Abstract
Airway wall thickening has been assumed to cause airway hyperresponsiveness, but a protective effect against airway narrowing has also been suggested. We investigated the relationship between airway wall thickness as assessed by helical computed tomography and two components of airway responsiveness, airway sensitivity and reactivity, in patients with stable asthma with (n = 23) and without (n = 22) inhaled steroid treatment. A cross-section of the apical bronchus of the right upper lobe was obtained. Airway wall area corrected by body surface area was measured as an index of wall thickness. Airway sensitivity and reactivity were measured by continuous inhalation of methacholine, on the basis of the methacholine respiratory resistance dose–response curve. The eosinophil count in sputum was determined in 16 patients [steroid (+) group] and 14 patients [steroid (−) group]. In both groups of patients, airway sensitivity was not related to airway reactivity. Airway sensitivity was related to eosinophil count [r = 0.57 in the steroid (+) group and r = 0.49 in the steroid (−) group], but not to airway wall thickness. In contrast, airway reactivity negatively correlated with airway wall thickness [r = −0.56 in the steroid (+) group and r = −0.55 in the steroid (−) group] but not with eosinophil count. Our results suggest that airway wall thickening attenuates airway reactivity in patients with asthma. These findings may have important implications in pathophysiology and in the treatment of airway remodeling.
Postmortem studies have shown thickened airway walls in patients dying of acute exacerbations of asthma (1–4). Wall thickening results from inflammatory changes, such as edema and inflammatory cell infiltration, and from structural changes, such as mucous gland hyperplasia, reticular basement membrane thickening, vascular proliferation, and airway smooth muscle (ASM) hypertrophy and hyperplasia. These structural changes are features of airway remodeling associated with chronic inflammation (5–8). Mathematical models indicate that modest airway wall thickening results in disproportionally severe airway narrowing due to ASM shortening (1, 9–11). Airway wall thickening may thus lead to airway hyperresponsiveness (AHR), an essential feature of asthma (12, 13).
Several studies provide evidence against this widely accepted theory. Okazawa and coworkers (14) have investigated the relationship between ASM shortening induced by carbachol administered through the main bronchus and airway dimensions, assessed by examining all measurable airways in excised canine lungs. They have found that ASM of airways with thicker walls was less likely to shorten. Physiological studies indicate that airways of patients with asthma are less distensible during forced inspiration (15, 16) or less collapsible during forced expiration (17) than those of normal subjects. The authors of these studies attributed such phenomena to increased stiffness of the asthmatic airway wall, possibly resulting from increased wall thickness. However, airway wall thickness was not assessed in these studies.
Computed tomography (CT) has been used to measure airway wall dimensions in patients with asthma (18–23). Patients have thicker airways on CT scans than do healthy control subjects (19–22), and the degree of thickening is related to the severity of disease (20, 21, 23) and airflow obstruction (21, 22). However, the relationship between airway wall thickness and inducible airway narrowing or AHR has rarely been investigated. Boulet and coworkers (18) and Little and coworkers (23) compared airway wall thickness, as assessed by CT, with AHR in patients with asthma. AHR was measured as the provocative concentration of methacholine (18) or histamine (23) that produced a 20% fall in FEV1 (PC20-FEV1). One study (18) demonstrated a positive correlation of AHR with airway wall thickness in a subgroup of patients, whereas the other (23) found no significant relationship.
We addressed these inconsistent findings by studying the relationship between airway wall thickness, assessed by our validated method for helical CT (21), and AHR, evaluated by continuous inhalation of methacholine (24), in patients with stable asthma. As compared with using PC20-FEV1 as the sole index of AHR, the latter method can separately evaluate the two distinct and possibly independent components of AHR: airway sensitivity, and airway reactivity or exaggerated airway narrowing (12, 13). Airway inflammation as assessed by induced sputum was also analyzed. Some of the results of these studies have been previously reported in the form of an abstract (25).
We studied 2 groups of adult patients with stable asthma diagnosed according to American Thoracic Society criteria (26): 23 patients receiving inhaled beclomethasone dipropionate [steroid (+) group], and 22 steroid-naive patients treated with inhaled short-acting β2-agonists alone [steroid (–) group]. No subject had ever smoked cigarettes, or had respiratory infections or exacerbation of asthma during the 8 weeks before enrollment.
The study was approved by the Ethics Committee of Kyoto University (Kyoto, Japan). Written informed consent was obtained from all participants.
Helical CT scanning was performed at full inspiration as described previously (21, 27). Briefly, a cross-section of the apical bronchus of the right upper lobe at its origin was examined to determine absolute airway thickness (T), airway wall area (WA), and percentage wall area (WA%, [WA/outer area of the bronchus] × 100). WA and T were normalized on the basis of body surface area (BSA). The CT data were transferred to a Power PC personal computer and automatically analyzed (27).
AHR was examined by continuous methacholine inhalation with simultaneous measurement of respiratory resistance (Rrs, cm H2O/L/second) (Astograph; Chest, Tokyo, Japan) (Figure 1)
Figure 1. Scheme of the continuous methacholine inhalation method with simultaneous measurement of respiratory resistance (Rrs) (24). After recording the baseline Rrs during inhalation of physiologic saline, methacholine was inhaled sequentially, beginning with the lowest concentration, at 1-minute intervals. Dmin, the cumulative dose of inhaled methacholine at the inflection point at which Rrs begins to increase, was used as the index of airway sensitivity. Inhalation of methacholine was continued until Rrs reached twice the baseline value. The slope of the methacholine respiratory resistance dose–response curve (SRrs) was used as the measure of airway reactivity. (See online supplement for details.)
[More] [Minimize]As separate studies, we examined the reproducibility of the method in eight patients by measuring Dmin and SRrs repeatedly at an interval of 7 days, and compared the doubling of Rrs with the change in FEV1 in seven patients.
After premedication with inhaled salbutamol, patients inhaled hypertonic (3%) saline solution for 15 minutes from an ultrasonic nebulizer, and sputum was induced (28). Adequate plugs of sputum were treated with 0.1% dithiothreitol (Sputasol; Oxoid, Hampshire, UK) followed by Dulbecco's phosphate-buffered saline. Eosinophil count (number per gram of sputum) was determined on centrifuged preparations stained with May–Grünwald–Giemsa.
Data were expressed as means ± SD or median (range) and analyzed with the StatView 4.5 program (SAS Institute, Cary, NC). An unpaired t test, the Mann–Whitney U test, or the χ2 test was used to compare groups. Spearman's rank correlation test or Pearson's correlation test was used to analyze relations between variables. p Values less than 0.05 were considered significant.
Of the eight patients with asthma examined, reproducibility of Dmin and Rrs measurements was good, as shown in Figure E1 (see the online supplement).
In seven patients, FEV1 was measured before challenge and immediately after the Rrs had doubled postchallenge. The change in FEV1 was –19.3 ± 6.9% (median, –19.7%; range, –8.0 to –31.4%), and the decrease in FEV1 exceeded 16% in six of seven patients. The doubling of baseline Rrs thus caused a relevant degree of airway narrowing in most of the patients.
The characteristics of the two groups of patients with asthma are summarized in Table 1
Inhaled Steroid (+) | Inhaled Steroid (–) | p Value | |
|---|---|---|---|
| n | 23 | 22 | |
| Age, yr | 59 ± 9 | 52 ± 18 | 0.11 |
| Sex, male/female | 12/11 | 6/16 | 0.088 |
| BSA, m2 | 1.6 ± 0.1 | 1.6 ± 0.2 | 0.58 |
| Disease duration, yr | 10 ± 14 | 4 ± 6 | 0.043 |
| Atopy*/nonatopy | 12/11 | 11/11 | 0.88 |
| Dose of BDP, μg/d | 800 (400–1,600) | 0 (0–0) | |
| FEV1, %pred | 92 ± 4 | 101 ± 16 | 0.095 |
| FEV1/FVC, % | 71 ± 10 | 78 ± 9 | 0.024 |
| FEV1 reversibility, %† | 8.0 ± 6.8 | 3.9 ± 4.3 | 0.041 |
| Sputum eosinophils, number/g‡ | 1.6 (0.0–35.2) | 0.2 (0.03–2.6) | 0.057 |
| Baseline Rrs, cm H2O/L/sec | 4.4 ± 1.8 | 3.9 ± 1.2 | 0.34 |
| Log Dmin, units | −0.17 ± 0.19 | 0.16 ± 0.13 | 0.16 |
| SRrs, cm H2O/Ll/sec/min) | 1.4 ± 0.9 | 1.8 ± 1.1 | 0.29 |
| WA/BSA, mm2/m2 | 21.2 ± 7.2 | 21.1 ± 7.6 | 0.98 |
| WA%, % | 64.5 ± 6.3 | 66.1 ± 7.0 | 0.45 |
| T/(BSA)1/2, mm/m | 1.3 ± 0.2 | 1.3 ± 0.3 | 0.97 |
In both groups of patients with asthma, log Dmin did not relate to SRrs (Figure 2)
Figure 2. Relationship between airway sensitivity and airway reactivity. Log Dmin was not significantly related to SRrs in either the steroid (+) group or the steroid (–) asthmatic group.
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Figure 3. Relationship between airway sensitivity and airway inflammation. Log Dmin was negatively related to sputum eosinophil count in both the steroid (+) and steroid (–) asthmatic groups.
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Figure 4. Relationship between airway sensitivity and airway wall thickness. Log Dmin was not significantly related to WA/BSA in both the steroid (+) and steroid (–) asthmatic groups.
[More] [Minimize]In contrast, irrespective of inhaled steroid treatment, SRrs was unrelated to the number of eosinophils in sputum (Figure 5)
Figure 5. Relationship between airway reactivity and airway inflammation. SRrs was unrelated to sputum eosinophil count in both steroid (+) and steroid (–) asthmatic groups.
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Figure 6. Relationship between airway reactivity and airway wall thickness. SRrs was negatively correlated with WA/BSA in both steroid (+) and steroid (–) asthmatic groups.
[More] [Minimize]In the steroid (+) group, the duration of inhaled steroid treatment was 3.1 years (0.7–5.0 years), and the cumulative dose of treatment (sum of [dose × duration]) was 1,950 (267–5,400) ([μg/day] × years). These indices, as well as the final dose of treatment as shown in Table 1, did not correlate with FEV1, log Dmin, SRrs, sputum eosinophil count, WA/BSA, WA%, or T/(BSA)1/2 in these patients (data not shown). Also, in both groups of patients with asthma, the sputum eosinophil count did not correlate with WA/BSA [r = 0.09, p = 0.74 in the steroid (+) group and r = –0.21, p = 0.45 in the steroid (–) group], WA% [r = 0.15, p = 0.55 in the steroid (+) group and r = 0.12, p = 0.66 in the steroid (–) group], or T/(BSA)1/2 [r = 0.08, p = 0.76 in the steroid (+) group and r = –0.14, p = 0.61 in the steroid (–) group].
This is the first study, to our knowledge, to investigate the relationship between airway wall thickness, as assessed by CT, and airway sensitivity and airway reactivity, measured separately in patients with asthma. We found consistent results in patients treated with inhaled steroid and those treated with β2-agonists alone. Airway reactivity negatively correlated with airway wall thickness. Airway sensitivity was related to the intensity of airway inflammation as assessed by sputum eosinophil count, but not to any index of airway wall thickness. It is suggested that airway wall thickening protects against excessive airway narrowing in asthma.
Our results may be against a widely accepted theory that airway wall thickening leads to AHR or excessive airway narrowing in patients with asthma (1, 9–11). Computational analyses of postmortem lungs by Pare, Hogg, and colleagues have indicated that a modest degree of airway wall thickening, associated with only a mild increase in baseline airway resistance, can exaggerate the airway closure induced by ASM shortening (1, 9–11). These elegant studies have provided the theoretical basis for the common notion mentioned above. However, when the elastic property of airway wall constituents is considered, this may not always be the case, as pointed out by Pare and coworkers (11). Evidence has been emerging to support this concept, as discussed below.
Airway sensitivity and airway reactivity are the two major components of AHR (12, 13). Several groups of investigators have examined these variables in patients with asthma and normal subjects. Orehek and coworkers (29) reported that the difference in airway reactivity between patients with asthma and normal subjects was greater than the difference in airway sensitivity. Airway sensitivity was measured as the dose of carbachol causing a 25% decrease in specific airway conductance, and airway reactivity was derived from the slope of the dose–response curve. There was no correlation between airway sensitivity and airway reactivity. They concluded that different mechanisms underlie the sensitivity and reactivity of the bronchial tree and considered hyperreactivity the main feature of the asthmatic response (29). A number of studies of the complete dose–response curves have shown a direct relationship between airway sensitivity, reactivity, and maximal airway narrowing (30, 31), but we found no correlation between airway sensitivity and airway reactivity, consistent with the results of Orehek and coworkers (29). A 1-year follow-up study of patients with asthma by Beaupre and Malo (32) showed that airway sensitivity measured as PC20-FEV1 to histamine paralleled clinical status, whereas airway reactivity assessed as the slope of the dose–response curve showed little change. They concluded that airway sensitivity (PC20-FEV1) is a more accurate index of the clinical status of patients with asthma than is airway reactivity (32). These results provide additional evidence that the origin(s) of airway sensitivity and airway reactivity may differ and suggest that airway reactivity may have more fixed or chronically determined components than airway sensitivity.
In 1989, Sterk and Bel (12) reviewed the mechanisms of airway sensitivity and airway reactivity or excessive airway narrowing. They proposed that airway sensitivity is determined by the strength of the stimulus that triggers the airways to narrow (“prejunctional” mechanisms). Determinants of airway sensitivity include epithelial damage and malfunction, neural control, inflammatory cell number/activity, interactions among these factors, and altered metabolism or absorption of inflammatory mediators. Our results and those of previous studies (33) support the view of Sterk and Bel (12) that the degree of cellular inflammation, as measured here by the sputum eosinophil count, is related to airway sensitivity, but conflict with the results of others (34). Epithelial damage and increased levels of several inflammatory mediators have also been associated with airway hypersensitivity. As for airway reactivity, Sterk and Bel regarded this variable to represent the responsiveness of the target organ (i.e., airways) to the stimulus given (“postjunctional” mechanisms) (12). Determinants of airway reactivity include ASM contractility, viscous and elastic loads on ASM shortening, swelling of the airway wall, and intralumenal exudate and secretions. ASM contractility, airway wall swelling, and intralumenal exudate and secretions were considered to heighten airway reactivity. In contrast, Sterk and Bel proposed that viscous and elastic loads on ASM shortening are responsible for the plateau of the dose–response curve in vivo. Only scant evidence was available at that time to indicate that individual components of airway structure, or the net effect of alterations in such components, can protect against ASM shortening in vivo. However, the involvement of viscous and elastic loads, as well as ASM contractility, might be in accordance with the possible “fixed” nature of airway reactivity (32).
Several observations suggest a protective effect of airway wall thickening against ASM shortening or airway narrowing. In animal models, airway wall thickening (14), or deposition of extracellular matrix such as collagen or fibronectin in the airway wall after repeated allergen exposure (35), was associated with attenuated ASM shortening. Such shortening was ascribed to stiffening of the airways. Postmortem studies show that the ASM layer of patients with asthma is thickened (1–4). This may result in AHR, if the contractility of ASM cells remains constant (11, 36). However, thickening of the ASM layer is partly attributed to increased deposition of extracellular matrix around individual ASM cells (4), which may act against ASM shortening. In addition, ASM cells may differentiate to a less contractile phenotype during proliferation (11, 36, 37). Physiological studies also indicate that the airways of patients with asthma may be less collapsible or less distensible than those of normal subjects. Brackel and coworkers (17) have investigated the mechanical properties of the airways with the use of an esophageal balloon and a Pitot static probe, positioned at several locations in the central airways. They obtained area-versus-transmural pressure curves during forced expiration. Airway compliance and specific airway compliance were significantly lower in patients with long-standing asthma than in healthy subjects. Brackel and coworkers concluded that airway remodeling may have resulted in stiffer dynamic elastic properties of the airway wall (17), although airway remodeling was not assessed by bronchial biopsy or CT. The change in anatomic dead space (ΔVd) with lung volume has been proposed as a noninvasive index of airway distensibility or stiffness (ΔVd as corrected by the change of end-inspiratory volume [ml/L]). Wilson and coworkers (15) have shown that patients with mild asthma have less distensible airways, with a ΔVd of 27.0 ml/L during inspiration as compared with 37.3 ml/L for control subjects (p = 0.014). Ward and coworkers (16) have attributed this phenomenon to reticular basement membrane thickening by showing an inverse correlation between ΔVd and reticular basement membrane thickness (r = –0.37, p = 0.03) in patients with mild to moderate asthma. Moreover, Milanese and coworkers (38) have found a negative correlation between reticular basement membrane thickness and AHR, as measured as the provocative dose of methacholine that produced a 20% fall in FEV1 (PD20-FEV1), in 11 patients with asthma (r = 0.77, p < 0.01), suggesting an airway stiffening consequent to this pathologic change. Available evidence thus indicates that the airways of patients with asthma may be stiffer than those of healthy subjects. Increased stiffness may result from mechanisms such as deposition of extracellular matrix in the airway wall, which may lead to airway wall thickening (39).
Our study demonstrates, for the first time, an inverse association between airway wall thickening and airway reactivity in patients with asthma. This suggests a protective role of airway wall thickening against excessive airway narrowing in human subjects in vivo. The relative roles of different connective tissue proteins are unclear, with respect to their responses to compressive stress (11). Moreover, elastosis or fragmentation of elastic fibers (40) or degenerative changes of cartilage (41) in remodeled asthmatic airways may decrease airway elasticity and thus increase deformability. Because CT cannot discern histologic details of the airway wall and we did not pathologically examine the airways, we are unable to discuss the contribution of individual components of airway remodeling to airway elasticity. We can at least state, however, that the “net” effect of airway wall thickening may be protection against ASM shortening. This assumption has important implications regarding the pathophysiologic role, and treatment, of airway wall thickening or remodeling. Airway wall thickening is related to chronic airflow obstruction (21, 22) and may progress with increased duration of disease (4, 21). Thickening of the airway wall might be associated with persistent airflow obstruction but infrequent exacerbations of the disease (5), considered a possible characteristic of long-standing asthma (42). Long-term treatment with inhaled steroid has been widely recommended for patients with persistent asthma. Such treatment may reduce airway wall thickness, as it does in animal models (43), or reticular basement membrane thickness in patients with asthma (44), which might possibly lead to airway hyperreactivity (5). In our patients treated with inhaled steroid, the total duration, final dose, and cumulative dose of treatment did not correlate with the indices of airway sensitivity, airway reactivity, or airway wall thickness. In addition, these patients did not differ from steroid-naive patients with regard to airway sensitivity, airway reactivity, or airway wall thickness. It may be difficult, however, to argue the effect of inhaled steroid on these indices by such cross-sectional analysis or comparison of two patient groups that differ in disease duration or pulmonary function.
Boulet and coworkers (18) showed a positive correlation between airway wall thickness, measured by CT, and AHR, based on PC20-FEV1. This correlation was observed in patients with asthma with fixed airflow obstruction (n = 13), but not in those without such obstruction (n = 11) or in healthy control subjects (n = 10). More recently, Little and coworkers (23) performed a similar study in 49 patients with optimally controlled asthma but failed to demonstrate a relationship between airway wall thickness and AHR. Both studies used PC20-FEV1 to estimate AHR, considered a marker of airway sensitivity, that may also reflect airway reactivity (13). In addition, the value of the study by Boulet and coworkers (18) may be limited by the small number of subjects. Unlike these previous studies, we separately evaluated airway sensitivity and airway reactivity.
We conclude that airway wall thickening may protect against excessive airway narrowing in patients with asthma. Airway sensitivity may be related to the degree of airway inflammation but not to airway wall thickness. These results have important implications regarding the pathophysiologic role, and therapeutic modulation, of airway wall thickening in asthma. Future studies should be conducted with large numbers of patients, to investigate the relationship between CT findings and clinical characteristics such as the frequency of exacerbations and variability in peak expiratory flow, and the long-term effect of antiinflammatory treatment, such as inhaled steroid, on CT findings, functional parameters, and their interrelations.
The authors are grateful to Ryuzo Tanaka, Miho Morimoto, Noboru Narai, and Hiroyuki Akazawa for radiological and technical support.
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