Peripheral Lung Resistance in Normal and Asthmatic Subjects1,2
ELIZABETH M. WAGNER, MARK C. LIU, GAIL G. WEINMANN, SOLBERT PERMUTT, and EUGENE R. BLEECKER
Introduction
Asthma is a clinical syndrome characterized by hyperresponsiveness of the tracheobronchial tree to a wide variety of stimuli (1). Airways hyperreactivity can be demonstrated in subjects with asymptomatic asthma even when baseline pulmonary function is within normal limits (2, 3). This heightened airways responsivenesshas been attributed to alterations in smooth muscle reactivity (4, 5), alterations in baseline airway caliber (6, 7), abnormalities in mucociliary and epithelial function (8, 9), altered neural control of airway tone (10, 11), and bronchial inflammation (12-16). Whatever the mechanism, it is generally agreed that the major symptoms during an asthmatic attack result from a narrowing of peripheral airways. There is indirect evidence that asymptomatic asthmatic subjects have physiologic abnormalities in peripheral airway function. Several pulmonary function tests including spirometry (17), frequency dependence of compliance (18), and density dependence of maximum expiratory flow (19, 20), have all demonstrated physiologic alterations in the small airways of asymptomatic asthmatic subjects. In addition, fluids obtained by bronchoalveolar lavage from asymptomatic asthmatic subjects show increased levels of inflammatory mediators compared to normal subjects (12-15). These findings suggest that there are physiologic and inflammatory abnormalities in subjects with asthma during asymptomatic periods. The present study was designed to quantify directly the pressureflow relationship of the peripheral lung in asthmatic subjects and relate this measure of resistance to their levels of airways reactivity. Toaccomplishthis we used a bronchoscope wedged in a subsegmental bronchus to measure the resistance of the peripherallung. Although the major airway 584
SUMMARY In obstructive lung disease, peripheral airways are a major site of pathologic abnormalIties. However, resistance to airflow In small airways In the periphery of the lung accounts for only a small fraction of total airway resistance. Consequently, abnormalities of small airway function may not be readily detected using routine pulmonary function testing. In the present study, resistance of the peripheral lung was examined directly In six normal subjects and nine mildly asthmatic subjects. There were no significant differences between the normal and asthmatic groups In pulmonary function assessed by spirometry (FEY1, FYC)and body plethysmography (specific airway conductance). Direct measurements of peripheral lung function were made using a flberoptlc bronchoscope wedged Into a subsegmental, right upper lobe bronchus. Using a double-lumen catheter Inserted Into the Instrument channel of the bronchoscope, pressures (PS)produced by three or more different levels of gas flow (V) (5% CO2 in air) between 50 and 500 mllmin were measured. All pressure measurements were made at a constant lung volume (i.e., functional residual capacity) confirmed by monitoring transpulmonary pressure with an esophageal balloon. The pressure-flow relationship in both normal and asthmatic subjects could be approximated by a straight line through the origin, demonstrating these airways to be relatively nondlstenslble. Peripheral lung resistance (Rp) was defined by PaN and averaged for three or more levels of flow. In contrast to spirometric results that showed no differences between the two groups, Rp was Increased more than sevenfold in asthmatic subjects (0.069 ± .017 em H20/mllmln) (mean ± SEM) compared to normal subjects (0.009 ± .002 em H20/ml/mln). Pretreatment of the asthmatic subjects (n 5) with aerosolized Isoproterenol did not decrease Rp to normal levels. These results demonstrate marked physiologic abnormalities in the peripheral lungs of asymptomatic asthmatic subjects and suggest that changes In the mechanical properties of these airways, although having a small Influence on overall pulmonary function, may contribute to Increased airway responsiveness In asthma.
,
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AM REV RESPIR DIS 1990; 141:584-588
component contributing to this measure is uncertain, there is considerable evidence to suggest that the airway responses measured with the wedged bronchoscope are similar to those of small airways (21, 22). In addition, the measurement of collateral . or peripheral lung resistance has also been used extensively in animal models to assess responsiveness of the peripherallung to various agonists (21-23). The technique has also been used in humans , to assess changes in collateral ventilation with age (24). Methods Subjects Six normal and nine asymptomatic asthmatic subjects were classified by clinical history and airways reactivity to inhaled methacholine. All asthmatic subjects were atopic and had skin test reactivity to one or more com-
mon allergens. This study was approved by the institutional review board and informed consent was obtained from all subjects.
Pulmonary Function Measurements of pulmonary function were performed in all subjects before and immedi-
(Received in original form March 21, 1989 and in revised form August 25, 1989) 1 From the Department of Medicine and the Department of Environmental Health Sciences, Division of Pulmonary and Critical Care Medicine, The Johns Hopkins University, Baltimore, Maryland. 2 Correspondence and requests for reprints should be addressedto Elizabeth M. Wagner,Ph.D., Division of Pulmonary and Critical Care Medicine, Johns Hopkins University at Francis Scott Key Medical Center, 4940 Eastern Avenue, Baltimore, MD 21224.
COLLATERAL RESISTANCE IN NORMAL AND ASTHMATIC SUBJECTS
ately after bronchoscopy. Airway resistance and thoracic gas volume were measured in a constant-volume body plethysmograph (Warren E. Collins, Braintree, MA) by the method of Dubois and colleagues (25). Resistance was converted to its reciprocal, conductance, and was expressed as specific airway conductance (sGaw),the ratio of conductance to thoracic gas volume (26). Spirograms were performed in triplicate using a Stead-Wells spirometer (Warren E. Collins). Forced expiratory volume in I s (FEVI), 3 s (FEV3), forced expiratory volume (FVC), and mean forced expiratory flow during the middle half of the FVC (FEF 2s - 7s.,.) were calculated.
Methacholine Airways Reactivity Methacholine challenge was performed using standard techniques (2) as part of the subject screening process on a visit sometime prior to bronchoscopy. In brief, the subjects were instructed to take five maximal inspirations from functional residual capacity (FRC). Methacholine solutions (J. T. Baker, Phillipsburg, NJ) were nebulized for 0.6 s by triggering a breath-activated solenoid valve timing circuit (dosimeter). This apparatus delivered an average of 1.028 ± 0.222 ml (mean ± SEM) with every five breaths. After first inhaling a control saline solution, all subjects received increasing concentrations of methacholine (0.625, 1.25, 2.5, 5.0, 10.0, and 25.0 mg/ml). During the 5-min intervals between inhalations, spirometry was performed. When a 20070 or greater fall in FEV 1 occurred, the challenge was terminated. Dose-response curvesfor methacholine werethen constructed whereby the cumulative dose of agonist required to produce a 20070 fall in FEV 1 was calculated (PD 2o FEV I). Protocol After baselinepulmonary function testingand 30 min before bronchoscopy, subjects were pretreated with atropine (0.6mg) administered intramuscularly; they then inhaled an aerosol of 4070 lidocaine (5 cc) administered using a DeVilbiss nebulizer no. 646 (DeVilbiss, Somerset, PAl. An esophageal balloon was passed through one naris to measure transpulmonary pressure in order to confirm that pressure measurements were made at a constant lung volume.A fiberoptic bronchoscope (5.5 outer diameter) was advanced into the lung and 6 to 12cc of lidocaine (2070) was administered through the bronchoscope directly to the airways to provide local anesthesia. With subjects in the supine position, the bronchoscope was wedged in a nondependent subsegmental bronchus of the right upper lobe. A double-lumen catheter (5FR) was inserted through the instrument channel of the bronchoscope. Air flow (5070 CO 2 in air) waspassed through a mass flow controller (Sierra, Carmel Valley, CAl through one lumen of the catheter. Back flow was prevented by a snugly fitting cap on the instrument channel port through which the catheter had been passed. Pressure at the tip of the bronchoscope (P a)
585 TABLE 1 PULMONARY FUNCTION OF NORMAL AND ASTHMATIC SUBJECTS Subjects
Number of subjects FEVl ' % predicted FVC, % predicted FEF25-75, % predicted FEV3/FVC0/o Rp (cm H20/ml/min) *
Significant at p
Normal
Asthmatic
6 ± ± ± ± ±
9
103.2 101.3 93.5 98.0 0.009
2.9 1.6 4.4 1.0 0.002
Significance
95.0 ± 4.2 95.4 ± 3.8 n.4 ± 6.6 97.0 ± 1.0 0.069 ± 0.017
NS, P = NS, P = * P = NS, P = * P =
0.088 0.124 0.047 0.226 0.013
< 0.05.
was measured through the second lumen of the catheter. The resistance of the peripheral lung distal to the wedged bronchoscope was assessed by making incremental increases in flow and recording the resultant pressure. At each flow, . subjects wereinstructed to breathhold at FRC and the steady-state pressure at the tip of the bronchoscope was measured. All measurements were made at FRC (confirmed by a constant transpulmonary pressure), because changes in lung volume have been demonstrated to affect peripheral lung resistance in man (27). Pressure measurements in the wedged segment were obtained in triplicate at each level of flow starting with a flow of 50 or 100 ml/min. Pressures produced by at least three levelsof flowweremeasured in each subject. To determine whether the pressure-flow relationship could be altered by pretreatment with a l3-sympathomimetic agent, five additional studies wereperformed in subjects with asthma several weeks after the initial study. These subjects inhaled 0.5 ml of 5070 aerosolized metaproterenol sulfate (Boehringer Ingelheim, Ridgefield, CT) before bronchoscopy. After the bronchoscope was wedged in a right upper lobe subsegmental bronchus, an aerosol of 2 mg/ml isoproterenol (Winthrop, New York, NY) was administered for 2 min at a flow of 200 ml/min directly into the peripheral segment. In pilot studies we determined that this dose of isoproterenol, administered to the peripheral airway segment, was sufficient to reverse histamine-induced increases in peripheral lung resistance. Pressure-flow data were obtained using the protocol described above.
Data Analysis The peripheral lung resistance (Rp) was defined as the Pa/Vaveraged for three or more levels of flow in each subject. Student's t test was used to determine the differences in pulmonary function between normal subjects and those with asthma (one-tailed), before and after bronchoscopy in each group (twotailed), and before and after isoproterenol pretreatment in subjects with asthma (twotailed). Spearman's rank correlation coefficient was used to assess the relationship between peripheral lung conductance (I/Rp) and methacholine sensitivity (28). For all compar-
isons, a p value ~ 0.05 was considered significant. All values are presented as the mean ± the standard error.
Results
Baseline pulmonary function assessed by spirometry showed no significant differences between normal subjects and those with asthma in FEV 1 OJo predicted, FVC OJo predicted or FEV 3/FVC OJo. However, FEF2 S - 7S '1., which is thought to reflect peripheral airway function, was significantly lower in subjects with asthma than in normal subjects (p = 0.047) (table 1). All subjects with asthma had increased levels of reactivity to methacholine, and the PD2 0 FEV 1 was 0.89 ± 0.15log units compared to normal subjects who either did not respond or responded at greater than 2.18 log units. Absolute values for pulmonary function in the two groups before bronchoscopy are presented in table 2. Bronchoscopy did not produce differential effects in the two groups. FEV., FEV 1 OJo, FEF2S - 7 S '1., and FEV 3 were unaltered by the procedure. However, a small but significant decrease in FVC [normal subjects: 5.22 ± 0.37 (p = .043); subjects with asthma: 4.38 ± 0.34 (p = 0.041)] and a significant increase in sGaw [normal and asthmatic subjects: 0.26 ± 0.04 (p = 0.024 and p = 0.020, respectively)] were measured in normal and asthmatic subjects after bronchoscopy. TABLE 2 PULMONARY FUNCTION BEFORE BRONCHOSCOPY Subjects Normal (n = 6) FEV1 FVC FEV1% FEF25- 75% FEV3 sGaw
4.45 ± 0.25 5.31 ± 0.37 84 ± 2 4.52 ± 0.20 5.20 ± 0.38 0.20 ± 0.02
Asthmatic (n = 9)
3.71 4.51 82 3.58 4.38 0.20
± 0.28 ± 0.35
± ± ± ±
2 0.35 0.35 0.02
586
WAGNER, LlU, WEINMANN, PERMUTT, AND BLEECKER
25
15
oLJ~~~~ o
100
200
300
400
500
V (ml/mln)
M
Fig. 1. Pressure (Pa)-tlow relationships of six normal subjects (closedsquares)and nine subjects with asymptomatic asthma (open squares).
Despite the general lack of baseline differences in pulmonary function of these normal subjects and those with mild asthma, there was a striking difference in peripheral lung resistance. The individual pressure-flow relationships for all subjects are presented in figure 1. It is of interest to note that the pressureflow curves appear relatively linear and seem to extrapolate through the origin. This observation suggests that the airways being studied are relatively nondistensible over the pressure range studied. This observation provides the rationale for our method of calculating Rp from the average of values measured at three or more different flows. The measurements of PB were acceptable only if esophageal pressure varied within a range of 3.5 em H 20 or less (SE ranged from 0.1 to 0.4 em H 20). The average Rp was 0.009 ± 0.002 em H 20/ml/min in the normal subjects compared to 0.069 ± 0.017 em H 2 0 / ml/min (p = 0.013) in asthmatic subjects. The relationship between Rp and whole lung sensitivity to methacholine in the nine asthmatic subjects is presented in figure 2. A positive correlation exists between peripheral airway conductance (I/Rp) and the methacholine sensitivity PD 20 FEV 1 (r = 0.383, NS) in the nine
subjects with asthma. However, when the one outlyer is excluded from the analysis, a significant correlation exists between peripheral lung conductance and methacholine PD 20 FEV 1 (r = 0.81, p < 0.05). Results of pressure-flow studies performed in the five asthmatic subjects who were studied after pretreatment with a l3-sympathomimetic are presented in figure 3. For each of the five subjects, the baseline pressure-flow relationship is compared to the pressure-flow data obtained after administration of aerosolized isoproterenol. Average baseline Rp for these five subjects with asthma was 0.086 ± 0.027 cm H 20/ml/min compared to 0.062 ± 0.019 after isoproterenol pretreatment (p = 0.392). Thus, pretreatment with this l3-sympathomimetic agent resulted in no change in Rp. Discussion The results of this study demonstrate a striking difference in peripheral lung resistance of subjects with asthma compared to normal subjects. More than a sevenfold increase in resistance was measured in these subjects with mild asthma compared to normal subjects, despite the fact that, in general, pulmonary function did not differ in the two groups. Although it is unclear what the major anatomic
component contributing to this peripherallung resistance measurement is, collateral channels (bronchoalveolar, interbronchiolar, and interalveolar pathways) in numerous animal studies have been shown to be similar to small airways, at least in terms of their pharmacologic responses (21-23). We believe the measure of Rp is dominated by the resistance of collateral channels although the small airways in series with these channels may also contribute. This contribution is likely greater in the asthmatics because in these subjects there was a significant reduction in FEF 2S - 7S .,. , a test presumed to measure small airway function. The observation that there was a correlation between whole lung methacholine reactivity and peripheral lung conductance suggests that either small airways responsive to methacholine challenge are being measured with Rp or that collateral channels serve as a marker of how small airways would respond. Based on the observations of the present study, small peripheral airways or collateral channels may playa functional role in the pathogenesis of asthma. Specifically, with regard to collateral channels, eliminating their functional role by significantly increasing collateral resistance would tend to increase air trapping and residual volume. These changes would exacerbate
• 00
Post-isoproterenol Pre-isoproterenol 25
0 0
100
200
300
400
15
500
V (ml/mln)
PB (em H2O)
V
15
PB (em H2O) 0 0
100
200
300
400
500
V (mllmln) 0 50
0
100
40
1/Rp
30
20
300
400
25
500
f
15
(ml/cm H20-min)
200
V (ml/mln)
..
PB (em H2O)
15
PB(em Hp>
o +----.---~----.--___, o Methacholine PD 2 0 (log)
0
0 0
Fig. 2. Correlation between methacholine PD20 (provocative dose that causes a 20% decrease in FEV1) and Peripheral lung conductance (1/Rp) of the nine subjects with asthma.
100
200
300
400
500
V (ml/mln) Fig. 3. Pressure (Pa)-tlow
('i) relationships of five SUbjects with
squares) isoproterenol pretreatment.
0
100
200
300
400
500
V (mllmln) asthma before (open squares) and after (closed
587
COLLATERAL RESISTANCE IN NORMAL AND ASTHMATIC SUBJECTS
the effects of small airway closure, an abnormality thought to accompany an acute asthmatic attack (29). Regardless of which air channel predominates in the measure of peripheral lung resistance, our results suggest that an important difference between the normal and asthmatic subjects is structural changes in peripheral airways. This conclusion is based on the observations made in the asthmatic subjects after pretreatment with isoproterenol. If the difference in resistance between normal subjects and subjects with asthma resulted from an increase in airway smooth muscle tone, then resistance measurements in subjects with asthma after isoproterenol pretreatment should have been substantially altered and should have approached the values obtained in the normal subjects. These results were not found. Rather, only minimal decreases in resistance were measured, reflecting a reduction in baseline smooth muscle tone in airways with substantially greater baseline resistance than that of normal subjects. Although it is possible that the dose of isoproterenol used was insufficient, we believe this explanation is unlikely because this local treatment reverses histamine-induced airway constriction in the lung periphery (unpublished observations). The results also exclude the possibility that the bronchoscopic procedure was acting as a smooth muscle stimulus to increase resistance in the group with asthma. This is supported by the fact that the postbronchoscopy pulmonary function was not differentially affected in the two groups. We can only speculate as to the physical changes that take place in the asthmatic airway. The measured increase in peripheral lung resistancecould be caused either by airway narrowing or by a decrease in the actual number of functional airways, perhaps reflecting airway closure. Our results cannot distinguish between these two mechanisms. However, several studies (12-15) demonstrate increased levels of inflammatory mediators in the bronchoalveolar lavage fluid of subjects with asthma compared to normal subjects. Roche and coworkers (30, 31)have recently reported that bronchial biopsies performed in subjectswith asymptomatic asthma not only show evidence of cellular inflammation with marked increases in eosinophils, but also structural abnormalities that are consistent with Collagen deposition in the airway wall. In addition, inflammatory events may produce mucosal swellingand edema that could result in decreased airway internal
diameters and increases in peripheral lung resistance. Thus, inflammation and associated structural abnormalities may be responsible for the functional changes reported in this study. Another significant finding in this study was the correlation between methacholine sensitivity and peripheral lung conductance in subjects with asthma. For the most part, the usual mechanisms that have been proposed to explain airways hyperreactivity have emphasized alterations in autonomic neural control of airway tone, changes in intrinsic bronchial smooth muscle function, and alterations in bronchial epithelial integrity and permeability. Although each of these potential mechanisms may play a role in the development of airways hyperresponsiveness, we believe that bronchial inflammation may be a common factor responsible for the development of airways hyperresponsiveness in asthma and may explain the relationship between methacholine sensitivity and peripheral lung conductance observed in this study. Bronchial inflammation, by causing mucus hypersecretion and mucosal edema, may cause a decrease in the baseline radius of small airways. Products of inflammation may be expected to extend toward the luminal surface reducing the size of peripheral airways. However, abnormalities associated with the development of bronchial inflammation should not be limited to the internal surface of the airways. Outward extension of these biochemical and cellular changes in the lung tissue surrounding the small airways would produce additional effects on airway size by altering the forces of interdependence (32) between airways and lung parenchyma. Tothe extent that these tethering forces are significant at FRe, any reduction will tend to decrease the baseline diameter of the airways. Changes in baseline diameter of small airways have been reported to result in substantial differences in the manifestation of agonist-induced constriction (33). Thus, if inflammatory changes in subjects with asthma result in a generalized decrease in the diameter of small airways, the response to a bronchoconstrictor agonist will be exaggerated and detected by changes in spirometry. In summary, the results of this study demonstrate marked physiologic abnormalities in the peripheral airways of subjects with mild, asymptomatic asthma. Although the sevenfold increase in the resistance to flow through the peripheral airways observed in the subjects with
asthma may not be sufficient to cause changes in pulmonary function, the physical changes within the small airways may be related to the presence of airways hyperractivity. Acknowledgment We thank Professor Wayne Mitzner for his insightful comments in preparing this manuscript, and Mary Johns, Elizabeth Rechsteiner, and Becky Stealey for their superb technical assistance. References 1. American Thoracic Society Board of Directors. Standards for the diagnosis and care of patients with chronic obstructive pulmonary disease(COPD) and asthma. Am Rev Respir Dis 1987; 136:225-44. 2. Chatham M, Bleecker ER, Norman P, Smith PL, Mason P. A screening test for airways reactivity. Chest 1982; 82:15-8. 3. YanK, Salome CM, Woolcock AJ. Prevalence and nature of bronchial hyperresponsiveness in subjects with chronic obstructive pulmonary disease. Am Rev Respir Dis 1985; 132:25-9. 4. Antonissen LA, Mitchell RW, Kroeger EA, Kepron W, Tse KS, Stephens NL. Mechanical alterations of airway smooth muscle in a canine asthmatic model. J Appl Physiol 1979; 46:681-7. 5. Vincenc KS, Black JL, Yan K, Armour CL, Donnelly PD, Woolcock AJ. Comparison of in vivo and in vitro responses to histamine in human airways. Am Rev Respir Dis 1983; 128:875-9. 6. Benson, MK. Bronchial hyperreactivity. Br J Dis Chest 1975; 69:227-39. 7. James AL, Pare PD, Hogg JC. The mechanics of airway narrowing in asthma. Am Rev Respir Dis 1989; 139:242-6. 8. Nadel JA, Holtzman MJ. Regulation of airway responsiveness and secretion: role of inflammation. In: Kay AB, Austin KF, Lichtenstein LM, eds. Asthma: physiology, immunopharmacology and treatment. Third international symposium. London: Academic Press, Inc., 1984; 129-56. 9. Laitinen LA, Heino M, Laitinen A, Kava T, Haahtela T. Damage of the airway epithelium and bronchial reactivity in patients with asthma. Am Rev Respir Dis 1985; 131:599-606. 10. Bleecker ER. Cholinergic and neurogenic mechanisms in obstructive airways disease, Am J Med 1986; 81(Suppl:93-102). 11. Nadel JA. Autonomic control of airwaysmooth muscle and airway secretions. In: Nadel JA, ed. Physiology and pharmacology of the airways. Vol 15. Lung biology in health and disease. New York: Marcel Dekker, Inc., 1980; 117-26. 12. Liu MC, Bleecker ER, Niv Y, et ale Asthma is characterized by elevated levelsof bronchospastic prostaglandins and histamine in the airway fluids obtained by bronchoalveolar lavage (abstract). Fed Proc 1988; 47:AI698. 13. Rankin JA, Kaliner M, Reynolds HY. Histamine levelsin bronchoalveolar lavage from patients with asthma, sarcoidosis and idiopathic pulmonary fibrosis. J Allergy Clin Immunol 1987; 79:371-7. 14. Casale TB, Wood D, Richerson HB, et al. Elevated bronchoalveolar lavagefluid histamine levels in allergic asthmatics are associated with methacholine bronchial hyperresponsiveness. J Clin Invest 1987; 79:1197-203. 15. Christiansen SC, Proud D, Cochrane CG. Detection of tissue kallikrein in the bronchoalveolar lavage fluid of asthmatic subjects. J Clin Invest 1987; 79:188-97.
588 16. Metzger WJ, Zavala D, Richerson HB, et al. Local allergen challenge and bronchoalveolar lavage of allergic asthmatic lungs. Am Rev Respir Dis 1978; 135:433-40. 17. McFadden ER Jr, Linden DA. A reduction in maximum mid-expiratory flow rate. A spirographic manifestation of small airway disease. Am J Med 1972; 52:725-37. 18. Woolcock AJ, Vincent NJ, Macklem PT. Frequency dependence of compliance as a test for obstruction in the small airways. J Clin Invest 1969; 48:1097-106. 19. Despas PJ, Leroux M, Macklem PT. Site of airway obstruction in asthma as determined by measuring maximal expiratory flow breathing air and a helium-oxygen mixture. J Clin Invest 1972; 51:3235-43. 20. McFadden ER Jr, Ingram RH Jr, Haynes RL, Wellman J J. Predominant site of flow limitation and mechanisms of postexertional asthma. J Appl Physiol 1977; 42:746-52. 21. Menkes HA, Macklem PT. Collateral flow. In: Fishman AP, 00. Handbook of physiology.The re-
WAGNER, LlU, WEINMANN, PERMUTT, AND BLEECKER
spiratory systemIII. Baltimore: WaverlyPress, 1986; 337-53. 22. Menkes HA, Traystman RJ. Collateral ventilation. Am Rev Respir Dis 1977; 116:287-309. 23. FreedAN, Bromberger-Barnea B, Menkes HA. Dry air-induced constriction in lung periphery: a canine model of exercise-induced asthma. J Appl Physiol 1985; 59:1986-90. 24. Terry PB, Traystman RJ, Newball HH, Batra G, Menkes HA. Collateral ventilation in man. N Engl J Med 1978; 298:10-5. 25. DuBois AB, Botelho SY, Comroe JH Jr. A new method for measuring airway resistancein man using a body plethysmograph: valuesin normal subjects and in patients with respiratory disease. J Clin Invest 1956; 35:327-35. 26. Briscoe WA, Dubois AB. The relationship between airway resistance, airway conductance and lung volume in subjects of different age and body size. J Clin Invest 1958; 37:1279-85. 27. Inners CR, Terry PB, Traystman RJ, Menkes HA. Effects of lung volume on collateral and airways resistance in man. J Appl Physiol 1979; 46:
67-73. 28. Snedecor OW, Cochran WG. Statistical methods. 6th 00. Ames: IowaState UniversityPress, 1967. 29. Permutt S. Physiologic changes in the acute asthmatic attack. In: Physiology, immunopharmacology and treatment. New York: Academic Press, Inc., 1973; 15-27. 30. Beasley R, Roche WR, Roberts JA, Holgate ST. Cellular events in the bronchii in mild asthma and after bronchoprovocation. Am Rev Respir Dis 1989; 139:806-17. 31. Roche WR, Beasley R, Williams JH, Holgate ST. Subepithelial fibrosis in the bronchii. Lancet 1989; 1:520-4. 32. Mead J, Takishima T, Leith D. Stress distribution in lungs: a model of pulmonary elasticity. J Appl Physiol 1970; 28:596-608. 33. Ding DJ, Martin JO, Macklem PT. Effects of lung volume on maximal methacholine-induced bronchoconstriction in normal humans. J Appl Physiol 1987; 62:1324-30.
ELIZABETH M. WAGNER, MARK C. LIU, GAIL G. WEINMANN, SOLBERT PERMUTT, and EUGENE R. BLEECKER
Introduction
Asthma is a clinical syndrome characterized by hyperresponsiveness of the tracheobronchial tree to a wide variety of stimuli (1). Airways hyperreactivity can be demonstrated in subjects with asymptomatic asthma even when baseline pulmonary function is within normal limits (2, 3). This heightened airways responsivenesshas been attributed to alterations in smooth muscle reactivity (4, 5), alterations in baseline airway caliber (6, 7), abnormalities in mucociliary and epithelial function (8, 9), altered neural control of airway tone (10, 11), and bronchial inflammation (12-16). Whatever the mechanism, it is generally agreed that the major symptoms during an asthmatic attack result from a narrowing of peripheral airways. There is indirect evidence that asymptomatic asthmatic subjects have physiologic abnormalities in peripheral airway function. Several pulmonary function tests including spirometry (17), frequency dependence of compliance (18), and density dependence of maximum expiratory flow (19, 20), have all demonstrated physiologic alterations in the small airways of asymptomatic asthmatic subjects. In addition, fluids obtained by bronchoalveolar lavage from asymptomatic asthmatic subjects show increased levels of inflammatory mediators compared to normal subjects (12-15). These findings suggest that there are physiologic and inflammatory abnormalities in subjects with asthma during asymptomatic periods. The present study was designed to quantify directly the pressureflow relationship of the peripheral lung in asthmatic subjects and relate this measure of resistance to their levels of airways reactivity. Toaccomplishthis we used a bronchoscope wedged in a subsegmental bronchus to measure the resistance of the peripherallung. Although the major airway 584
SUMMARY In obstructive lung disease, peripheral airways are a major site of pathologic abnormalIties. However, resistance to airflow In small airways In the periphery of the lung accounts for only a small fraction of total airway resistance. Consequently, abnormalities of small airway function may not be readily detected using routine pulmonary function testing. In the present study, resistance of the peripheral lung was examined directly In six normal subjects and nine mildly asthmatic subjects. There were no significant differences between the normal and asthmatic groups In pulmonary function assessed by spirometry (FEY1, FYC)and body plethysmography (specific airway conductance). Direct measurements of peripheral lung function were made using a flberoptlc bronchoscope wedged Into a subsegmental, right upper lobe bronchus. Using a double-lumen catheter Inserted Into the Instrument channel of the bronchoscope, pressures (PS)produced by three or more different levels of gas flow (V) (5% CO2 in air) between 50 and 500 mllmin were measured. All pressure measurements were made at a constant lung volume (i.e., functional residual capacity) confirmed by monitoring transpulmonary pressure with an esophageal balloon. The pressure-flow relationship in both normal and asthmatic subjects could be approximated by a straight line through the origin, demonstrating these airways to be relatively nondlstenslble. Peripheral lung resistance (Rp) was defined by PaN and averaged for three or more levels of flow. In contrast to spirometric results that showed no differences between the two groups, Rp was Increased more than sevenfold in asthmatic subjects (0.069 ± .017 em H20/mllmln) (mean ± SEM) compared to normal subjects (0.009 ± .002 em H20/ml/mln). Pretreatment of the asthmatic subjects (n 5) with aerosolized Isoproterenol did not decrease Rp to normal levels. These results demonstrate marked physiologic abnormalities in the peripheral lungs of asymptomatic asthmatic subjects and suggest that changes In the mechanical properties of these airways, although having a small Influence on overall pulmonary function, may contribute to Increased airway responsiveness In asthma.
,
=
AM REV RESPIR DIS 1990; 141:584-588
component contributing to this measure is uncertain, there is considerable evidence to suggest that the airway responses measured with the wedged bronchoscope are similar to those of small airways (21, 22). In addition, the measurement of collateral . or peripheral lung resistance has also been used extensively in animal models to assess responsiveness of the peripherallung to various agonists (21-23). The technique has also been used in humans , to assess changes in collateral ventilation with age (24). Methods Subjects Six normal and nine asymptomatic asthmatic subjects were classified by clinical history and airways reactivity to inhaled methacholine. All asthmatic subjects were atopic and had skin test reactivity to one or more com-
mon allergens. This study was approved by the institutional review board and informed consent was obtained from all subjects.
Pulmonary Function Measurements of pulmonary function were performed in all subjects before and immedi-
(Received in original form March 21, 1989 and in revised form August 25, 1989) 1 From the Department of Medicine and the Department of Environmental Health Sciences, Division of Pulmonary and Critical Care Medicine, The Johns Hopkins University, Baltimore, Maryland. 2 Correspondence and requests for reprints should be addressedto Elizabeth M. Wagner,Ph.D., Division of Pulmonary and Critical Care Medicine, Johns Hopkins University at Francis Scott Key Medical Center, 4940 Eastern Avenue, Baltimore, MD 21224.
COLLATERAL RESISTANCE IN NORMAL AND ASTHMATIC SUBJECTS
ately after bronchoscopy. Airway resistance and thoracic gas volume were measured in a constant-volume body plethysmograph (Warren E. Collins, Braintree, MA) by the method of Dubois and colleagues (25). Resistance was converted to its reciprocal, conductance, and was expressed as specific airway conductance (sGaw),the ratio of conductance to thoracic gas volume (26). Spirograms were performed in triplicate using a Stead-Wells spirometer (Warren E. Collins). Forced expiratory volume in I s (FEVI), 3 s (FEV3), forced expiratory volume (FVC), and mean forced expiratory flow during the middle half of the FVC (FEF 2s - 7s.,.) were calculated.
Methacholine Airways Reactivity Methacholine challenge was performed using standard techniques (2) as part of the subject screening process on a visit sometime prior to bronchoscopy. In brief, the subjects were instructed to take five maximal inspirations from functional residual capacity (FRC). Methacholine solutions (J. T. Baker, Phillipsburg, NJ) were nebulized for 0.6 s by triggering a breath-activated solenoid valve timing circuit (dosimeter). This apparatus delivered an average of 1.028 ± 0.222 ml (mean ± SEM) with every five breaths. After first inhaling a control saline solution, all subjects received increasing concentrations of methacholine (0.625, 1.25, 2.5, 5.0, 10.0, and 25.0 mg/ml). During the 5-min intervals between inhalations, spirometry was performed. When a 20070 or greater fall in FEV 1 occurred, the challenge was terminated. Dose-response curvesfor methacholine werethen constructed whereby the cumulative dose of agonist required to produce a 20070 fall in FEV 1 was calculated (PD 2o FEV I). Protocol After baselinepulmonary function testingand 30 min before bronchoscopy, subjects were pretreated with atropine (0.6mg) administered intramuscularly; they then inhaled an aerosol of 4070 lidocaine (5 cc) administered using a DeVilbiss nebulizer no. 646 (DeVilbiss, Somerset, PAl. An esophageal balloon was passed through one naris to measure transpulmonary pressure in order to confirm that pressure measurements were made at a constant lung volume.A fiberoptic bronchoscope (5.5 outer diameter) was advanced into the lung and 6 to 12cc of lidocaine (2070) was administered through the bronchoscope directly to the airways to provide local anesthesia. With subjects in the supine position, the bronchoscope was wedged in a nondependent subsegmental bronchus of the right upper lobe. A double-lumen catheter (5FR) was inserted through the instrument channel of the bronchoscope. Air flow (5070 CO 2 in air) waspassed through a mass flow controller (Sierra, Carmel Valley, CAl through one lumen of the catheter. Back flow was prevented by a snugly fitting cap on the instrument channel port through which the catheter had been passed. Pressure at the tip of the bronchoscope (P a)
585 TABLE 1 PULMONARY FUNCTION OF NORMAL AND ASTHMATIC SUBJECTS Subjects
Number of subjects FEVl ' % predicted FVC, % predicted FEF25-75, % predicted FEV3/FVC0/o Rp (cm H20/ml/min) *
Significant at p
Normal
Asthmatic
6 ± ± ± ± ±
9
103.2 101.3 93.5 98.0 0.009
2.9 1.6 4.4 1.0 0.002
Significance
95.0 ± 4.2 95.4 ± 3.8 n.4 ± 6.6 97.0 ± 1.0 0.069 ± 0.017
NS, P = NS, P = * P = NS, P = * P =
0.088 0.124 0.047 0.226 0.013
< 0.05.
was measured through the second lumen of the catheter. The resistance of the peripheral lung distal to the wedged bronchoscope was assessed by making incremental increases in flow and recording the resultant pressure. At each flow, . subjects wereinstructed to breathhold at FRC and the steady-state pressure at the tip of the bronchoscope was measured. All measurements were made at FRC (confirmed by a constant transpulmonary pressure), because changes in lung volume have been demonstrated to affect peripheral lung resistance in man (27). Pressure measurements in the wedged segment were obtained in triplicate at each level of flow starting with a flow of 50 or 100 ml/min. Pressures produced by at least three levelsof flowweremeasured in each subject. To determine whether the pressure-flow relationship could be altered by pretreatment with a l3-sympathomimetic agent, five additional studies wereperformed in subjects with asthma several weeks after the initial study. These subjects inhaled 0.5 ml of 5070 aerosolized metaproterenol sulfate (Boehringer Ingelheim, Ridgefield, CT) before bronchoscopy. After the bronchoscope was wedged in a right upper lobe subsegmental bronchus, an aerosol of 2 mg/ml isoproterenol (Winthrop, New York, NY) was administered for 2 min at a flow of 200 ml/min directly into the peripheral segment. In pilot studies we determined that this dose of isoproterenol, administered to the peripheral airway segment, was sufficient to reverse histamine-induced increases in peripheral lung resistance. Pressure-flow data were obtained using the protocol described above.
Data Analysis The peripheral lung resistance (Rp) was defined as the Pa/Vaveraged for three or more levels of flow in each subject. Student's t test was used to determine the differences in pulmonary function between normal subjects and those with asthma (one-tailed), before and after bronchoscopy in each group (twotailed), and before and after isoproterenol pretreatment in subjects with asthma (twotailed). Spearman's rank correlation coefficient was used to assess the relationship between peripheral lung conductance (I/Rp) and methacholine sensitivity (28). For all compar-
isons, a p value ~ 0.05 was considered significant. All values are presented as the mean ± the standard error.
Results
Baseline pulmonary function assessed by spirometry showed no significant differences between normal subjects and those with asthma in FEV 1 OJo predicted, FVC OJo predicted or FEV 3/FVC OJo. However, FEF2 S - 7S '1., which is thought to reflect peripheral airway function, was significantly lower in subjects with asthma than in normal subjects (p = 0.047) (table 1). All subjects with asthma had increased levels of reactivity to methacholine, and the PD2 0 FEV 1 was 0.89 ± 0.15log units compared to normal subjects who either did not respond or responded at greater than 2.18 log units. Absolute values for pulmonary function in the two groups before bronchoscopy are presented in table 2. Bronchoscopy did not produce differential effects in the two groups. FEV., FEV 1 OJo, FEF2S - 7 S '1., and FEV 3 were unaltered by the procedure. However, a small but significant decrease in FVC [normal subjects: 5.22 ± 0.37 (p = .043); subjects with asthma: 4.38 ± 0.34 (p = 0.041)] and a significant increase in sGaw [normal and asthmatic subjects: 0.26 ± 0.04 (p = 0.024 and p = 0.020, respectively)] were measured in normal and asthmatic subjects after bronchoscopy. TABLE 2 PULMONARY FUNCTION BEFORE BRONCHOSCOPY Subjects Normal (n = 6) FEV1 FVC FEV1% FEF25- 75% FEV3 sGaw
4.45 ± 0.25 5.31 ± 0.37 84 ± 2 4.52 ± 0.20 5.20 ± 0.38 0.20 ± 0.02
Asthmatic (n = 9)
3.71 4.51 82 3.58 4.38 0.20
± 0.28 ± 0.35
± ± ± ±
2 0.35 0.35 0.02
586
WAGNER, LlU, WEINMANN, PERMUTT, AND BLEECKER
25
15
oLJ~~~~ o
100
200
300
400
500
V (ml/mln)
M
Fig. 1. Pressure (Pa)-tlow relationships of six normal subjects (closedsquares)and nine subjects with asymptomatic asthma (open squares).
Despite the general lack of baseline differences in pulmonary function of these normal subjects and those with mild asthma, there was a striking difference in peripheral lung resistance. The individual pressure-flow relationships for all subjects are presented in figure 1. It is of interest to note that the pressureflow curves appear relatively linear and seem to extrapolate through the origin. This observation suggests that the airways being studied are relatively nondistensible over the pressure range studied. This observation provides the rationale for our method of calculating Rp from the average of values measured at three or more different flows. The measurements of PB were acceptable only if esophageal pressure varied within a range of 3.5 em H 20 or less (SE ranged from 0.1 to 0.4 em H 20). The average Rp was 0.009 ± 0.002 em H 20/ml/min in the normal subjects compared to 0.069 ± 0.017 em H 2 0 / ml/min (p = 0.013) in asthmatic subjects. The relationship between Rp and whole lung sensitivity to methacholine in the nine asthmatic subjects is presented in figure 2. A positive correlation exists between peripheral airway conductance (I/Rp) and the methacholine sensitivity PD 20 FEV 1 (r = 0.383, NS) in the nine
subjects with asthma. However, when the one outlyer is excluded from the analysis, a significant correlation exists between peripheral lung conductance and methacholine PD 20 FEV 1 (r = 0.81, p < 0.05). Results of pressure-flow studies performed in the five asthmatic subjects who were studied after pretreatment with a l3-sympathomimetic are presented in figure 3. For each of the five subjects, the baseline pressure-flow relationship is compared to the pressure-flow data obtained after administration of aerosolized isoproterenol. Average baseline Rp for these five subjects with asthma was 0.086 ± 0.027 cm H 20/ml/min compared to 0.062 ± 0.019 after isoproterenol pretreatment (p = 0.392). Thus, pretreatment with this l3-sympathomimetic agent resulted in no change in Rp. Discussion The results of this study demonstrate a striking difference in peripheral lung resistance of subjects with asthma compared to normal subjects. More than a sevenfold increase in resistance was measured in these subjects with mild asthma compared to normal subjects, despite the fact that, in general, pulmonary function did not differ in the two groups. Although it is unclear what the major anatomic
component contributing to this peripherallung resistance measurement is, collateral channels (bronchoalveolar, interbronchiolar, and interalveolar pathways) in numerous animal studies have been shown to be similar to small airways, at least in terms of their pharmacologic responses (21-23). We believe the measure of Rp is dominated by the resistance of collateral channels although the small airways in series with these channels may also contribute. This contribution is likely greater in the asthmatics because in these subjects there was a significant reduction in FEF 2S - 7S .,. , a test presumed to measure small airway function. The observation that there was a correlation between whole lung methacholine reactivity and peripheral lung conductance suggests that either small airways responsive to methacholine challenge are being measured with Rp or that collateral channels serve as a marker of how small airways would respond. Based on the observations of the present study, small peripheral airways or collateral channels may playa functional role in the pathogenesis of asthma. Specifically, with regard to collateral channels, eliminating their functional role by significantly increasing collateral resistance would tend to increase air trapping and residual volume. These changes would exacerbate
• 00
Post-isoproterenol Pre-isoproterenol 25
0 0
100
200
300
400
15
500
V (ml/mln)
PB (em H2O)
V
15
PB (em H2O) 0 0
100
200
300
400
500
V (mllmln) 0 50
0
100
40
1/Rp
30
20
300
400
25
500
f
15
(ml/cm H20-min)
200
V (ml/mln)
..
PB (em H2O)
15
PB(em Hp>
o +----.---~----.--___, o Methacholine PD 2 0 (log)
0
0 0
Fig. 2. Correlation between methacholine PD20 (provocative dose that causes a 20% decrease in FEV1) and Peripheral lung conductance (1/Rp) of the nine subjects with asthma.
100
200
300
400
500
V (ml/mln) Fig. 3. Pressure (Pa)-tlow
('i) relationships of five SUbjects with
squares) isoproterenol pretreatment.
0
100
200
300
400
500
V (mllmln) asthma before (open squares) and after (closed
587
COLLATERAL RESISTANCE IN NORMAL AND ASTHMATIC SUBJECTS
the effects of small airway closure, an abnormality thought to accompany an acute asthmatic attack (29). Regardless of which air channel predominates in the measure of peripheral lung resistance, our results suggest that an important difference between the normal and asthmatic subjects is structural changes in peripheral airways. This conclusion is based on the observations made in the asthmatic subjects after pretreatment with isoproterenol. If the difference in resistance between normal subjects and subjects with asthma resulted from an increase in airway smooth muscle tone, then resistance measurements in subjects with asthma after isoproterenol pretreatment should have been substantially altered and should have approached the values obtained in the normal subjects. These results were not found. Rather, only minimal decreases in resistance were measured, reflecting a reduction in baseline smooth muscle tone in airways with substantially greater baseline resistance than that of normal subjects. Although it is possible that the dose of isoproterenol used was insufficient, we believe this explanation is unlikely because this local treatment reverses histamine-induced airway constriction in the lung periphery (unpublished observations). The results also exclude the possibility that the bronchoscopic procedure was acting as a smooth muscle stimulus to increase resistance in the group with asthma. This is supported by the fact that the postbronchoscopy pulmonary function was not differentially affected in the two groups. We can only speculate as to the physical changes that take place in the asthmatic airway. The measured increase in peripheral lung resistancecould be caused either by airway narrowing or by a decrease in the actual number of functional airways, perhaps reflecting airway closure. Our results cannot distinguish between these two mechanisms. However, several studies (12-15) demonstrate increased levels of inflammatory mediators in the bronchoalveolar lavage fluid of subjects with asthma compared to normal subjects. Roche and coworkers (30, 31)have recently reported that bronchial biopsies performed in subjectswith asymptomatic asthma not only show evidence of cellular inflammation with marked increases in eosinophils, but also structural abnormalities that are consistent with Collagen deposition in the airway wall. In addition, inflammatory events may produce mucosal swellingand edema that could result in decreased airway internal
diameters and increases in peripheral lung resistance. Thus, inflammation and associated structural abnormalities may be responsible for the functional changes reported in this study. Another significant finding in this study was the correlation between methacholine sensitivity and peripheral lung conductance in subjects with asthma. For the most part, the usual mechanisms that have been proposed to explain airways hyperreactivity have emphasized alterations in autonomic neural control of airway tone, changes in intrinsic bronchial smooth muscle function, and alterations in bronchial epithelial integrity and permeability. Although each of these potential mechanisms may play a role in the development of airways hyperresponsiveness, we believe that bronchial inflammation may be a common factor responsible for the development of airways hyperresponsiveness in asthma and may explain the relationship between methacholine sensitivity and peripheral lung conductance observed in this study. Bronchial inflammation, by causing mucus hypersecretion and mucosal edema, may cause a decrease in the baseline radius of small airways. Products of inflammation may be expected to extend toward the luminal surface reducing the size of peripheral airways. However, abnormalities associated with the development of bronchial inflammation should not be limited to the internal surface of the airways. Outward extension of these biochemical and cellular changes in the lung tissue surrounding the small airways would produce additional effects on airway size by altering the forces of interdependence (32) between airways and lung parenchyma. Tothe extent that these tethering forces are significant at FRe, any reduction will tend to decrease the baseline diameter of the airways. Changes in baseline diameter of small airways have been reported to result in substantial differences in the manifestation of agonist-induced constriction (33). Thus, if inflammatory changes in subjects with asthma result in a generalized decrease in the diameter of small airways, the response to a bronchoconstrictor agonist will be exaggerated and detected by changes in spirometry. In summary, the results of this study demonstrate marked physiologic abnormalities in the peripheral airways of subjects with mild, asymptomatic asthma. Although the sevenfold increase in the resistance to flow through the peripheral airways observed in the subjects with
asthma may not be sufficient to cause changes in pulmonary function, the physical changes within the small airways may be related to the presence of airways hyperractivity. Acknowledgment We thank Professor Wayne Mitzner for his insightful comments in preparing this manuscript, and Mary Johns, Elizabeth Rechsteiner, and Becky Stealey for their superb technical assistance. References 1. American Thoracic Society Board of Directors. Standards for the diagnosis and care of patients with chronic obstructive pulmonary disease(COPD) and asthma. Am Rev Respir Dis 1987; 136:225-44. 2. Chatham M, Bleecker ER, Norman P, Smith PL, Mason P. A screening test for airways reactivity. Chest 1982; 82:15-8. 3. YanK, Salome CM, Woolcock AJ. Prevalence and nature of bronchial hyperresponsiveness in subjects with chronic obstructive pulmonary disease. Am Rev Respir Dis 1985; 132:25-9. 4. Antonissen LA, Mitchell RW, Kroeger EA, Kepron W, Tse KS, Stephens NL. Mechanical alterations of airway smooth muscle in a canine asthmatic model. J Appl Physiol 1979; 46:681-7. 5. Vincenc KS, Black JL, Yan K, Armour CL, Donnelly PD, Woolcock AJ. Comparison of in vivo and in vitro responses to histamine in human airways. Am Rev Respir Dis 1983; 128:875-9. 6. Benson, MK. Bronchial hyperreactivity. Br J Dis Chest 1975; 69:227-39. 7. James AL, Pare PD, Hogg JC. The mechanics of airway narrowing in asthma. Am Rev Respir Dis 1989; 139:242-6. 8. Nadel JA, Holtzman MJ. Regulation of airway responsiveness and secretion: role of inflammation. In: Kay AB, Austin KF, Lichtenstein LM, eds. Asthma: physiology, immunopharmacology and treatment. Third international symposium. London: Academic Press, Inc., 1984; 129-56. 9. Laitinen LA, Heino M, Laitinen A, Kava T, Haahtela T. Damage of the airway epithelium and bronchial reactivity in patients with asthma. Am Rev Respir Dis 1985; 131:599-606. 10. Bleecker ER. Cholinergic and neurogenic mechanisms in obstructive airways disease, Am J Med 1986; 81(Suppl:93-102). 11. Nadel JA. Autonomic control of airwaysmooth muscle and airway secretions. In: Nadel JA, ed. Physiology and pharmacology of the airways. Vol 15. Lung biology in health and disease. New York: Marcel Dekker, Inc., 1980; 117-26. 12. Liu MC, Bleecker ER, Niv Y, et ale Asthma is characterized by elevated levelsof bronchospastic prostaglandins and histamine in the airway fluids obtained by bronchoalveolar lavage (abstract). Fed Proc 1988; 47:AI698. 13. Rankin JA, Kaliner M, Reynolds HY. Histamine levelsin bronchoalveolar lavage from patients with asthma, sarcoidosis and idiopathic pulmonary fibrosis. J Allergy Clin Immunol 1987; 79:371-7. 14. Casale TB, Wood D, Richerson HB, et al. Elevated bronchoalveolar lavagefluid histamine levels in allergic asthmatics are associated with methacholine bronchial hyperresponsiveness. J Clin Invest 1987; 79:1197-203. 15. Christiansen SC, Proud D, Cochrane CG. Detection of tissue kallikrein in the bronchoalveolar lavage fluid of asthmatic subjects. J Clin Invest 1987; 79:188-97.
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