Greater parallel heterogeneity of airway narrowing and airway closure in asthma measured by high-resolution CT.

BACKGROUND: Heterogeneous airway narrowing and closure are characteristics of asthma. However, they have never been quantified by direct measurements of parallel sister airways obtained from image data, and the anatomical basis of these processes remains unknown. METHODS: Seven normal and nine asthmatic subjects underwent high-resolution CT, before and after methacholine challenge. Mean lumen areas of the entire airways were measured in 28 and 24 parallel sister airway pairs (a pair of airways arising from the same bifurcation) respectively (range 1.0-8.7 mm diameter). Heterogeneous narrowing was defined as the median difference in percentage narrowing between parallel sister airways. Forced oscillatory respiratory resistance (Rrs) and spirometry were measured before and after methacholine challenge conducted while supine. RESULTS: The airways of asthmatics were smaller at baseline, and following bronchoconstriction there were similar decreases in FEV1, increases in Rrs and mean narrowing of airways for asthmatic and non-asthmatic groups. Non-asthmatics required higher doses of methacholine than asthmatics to achieve the same changes. However, parallel heterogeneity (median (IQR) 33% (27-53%) vs 11% (9-18%), p<0.001) and airway closure (24.1% and 7.7%, p=0.001, χ(2)) were greater in asthmatics versus non-asthmatics. CONCLUSION: We found clear evidence of differences in airway behaviour in the asthmatic group. Asthmatic airways were narrower at baseline and responded to inhaled methacholine by more heterogeneous narrowing of parallel sister airways and greater airway closure.

Small airways function declines after allogeneic haematopoietic stem cell transplantation.

Bronchiolitis obliterans (BO) following allogeneic haematopoietic stem cell transplantation (HSCT) affects peripheral airways. Detection of BO is presently delayed by the low sensitivity of spirometry. We examined the relationship between peripheral airway function and time since HSCT, and compared it with spirometry and clinical indices in 33 clinically stable allogeneic HSCT recipients. The following measurements were performed: lung function, exhaled nitric oxide, forced oscillatory respiratory system resistance and reactance, acinar (S(acin)) and conductive airways ventilation heterogeneity and lung clearance index (LCI) measured by multiple breath nitrogen washout. 22 patients underwent repeat visits from which short-term changes were examined. Median time post HSCT was 12 months. Eight patients were clinically diagnosed as having BO. In multivariate analysis, time since HSCT was predicted by S(acin) and forced expiratory volume in 1 s % predicted. 20 patients had abnormal S(acin) with normal spirometry, whereas none had airflow obstruction with normal S(acin). S(acin) and LCI were the only measures to change significantly between two visits, with both worsening. Change in S(acin) was the only parameter to correlate with change in chronic graft-versus-host disease grade. In conclusion, peripheral airways ventilation heterogeneity worsens with time after HSCT. S(acin) may be more sensitive than spirometry in detecting BO at an early stage, which needs confirmation in a prospective study.

Increased airway closure is a determinant of airway hyperresponsiveness.

In order to investigate whether increased airway closure is a component of airway hyperresponsiveness (AHR), airway closure was compared during induced bronchoconstriction in 62 asthmatic, 41 nonasthmatic nonobese (control) and 20 nonasthmatic obese (obese) subjects. Airway closure and airway narrowing were measured by spirometry as percentage change in forced vital capacity (%DeltaFVC) and change in forced expiratory ratio (DeltaFER), respectively. Multiple regression analyses were used to assess the determinants of AHR, assessed by the dose response slope (DRS). The DRS was significantly increased in asthmatics compared with controls but did not differ between obese and controls. The spirometric predictors of logDRS were baseline FER, DeltaFER, body mass index (BMI) and %DeltaFVC. There was a negative relationship between BMI and logDRS in the regression, suggesting a protective effect. The present findings suggest that the extent of airway closure during induced bronchoconstriction is a determinant of airway hyperresponsiveness, independent of the level of airway narrowing. However, after adjusting for airway closure, obesity appears to protect against airway hyperresponsiveness.

Effect of obesity on breathlessness and airway responsiveness to methacholine in non-asthmatic subjects.

BACKGROUND: Obesity is associated with increased prevalence and incidence of asthma, but the mechanism is unknown. Obesity reduces lung volumes, which can increase airway responsiveness, and increases resistive and elastic work of breathing, which can increase dyspnea. OBJECTIVE: To determine if the intensity of dyspnea due to airway narrowing or if airway responsiveness is increased in obese, non-asthmatic subjects. SUBJECTS: Twenty-three obese (BMI (body mass index) > or =30 kg m(-2)) and 26 non-obese (BMI <30 kg m(-2)) non-asthmatic subjects, aged between 18 and 70 years. METHODS: High-dose methacholine challenge was used to determine the sensitivity and the maximal response to methacholine. Respiratory system resistance (Rrs) and reactance were measured, using the forced oscillation technique, as indicators of resistive and elastic loads during challenge. Perception of dyspnea was measured by the Borg score during challenge. Static lung volumes were measured by body plethysmography. RESULTS: Static lung volumes were reduced in the obese subjects. There were no significant differences in the sensitivity or maximal response to methacholine between obese and non-obese subjects. The magnitude of change in Rrs was similar in both groups, but obese subjects had more negative reactance after challenge (P=0.002) indicating a greater elastic load. The intensity of dyspnea was greater in obese subjects (P=0.03). CONCLUSIONS: Obesity reduces lung volumes, but does not alter the sensitivity or maximal response to methacholine. However, obese subjects have enhanced perception of dyspnea, associated with greater apparent stiffness of the respiratory system, and may therefore be at greater risk of symptoms.

Airway dimensions measured from micro-computed tomography and high-resolution computed tomography.

Volume averaging results in both over- and underestimation of airway dimensions when they are measured by high-resolution computed tomography (HRCT). The current authors calibrated computerised measurements of airway dimensions from HRCT against a novel three-dimensional micro-computed tomography (CT) standard, which has a 50-fold greater resolution, as well as against traditional morphometry. Inflation-fixed porcine lung cubes were scanned by HRCT and micro-CT. A total of 59 lumen area (Ai), 30 wall area (A(aw)) and 11 lumen volume (Vi) measurements were made. Ai was measured from the cut surface of 11 airways by morphometry. Airways in scanned images were matched using branching points. After calibration, the errors of Ai, A(aw) and Vi HRCT measurements were determined. The current authors found a systematic, size-dependent underestimation of Ai and overestimation of A(aw) from HRCT measurements. This was used to calibrate an HRCT measurement algorithm. The 95% limits of agreement of subsequent measurements were +/-3.2 mm2 for Ai, +/-4.3 mm2 for A(aw), and +/-11.2 mm3 for Vi with no systematic error. Morphometric measurements agreed with micro-CT (+/-2.5 mm2) without systematic error. In conclusion, micro-computed tomography image data from inflation-fixed airways can be used as calibration standards for three-dimensional lumen volume measurements from high-resolution computed tomography, while morphometry is acceptable for two-dimensional measurements. The image dataset could be used to validate other developmental three-dimensional segmentation algorithms.

The effects of body weight on airway calibre.

Increased wheeze and asthma diagnosis in obesity may be due to reduced lung volume with subsequent airway narrowing. Asthma (wheeze and airway hyperresponsiveness), functional residual capacity (FRC) and airway conductance (Gaw) were measured in 276 randomly selected subjects aged 28-30 yrs. Data were initially adjusted for smoking and asthma before examining relationships between weight and FRC (after adjustment for height), and between body mass index (BMI = weight.height(-2)) and Gaw (after adjustment for FRC) by multiple linear regression, separately for females and males. For males and females, BMI (+/-95% confidence interval) was 27.0+/-4.6 kg.m(-2) and 25.6+/-6.0 kg.m(-2) respectively, Gaw was 0.64+/-0.04 L.s(-1).cmH2O(-1) and 0.57+/-0.03 L.s(-1).cmH2O(-1), and FRC was 85.3+/-3.4 and 84.0+/-2.9% of predicted. Weight correlated independently with FRC in males and females. BMI correlated independently and inversely with Gaw in males, but only weakly in females. In conclusion, obesity is associated with reduced lung volume, which is linked with airway narrowing. However, in males, airway narrowing is greater than that due to reduced lung volume alone. The mechanisms causing airway narrowing and sex differences in obesity are unknown.

Low lung volume alters contractile properties of airway smooth muscle in sheep.

Breathing at volumes lower than functional residual capacity (FRC) can induce changes in nonasthmatic airways consistent with the behaviour of asthmatic airways. This study investigated the chronic effect of breathing at volumes lower than FRC on the contractility of airway smooth muscle and myosin light chain kinase (MLCK) content and activity. Sheep of three age groups (neonate, adolescent and adult) had their FRC reduced by approximately 25%, for 4 weeks using a leather corset. Contractile responses to carbachol were then recorded in isolated tracheal strips and bronchial rings. MLCK content and activity were assessed by immunoblotting. The rate of stress generation increased in the bronchial smooth muscle of both adult and adolescent but not neonatal corseted sheep: adolescent corseted versus control, 65.0 +/- 4.1 versus 103.4 +/- 7.0 s (to reach 50% maximum stress), respectively; and adult corseted versus control, 57.0 +/- 6.4 versus 93.4 +/- 8.2 s, respectively. This was not due to increases in either bronchial or tracheal smooth muscle amount or MLCK content and activity. The present results indicate that chronic breathing at low lung volumes increases the rate of stress generation in airway smooth muscle.

Airway re-narrowing following deep inspiration in asthmatic and nonasthmatic subjects.

After bronchoconstriction, deep inspiration (DI) causes dilatation followed by airway re-narrowing. Re-narrowing may be faster in asthmatic than nonasthmatic subjects. This study investigated the relationship between re-narrowing and the magnitude of both DI-induced dilatation and the volume-dependence of respiratory system resistance (Rrs) during tidal breathing. In 25 asthmatic and 18 nonasthmatic subjects the forced oscillation technique was used to measure Rrs at baseline and after methacholine challenge, during 1 min of tidal breathing, followed by DI to total lung capacity (TLC) and passive return to functional residual capacity (FRC). Dilatation was measured as the decrease in Rrs between end tidal inspiration and TLC, re-narrowing as Rrs at FRC immediately after DI, as per cent Rrs at end-tidal expiration, and volume dependent tidal fluctuation as the difference between mean Rrs at end-expiration and end-inspiration. Asthmatic subjects had greater re-narrowing, less dilatation, and greater tidal fluctuations both at baseline and after challenge. Re-narrowing correlated with baseline tidal fluctuation and inversely with dilatation. Both baseline tidal fluctuation and dilatation were significant independent predictors of re-narrowing. Following deep inspiration-induced dilatation, faster airway re-narrowing in asthmatic than nonasthmatic subjects is associated not only with reduced deep inspiration-induced dilatation but also with some property of the airways that is detectable prior to challenge as an increased volume dependence of resistance.

Improved safety with equivalent asthma control in adults with chronic severe asthma on high-dose fluticasone propionate.

OBJECTIVE: High-dose inhaled corticosteroids (ICS) have been associated with the same side-effects as oral corticosteroids. Beclomethasone dipropionate (BDP) and budesonide (BUD) in doses greater than 2000 microg/day are used regularly in severe asthma, despite the fact that safety and efficacy data at such high doses are limited. Fluticasone propionate (FP) has been promoted as being twice as potent clinically as BDP or BUD at doses of 2000 microg/day or less with a similar safety profile. The aim of this study was to compare the efficacy and safety of FP with BDP and BUD in 133 symptomatic adult asthmatics requiring at least 1750 microg/day of BDP or BUD. METHODOLOGY: Patients fulfilling the entry criteria were randomized to receive either their regular ICS medication or FP at approximately half the microgram dose for 6 months in an open, parallel group study. The primary efficacy measure was based on morning peak expiratory flow measurements recorded by patients on daily record cards, while determination of safety was based on a number of endpoints including changes in bone turnover indices, the incidence of topical side-effects and assessments of quality of life. RESULTS: It was shown that patients who were switched to FP, but not those continuing with BDP or BUD, had significant increases in levels of morning serum cortisol and the urine cortisol:creatinine ratio while maintaining asthma control. Serum osteocalcin and the pyridinoline:creatinine ratio, as well as the deoxypyridinoline:creatinine ratio, were also shown to increase only in the FP group. Subjective assessments such as quality of life score, the incidence and ease of bruising, and reports of hoarseness also favoured the FP group. CONCLUSIONS: It is concluded that, at the doses studied and with the delivery devices used clinically, FP is at least as effective as BDP/BUD in the management of severe asthma and may offer clinical advantages with respect to steroid-related adverse effects.

Relationship between airway microvascular leakage, edema, and baseline airway functions.

This study was designed to examine the relationship among microvascular leakage, edema, and baseline airway function. Microvascular leakage was induced in the airways of anesthetized, tracheostomized New Zealand White rabbits (n = 22) by using nebulized N-formyl-methionyl-leucyl-phenylalanine (10 mg) and was measured in the trachea by using the Evans blue dye technique. Airway wall thickness was assessed morphometrically in the right main bronchus after Formalin fixation at a pressure of 25 cmH2O. Areas calculated included the mucosal wall area, the adventitial wall area, the total wall area, and the percentage of total wall area consisting of blood vessels. A neutrophil count was also performed by analyzing numbers of cells in both the mucosal wall area and the adventitial wall area. Airway function was assessed before and 30 min after challenge with N-formyl-methionyl-leucyl-phenylalanine by determining airway resistance, functional residual capacity, specific airway resistance, and flow-volume and pressure-volume curves (after paralysis of the animals with suxamethonium). The concentration of Evans blue dye in tracheal tissue ranged from 31.3 to 131.2 micrograms. There was a significant correlation between this concentration and both the adventitial wall area (P < 0.01) and mucosal neutrophil numbers (P < 0.005). There was no correlation between Evans blue concentration and either blood vessel area or changes in respiratory physiology parameters before and after challenge. There was no significant difference between any respiratory physiology measurements before and after challenge. We conclude that an increase in microvascular leakage correlates with airway edema in the adventitia; however, these airway changes have no significant effect on airway elastic or resistive properties.

Tachykinins do not cause plasma leakage in the rabbit trachea.

The aim of this study was to determine if neuropeptides mediate increases in airway microvascular leakage in the rabbit. New Zealand white rabbits were anaesthetised and challenged with either capsaicin (50 mg/kg s.c. or i.p.) or substance P (10(-3) or 10(-4) nebulised or 1 microgram/kg i.v.) or vagally stimulated with a range of pulse trains (5-15 Hz, 5-15 V, 0.2-2 msec). Microvascular leakage was assessed using the Evans blue dye technique. A further two groups of rabbits were challenged with nebulised metabisulphite (100 mg) or phosphate buffered saline to serve as positive and negative controls. Challenge with capsaicin, substance P or vagal stimulation did not significantly increase tracheal or bronchial Evans blue concentrations above negative control levels. We conclude that neuropeptides do not mediate increases in microvascular leakage in the major airways of the rabbit.

Assessment and management of dyspnoea.

The terms 'dyspnoea' or 'breathlessness' refer to an individual's subjective awareness of discomfort related to the act of breathing. Elevations in CO2 above normal levels have been shown to cause breathlessness although it is unlikely to be the sole cause of breathlessness in a clinical setting. Several studies suggest that supplemental O2 during exercise will diminish the sensation of breathlessness although not all work has confirmed this finding. Much about the role of gas exchange in dyspnogenesis remains controversial. Phrenic blockade can abolish dyspnoea in response to breath-holding, while work in quadriplegics suggests that the intercostal muscles are not involved. A separate and direct pathway from the respiratory centre to the sensory cortex has also be implicated. Threshold discrimination has established that patients with chronic airflow limitation (CAL) have a blunted response to the addition of resistive loads to breathing, while category scaling methods (e.g. the Borg scale) have added descriptive terms to these physiological measures. Questionnaires often appear limited by their subjectivity and lack of correlation with physiological changes, but remain a useful tool in the clinical setting. In regard to therapy of dyspnoea high fat diets have a theoretical advantage in the CAL group but are generally not well tolerated. Resistive training devices and exercise training in CAL have been widely researched but in general, measures of lung remain unaltered and many of the studies would suggest that they have little, if any, inpact on functional status. Beta-agonists have been widely shown to be useful in CAL patients, despite the fact that bronchodilatation is not always demonstrable. Anticholinergics have be shown to be effective bronchodilators, but whether there is an improvement in dyspnoea above that expected from improvement in lung function is unclear. Animal studies and work in normal individuals would suggest that methylxanthines have a theoretical role in CAL possibly by increasing diaphragmatic muscle strength and decaying fatigue, but toxicity and lack of clear benefit in this group suggest that they should not be used as monotherapy. There is little evidence to support the use of opioids in chronic CAL although their role in the acute dyspnoea of end-stage CAL remains defined. The use of benzodiazepines has also been disappointing. Bullectomy remains widely accepted in clinical practice. New techniques such as 'reduction surgery' for diffuse emphysema are showing promise, although still in need of further testing and validation.

Role of platelet activating factor, leukotrienes and polymorphs in the FMLP induced increase in microvascular leakage in rabbit trachea.

This study was designed to determine the time course of N-formyl-methionyl-leucyl-phenylalanine (FMLP) induced microvascular leakage in the airways of anaesthetized, tracheostomized New Zealand white rabbits. We have previously shown that FMLP increases microvascular leakage in the rabbit trachea at 30 min post challenge. A further aim was to determine the mechanisms underlying this response. Microvascular leakage was assessed using the albumin binding dye, Evans blue which was injected intravenously (50 mg/kg) immediately prior to FMLP challenge (10 mg nebulised for 2 min) or the control (dimethylsulphoxide/saline). Microvascular leakage was assessed in the trachea and bronchi at 15 min, 22.5 min, 45 min and 120 minutes with n = 6 for each group. The only significant difference between control and FMLP challenged groups was at 45 min in the bronchi (FMLP 77.6 +/- 12.2, control 33.4 +/- 5.7, P < 0.05). To determine the mechanism underlying FMLP-induced increases in microvascular leakage rabbits were treated with one of the following: (i) nitrogen mustard (1.75 mg/kg intravenously), which depletes circulating polymorphs (n = 6); (ii) the platelet activating factor (PAF) receptor antagonist WEB 2086 (10 mg/kg; n = 5); (iii) the Cys-LTR1 receptor antagonist ICI198615 (nebulised 10(-4) mol/L), or challenged with the thromboxane agonist U46619. Tracheal Evans blue concentration was assessed at 30 min after FMLP challenge and compared with the appropriate control. Treatment with WEB 2086 significantly (P < 0.05) reduced tracheal microvascular leakage (FMLP 76.3 +/- 13.1 WEB 2086/FMLP 31.4 +/- 3.7 micrograms/g trachea) as did ICI198615 (FMLP 69.8 +/- 11.5 ICI198615/FMLP 30.0 +/- 5.7). In conclusion FMLP induced an increase in tracheal microvascular leakage which was significant in the bronchi at 45 min and this increase was mediated by platelet activating factor and the sulphidopeptide leukotrienes C4 and D4.

The effect of nitrous oxide on the measurement of single-breath transfer factor.

One hour after a bone marrow biopsy and inhalation of Entonox gas (50% nitrous oxide (N2O) and 50% oxygen), a patient had a markedly reduced transfer factor of the lung for carbon monoxide (TL,CO). Three hours after Entonox, the patient had a normal TL,CO. Since carbon monoxide (CO) and N2O have similar spectral wavelengths, it was proposed that residual N2O in the lungs was interfering with the infra-red analysers used to detect CO concentrations. Experiments were performed to verify the "interference" effect and its duration. Five healthy volunteers performed serial triplicate TL,CO measurements over 3 h on two randomized days (Control vs N2O). The first triplicate TL,CO on each day served as a baseline measurement. Following the baseline measurement on the N2O day, each subject inhaled Entonox for 10 min. To serve as a control for the infrared effect, the identical protocol was repeated using a gas chromatography method for TL,CO determination. The infra-red method showed a marked reduction (> 50%) in TL,CO 30 min after N2O inhalation. This reduction did not return to baseline levels for at least 2 h. In comparison, the gas chromatography method showed no significant reduction in TL,CO. In a group of healthy nonsmoking subjects, N2O markedly affected the measurement of the transfer factor of the lungs for carbon monoxide using infra-red analysers. The time course over which the measurement was reduced was at least 2 h for a 10 min inhalation period. The effect was entirely due to a measurement error associated with infra-red technology.

Use of the Sensormedics MCT Accelerator in sputum induction.

Airway inflammation can be studied by obtaining sputum induced by inhalation of 3% saline. The Sensormedics MCT accelerator is an oral asymmetrical high frequency oscillator which safely enhances clearance of airway secretions. The aim of this study was to determine if use of the MCT Accelerator enhances sputum production in non-asthmatic non-atopic, atopic and asthmatic subjects. Fifteen subjects were studied over 3 days. On day 1 skin prick testing to common aeroallergens and methacholine bronchial reactivity were performed. On days 2 and 3, separated by 7 days, sputum was induced by inhalation of 3% saline alone for 30 mins or via the nebulizer port of the MCT Accelerator. The cellular profile and volume of sputum were analysed. The use of the MCT Accelerator did not alter the cellular profile of the induced sputum nor was there an increase in volume. In conclusion the induction of sputum by inhalation of 3% saline was not altered by use of the MCT Accelerator.

Evidence for opioid modulation and generation of prostaglandins in sulphur dioxide (SO)2-induced bronchoconstriction.

BACKGROUND: Inhalation of sulphur dioxide (SO2) provokes bronchoconstriction in asthmatic subjects. Cholinergic mechanisms contribute, but other mechanisms remain undefined. The effect of morphine, an opioid agonist, on the cholinergic component of SO2-induced bronchoconstriction was investigated, and the effect of indomethacin, a cyclooxygenase inhibitor, on SO2-induced bronchoconstriction and tachyphylaxis was studied. METHODS: In the first study 16 asthmatic subjects inhaled either ipratropium bromide or placebo 60 minutes before an SO2 challenge on days 1 and 2. On day 3 an SO2 challenge was performed immediately after intravenous morphine. In the second study 15 asthmatic subjects took either placebo or indomethacin for three days before each study day when two SO2 challenges were performed 30 minutes apart. The response was measured as the cumulative dose causing a 35% fall in specific airways conductance (sGaw; PDsGaw35). RESULTS: Ipratropium bromide significantly inhibited SO2 responsiveness, reducing PDsGaw35 by 0.89 (95% CI 0.46 to 1.31) doubling doses. This effect persisted after correction for bronchodilatation induced by ipratropium bromide. The effect of ipratropium bromide and morphine on SO2 responsiveness also correlated (r2 = 0.71). In the second study SO2 tachyphylaxis developed with PDsGaw35 on repeated testing, being reduced by 0.62 (95% CI 0.17 to 1.07) doubling doses. Indomethacin attenuated baseline SO2 responsiveness, increasing PDsGaw35 by 0.5 (95% CI 0.06 to 0.93) doubling doses. CONCLUSIONS: These results suggest that opioids modulate the cholinergic component of SO2 responsiveness and that cyclooxygenase products contribute to the immediate response to SO2.

Comparison of sulphur dioxide and metabisulphite airway reactivity in subjects with asthma.

BACKGROUND: In asthmatic subjects bronchoconstriction is induced by inhalation of the common food preservatives sulphur dioxide (SO2) and metabisulphite (MBS). SO2 and MBS challenges share many similarities, but it is not known whether they are equivalent. In this study of subjects with mild clinical asthma equivalence was assessed by comparing SO2 and MBS reactivity by estimating the total dose of SO2 inhaled during SO2 and MBS challenges, and by calculating SO2 uptake during both challenges. In addition, as the MBS solutions inhaled were acidic and hyperosmolar, the effect of these factors on MBS responsiveness was investigated. METHODS: Fifteen subjects were challenged on separate days with doubling (0.5 to 8.0 ppm) concentrations of SO2 gas inhaled during three minute periods of isocapnic hyperventilation and MBS administered in doses ranging from 0.1 to 12.8 mumol using the Wright protocol. On two other days SO2 and MBS challenges were preceded by a challenge with phosphate buffered saline (PBS) solutions of pH and osmolarity similar to MBS solutions. Response was measured as the dose or concentration causing a 20% fall in FEV1 (PD20 or PC20). RESULTS: All subjects reacted to MBS and 14 responded to SO2. Geometric mean histamine PD20 was 1.61 mumol (95% confidence interval 0.72 to 3.60). MBS and SO2 airway responsiveness were not significantly related. Estimates of the mean concentration of SO2 inhaled during SO2 and MBS challenges differed, as did estimates of the mean SO2 uptake during both challenges. MBS and SO2 reactivity were not affected by prior challenge with PBS solutions. CONCLUSIONS: SO2 and MBS challenges are not comparable. MBS reactivity was not affected by the hyperosmolar, acidic nature of its solutions.