine the structural and elastic properties of the constituent network of elastin and collagen of the alveoli that form the mammalian lung. The role of the surface tension of pulmonary surfactant of the lung is also examined using an area-dependent relationship inferred from experimental studies. The pressure-volume (PV) curves of the resulting model are found to compare favourably with measured pressure-volume curves for whole lungs filled with air (surface tension included) and saline (no surface tension effects).

Large lungs after childhood asthma: a consequence of enlarged airspaces

The long-term effects of asthma on lung volume were investigated in 18 asthmatic patients during periods of remission from airway obstruction; ten had developed asthma in childhood (before 8 years of age) and eight after the age of 18 years. Subjects whose asthma began in childhood had an increased total lung capacity (TLC) on average; adult-onset asthmatics had normal mean lung volumes. Exponential analysis of static pressure-volume curves in these subjects showed that the childhood-onset asthmatics had lungs of increased distensibility, while adult-onset asthmatics had lungs of normal distensibility. In a further ten adults selected for having had asthma in the past and an increased vital capacity, all had developed asthma in childhood, had lungs of increased distensibility and an increased TLC. Increased distensibility and an increased TLC were closely associated within individuals. The close relationship between distensibility and the size of peripheral air spaces demonstrated in mammalian lungs, suggests that asthma in childhood may increase TLC and distensibility because of an increase in the size of peripheral airspaces.

Exponential analysis of the lung pressure-volume curve in newborn mammals

The compliance of the lung (per unit of lung weight) is less in newborn mammals than in adults. This could result from a smaller volume of airspaces per unit weight and/or a lower lung distensibility. The isolated role of lung distensibility was evaluated by using a mathematical description of the pressure-volume (P-V) curve during lung deflation. Deflation limbs of static P-V curves in newborns of six species (four experimentally obtained and two taken from the literature) ranging from total lung capacity to the resting volume (Vr) were fitted by a monoexponential function of the type V = B - Ae-KP, where B equals Vmax at infinite P, A equals the difference between Vmax and V at P = O, and K is a constant representing lung distensibility. Unlike in adults, in newborns the monoexponential fitting provided an adequate description of the P-V curve for only a relatively small range of transpulmonary pressure (from P at Vr to 10-15 cm H2O). The K value of this portion of the curve was similar among species but higher than in adult mammals, averaging 0.240 cm H2O-1. This suggests a similar lung structure in the different species. Since lung distensibility in newborns is larger than in adults, the fact that a unit mass of lung in the newborn is less compliant should be due to the smaller volume of its airspaces.

Total respiratory compliance as a function of lung volume in patients with mechanical ventilation

In normal subjects breathing spontaneously, compliance has been shown to be influenced by the lung volume from which deflation started. We wondered whether this would also be true for patients with acute respiratory failure who required mechanical ventilation and we accordingly studied 15 such patients. Chest inflation was performed using a continuous flow device (ATM-PV 102), and total compliance was measured as the slope of the pressure-volume relationship during deflation. As inflated lung volumes were increased by 10 to 15, 20 and 25 ml/kg body weight above FRC, mean compliance increased to 38.2, 45.5, 52.2 and 59.3 ml/cmH2O respectively. Each of these increases in mean compliance was statistically significant (p less than 0.001). Increasing the inflated lung volume to 30 ml/kg produced no further significant increase in mean compliance. This study showed that, in patients with acute respiratory failure requiring mechanical ventilation, compliance measured during deflation is a function of the inflated lung volume. We recommend that the compliance should be measured from a constant and high lung volume, equal to 25 ml/kg body weight above FRC.

Analysis of the pressure-volume relationship of excised lungs

The pressure-volume relationship of excised lungs is explicitly defined in the form of a mathematical model. In the model, lung volume (V) is given by the function V = VmaxF(Ptp,T*)H(Ptp). Vmax is maximum lung volume. F, which describes the recruitment of air-filled units, is a function of transpulmonary pressure (Ptp) and surface tension (T*), whereas H, which is also a function of transpulmonary pressure, describes the expansion of recruited units against tissue forces. F is shown to be the integral of the normalized distribution function of the lung units and remains constant so long as the number of air-filled units does not change. H, on the other hand, is shown to be the product of the elastic properties of the tissues and is responsible for the characteristic non-linear sigmoid shape of lung deflation curves. Results obtained with the model are consistent with the hypothesis that tissue elasticity, tissue hysteresis, area dependent surface tension, and recruitment share responsibility for the characteristic hysteresis of excised lungs.

Ventilation inhomogeneity: alveolar mechanics and gas distribution

The effects of regional lung differences in alveolar mechanics on the transpulmonary pressure-volume (Ptp-V) relationship and the single-breath washout (SBW) of nitrogen were investigated by mathematical modeling and postmorten human lung experiments. Regional nonuniformity in alveolar collapse and re-opening were associated with differences in gravitational stress or elasticity. Model simulations predict that neither type of regional nonuniformity qualitatively affects the shape of the Ptp-V curve, but does affect the terminal (or small-volume) portion of the SBW. Comparisons of characteristics of the Ptp-V and SBW curves indicate that regional nonuniformity in alveolar collapse is an important mechanism associated with ventilation inhomogeneity.

Exponential analysis of lobar pressure-volume characteristics

In 19 excised left human lungs, seven of which were emphysema-free, lobar pressure-volume (P-V) characteristics were analysed by fitting to the P-V data an exponential of the form V=A-Be-KPL (where V is the volume at a transpulmonary pressure (PL), A is the theoretical lung volume at infinite PL, B is the difference between A and the intercept of the fitted exponential on the volume axis and K is a shape constant). In the emphysema-free lungs there was no difference in the B/A ratio expressed as a percentage, indicating an identical position of the P-V curves of the upper and lower lobes. However the upper lobes had significantly (p less than 0.025) higher values for K than the lower lobes indicating greater compliance. In 10 emphysematous lungs the upper lobes were more severely involved than the lower lobes. In these lungs the difference in K between upper and lower lobes had been abolished suggesting that the less emphysematous lower lobes had increased compliance relatively more than the upper lobes.

Use of lung pressure-volume curves and helium-sulphur hexafluoride washout to detect emphysema in subjects with mild airflow obstruction

Mild abnormalities of peripheral lung function can be detected by simple methods, but it remains difficult to determine when these changes are the result of emphysema rather than disease of the airways. We have compared the value of measurements of lung distensibility and a multibreath test of helium (He) and sulphur hexafluoride (SF₆) washout in distinguishing between six men with mild impairment of airway function caused by asthma (group A) and six men with similar airway function but probable widespread emphysema (group E). In group E there were striking abnormalities in the static pressure-volume curve of the lungs (reduced lung recoil pressures, increased chord compliance, increased shape factor) and the relation between maximum expiratory flow and lung recoil pressure fell within the normal range. In group A there were only minor abnormalities in lung distensibility and maximum expiratory flow was reduced at a standard lung recoil pressure. In addition carbon monoxide transfer coefficient was reduced in group E but normal in group A. Normal values for He-SF₆ washout were similar to those previously described. Differences in He-SF₆ washout between group A and group E men were small and in part accounted for by differences in functional residual capacity. In subjects with lung disease, end-tidal He and SF₆ concentrations during washout were erratic and it was sometimes impossible to define a crossover point. We conclude that in our hands this technique is less useful for detecting acinar disease than are measurements of lung distensibility or carbon monoxide transfer. Considerable changes in lung distensibility may occur at an early stage in the natural history of emphysema and are readily distinguishable from the small changes that occur in mild asthma.

Quantitative comparisons of mammalian lung pressure volume curves

The deflation pressure-volume curves of the lungs of a wide range of mammalian species were studied to compare their mechanical properties. A monoexponential mathematical function of the form V = Vmax - (Vmax - Vo)e - kp was fitted to the deflation data. It was found that the bulk stiffness index k (approximately 0.12 cm H2O-1) varied little over the 10(5) fold range of animal body weight. This range of k was far smaller than found in man in the presence of pulmonary parenchymal disease. It was concluded that the intrinsic stiffness characteristics of most mammalian lungs are similar.

Lung mechanics after cardiac valve replacement

Fourteen patients undergoing single aortic or mitral valve replacement had measurements made of lung volumes, static pressure-volume (P-V) relationships, and conductance-pressure relationships during deflation before operation and again between one and two years later. At follow-up, total lung capacity (TLC), functional residual capacity (FRC), residual volume (RV), and static tidal compliance (slope of static P-V deflation line for one litre above FRC) had increased significantly, in association with a decrease in heart size. There was a change in the shape and position of some P-V curves both in the aortic and mitral patients. In the patients with aortic disease P-V deflation curves shifted to the left after operation. In the patients with mitral disease the P-V deflation curves before operation crossed those measured after operation, so that at high lung volumes recoil became less after operation, but at low lung volumes recoil increased. Conductance had increased at high lung volumes. The data suggest that in longstanding pulmonary congestion, airways are more rigid making them less distensible at high and less compressible at low transpulmonary pressures than after operation when congestion has been at least partly relieved.

Lung distensibility and airway function in intermediate alpha 1-antitrypsin deficiency (Pi MZ)

We examined the role of intermediate alpha 1-antitrypsin deficiency in predisposing to abnormalities of lung distensibility and airway function in 20 heterozygotes (Pi MZ) who were individually matched with a control Pi M subject of similar age, height, and smoking habits drawn from the same male, working population. There were no significant differences between the heterozygotes and their controls in the results of spirometry, maximum expiratory flow-volume curves (breathing air), single breath nitrogen test, arterialised capillary blood oxygen pressure, or single breath carbon monoxide transfer. Additional studies were made in 12 of the pairs of Pi MZ and Pi M subjects. Comparison of maximum expiratory flow-volume curves breathing air and 80% helium-20% oxygen showed no differences between the Pi MZ and Pi M subjects. Although airway function was similar in the two groups, four of 12 Pi MZ subjects showed abnormalities of the pressure-volume curve of the lung (reduction in lung recoil pressure, abnormal shape factor, increase in functional residual capacity). Abnormalities of washout of a helium-sulphur hexafluoride gas mixture, of a type previously described as characteristic of emphysema, were found in two of the men with abnormal pressure-volume curves. The results suggest that Pi MZ subjects have an increased susceptibility to alveolar abnormalities without increased abnormalities of airway function; this may explain the increased frequency of emphysema at necropsy despite many studies showing no predisposition to abnormal airway function in life. The functional changes we observed would be unlikely to cause symptoms. The risk of disablement from chronic lung disease appears to be only slightly enhanced by intermediate alpha 1-antitrypsin deficiency.

Trapped gas at maximum lung volume in intact isolated rat lungs

Excised rat lungs were ventilated with air in a liquid filled plethysmograph that was enclosed in a large pressure chamber, C(B). The lungs were inflated then deflated by removing or adding saline to the plethysmograph while the trachea was attached to a cannula extending through the plethysmograph base. In this system, tracheal pressure, Pao, was equal to gas pressure inside C(B). The gas pressure was held constant at either ambient pressure (Pamb), Pamb + 350 Torr, or Pamb - 350 Torr. When excised lungs were ventilated slowly from their atelectatic state for 10 inflation-deflation cycles with Pao equal to any one of the three pressures, an equivalent amount of gas was trapped in the lungs. If after 10 cycles, however, lungs containing trapped gas were inflated and held at maximum lung volume, the trapped gas spaces could be made to expand in response to rarefaction or compression of the tracheal gas. The amount of expansion and contraction of the trapped gas spaces demonstrates that trapped gas is likely trapped between menisci of a foam that occlude the alveoli or small airways.

The compliance curve for the flow limiting segments of the airway. II. Experiments with human subjects

Maximum effort flow-static recoil curves were obtained in 5 healthy subjects breathing air, He/O2, and SF6/O2 mixtures. In 4 of them maximum effort flows corresponded to really maximal flows and their curves were transformed into compliance curves for the flow limiting segments of the airway and analyzed from the point of view of a previously presented lung model (Pedersen and Nielsen 1976). The results showed, that viscosity dependent pressure losses from the alveoli to the flow limiting segments were minimal for air and SF6/O2, but not for He/O2. When viscosity dependent pressure lossess could be negleted, then expiration of gases of different densities gave almost identical compliance curves for the flow limiting segments. This supported the applicability of the model. The calculated compliance curves for the flow limiting segments were compared with data from the literature, and the findings indicated that flow limitation during expirations with just maximal flows throughout began in the extrapulmonary airways and moved upstream during the expiration.

Shape changes in static V-P loops from children's lungs related to growth

Sixty-eight sets of deflation data from the left lungs of children who had died from non-pulmonary causes were analysed by the exponential equation model to derive theoretical values of Pst(1) for 50, 60, and 90% of the observed maximum inflation volume. The resultant P50, 60, and 90 values were compared to the same values measured from graphic displays of the V-P data. The graphically derived Pst(1) data at 50, 60, and 90% of TLC were then plotted against the crown-heel length to demonstrate a shape change related to physical growth. It was found that the form of the deflation curve in preterm infants did not fit an exponential model as satisfactorily as did the deflation curve from older children. The exponential model should be used with caution in small infants where Pst(1) values below P90 are sought. A maximum inflation pressure of +30 cm H2O was found to produce a Vmax within 98% of the hypothetical VINF value.

Post-mortem studies of the semistatic volume-pressure characteristics of infants' lungs

Semistatic air volume-pressure (V-P) loops were recorded from 172 isolated lungs obtained from infants and children at necropsy. Technically unsatisfactory V-P loop behaviour and the presence of pathological changes led to the exclusion of 98 lungs. A further 10 lungs were excluded as it was decided to study the period of growth up to 90 cm crown-heel length. A total of 64 left lungs remained which were designated "normal". Control data were collected in an attempt to quantify the effects of storage and variations in inflation technique. A maximum inflation pressure of +30 cmH2O was used as a standard Pmax. The resulting maximum inflation volume (Vmax) data showed a cubic relation to the crown-heel length which deviated from the values obtained by the use of in-vivo regression equations. The extent and pattern of the deviation suggest that the technical factors are not a major contributory cause, and other explanations are discussed. The low-pressure proportional data from the deflation limb of the V-P curve are plotted against the crown-hell length, and the resulting curve is analysed. There is a sharp fall in the proportion of Vmax retained in the lung at +5 cmH2O and +2-5 cmH2O with increasing body length, but proportional volume at zero transpulmonary pressure follows the same growth-related pattern as Vmax.

Effect of papain-induced emphysema on the distrubtion of pleural surface pressure

Distribution of transpulmonary pressure (Ptp) at FRC, during spontaneous breathing, and during inflation of the relaxed respiratory system was studied in the supine, lateral and head-up postures after papain had been injected intratracheally into dogs. Functional and morphological changes of the lung resembling those of panlobular emphysema occurred in papain-treated dogs. In all postures the relationship between lung height and Ptp at FRC was steeper and shifted to the left of that obtaining for normal dogs. During artifical inflation changes of Ptp were larger in the dependent than in the upper lung and the vertical gradient of Ptp eventually disappeared, as observed in normal dogs. During spontaneous breathing changes of Ptp were uniform in the horizontal postures, but larger over the upper regions in the head-up posture; whereas they are uniform in all postures in normal dogs. From local Ptp values and lung P-V curve, distribution of specific lung volumes at FRC and ventilation during spontaneous breathing were assessed for both normal and papain-treated head-up dogs: the results agreed with those obtained using radioactive gases in normal and in elderly man or emphysematous subjects with no apparently localized lesions.

Effect of negative abdominal pressure on regional lung volumes in supine dogs

Using 133Xe we measured regional lung volumes from apex to base in supine dogs during the application of negative abdominal pressure (-50 cm H2O). Changes in rib cage shape were monitored with magnetometers. Negative abdominal pressure caused a decrease of 5% in the cross-sectional area of the upper rib cage and a decrease of 19% in the cross-sectional area of the lower rib cage. These changes were similar to those observed when the dog was tilted to the erect position. Over the 10 cm of lung examined, control measurements showed a mean apex-to-base gradient of 1.4% TLC/cm, apical regions being more expanded than basal. Negative abdominal pressure increased this gradient to a mean of 2.1% TLC/cm. This change was in qualitative agreement with measurements of costal pleural surface pressure made in similar experiments by others. However, the changes we observed tended to be smaller than those predicted on the basis of costal pleural surface pressure. Since regional volumes represented the average of the pressures operating on lung regions, it was suggested that during negative abdominal pressure this average differed from pressure measured at the costal surface.

Stress-strain relationships during uniform and non uniform expansion of isolated lungs

Stress-strain relationships in various directions were determined on isolated rabbit and cat lungs both during unrestrained and restrained expansion, the latter approximating the condition of cylindrical deformation. Under this condition the distortion due to the weight supported at the lung surface was also studied. Experimental data did not fit the equilibrium equations for a continuum solid, except for relatively small distortions of lung parenchyma. On the contrary, over the full range of lung distortions they fitted a model made up by a three-dimensional network of interconnected springs. While distortions produced even at relatively low lung volumes caused substantial changes of surface pressure distribution, the distorting effects of the weight were negligible under this respect. These results support the conclusion that pleural surface pressure distribution in rabbits, cats and dogs is mainly due to the lung distortion caused by the mismatching between lung and chest wall inherent shapes, while that caused by the lung weight plays only a minor role.