Airway narrowing in healthy humans inhaling methacholine without deep inspirations demonstrated by HRCT

Effect of Respiratory Tract Disease on Particle Deposition

This chapter describes the effects that respiratory disease has on particle deposition in the lungs. The geometry of airways, breathing patterns, and regional ventilation are all affected by various lung diseases, including COPD, asthma, and cystic fibrosis, and in turn modify total and regional deposition from normal. Total particle deposition in the lung is increased by airways obstruction and increased ventilation at rest compared to healthy individuals. Regional particle deposition is 1) shifted from distal to more proximal bronchial airways by airway obstruction, and 2) becomes more heterogeneous due to uneven lung ventilation. The net effect of the changes in total and regional particle deposition from normal is to greatly enhance bronchial airway surface doses for particle deposition while leaving unventilated lung regions inaccessible to the particles. As a result, both therapeutic aerosol delivery and the adverse effects of pollutant particles may be altered with progression of lung disease.

Fetal Origins of Asthma: A Longitudinal Study from Birth to Age 36 Years

Rationale: Deficits in infant lung function-including the ratio of the time to reach peak tidal expiratory flow to the total expiratory time (tptef/te) and maximal expiratory flow at FRC (V̇maxFRC)-have been linked to increased risk for childhood asthma.Objectives: To examine the individual and combined effects of tptef/te and V̇maxFRC in infancy on risk for asthma and abnormalities of airway structure into mid-adult life.Methods: One hundred eighty participants in the Tucson Children's Respiratory Study birth cohort had lung function measured by the chest-compression technique in infancy (mean age ± SD: 2.0 ± 1.2 mo). Active asthma was assessed in up to 12 questionnaires between ages 6 and 36 years. Spirometry and chest high-resolution computed tomographic (HRCT) imaging were completed in a subset of participants at age 26. The relations of infant tptef/te and V̇maxFRC to active asthma and airway structural abnormalities into adult life were tested in multivariable mixed models.Measurements and Main Results: After adjustment for covariates, a 1-SD decrease in infant tptef/te and V̇maxFRC was associated with a 70% (P = 0.001) and 55% (P = 0.005) increased risk of active asthma, respectively. These effects were partly independent, and two out of three infants who were in the lowest tertile for both tptef/te and V̇maxFRC developed active asthma by mid-adult life. Infant V̇maxFRC predicted reduced airflow and infant tptef/te reduced HRCT airway caliber at age 26.Conclusions: These findings underscore the long-lasting effects of the fetal origins of asthma, support independent contributions by infant tptef/te and V̇maxFRC to development of asthma, and link deficits at birth in tptef/te with HRCT-assessed structural airway abnormalities in adult life.

Dynamic airway constriction in rats: heterogeneity and response to deep inspiration

Airway narrowing due to hyperresponsiveness severely limits gas exchange in patients with asthma. Imaging studies in humans and animals have shown that bronchoconstriction causes patchy patterns of ventilation defects throughout the lungs, and several computational models have predicted that these regions are due to constriction of smaller airways. However, these imaging approaches are often limited in their ability to capture dynamic changes in small airways, and the patterns of constriction are heterogeneous. To directly investigate regional variations in airway narrowing and the response to deep inspirations (DIs), we utilized tantalum dust and microfocal X-ray imaging of rat lungs to obtain dynamic images of airways in an intact animal model. Airway resistance was simultaneously measured using the flexiVent system. Custom-developed software was used to track changes in airway diameters up to generation 19 (~0.3-3 mm). Changes in diameter during bronchoconstriction were then measured in response to methacholine (MCh) challenge. In contrast with the model predictions, we observed significantly greater percent constriction in larger airways in response to MCh challenge. Although there was a dose-dependent increase in total respiratory resistance with MCh, the percent change in airway diameters was similar for increasing doses. A single DI following MCh caused a significant reduction in resistance but did not cause a significant increase in airway diameters. Multiple DIs did, however, cause significant increases in airway diameters. These measurements allowed us to directly quantify dynamic changes in airways during bronchoconstriction and demonstrated greater constriction in larger airways.

Elevation in lung volume and preventing catastrophic airway closure in asthmatics during bronchoconstriction

Background

Asthma exacerbations cause lung hyperinflation, elevation in load to inspiratory muscles, and decreased breathing capacity that, in severe cases, may lead to inspiratory muscle fatigue and respiratory failure. Hyperinflation has been attributed to a passive mechanical origin; a respiratory system time-constant too long for full exhalation. However, because the increase in volume is also concurrent with activation of inspiratory muscles during exhalation it is unclear whether hyperinflation in broncho-constriction is a passive phenomenon or is actively controlled to avoid airway closure.

Methods

Using CT scanning, we measured the distensibility of individual segmental airways relative to that of their surrounding parenchyma in seven subjects with asthma and nine healthy controls. With this data we tested whether the elevation of lung volume measured after methacholine (MCh) provocation was associated with airway narrowing, or to the volume required to preventing airway closure. We also tested whether the reduction in FVC post-MCh could be attributed to gas trapped behind closed segmental airways.

Findings

The changes in lung volume by MCh in subjects with and without asthma were inversely associated with their reduction in average airway lumen. This finding would be inconsistent with hyperinflation by passive elevation of airway resistance. In contrast, the change in volume of each subject was associated with the lung volume estimated to cause the closure of the least stable segmental airway of his/her lungs. In addition, the measured drop in FVC post MCh was associated with the estimated volume of gas trapped behind closed segmental airways at RV.

Conclusions

Our data supports the concept that hyperinflation caused by MCh-induced bronchoconstriction is the result of an actively controlled process where parenchymal distending forces on airways are increased to counteract their closure. To our knowledge, this is the first imaging-based study that associates inter-subject differences in whole lung behavior with the interdependence between individual airways and their surrounding parenchyma.

[Guidelines for methacholine provocation testing]

Bronchial challenge with the direct bronchoconstrictor agent methacholine is commonly used for the diagnosis of asthma. The "Lung Function" thematic group of the French Pulmonology Society (SPLF) elaborated a series of guidelines for the performance and the interpretation of methacholine challenge testing, based on French clinical guideline methodology. Specifically, guidelines are provided with regard to the choice of judgment criteria, the management of deep inspirations, and the role of methacholine bronchial challenge in the care of asthma, exercise-induced asthma, and professional asthma.

Reduced Baseline Airway Caliber Relates to Larger Airway Sensitivity to Rostral Fluid Shift in Asthma

Background: We have previously shown that when asthmatics go supine, fluid shifts out of the legs, accumulates in the thorax, and exacerbates lower airway narrowing. In the retrospective analysis of our previous work presented here, we test the hypothesis that the sensitivity of this process relates inversely to baseline caliber of the lower airways.

Methods: Eighteen healthy (six women) and sixteen asthmatic subjects (nine women) sat for 30 min, and then lay supine for 30 min. While supine, lower body positive pressure (LBPP, 40 mm Hg) was applied to displace fluid from the legs similar in amount to the overnight fluid shift. Respiratory resistance and reactance at 5 Hz (R5 and X5) and leg and thoracic fluid volumes (LFV and TFV) were measured at the beginning and end of the supine period.

Results: With LBPP, healthy, and asthmatic subjects had similar changes in the LFV and TFV (p = 0.3 and 0.1, respectively). Sensitivity to fluid shift, defined by ΔR5/ΔTFV, was larger in the asthmatics than in the healthy subjects (p = 0.0001), and correlated with baseline R5 in the supine position in the asthmatics (p = 0.7, p = 0.003). No such association was observed in the healthy subjects (p = 0.6). In the asthmatics, women showed a greater reduction in X5 than men with LBPP (p = 0.009).

Conclusions: Smaller baseline airway caliber, as assessed by larger R5, was associated with increased sensitivity to fluid shift in the supine position. We conclude that asthmatics with narrower small airways such as obese asthma patients, women with asthma and those with severe asthma may be more sensitive to the effects fluid shift while supine as during sleep.

Relationship between atrial septal defects and asthma-like dyspnoea: the impact of transcatheter closure

BACKGROUND

Patients with atrial septal defects (ASD) are often misdiagnosed as asthma patients and accordingly receive erroneous bronchodilator treatment. In order to characterise their symptoms of dyspnoea to explain this clinical observation, we investigated the prevalence of asthma-like symptoms in patients with secundum ASD who then underwent successful percutaneous closure.

METHODS

A total of 80 ASD patients (74 % female, mean age 46.7 ± 16.8 years, median follow-up 3.0 [2.0-5.0] years) retrospectively completed dyspnoea questionnaires determining the presence and extent of cough, wheezing, chest tightness, effort dyspnoea and bronchodilator use on a 7-point scale (0 = none, 6 = maximum) before and after ASD closure. The Mini Asthma Quality of Life (Mini-AQLQ) and Asthma Control Questionnaire with bronchodilator use (ACQ6) were administered.

RESULTS

A total of 48 (60 %) patients reported cough, 27 (34 %) wheezing, 26 (33 %) chest tightness and 62 (78 %) effort dyspnoea. Symptom resolution or reduction was found in 64 (80 %) patients after ASD closure. Asthma symptom scores decreased significantly on the Mini-AQLQ and ACQ6 (both p < 0.001). The number of patients using bronchodilators decreased from 16 (20 %) to 8 (10 %) patients after ASD closure (p = 0.039) with less frequent use of bronchodilators (p = 0.015).

CONCLUSIONS

A high prevalence of asthma-like symptoms and bronchodilator use is present in ASD patients, which exceeds the low prevalence of bronchial asthma in this study population. Future prospective research is required to confirm this phenomenon. The presence of an ASD should be considered in the differential diagnosis of patients with asthma-like symptoms, after which significant symptom relief can be achieved by ASD closure.

[Mechanisms of non-specific airway hyperresponsiveness: Methacholine-induced alterations in airway architecture]

Multiple mechanisms drive non-specific airway hyperresponsiveness in asthma. At the organ level, methacholine inhalation induces a complex bronchomotor response involving both bronchoconstriction and, to some extent, paradoxical bronchodilatation. This response is heterogeneous both serially, along a single bronchial axis, and in parallel, among lung regions. The bronchomotor response to methacholine induces contraction of distal airways as well as focal airway closure in select lung territories, leading to anatomically defined ventilation defects and decreased vital capacity. In addition, loss of the bronchoprotector and bronchodilator effects of deep inspirations is a key contributor to airway hyperresponsiveness in asthma.

Particle deposition in a child respiratory tract model: in vivo regional deposition of fine and ultrafine aerosols in baboons

To relate exposure to adverse health effects, it is necessary to know where particles in the submicron range deposit in the respiratory tract. The possibly higher vulnerability of children requires specific inhalation studies. However, radio-aerosol deposition experiments involving children are rare because of ethical restrictions related to radiation exposure. Thus, an in vivo study was conducted using three baboons as a child respiratory tract model to assess regional deposition patterns (thoracic region vs. extrathoracic region) of radioactive polydisperse aerosols ([d16-d84], equal to [0.15 µm-0.5 µm], [0.25 µm-1 µm], or [1 µm-9 µm]). Results clearly demonstrated that aerosol deposition within the thoracic region and the extrathoraic region varied substantially according to particle size. High deposition in the extrathoracic region was observed for the [1 µm-9 µm] aerosol (72% ± 17%). The [0.15 µm-0.5 µm] aerosol was associated almost exclusively with thoracic region deposition (84% ± 4%). Airborne particles in the range of [0.25 µm-1 µm] showed an intermediate deposition pattern, with 49% ± 8% in the extrathoracic region and 51% ± 8% in the thoracic region. Finally, comparison of baboon and human inhalation experiments for the [1 µm-9 µm] aerosol showed similar regional deposition, leading to the conclusion that regional deposition is species-independent for this airborne particle sizes.

Dilatation of the constricted human airway by tidal expansion of lung parenchyma

RATIONALE

In the normal lung, breathing and deep inspirations potently antagonize bronchoconstriction, but in the asthmatic lung this salutary effect is substantially attenuated or even reversed. To explain these findings, the prevailing hypothesis focuses on contracting airway smooth muscle and posits a nonlinear dynamic interaction between actomyosin binding and the tethering forces imposed by tidally expanding lung parenchyma.

OBJECTIVE

This hypothesis has never been tested directly in bronchial smooth muscle embedded within intraparenchymal airways. Our objective here is to fill that gap.

METHODS

We designed a novel system to image contracting intraparenchymal human airways situated within near-normal lung architecture and subjected to dynamic parenchymal expansion that simulates breathing.

MEASUREMENTS AND MAIN RESULTS

Reversal of bronchoconstriction depended on the degree to which breathing actually stretched the airway, which in turn depended negatively on severity of constriction and positively on the depth of breathing. Such behavior implies positive feedbacks that engender airway instability.

OVERALL CONCLUSIONS

These findings help to explain heterogeneity of airflow obstruction as well as why, in people with asthma, deep inspirations are less effective in reversing bronchoconstriction.

Effect of airway smooth muscle tone on airway distensibility measured by the forced oscillation technique in adults with asthma

Airway distensibility appears to be unaffected by airway smooth muscle (ASM) tone, despite the influence of ASM tone on the airway diameter-pressure relationship. This discrepancy may be because the greatest effect of ASM tone on airway diameter-pressure behavior occurs at low transpulmonary pressures, i.e., low lung volumes, which has not been investigated. Our study aimed to determine the contribution of ASM tone to airway distensibility, as assessed via the forced oscillation technique (FOT), across all lung volumes with a specific focus on low lung volumes. We also investigated the accompanying influence of ASM tone on peripheral airway closure and heterogeneity inferred from the reactance versus lung volume relationship. Respiratory system conductance and reactance were measured using FOT across the entire lung volume range in 22 asthma subjects and 19 healthy controls before and after bronchodilator. Airway distensibility (slope of conductance vs. lung volume) was calculated at residual volume (RV), functional residual capacity (FRC), and total lung capacity. At baseline, airway distensibility was significantly lower in subjects with asthma at all lung volumes. After bronchodilator, distensibility significantly increased at RV (64.8%, P < 0.001) and at FRC (61.8%, P < 0.01) in subjects with asthma but not in control subjects. The increased distensibility at RV and FRC in asthma were not associated with the accompanying changes in the reactance versus lung volume relationship. Our findings demonstrate that, at low lung volumes, ASM tone reduces airway distensibility in adults with asthma, independent of changes in airway closure and heterogeneity.

Mechanisms of remodeling in asthmatic airways

Asthma is a chronic inflammatory airway disorder characterized by airway hyperresponsiveness and reversible airflow obstruction. Subgroups of asthma patients develop airflow obstruction that is irreversible or only partially reversible and experience an accelerated rate of lung function decline. The structural changes in the airways of these patients are referred to as airway remodeling. All elements of the airway wall are involved, and remodeled airway wall thickness is substantially increased compared to normal control airways. Airway remodeling is thought to contribute to the subphenotypes of irreversible airflow obstruction and airway hyperresponsiveness, and it has been associated with increased disease severity. Reversal of remodeling is therefore of paramount therapeutic importance, and mechanisms responsible for airway remodeling are feasible therapeutic targets for asthma treatment. This paper will focus on our current understanding of the mechanisms of airway remodeling in asthma and potential targets for future intervention.

Bronchodilation response to deep inspirations in asthma is dependent on airway distensibility and air trapping

In healthy individuals, deep inspirations (DIs) have a potent bronchodilatory ability against methacholine (MCh)-induced bronchoconstriction. This is variably attenuated in asthma. We hypothesized that inability to bronchodilate with DIs is related to reduced airway distensibility. We examined the relationship between DI-induced bronchodilation and airway distensibility in 15 asthmatic individuals with a wide range of baseline lung function [forced expired volume in 1 s (FEV(1)) = 60-99% predicted]. After abstaining from DIs for 20 min, subjects received a single-dose MCh challenge and then asked to perform DIs. The effectiveness of DIs was assessed by the ability of the subjects to improve FEV(1). The same subjects were studied by two sets of high-resolution CT scans, one at functional residual capacity (FRC) and one at total lung capacity (TLC). In each subject, the areas of 21-41 airways (0.8-6.8 mm diameter at FRC) were matched and measured, and airway distensibility (increase in airway diameter from FRC to TLC) was calculated. The bronchodilatory ability of DIs was significantly lower in individuals with FEV(1) <75% predicted than in those with FEV(1) ≥75% predicted (15 ± 11% vs. 46 ± 9%, P = 0.04) and strongly correlated with airway distensibility (r = 0.57, P = 0.03), but also with residual volume (RV)/TLC (r = -0.63, P = 0.01). In multiple regression, only RV/TLC was a significant determinant of DI-induced bronchodilation. These relationships were lost when the airways were examined after maximal bronchodilation with albuterol. Our data indicate that the loss of the bronchodilatory effect of DI in asthma is related to the ability to distend the airways with lung inflation, which is, in turn, related to the extent of air trapping and airway smooth muscle tone. These relationships only exist in the presence of airway tone, indicating that structural changes in the conducting airways visualized by high-resolution CT do not play a pivotal role.

Paradoxical conducting airway responses and heterogeneous regional ventilation after histamine inhalation in rabbit studied by synchrotron radiation CT

We studied both central conducting airway response and changes in the distribution of regional ventilation induced by inhaled histamine in healthy anesthetized and mechanically ventilated rabbit using a novel xenon-enhanced synchrotron radiation computed tomography (CT) imaging technique, K-edge subtraction imaging (KES). Images of specific ventilation were obtained using serial KES during xenon washin, in three axial lung slices, at baseline and twice after inhalation of histamine aerosol (50 or 125 mg/ml) in two groups of animals (n = 6 each). Histamine inhalation caused large clustered areas of poor ventilation, characterized by a drop in average specific ventilation (sV(m)), but an increase in sV(m) in the remaining lung zones indicating ventilation redistribution. Ventilation heterogeneity, estimated as coefficient of variation (CV) of sV(m) significantly increased following histamine inhalation. The area of ventilation defects and CV were significantly larger with the higher histamine dose. In conducting airways, histamine inhalation caused a heterogeneous airway response combining narrowing and dilatation in individual airways of different generations, with the probability for constriction increasing peripherally. This finding provides further in vivo evidence that airway reactivity in response to inhaled histamine is complex and that airway response may vary substantially with location within the bronchial tree.

Airway nerves and dyspnea associated with inflammatory airway disease

The neurobiology of dyspnea is varied and complex, but there is little doubt that vagal nerves within the airways are capable of causing or modulating some dyspneic sensations, especially those associated with inflammatory airway diseases. A major contributor to the dyspnea associated with inflammatory airway disease is explained by airway narrowing and increases in the resistance to airflow. The autonomic (parasympathetic) airway nerves directly contribute to this by regulating bronchial smooth muscle tone and mucus secretion. In addition, a component of the information reaching the brainstem via airway mechanosensing and nociceptive afferent nerves likely contributes to the overall sensations of breathing. The airway narrowing can lead to activation of low threshold mechanosensitive stretch receptors, and vagal and spinal C-fibers as well as some rapidly adapting stretch receptor in the airways that are directly activated by various aspects of the inflammatory response. Inflammatory mediators can induce long lasting changes in afferent nerve activity by modulating the expression of key genes. The net effect of the increase in afferent traffic to the brainstem modulates synaptic efficacy at the second-order neurons via various mechanisms collectively referred to as central sensitization. Many studies have shown that stimuli that activate bronchopulmonary afferent nerves can lead to dyspnea in healthy subjects. A logical extension of the basic research on inflammation and sensory nerve function is that the role of vagal sensory nerve in causing or shaping dyspneic sensations will be exaggerated in those suffering from inflammatory airway disease.

Interpretation of the "positive" methacholine challenge

BACKGROUND

A methacholine challenge may be used in confirming the diagnosis of asthma, occupational asthma, or reactive airways dysfunction syndrome (RADS) through identification of bronchial hyperreactivity (BHR). While sensitivity of the test in diagnosing clinically significant asthma is excellent, specificity of the test is poor. Since there are many conditions which have been associated with BHR, a positive test must be interpreted cautiously.

METHODS

This paper reviews potential causes of a positive methacholine challenge other than asthma or RADS which have been reported in the medical literature.

RESULTS

Factors which may be associated with a positive methacholine test include test methodology, normal variation of BHR in the general population, and numerous medical conditions.

CONCLUSIONS

In cases of inhalation exposure evaluations, alternative explanations must be considered when determining whether a causal association exists between the exposure and a positive methacholine test result.

Airway constriction measured from tantalum bronchograms in conscious mice in response to methacholine

A single-projection X-ray technique showed an increase in functional residual capacity (FRC) in conscious mice in response to aerosolized methacholine (MCh) with little change in airway resistance (Raw) measured using barometric plethysmography (Lai-Fook SJ, Houtz PK, Lai Y-L. J Appl Physiol 104: 521-533, 2008). The increase in FRC presumably prevented airway constriction by offsetting airway contractility. We sought a more direct measure of airway constriction. Anesthetized Balb/c mice were intubated with a 22-G catheter, and tantalum dust was insufflated into the lungs to produce a well-defined bronchogram. After overnight recovery, the conscious mouse was placed in a sealed box, and bronchograms were taken at maximum and minimum points of the box pressure cycle before (control) and after 1-min exposures to 25, 50, and 100 mg/ml MCh aerosol. After overnight recovery, each mouse was studied under both room and body temperature box air conditions to correct for gas compression effects on the control tidal volume (Vt) and to determine Vt and Raw with MCh. Airway diameter (D), FRC, and Vt were measured from the X-ray images. Compared with control, D decreased by 24%, frequency decreased by 35%, FRC increased by 120%, and Raw doubled, to reach limiting values with 100 mg/ml MCh. Vt was unchanged with MCh. The limiting D occurred near zero airway elastic recoil, where the maximal contractility was relatively small. The conscious mouse adapted to MCh by breathing at a higher lung volume and reduced frequency to reach a limit in constriction.

Investigation of hyperpolarized 3He magnetic resonance imaging utility in examining human airway diameter behavior in asthma through comparison with high-resolution computed tomography

RATIONALE AND OBJECTIVES

Application of a previously developed model-based algorithm on hyperpolarized (HP) (3)He magnetic resonance (MR) dynamic projection images of phantoms was extended to investigate the utility of HP (3)He MR imaging (MRI) in quantifying airway caliber changes associated with asthma.

MATERIALS AND METHODS

Airways of seven volunteers were imaged and measured using HP (3)He MRI and multidetector-row computed tomography (MDCT) before and after a methacholine (MCh) challenge. MDCT data were obtained at functional residual capacity and 1 L above functional residual capacity.

RESULTS

Comparison of the resultant data showed that HP (3)He MRI did not match MDCT in measuring the ratios of airway calibers before and after the MCh challenge in 37% to 43% of the airways from the first six generations at the two lung volumes tested. However, MDCT did yield the observation that 49% to 69% of these airways displayed bronchodilation following MCh challenge.

CONCLUSION

The current implementation of HP (3)He MRI did not match the MCh-induced postchallenge-to-prechallenge airway caliber ratios as measured with MDCT. Elevated parenchymal tethering due to bronchoconstriction-induced hyperinflation was proposed as a possible explanation for this airway dilation.

Different effects of deep inspirations on central and peripheral airways in healthy and allergen-challenged mice

BACKGROUND

Deep inspirations (DI) have bronchodilatory and bronchoprotective effects in healthy human subjects, but these effects appear to be absent in asthmatic lungs. We have characterized the effects of DI on lung mechanics during mechanical ventilation in healthy mice and in a murine model of acute and chronic airway inflammation.

METHODS

Balb/c mice were sensitized to ovalbumin (OVA) and exposed to nebulized OVA for 1 week or 12 weeks. Control mice were challenged with PBS. Mice were randomly selected to receive DI, which were given twice during the minute before assessment of lung mechanics.

RESULTS

DI protected against bronchoconstriction of central airways in healthy mice and in mice with acute airway inflammation, but not when OVA-induced chronic inflammation was present. DI reduced lung resistance induced by methacholine from 3.8 +/- 0.3 to 2.8 +/- 0.1 cmH2O.s.mL-1 in healthy mice and 5.1 +/- 0.3 to 3.5 +/- 0.3 cmH2O.s.mL-1 in acute airway inflammation (both P < 0.001). In healthy mice, DI reduced the maximum decrease in lung compliance from 15.9 +/- 1.5% to 5.6 +/- 0.6% (P < 0.0001). This protective effect was even more pronounced in mice with chronic inflammation where DI attenuated maximum decrease in compliance from 44.1 +/- 6.6% to 14.3 +/- 1.3% (P < 0.001). DI largely prevented increased peripheral tissue damping (G) and tissue elastance (H) in both healthy (G and H both P < 0.0001) and chronic allergen-treated animals (G and H both P < 0.0001).

CONCLUSION

We have tested a mouse model of potential value for defining mechanisms and sites of action of DI in healthy and asthmatic human subjects. Our current results point to potent protective effects of DI on peripheral parts of chronically inflamed murine lungs and that the presence of DI may blunt airway hyperreactivity.

Comparison of airway diameter measurements from an anthropomorphic airway tree phantom using hyperpolarized 3He MRI and high-resolution computed tomography

An anthropomorphic airway tree phantom was imaged with both hyperpolarized (HP) 3He MRI using a dynamic projection scan and computed tomography (CT). Airway diameter measurements from the HP 3He MR images obtained using a newly developed model-based algorithm were compared against their corresponding CT values quantified with a well-established method. Of the 45 airway segments that could be evaluated with CT, only 14 airway segments (31%) could be evaluated using HP 3He MRI. No airway segments smaller than approximately 4 mm in diameter and distal to the fourth generation were adequate for analysis in MRI. For the 14 airway segments measured, only two airway segments yielded a non-equivalent comparison between the two imaging modalities, while eight more had inconclusive comparison results, leaving only four airway segments (29%) that satisfied the designed equivalence criteria. Some of the potential problems in airway diameter quantification described in the formulation of the model-based algorithm were observed in this study. These results suggest that dynamic projection HP 3He MRI may have limited utility for measuring airway segment diameters, particularly those of the central airways.

Heterogeneity of bronchoconstriction does not distinguish mild asthmatic subjects from healthy controls when supine

Heterogeneity is a fundamental property of airway constriction; however, whether it is a distinguishing feature of mild asthma is not clear. We used computerized tomography and the forced oscillation technique to compare lung heterogeneity between 18 mildly asthmatic and 19 healthy control subjects at similar levels of bronchoconstriction while subjects were supine. We also assessed the effects of deep inhalation and albuterol on supine lung mechanics. Measures of heterogeneity included lung attenuation, from which we derived a novel index of air-space size, and the frequency dependence of respiratory system resistance between 1 and 20 Hz. We found that asthmatic subjects had airways hyperresponsiveness to methacholine in the sitting position compared with controls, but both groups had similar falls in forced expiratory volume in 1 s after inhaling methacholine while supine. There were no baseline differences between the groups in the frequency dependence of resistance, or lung attenuation, before methacholine, and both groups responded similarly with an increase in air-space size (+9.2% vs. +3.4%), air-space size heterogeneity (+9.8% vs. +4.2%), and frequency dependence of resistance (+76% vs. +86%) after methacholine. Deep inhalation did not affect resistance in either group, but albuterol significantly reduced resistance in both groups. We conclude that both computerized tomography and the forced oscillation technique demonstrate increased heterogeneity of airway narrowing during induced bronchoconstriction while supine and that this heterogeneity is equivalent between subjects with mild asthma and healthy controls when bronchoconstricted to the same degree. Thus heterogeneity appears to be a fundamental feature of bronchoconstriction and is not unique to mild asthma.

End-expiratory and tidal volumes measured in conscious mice using single projection x-ray images

The evaluation of airway resistance (R(aw)) in conscious mice requires both end-expiratory (V(e)) and tidal volumes (V(t)) (Lai-Fook SJ and Lai YL. J Appl Physiol 98: 2204-2218, 2005). In anesthetized BALB/c mice we measured lung area (A(L)) from ventral-to-dorsal x-ray images taken at FRC (V(e)) and after air inflation with 0.25 and 0.50 ml (DeltaV(L)). Total lung volume (V(L)) described by equation: V(L) = DeltaV(L) + V(FRC) = KA(L)(1.5) assumed uniform (isotropic) inflation. Total V(FRC) averaged 0.55 ml, consisting of 0.10 ml tissue, 0.21 ml blood and 0.24 ml air. K averaged 1.84. In conscious mice in a sealed box, we measured the peak-to-peak box pressure excursions (DeltaP(b)) and x-rays during several cycles. K was used to convert measured A(L)(1.5) to V(L) values. We calculated V(e) and V(t) from the plot of V(L) vs. cos(alpha - phi). Phase angle alpha was the minimum point of the P(b) cycle to the x-ray exposure. Phase difference between the P(b) and V(L) cycles (phi) was measured from DeltaP(b) values using both room- and body-temperature humidified box air. A similar analysis was used after aerosol exposures to bronchoconstrictor methacholine (Mch), except that phi depended also on increased R(aw). In conscious mice, V(e) (0.24 ml) doubled after Mch (50-125 mg/ml) aerosol exposure with constant V(t), frequency (f), DeltaP(b), and R(aw). In anesthetized mice, in addition to an increased V(e), repeated 100 mg/ml Mch exposures increased both DeltaP(b) and R(aw) and decreased f to apnea in 10 min. Thus conscious mice adapted to Mch by limiting R(aw), while anesthesia resulted in airway closure followed by diaphragm fatigue and failure.

Complex airway behavior and paradoxical responses to bronchoprovocation

Heterogeneity of airway constriction and regional ventilation in asthma are commonly studied under the paradigm that each airway's response is independent from other airways. However, some paradoxical effects and contradictions in recent experimental and theoretical findings suggest that considering interactions among serial and parallel airways may be necessary. To examine airway behavior in a bronchial tree with 12 generations, we used an integrative model of bronchoconstriction, including for each airway the effects of pressure, tethering forces, and smooth muscle forces modulated by tidal stretching during breathing. We introduced a relative smooth muscle activation factor (Tr) to simulate increasing and decreasing levels of activation. At low levels of Tr, the model exhibited uniform ventilation and homogeneous airway narrowing. But as Tr reached a critical level, the airway behavior suddenly changed to a dual response with a combination of constriction and dilation. Ventilation decreased dramatically in a group of terminal units but increased in the rest. A local increase of Tr in a single central airway resulted in full closure, while no central airway closed under global elevation of Tr. Lung volume affected the response to both local and global stimulation. Compared with imaging data for local and global stimuli, as well as for the time course of airway lumen caliber during bronchoconstriction recovery, the model predictions were similar. The results illustrate the relevance of dynamic interactions among serial and parallel pathways in airway interdependence, which may be critical for the understanding of pathological conditions in asthma.

The effects of systemic lidocaine on airway tone and pulmonary function in asthmatic subjects

BACKGROUND

To prevent reflex-induced bronchoconstriction in patients with asthma, local anesthetics are commonly administered by aerosol or IV as adjunct medication. Lidocaine attenuates responsiveness to a neurally active stimulus that increases tone, but there is scant information about the effect of lidocaine on baseline airway tone. Therefore we examined the effects of IV lidocaine on baseline airway tone in asthmatic subjects.

METHODS

Small, medium, and large airways (2-5, 5-8, >8 mm diameter) were analyzed by computed tomography in 15 asthmatic volunteers under baseline conditions and during infusion of lidocaine. Changes in luminal airway diameter and wall thickness from baseline to during lidocaine infusion, and the change in pulmonary function induced by lidocaine, were analyzed.

RESULTS

Lidocaine caused a significant decrease in the forced expiratory volume in 1 s pulmonary function measure (7 +/- 2%, P = 0.006). There was also a small but significant decrease in the airway luminal diameter at total lung capacity during lidocaine infusion compared to baseline (-3 +/- 0.5%, P < 0.001). Moreover, there was a significant correlation between the change in forced expiratory volume in 1 s and the change in airway luminal diameter at total lung capacity (r2 = 0.47, P = 0.01).

CONCLUSION

Lidocaine, which reduces airway responsiveness to drugs that cause bronchospasm through sensory nerve activation, did not reduce baseline airway tone. Instead, even when administered IV, lidocaine significantly increased airway tone and caused airway narrowing. Therefore, while the administration of lidocaine can prevent intubation-induced bronchospasm, the airways should be constantly monitored by auscultation even during IV lidocaine administration.

Measurement of the internal diameter of plastic tubes from projection MR images using a model-based least-squares fit approach

Hyperpolarized (HP) 3He MRI is an emerging tool in the diagnosis and evaluation of pulmonary diseases involving bronchoconstriction, such as asthma. Previously, airway diameters from dynamic HP 3He MR images of the lung were assessed manually and subjectively, and were thus prone to uncertainties associated with human error and partial volume effects. A model-based algorithm capable of fully utilizing pixel intensity profile information and attaining subpixel resolution has been developed to measure surrogate airway diameters from HP 3He MR static projection images of plastic tubes. This goal was achieved by fitting ideal pixel intensity profiles for various diameter (6.4 to 19.1 mm) circular tubes to actual pixel intensity data. A phantom was constructed from plastic tubes of various diameters connected in series and filled with water mixed with contrast agent. Projection MR images were then taken of the phantom. The favorable performance of the model-based algorithm compared to manual assessment demonstrates the viability of our approach. The manual and algorithm approaches yielded diameter measurements that generally stayed within 1 x the pixel dimension. However, inconsistency of the manual approach can be observed from the larger standard deviations of its measured values. The method was then extended to HP 3He MRI, producing encouraging results at tube diameters characteristic of airways beyond the second generation, thereby justifying their application to lung airway imaging and measurement. Potential obstacles when measuring airway diameters using this method are discussed.

The structural basis of airways hyperresponsiveness in asthma

We hypothesized that structural airway remodeling contributes to airways hyperresponsiveness (AHR) in asthma. Small, medium, and large airways were analyzed by computed tomography in 21 asthmatic volunteers under baseline conditions (FEV1= 64% predicted) and after maximum response to albuterol (FEV1= 76% predicted). The difference in pulmonary function between baseline and albuterol was an estimate of AHR to the baseline smooth muscle tone (BSMT). BSMT caused an increase in residual volume (RV) that was threefold greater than the decrease in forced vital capacity (FVC) because of a simultaneous increase in total lung capacity (TLC). The decrease in FVC with BSMT was the major determinant of the baseline FEV1( P < 0.0001). The increase in RV correlated inversely with the relaxed luminal diameter of the medium airways ( P = 0.009) and directly with the wall thickness of the large airways ( P = 0.001). The effect of BSMT on functional residual capacity (FRC) controlled the change in TLC relative to the change in RV. When the FRC increased with RV, TLC increased and FVC was preserved. When the relaxed large airways were critically narrowed, FRC and TLC did not increase and FVC fell. With critical large airways narrowing, the FRC was already elevated from dynamic hyperinflation before BSMT and did not increase further with BSMT. FEV1/FVC in the absence of BSMT correlated directly with large airway luminal diameter and inversely with the fall in FVC with BSMT. These findings suggest that dynamic hyperinflation caused by narrowing of large airways is a major determinant of AHR in asthma.

Mechanisms of limited airway dimension with lung inflation

Airways distend with each inspiration, while a sigh or deep inspiration (DI) leads to a significant or a maximum distension of the airways. Distension of the airways is thought to play an important role in maintaining airway patency. Limited distension of the airways with lung inflation may be a major factor in certain lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). High resolution computed tomography (HRCT) has gained wide acceptance as a diagnostic and investigational radiological tool for the evaluation of airway function. HRCT has been used to measure dynamic changes in airway caliber in vivo that are not detectable by conventional global lung measurements such as airway and lung resistance. HRCT is uniquely capable of imaging and quantifying airway size at different lung volumes. The current paper reviews the use of HRCT to examine the role of lung inflation on airway distension in animal models, and discusses potential mechanisms for limited distension of the airways with lung inflation in individuals with asthma and COPD.

Bronchial reactivity in hyperresponsive patients and healthy individuals: demonstration with high resolution computed tomography

OBJECTIVE

High resolution computed tomography (HRCT) was used to assess the extent of bronchial reactivity after inhalative bronchoprovocation and dilation in hyperresponsive patients and healthy subjects.

PATIENTS AND METHODS

Patients with mild intermittent asthma, 15 with a >20% decrease in FEV1 and a >10 mmHg (PC20+) in PaO2, 12 with a <20% decrease in FEV1 and a >10 mmHg (PC20-) in PaO2 after provocation, and eight healthy humans were included in the study. Changes in cross-sectional area in a total of 1256 bronchi and in bronchial wall area (792 bronchi) were evaluated after histamine-triggered bronchoprovocation and salbutamol-induced bronchodilation at high lung volumes (FVC 80%). Data were compared with the results of pulmonary function tests (FEV1, PaO2, PaCO2).

RESULTS

In all groups, a significant decrease in bronchial cross-sectional area (P<0.001) and a significant increase in bronchial wall area (P<0.001) were observed subsequent to bronchoprovocation. After bronchodilation, the increase in cross-sectional area (P<0.001) and the further increase in airway wall area (P<0.01) were significant in all groups. In PC20+ and PC20- asthmatics, significant differences (P<0.05) in PaO2, >10 mmHg between baseline and provocation were observed. In healthy persons, the PaO2 decrease was <10 mmHg (P>0.05). After histamine provocation, the decrease in FEV1 was measured in the PC20+ group, whereas a <20% FEV1 decrease was found in the PC20- and the control groups, respectively. No significant correlations were observed between radiological data and the results of pulmonary function tests.

CONCLUSIONS

HRCT demonstrated bronchial reactivity in hyperresponsive patients and, unexpectedly, in healthy subjects. The applied pulmonary function tests failed to characterize bronchial reactions in the healthy subjects. Based on these results, HRCT is a useful tool by which to achieve a comprehensive understanding of the pathophysiological processes in asthmatic patients.

Deep inspiration avoidance and methacholine response in normal subjects and patients with asthma

BACKGROUND

Deep inspiration (DI) avoidance and time intervals between inhalation and measurement of FEV1 may influence methacholine challenges.

OBJECTIVES

(1) To compare the degree of airway response to methacholine when the initial FEV1 measurements are obtained either 30 s or 3 min after inhalation, (2) to evaluate a simplified method to study the influence of DI avoidance before inhalation on the fall in FEV1, and (3) to determine if methacholine has a cumulative effect.

PARTICIPANTS/METHODS

Twenty-five patients with asthma and 21 normal subjects without asthma. Four methacholine inhalation tests (MITs) were performed: two standard tidal-breathing MITs, with the first FEV1 measured 30 s (test A) and 3 min (test B) after the end of inhalation; a single-dose MIT, using the last concentration from test B, with no control of DI and the first FEV1 obtained 3 min after inhalation (test C); and an identical single-dose MIT preceded by 20-min of DI avoidance (test D). We compared the provocative concentration of methacholine causing a 20% fall in FEV1 (PC20) from tests A and B (aim 1), the percentage fall in FEV1 from tests C and D (aim 2), and the percentage fall in FEV1 from tests B and C (aim 3).

RESULTS

Mean PC20 values from tests A and B were 1.5 mg/mL and 1.0 mg/mL (p = 0.002) in patients with asthma, and 69.8 mg/mL and 29.9 mg/mL (p < 0.0001) in control subjects, respectively. The mean falls in FEV1 for tests C and D were 22.0% and 24.5% (p > 0.05) in patients with asthma, and 22.1% and 38.9% (p = 0.0005) in control subjects, respectively. The mean falls in FEV2 for tests B and C were 30.2% and 22.0% (p = 0.01) in patients with asthma, and 27.5% and 22.1% (p > 0.05) in control subjects, respectively.

CONCLUSIONS

In both groups, the longer the time interval between the end of inhalation and the first FEV2 measurement, the greater the fall in FEV2 (lower PC20). DI avoidance before inhalation does not enhance the fall in FEV2 in subjects with asthma, while it does in control subjects. Methacholine has a slight cumulative effect that is significant in patients with asthma (p = 0.007).

Morphometric changes after thermal and methacholine bronchoprovocations

To determine whether there are distinctions in the location and pattern of response between different bronchoprovocations, we performed high-resolution computer-assisted tomography in 10 asthmatic subjects before and after isocapnic hyperventilation of frigid air (HV) and methacholine (Meth). The luminal areas of the trachea, main stem, lobar, and segmental bronchi were computed before and after each provocation and blindly compared. Both stimuli reduced the 1-s forced expiratory volume similarly (percent change in 1-s forced expiratory volume HV = 28.1 +/- 5.5%, Meth = 25.8 +/- 5.2%; P = 0.69) but did so in different fashions. Each provocation was associated with the development of both bronchial narrowing and dilation; however, more airways constricted with HV (67.7%) than with Meth (47.0%; P < 0.001). Furthermore, there was little concordance between either the magnitude or direction of change between stimuli in any region of the lung (r = 0.25). In general, the frequency of narrowing increased with branching. Constriction became more prominent in the lobar regions and increased further in the segmental branches, but a wide range of intensity existed. These data demonstrate that provocational stimuli evoke complex morphometric changes within the tracheobronchial tree and that different agonists produce different patterns. Thermal stimuli chiefly influence the segmental level, whereas the response to Meth develops more distally. Even within this distribution, the same airway does not respond in an identical fashion to different stimuli, so there does not appear to be a uniform trigger zone.

Intraluminal pressure oscillation enhances subsequent airway contraction in isolated bronchial segments

A period of deep inspiration in humans has been shown to attenuate subsequent bronchoconstriction, a phenomenon termed bronchoprotection. The bronchoprotective effect of deep inspiration may be caused though a depression in the force production of airway smooth muscle (ASM). We determined the response of whole airway segments and isolated ASM to a period of cyclic stretches. Isovolumetric contraction to electrical field stimulation (EFS) was assessed in porcine bronchial segments before and after intraluminal pressure oscillation from 5 to 25 cmH2O for 10 min at 0.5 Hz. Morphometry showed that this pressure oscillation stretched ASM length by 21%. After pressure oscillation, the response to EFS was not reduced but instead was modestly enhanced ( P < 0.01). Airway responses to EFS returned to preoscillation levels 10 min after the end of oscillation. The increase in EFS response after pressure oscillation was not altered by the addition of indomethacin. In a separate experiment, we assessed isometric force in isolated ASM strips before and after length oscillation. The amplitude, frequency, and duration of length oscillation were similar to those induced in bronchial segments. In contrast to bronchial segments, length oscillation of ASM produced a significant depression in isometric force induced by EFS ( P < 0.01). These results suggest that the response of ASM to length oscillation is modified by the airway wall. They also suggest that the phenomenon of bronchoprotection reported in some in vivo studies may not be an intrinsic property of the airway.

Heart rate variability biofeedback increases baroreflex gain and peak expiratory flow

OBJECTIVE

We evaluated heart rate variability biofeedback as a method for increasing vagal baroreflex gain and improving pulmonary function among 54 healthy adults.

METHODS

We compared 10 sessions of biofeedback training with an uninstructed control. Cognitive and physiological effects were measured in four of the sessions.

RESULTS

We found acute increases in low-frequency and total spectrum heart rate variability, and in vagal baroreflex gain, correlated with slow breathing during biofeedback periods. Increased baseline baroreflex gain also occurred across sessions in the biofeedback group, independent of respiratory changes, and peak expiratory flow increased in this group, independently of cardiovascular changes. Biofeedback was accompanied by fewer adverse relaxation side effects than the control condition.

CONCLUSIONS

Heart rate variability biofeedback had strong long-term influences on resting baroreflex gain and pulmonary function. It should be examined as a method for treating cardiovascular and pulmonary diseases. Also, this study demonstrates neuroplasticity of the baroreflex.

Understanding airway pathophysiology with computed tomograpy

Conventional pulmonary function tests are limited in the mechanistic insight that they can provide by the fact that they can only provide average measures of lung function. For example, a measurement of decreased expiratory flow assessed with conventional spirometry could result from narrowed large airways, narrowed small airways, closed airways, altered elasticity, or regional heterogeneities in parenchyma or airways. To examine specific mechanisms and pathology in the airways, a method is required that can actually look at specific individual airways. Over the past decade, several more direct methods of assessing specific mechanisms and structural alterations in normal airways and airway pathology in asthma have become available for such purposes. One such method is high-resolution computed tomography (HRCT), a method that allows the study of multiple individual airways during either contraction to closure or relaxation in real time, as well as changes in airway size with changes in lung volume. Although other imaging modalities have the potential to image airways in vivo, none presently has the convenience and the accessibility coupled with the resolution required to visualize the parenchymal airways in vivo. Although HRCT may never be widely utilized for routine measurements or screening, because of radiation exposure, cost issues, and a limited ability to follow changes over extended time periods, the method has distinct and unique advantages in quantifying the behavior of airways in vivo. In this mini-review, we focus on these capabilities of HRCT by briefly reviewing highlights of experimental results from several canine and human studies.

Absence of deep inspiration-induced bronchoprotection against inhaled allergen

Deep inspiration-induced bronchoprotection appears to be a major mechanism through which airway obstruction by spasmogens is avoided. Loss of bronchoprotection is associated with airway hyper-responsiveness. Individuals with allergic rhinitis and no airway hyperresponsiveness develop obstruction after allergen inhalation. To test the hypothesis that deep inspiration-induced bronchoprotection is not active against allergic reactions, we performed four single-dose bronchial challenges, two with methacholine and two with allergen, on 10 subjects with allergic rhinitis. Without deep inspirations, the methacholine-induced reduction in FEV1 from baseline was 36.9 +/- 3.6% (mean +/- SEM); this was attenuated to 15.0 +/- 2.0 when five deep inspirations preceded methacholine inhalation (p = 0.0001). When allergen was inhaled, the reduction in FEV1 was 24.7 +/- 2.9% and 28.8 +/- 6.4% without and with deep inspirations, respectively. We conclude that bronchoprotection by deep inspirations is absent against allergic reactions. Understanding the cause of this phenomenon may shed light into the pathogenesis of airway hyperresponsiveness in allergic asthma.

Bronchodilation and bronchoprotection by deep inspiration and their relationship to bronchial hyperresponsiveness

Bronchial hyperresponsiveness (BHR) is a cardinal feature of asthma. Airway inflammation and BHR are probably linked, but the mechanisms underlying this relationship remain elusive. BHR is closely associated with defects in the beneficial responses to lung inflation. These responses, which become apparent by the fact that healthy individuals can develop severe airway obstruction if they are exposed to methacholine in the absence of deep inspirations, include bronchodilation and bronchoprotection. Bronchodilation refers to the effect of lung inflation after the induction of airway smooth muscle tone, while bronchoprotection is used to indicate the effect prior to inhalation of a spasmogen. Mild asthmatics who manifest BHR lack bronchoprotection by lung inflation. In contrast, many of them are able to bronchodilate. In more severe disease, both functions are impaired. The lack of bronchoprotection is also found in individuals with rhinitis and BHR, but no asthma. These and other observations suggest that the mechanisms of bronchodilation and bronchoprotection may be distinct, although overlap is possible. We believe that the loss of bronchoprotection is pertinent to the phenomenon of hyperresponsiveness, but that both the bronchodilatory and bronchoprotective functions of deep inspiration interact to produce the asthmatic phenotype. In this review, we describe the phenomena of lung inflation-induced bronchodilation and bronchoprotection and detail potential mechanical and neurohumoral mechanisms accounting for these physiologic functions. In addition, possible mechanisms leading to the impairment of these functions in subjects with BHR are discussed.

Psychological aspects of asthma

Asthma can be affected by stress, anxiety, sadness, and suggestion, as well as by environmental irritants or allergens, exercise, and infection. It also is associated with an elevated prevalence of anxiety and depressive disorders. Asthma and these psychological states and traits may mutually potentiate each other through direct psychophysiological mediation, nonadherence to medical regimen, exposure to asthma triggers, and inaccuracy of asthma symptom perception. Defensiveness is associated with inaccurate perception of airway resistance and stress-related bronchoconstriction. Asthma education programs that teach about the nature of the disease, medications, and trigger avoidance tend to reduce asthma morbidity. Other promising psychological interventions as adjuncts to medical treatment include training in symptom perception, stress management, hypnosis, yoga, and several biofeedback procedures.

Mild intermittent asthma: CT assessment of bronchial cross-sectional area and lung attenuation at controlled lung volume

PURPOSE

To evaluate, with thin-section computed tomography (CT), changes in bronchial cross-sectional area and lung attenuation induced by bronchial stimulation in patients with mild intermittent asthma, at a given lung volume monitored with pneumotachography.

MATERIALS AND METHODS

Twelve patients with mild intermittent asthma who were nonsmokers (National Institutes of Health staging) and six nonsmoking healthy volunteers, age and sex ratio-matched, were examined by using helical thin-collimation CT at the level of basal bronchi at 65% of total lung capacity. Three sets of acquisitions were obtained: at baseline and after inhalation of methacholine and then salbutamol. Cross-sectional areas of bronchi greater than 4 mm(2) were segmented and calculated from CT images. Lung attenuation was measured in the anterior, lateral, and posterior areas of the right lung parenchyma. Gas trapping was evaluated by using thin-section CT at residual volume in six of the patients with asthma. Statistical analysis included two factors repeated-measurement analysis of variance and Mann-Whitney and Kruskal-Wallis nonparametric tests.

RESULTS

Bronchial cross-sectional areas and lung attenuation did not vary significantly compared with baseline values following bronchial challenge in healthy volunteers or patients with asthma. However, in patients with asthma, bronchial cross-sectional areas were significantly smaller than in healthy volunteers, except after inhalation of salbutamol. Lung attenuation and anteroposterior attenuation gradient were significantly higher in patients with asthma than in healthy patients (P <.001). Air-trapping scores were significantly higher after methacholine challenge.

CONCLUSION

Helical thin-collimation CT at controlled lung volume and at full expiration associated with bronchial challenge may help evaluate bronchoreactivity and inflammation in mild intermittent asthma.

The role of nerves in asthma

Asthma is a syndrome characterized by reversible episodes of wheezing, cough, and sensations of chest tightness and breathlessness. These symptoms are secondary to changes in the activity of the nervous system. The mechanisms by which the nervous system is altered such that the symptoms of asthma occur have not yet been elucidated. Airway inflammation associated with asthma may affect neuronal activity at several points along the neural reflex pathway, including the function of the primary afferent (sensory) nerves, integration within the central nervous system, synaptic transmission within autonomic ganglia, and transmission at the level of the postganglionic neuroeffector junction. We provide a brief overview of these interactions and the relevance they may have to asthma.

Decreased pulmonary vascular permeability in aquaporin-1-null humans

The molecular determinants of water permeability in the human lung are incompletely defined. Aquaporins (AQP) are water-specific membrane channel proteins. AQP1 is present in endothelial cells in the lung, including those in the vascular plexus around the airways. Rare individuals have been identified who are deficient in AQP1. High-resolution computed tomography scans of the lung were used to evaluate the response to i.v. fluid challenge in two unrelated AQP1-null individuals and five normal controls. The airways and pulmonary vessels were measured at baseline and after i.v. administration of 3 liters of saline. Increases in airway wall thickness after fluid administration reflect peribronchiolar edema formation. Both control and AQP1 null subjects had approximately a 20% increase in pulmonary vessel area in response to saline infusion, suggesting similar degrees of volume loading. Control subjects had a 44% increase in the thickness of the airway wall, consistent with peribronchiolar edema formation. In marked contrast, airway wall thickness did not change in AQP1-null subjects in response to saline infusion. These studies indicate that AQP1 is a determinant of vascular permeability in the lung, and demonstrate a role for aquaporins in human pulmonary physiology.

Effects of tidal volume stretch on airway constriction in vivo

Tidal stresses are thought to be involved in maintaining airway patency in vivo. The present study examined the effects of normal stresses exerted by the lung parenchyma during tidal ventilation on recovery from agonist-induced airway constriction. In seven anesthetized dogs, one lung was selectively ventilated with a Univent endotracheal tube (Vitaid, Lewiston, NY). Airway tone was increased either transiently (intravenous bolus) or continuously (intravenous infusion) with methacholine (MCh). During one-lung ventilation, changes in the airway size of both lungs were measured for up to 40 min during recovery from constriction by using high-resolution computed tomography. After recovery to baseline, the alternate lung was ventilated, and the protocol was repeated. The absence of tidal stresses led to an attenuated recovery from either transient or steady-state airway constriction. The effectiveness or lack thereof of normal tidal stress in stabilizing airway size may be one factor that contributes to the lack of reversal with tidal breathing and deep inspiration seen in asthmatic subjects.

Can peak expiratory flow measurements estimate small airway function in asthmatic children?

BACKGROUND

Asthma is characterized in part by small airways dysfunction. Peak expiratory flow (PEF) measurement has been suggested by all international guidelines as an important tool in asthma management. The correlation between PEF and FEV(1) but not with forced expired flow at 50% of vital capacity (FEF(50)) is well-established.

STUDY OBJECTIVE

To determine the value of PEF measurement as a predictor of small airways status as expressed by FEF(50).

DESIGN

Analysis of the association between PEF and FEF(50) in single and multiple determinations.

PATIENTS

One hundred eleven asthmatic children (mean age, 11.8 years), grouped in the following way according to FEV(1) values: within normal range (n = 46); mildly reduced FEV(1) (n = 44); and moderately/severely reduced FEV(1) (n = 21).

RESULTS

Overall, FEF(50) and PEF were significantly correlated (r = 0.49; p < 0.0001). However, in 41.6% of the patients, the actual FEF(50) differed by > 20% from the calculated FEF(50). PEF has a high specificity (82.4%) but a poor sensitivity (51.7%) to detect FEF(50) status. PEF was better able to reflect abnormal FEF(50) in the patients with more severe asthma and to reflect normal FEF(50) values in the healthier patients. In patients with multiple measurements (n = 40), the correlation between FEF(50) and PEF was significantly better than that derived from a single determination (multiple measurements r = 0.77; single measurement, r = 0.49).

CONCLUSIONS

Although PEF is an important tool in the management of asthmatic patients, it does not yield a complete picture because it is not sensitive in detecting small airways function. It is best used at home along with regular spirometry measurements at the clinic. PEF may serve as a better index of changes in small airways function once an individual regression is determined.

High-resolution computed tomographic evaluation of airway distensibility and the effects of lung inflation on airway caliber in healthy subjects and individuals with asthma

The effects of a deep inspiration (DI) in individuals with asthma differ from those observed in healthy subjects. It has been postulated that the beneficial effect of lung inflation is mediated by airway stretch. One hypothesis to explain the defects in the function of lung inflation in asthma is that a DI may be unable to stretch the airways. This may result from attenuation of the tethering forces between the airways and the surrounding parenchyma. In the current study, we used high-resolution computed tomography (HRCT) to examine the ability of a DI to distend the airways of subjects with asthma (n = 10) compared with healthy subjects (n = 9) at baseline and after increasing airway tone with methacholine (MCh). We found that both at baseline and after the induction of smooth muscle tone with MCh, a DI distended the airways of healthy and asthmatic subjects to a similar extent, indicating that abnormal interdependence between the lung parenchyma and the airways is unlikely to play a major role in the loss or attenuation of the beneficial effect of lung inflation that characterizes asthma. Furthermore, we observed that after constriction had already been induced by MCh, following a DI, bronchodilation occurred in the healthy subjects but further bronchoconstriction occurred in the subjects with asthma. Our findings suggest that an abnormal excitation contraction mechanism in the airway smooth muscle of subjects with mild asthma counteracts the bronchodilatory effect of a DI. Therefore, the mechanism for reduced bronchodilation after DIs in subjects with mild asthma could be intrinsic to the airway smooth muscle.

Airway closure with high PEEP in vivo

When airway smooth muscle is contracted in vitro, the airway lumen continues to narrow with increasing concentrations of agonist until complete airway closure occurs. Although there remains some controversy regarding whether airways can close in vivo, recent work has clearly demonstrated that, if the airway is sufficiently stimulated with contractile agonists, complete closure of even large cartilaginous conducting airways can readily occur with the lung at functional residual capacity (Brown RH and Mitzner W. J Appl Physiol 85: 2012-2017, 1998). This result suggests that the tethering of airways in situ by parenchymal attachments is small at functional residual capacity. However, at lung volumes above functional residual capacity, the outward tethering of airways should increase, because both the parenchymal shear modulus and tethering forces increase in proportion to the transpulmonary pressure. In the present study, we tested whether we could prevent airway closure in vivo by increasing lung volume with positive end-expiratory pressure (PEEP). Airway smooth muscle was stimulated with increasing methacholine doses delivered directly to airway smooth muscle at three levels of PEEP (0, 6, and 10 cmH(2)O). Our results show that increased lung volume shifted the airway methacholine dose-response curve to the right, but, in many airways in most animals, airway closure still occurred even at the highest levels of PEEP.