H W Mitchell

University of Western Australia, Perth City, Western Australia, Australia

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Publications (62)232.03 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: During deep inspirations (DI) a distending force is applied to airway smooth muscle (ASM, i.e. stress) and the muscle is lengthened (i.e. strain), which produces a transient reversal of bronchoconstriction (i.e. bronchodilation). The aim of the present study was to determine whether an increase in ASM stress or the accompanying increase in strain mediates the bronchodilatory response to DI. We used whole porcine bronchial segments in vitro and a servo-controlled syringe pump that applied fixed-transmural pressure (Ptm) or fixed-volume oscillations, simulating tidal breathing and DI. The relationship between ASM stress and strain during oscillation was altered by increasing doses of acetylcholine, which stiffened the airway wall, or by changing the rate-of-inflation during DI, which utilised the viscous properties of the intact airway. Bronchodilation to DI was positively correlated with ASM strain (range of r values from 0.81 to 0.95) and negatively correlated with stress (range of r values from -0.42 to -0.98). Fast fixed-Ptm DI produced greater bronchodilation than slow DI, despite less ASM strain. Fast fixed-volume DI produced greater bronchodilation than slow DI, despite identical ASM strain. We show that ASM strain, rather than stress, is the critical determinant of bronchodilation and unexpectedly, that the rate-of-inflation during DI also impacts on bronchodilation, independent of the magnitudes of either stress or strain.
    Journal of Applied Physiology 05/2013; · 3.48 Impact Factor
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    ABSTRACT: The present study presents preliminary findings on how structural/functional abnormalities of the airway wall relate to excessive airway narrowing and reduced bronchodilatory response to deep inspiration (DI) in subjects with a history of asthma. Bronchial segments were acquired from subjects undergoing surgery, mostly to remove pulmonary neoplasms. Subjects reported prior doctor-diagnosed asthma (n=5) or had no history of asthma (n=8). In vitro airway narrowing in response to acetylcholine was assessed to determine maximal bronchoconstriction and sensitivity, under static conditions and during simulated tidal and DI manoeuvres. Fixed airway segments were sectioned for measurement of airway wall dimensions, particularly the ASM layer. Airways from subjects with a history of asthma had increased airway smooth muscle (ASM, P=0.014), greater maximal airway narrowing under static conditions (P=0.003) but no change in sensitivity. Maximal airway narrowing was positively correlated with the area of the ASM layer (r=0.58, P=0.039). In tidally oscillating airways, DI produced bronchodilation in airways from the control group (P=0.0001) and the group with a history of asthma (P=0.001). While bronchodilation to DI was reduced with increased airway narrowing (P=0.02), when the level of airway narrowing was matched, there was no difference in magnitude of bronchodilation to DI between groups. Results suggest that greater ASM mass in asthma contributes to exaggerated airway narrowing in vivo. In comparison, the airway wall in asthma may have a normal response to mechanical stretch during DI. We propose that increased maximal airway narrowing and the reduced bronchodilatory response to DI in asthma are independent.
    Journal of Applied Physiology 03/2013; · 3.48 Impact Factor
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    ABSTRACT: In a healthy human, deep inspirations produce bronchodilation of contracted airways, which probably occurs due to the transient distension of the airway smooth muscle (ASM). We hypothesised that deep expiratory manoeuvres also produce bronchodilation due to transient airway wall and ASM compression. We used porcine bronchial segments to assess the effects of deep inspirations, and maximal and partial expiration (submaximal) on airway calibre. Respiratory manoeuvres were simulated by varying transmural pressure using a hydrostatic pressure column: deep inspiration 5 to 30 cmH(2)O, maximal expiration 30 to -15 cmH(2)O, partial expiration 10 to -15 cmH(2)O; amidst a background of tidal oscillations, 5 to 10 cmH(2)O at 0.25 Hz. Changes in luminal cross-sectional area in carbachol-contracted airways were measured using video endoscopy. Deep inspirations produce an immediate bronchodilation (∼40-60%, p=0.0076) that lasts for up to 1 min (p=0.0479). In comparison, after maximal expiration there was no immediate change in airway calibre; however, a delayed bronchodilatory response was observed from 4 s after the manoeuvre (p=0.0059) and persisted for up to 3 min (p=0.0182). Partial expiration had little or no effect or airway calibre. The results observed demonstrate that the airway wall dilates to deep inspiration manoeuvres but is unresponsive to deep expiratory manoeuvres.
    European Respiratory Journal 01/2012; 40(2):455-61. · 6.36 Impact Factor
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    ABSTRACT: The primary functional abnormality in asthma is airway hyperresponsiveness (AHR)-excessive airway narrowing to bronchoconstrictor stimuli. Our understanding of the underlying mechanism(s) producing AHR is incomplete. While structure-function relationships have been evoked to explain AHR (e.g., increased airway smooth muscle (ASM) mass in asthma) more recently there has been a focus on how the dynamic mechanical environment of the lung impacts airway responsiveness in health and disease. The effects of breathing movements such as deep inspiration reveal innate protective mechanisms in healthy individuals that are likely mediated by dynamic ASM stretch but which may be impaired in asthmatic patients and thereby facilitate AHR. This perspective considers the evidence for and against a role of dynamic ASM stretch in limiting the capacity of airways to narrow excessively. We propose that lung function measured after bronchial provocation in the laboratory and changes in lung function perceived by the patient in everyday life may be quite different in their dependence on dynamic ASM stretch.
    Journal of Allergy 01/2012; 2012:157047.
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    ABSTRACT: In healthy individuals, deep inspiration produces bronchodilation and reduced airway responsiveness, which may be a response of the airway wall to mechanical stretch. The aim of this study was to examine the in vitro response of isolated human airways to the dynamic mechanical stretch associated with normal breathing. Human bronchial segments (n = 6) were acquired from patients without airflow obstruction undergoing lung resection for pulmonary neoplasms. The side branches were ligated and the airways were mounted in an organ bath chamber. Airway narrowing to cumulative concentrations of acetylcholine (3 × 10(-6) M to 3 × 10(-3) M) was measured under static conditions and in the presence of "tidal" oscillations with intermittent "deep inspiration." Respiratory maneuvers were simulated by varying transmural pressure using a motor-controlled syringe pump (tidal 5 to 10 cmH(2)O at 0.25 Hz, deep inspiration 5 to 30 cmH(2)O). Airway narrowing was determined from decreases in lumen volume. Tidal oscillation had no effect on airway responses to acetylcholine which was similar to those under static conditions. Deep inspiration in tidally oscillating, acetylcholine-contracted airways produced potent, transient (<1 min) bronchodilation, ranging from full reversal in airway narrowing at low acetylcholine concentrations to ∼50% reversal at the highest concentration. This resulted in a temporary reduction in maximal airway response (P < 0.001), without a change in sensitivity to acetylcholine. Our findings are that the mechanical stretch of human airways produced by physiological transmural pressures generated during deep inspiration produces bronchodilation and a transient reduction in airway responsiveness, which can explain the beneficial effects of deep inspiration in bronchial provocation testing in vivo.
    Journal of Applied Physiology 02/2011; 110(6):1510-8. · 3.48 Impact Factor
  • Journal of Applied Physiology 09/2010; 109(3):938-9; author reply 940-1. · 3.48 Impact Factor
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    ABSTRACT: Previous histological and imaging studies have shown the presence of variability in the degree of bronchoconstriction of airways sampled at different locations in the lung (i.e., heterogeneity). Heterogeneity can occur at different airway generations and at branching points in the bronchial tree. Whilst heterogeneity has been detected by previous experimental approaches, its spatial relationship either within or between airways is unknown. In this study, distribution of airway narrowing responses across a portion of the porcine bronchial tree was determined in vitro. The portion comprised contiguous airways spanning bronchial generations (#3-11), including the associated side branches. We used a recent optical imaging technique, anatomical optical coherence tomography, to image the bronchial tree in three dimensions. Bronchoconstriction was produced by carbachol administered to either the adventitial or luminal surface of the airway. Luminal cross sectional area was measured before and at different time points after constriction to carbachol and airway narrowing calculated from the percent decrease in luminal cross sectional area. When administered to the adventitial surface, the degree of airway narrowing was progressively increased from proximal to distal generations (r = 0.80 to 0.98, P < 0.05 to 0.001). This 'serial heterogeneity' was also apparent when carbachol was administered via the lumen, though it was less pronounced. In contrast, airway narrowing was not different at side branches, and was uniform both in the parent and daughter airways. Our findings demonstrate that the bronchial tree expresses intrinsic serial heterogeneity, such that narrowing increases from proximal to distal airways, a relationship that is influenced by the route of drug administration but not by structural variations accompanying branching sites.
    Respiratory research 01/2010; 11:9. · 3.64 Impact Factor
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    ABSTRACT: Regulation of airway caliber by lung volume or bronchoconstrictor stimulation is dependent on physiological, structural, and mechanical events within the airway wall, including airway smooth muscle (ASM) contraction, deformation of the mucosa and cartilage, and tensioning of elastic matrices linking wall components. Despite close association between events in the airway wall and the resulting airway caliber, these have typically been studied separately: the former primarily using histological approaches, the latter with a range of imaging modalities. We describe a new optical technique, anatomical optical coherence tomography (aOCT), which allows changes at the luminal surface (airway caliber) to be temporally related to corresponding dynamic movements within the airway wall. A fiber-optic aOCT probe was inserted into the lumen of isolated, liquid-filled porcine airways. It was used to image the response to ASM contraction induced by neural stimulation and to airway inflation and deflation. Comparisons with histology indicated that aOCT provided high-resolution images of the airway lumen including mucosal folds, the entire inner wall (mucosa and ASM), and partially the cartilaginous outer wall. Airway responses assessed by aOCT revealed several phenomena in "live" airways (i.e., not fixed) previously identified by histological investigations of fixed tissue, including a geometric relationship between ASM shortening and luminal narrowing, and sliding and bending of cartilage plates. It also provided direct evidence for distensibility of the epithelial membrane and anisotropic behavior of the airway wall. Findings suggest that aOCT can be used to relate changes in airway caliber to dynamic events in the wall of airways.
    Journal of Applied Physiology 11/2009; 108(2):401-11. · 3.48 Impact Factor
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    ABSTRACT: In adults, respiratory movements, such as tidal and deep breaths, reduce airway smooth muscle force and cause bronchodilation. Evidence suggests that these beneficial effects of oscillatory strain do not occur in children, possibly because of reduced coupling of the airways to lung tissue or maturational differences in the intrinsic response of the airways to oscillatory strain. The bronchodilator effects of oscillatory strain were compared in isolated airway segments from immature (3-4 weeks and 8-10 weeks old) and mature (18-20 weeks old) pigs. The lumen of fluid-filled bronchi was volume-oscillated to simulate tidal breaths and 0.5x, 2x and 4x tidal volumes. Contractions to acetylcholine and electrical field stimulation were recorded from the lumen pressure and were compared under oscillating and static conditions. Airway stiffness was determined from the amplitude of the lumen pressure cycles and the volume of oscillation. Volume oscillation reduced contractions to acetylcholine and electrical field stimulation in an amplitude-dependent manner and the percentage reduction was the same for the different stimuli across all age groups. There was no difference in the relaxed dynamic stiffness of airways from the different age groups. The intrinsic response of the airway wall to equivalent dynamic strain did not differ in airways from pigs of different ages. These findings suggest that mechanisms external to the airway wall may produce age-related differences in the response to lung inflation during development.
    Respirology 08/2009; 14(7):991-8. · 2.78 Impact Factor
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    ABSTRACT: Airway dimensions are difficult to quantify bronchoscopically because of optical distortion and a limited ability to gauge depth. Anatomical optical coherence tomography (aOCT), a novel imaging technique, may overcome these limitations. This study evaluated the accuracy of aOCT against existing techniques in phantom, excised pig and in vivo human airways. Three comparative studies were performed: 1) micrometer-derived area measurements in 10 plastic tubes were compared with aOCT-derived area; 2) aOCT-derived airway compliance curves from excised pig airways were compared with curves derived using an endoscopic technique; and 3) airway dimensions from the trachea to subsegmental bronchi were measured using aOCT in four anaesthetised patients during bronchoscopy and compared with computed tomography (CT) measurements. Measurements in plastic tubes revealed aOCT to be accurate and reliable. In pig airways, aOCT-derived compliance measurements compared closely with endoscopic data. In human airways, dimensions measured with aOCT and CT correlated closely. Bland-Altman plots showed that aOCT diameter and area measurements were higher than CT measurements by 7.6% and 15.1%, respectively. Airway measurements using aOCT are accurate, reliable and compare favourably with existing imaging techniques. Using aOCT with conventional bronchoscopy allows real-time measurement of airway dimensions and could be useful clinically in settings where knowledge of airway calibre is required.
    European Respiratory Journal 07/2009; 35(1):34-41. · 6.36 Impact Factor
  • H W Mitchell
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    ABSTRACT: Past and contemporary views of airway smooth muscle (ASM) have led to a high level of understanding of the control and intracellular regulation of force or shortening of ASM and of its possible role in airway disease. As well as the multitude of cellular mechanisms that regulate ASM contraction, a number of structural and mechanical factors, which are only present at the airway and lung level, provide overriding control over ASM. With new knowledge about the cellular physiology and biology of ASM, there is increasing need to understand how ASM contraction is regulated and expressed at these airway and system levels.
    Pulmonary Pharmacology &amp Therapeutics 01/2009; 22(5):363-9. · 2.54 Impact Factor
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    ABSTRACT: Airway relaxation in response to isoprenaline, sodium nitroprusside (SNP) and electrical field stimulation (EFS) was compared under static and dynamic conditions. The capacity of relaxants to reduce airway stiffness and, thus, potentially contribute to bronchodilation was also investigated. Relaxation responses were recorded in fluid filled bronchial segments from pigs under static conditions and during volume oscillations simulating tidal and twice tidal manoeuvres. Bronchodilation was assessed from the reduction in carbachol-induced lumen pressure, at isovolume points in pressure cycles produced by volume oscillation, and stiffness was assessed from cycle amplitudes. Under static conditions, all three inhibitory stimuli produced partial relaxation of the carbachol-induced contraction. Volume oscillation alone also reduced the contraction in an amplitude-dependent manner. However, maximum relaxation was observed when isoprenaline or SNP were combined with volume oscillation, virtually abolishing contraction at the highest drug concentrations. The proportional effects of isoprenaline and EFS were not different under static or oscillating conditions, whereas relaxation to SNP was slightly greater in oscillating airways. All three inhibitory stimuli also strongly reduced carbachol-induced airway stiffening. The current authors conclude that bronchoconstriction is strongly suppressed by combining the inhibitory stimulation of airway smooth muscle with cyclical mechanical strains. The capacity of airway smooth muscle relaxants to also reduce stiffness may further contribute to bronchodilation.
    European Respiratory Journal 12/2008; 33(4):844-51. · 6.36 Impact Factor
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    ABSTRACT: Contraction of airway smooth muscle (ASM) is regulated by the physiological, structural and mechanical environment in the lung. We review two in vitro techniques, lung slices and airway segment preparations, that enable in situ ASM contraction and airway narrowing to be visualized. Lung slices and airway segment approaches bridge a gap between cell culture and isolated ASM, and whole animal studies. Imaging techniques enable key upstream events involved in airway narrowing, such as ASM cell signalling and structural and mechanical events impinging on ASM, to be investigated.
    Pulmonary Pharmacology &amp Therapeutics 12/2008; 22(5):398-406. · 2.54 Impact Factor
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    ABSTRACT: Deep inspirations (DIs) are large periodic breathing maneuvers that regulate airway caliber and prevent airway obstruction in vivo. This study characterized the intrinsic response of the intact airway to DI, isolated from parenchymal attachments and other in vivo interactions. Porcine isolated bronchial segments were constricted with carbachol and subjected to transmural pressures of 5-10 cmH2O at 0.25 Hz (tidal breathing) interspersed with single DIs of amplitude 5-20 cmH2O, 5-30 cmH2O, or 5-40 cmH2O (6-s duration) or DI of amplitude 5-30 cmH2O (30-s duration). Tidal breathing was ceased after DI in a subset of airways and in control airways in which no DI was performed. Luminal cross-sectional area was measured using a fiber-optic endoscope. Bronchodilation by DI was amplitude dependent; 5-20 cmH2O DIs produced less dilation than 5-30 cmH2O and 5-40 cmH2O DIs (P=0.003 and 0.012, respectively). Effects of DI duration were not significant (P=0.182). Renarrowing after DI followed a monoexponential decay function to pre-DI airway caliber with time constants between 27.4+/-4.3 and 36.3+/-6.9 s. However, when tidal breathing was ceased after DI, further bronchoconstriction occurred within 30s. This response was identical in both the presence and absence of DI (P=0.919). We conclude that the normal bronchodilatory response to DI occurs as a result of the direct mechanical effects of DI on activated ASM in the airway wall. Further bronchoconstriction occurs by altering the airway wall stress following DI, demonstrating the importance of continual transient strains in maintaining airway caliber.
    Journal of Applied Physiology 07/2008; 105(2):479-85. · 3.48 Impact Factor
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    Howard Mitchell, Peter Noble, Peter McFawn
    Journal of Applied Physiology 11/2007; 103(4):1461; author reply 1466. · 3.48 Impact Factor
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    Peter B Noble, Peter K McFawn, Howard W Mitchell
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    ABSTRACT: In vivo, breathing movements, including tidal and deep inspirations (DIs), exert a number of beneficial effects on respiratory system responsiveness in healthy humans that are diminished or lost in asthma, possibly as a result of reduced distension (strain) of airway smooth muscle (ASM). We used bronchial segments from pigs to assess airway responsiveness under static conditions and during simulated tidal volume oscillations with and without DI and to determine the roles of airway stiffness and ASM strain on responsiveness. To simulate airway dilations during breathing, we cycled the luminal volume of liquid-filled segments. Volume oscillations (15 cycles/min) were set so that, in relaxed airways, they produced a transmural pressure increase of approximately 5-10 cmH(2)O for tidal maneuvers and approximately 5-30 cmH(2)O for DIs. ACh dose-response curves (10(-7)-3 x 10(-3) M) were constructed under static and dynamic conditions, and maximal response and sensitivity were determined. Airway stiffness was measured from tidal trough-to-peak pressure and volume cycles. ASM strain produced by DI was estimated from luminal volume, airway length, and inner wall area. DIs produced substantial ( approximately 40-50%) dilation, reflected by a decrease in maximal response (P < 0.001) and sensitivity (P < 0.05). However, the magnitude of bronchodilation decreased significantly in proportion to airway stiffening caused by contractile activation and an associated reduction in ASM strain. Tidal oscillations, in comparison, had little effect on responsiveness. We conclude that DI regulates airway responsiveness at the airway level, but this is limited by airway stiffness due to reduced ASM strain.
    Journal of Applied Physiology 09/2007; 103(3):787-95. · 3.48 Impact Factor
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    ABSTRACT: Excessive airway obstruction is the cause of symptoms and abnormal lung function in asthma. As airway smooth muscle (ASM) is the effecter controlling airway calibre, it is suspected that dysfunction of ASM contributes to the pathophysiology of asthma. However, the precise role of ASM in the series of events leading to asthmatic symptoms is not clear. It is not certain whether, in asthma, there is a change in the intrinsic properties of ASM, a change in the structure and mechanical properties of the noncontractile components of the airway wall, or a change in the interdependence of the airway wall with the surrounding lung parenchyma. All these potential changes could result from acute or chronic airway inflammation and associated tissue repair and remodelling. Anti-inflammatory therapy, however, does not "cure" asthma, and airway hyperresponsiveness can persist in asthmatics, even in the absence of airway inflammation. This is perhaps because the therapy does not directly address a fundamental abnormality of asthma, that of exaggerated airway narrowing due to excessive shortening of ASM. In the present study, a central role for airway smooth muscle in the pathogenesis of airway hyperresponsiveness in asthma is explored.
    European Respiratory Journal 06/2007; 29(5):834-60. · 6.36 Impact Factor
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    ABSTRACT: Mucosal trypsin, a protease-activated receptor (PAR) stimulant, may have an endogenous bronchoprotective role on airway smooth muscle. To test this possibility the effects of lumenal trypsin on airway tone in segments of pig bronchus were tested. Bronchial segments from pigs were mounted in an organ chamber containing Kreb's solution. Contractions were assessed from isovolumetric lumen pressure induced by acetylcholine (ACh) or carbachol added to the adventitia. Trypsin, added to the airway lumen (300 microg x mL(-1)), had no immediate effect on smooth muscle tone but suppressed ACh-induced contractions after 60 min, for at least 3 h. Synthetic activating peptides (AP) for PAR1, PAR2 or PAR3 were without effect, but PAR4 AP caused rapid, weak suppression of contractions. Lumenal thrombin was without effect and did not prevent the effects of trypsin. Effects of trypsin were reduced by N(omega)-nitro-L-arginine methyl ester but not indomethacin. Trypsin, thrombin and PAR4 AP released prostaglandin E2. Adventitially, trypsin, thrombin and PAR4 AP (but not PAR2 AP) relaxed carbachol-toned airways after <3 min. The findings of this study show that trypsin causes delayed and persistent bronchoprotection by interacting with airway cells accessible from the lumen. The signalling mechanism may involve nitric oxide synthase but not prostanoids or protease-activated receptors.
    European Respiratory Journal 01/2006; 27(1):20-8. · 6.36 Impact Factor
  • P B Noble, A Sharma, P K McFawn, H W Mitchell
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    ABSTRACT: The bronchial mucosa contributes to elastic properties of the airway wall and may influence the degree of airway expansion during lung inflation. In the deflated lung, folds in the epithelium and associated basement membrane progressively unfold on inflation. Whether the epithelium and basement membrane also distend on lung inflation at physiological pressures is uncertain. We assessed mucosal distensibility from strain-stress curves in mucosal strips and related this to epithelial length and folding. Mucosal strips were prepared from pig bronchi and cycled stepwise from a strain of 0 (their in situ length at 0 transmural pressure) to a strain of 0.5 (50% increase in length). Mucosal stress and epithelial length in situ were calculated from morphometric data in bronchial segments fixed at 5 and 25 cmH(2)O luminal pressure. Mucosal strips showed nonlinear strain-stress properties, but regions at high and low stress were close to linear. Stresses calculated in bronchial segments at 5 and 25 cmH(2)O fell in the low-stress region of the strain-stress curve. The epithelium of mucosal strips was deeply folded at low strains (0-0.15), which in bronchial segments equated to < or =10 cmH(2)O transmural pressure. Morphometric measurements in mucosal strips at greater strains (0.3-0.4) indicated that epithelial length increased by approximately 10%. Measurements in bronchial segments indicated that epithelial length increased approximately 25% between 5 and 25 cmH(2)O. Our findings suggest that, at airway pressures <10 cmH(2)O, airway expansion is due primarily to epithelial unfolding but at higher pressures the epithelium also distends.
    Journal of Applied Physiology 12/2005; 99(6):2061-6. · 3.48 Impact Factor
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    P B Noble, A Sharma, P K McFawn, H W Mitchell
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    ABSTRACT: During bronchoconstriction elastic after-loads arise due to distortion of lung parenchyma by the narrowing airway. In the present study, the functional effect of parenchymal elastic after-load on airway narrowing was determined. Airway narrowing was measured in vivo over a range of transpulmonary pressures and compared with in vitro narrowing measured at corresponding transmural pressures. Bronchi were generation 10 with internal diameters of approximately 4 mm. In vivo luminal narrowing was measured by videobronchoscopy in anaesthetised and ventilated pigs. In vitro luminal narrowing was measured by videoendoscopy in isolated bronchial segments. Airways were activated by maximum vagal nerve stimulation and maximum electrical field stimulation in vivo and in vitro, respectively. At 5 cmH2O, stimulation produced a 35.9+/-3.2% (n = 6) and a 36.5+/-2.4% (n = 11) decrease in lumen diameter in vivo and in vitro, respectively. At 30 cmH2O, luminal narrowing fell to 23.7+/-2.0% in vivo and 23.4+/-2.5% in vitro. There was no difference between luminal narrowing in vivo and in vitro at any pressure. In conclusion, these findings suggest that in mid-sized, cartilaginous bronchi, parenchymal elastic after-loads do not restrict airway narrowing.
    European Respiratory Journal 12/2005; 26(5):804-11. · 6.36 Impact Factor

Publication Stats

726 Citations
44 Downloads
3k Views
232.03 Total Impact Points

Institutions

  • 1988–2011
    • University of Western Australia
      • • School of Electrical, Electronic and Computer Engineering
      • • School of Anatomy, Physiology and Human Biology
      • • School of Medicine and Pharmacology
      • • School of Chemistry and Biochemistry
      Perth City, Western Australia, Australia
  • 2008
    • Boston University
      • Department of Biomedical Engineering
      Boston, MA, United States
  • 2001
    • Murdoch University
      Perth City, Western Australia, Australia