Duration of deep inspiration and subsequent airway constriction in vivo

Prestretched airway smooth muscle response to length oscillation

Airway smooth muscle (ASM) hyperconstriction is the cause of many respiratory diseases including asthma. In vitro testing has demonstrated that the active forces of ASM are reduced by length oscillation (LO) mimicking tidal breathing. In a previous study, we demonstrated that this force reduction can be further enhanced when superimposing oscillations (with certain frequencies and amplitudes) on this LO In contrast, it has been reported that pressurizing the lung may help in relieving asthmatic airway constrictions. Ultimately, this pressurizing stretches the ASM and may disturb the acto-myosin cross-bridges in a manner similar to LO; however, it is of a static rather than dynamic nature. This research investigates the effect of combining both prestretch- and LO-applications on contracted porcine ASM Isolated porcine ASM relaxation was tested with a 0.56%, 2%, or 4% stretch of its reference length (L_ref) in addition to LO These oscillations are composed of a main wave mimicking the normal breathing (frequency of 0.33 Hz and amplitude of 4% L_ref) and superimposed oscillations (frequencies of 20, 30, 40, 60 and 80 Hz and amplitude of 1% L_ref). The oscillations were maintained for 10 min. The results demonstrate that a prestretch of 0.56% and 2% L_ref does enhance the contracted ASM relaxation at certain superimposed length oscillations frequencies while of 4% L_ref does not.

Phosphodiesterase V inhibition reduces airway responsiveness, but does not improve the beneficial effect of deep inspiration

BACKGROUND

Deep inspirations (DIs) can prevent (bronchoprotection; BP) and reverse (bronchodilation; BD) methacholine (Mch)-induced bronchoconstriction, but this effect is reduced or absent in people with asthma or airways hyperresponsiveness (AHR). The mechanisms of this defect are unknown.

OBJECTIVE

To indirectly examine the role of guanosine 3',5'-cyclic monophosphate (cGMP) by testing the hypothesis that the phosphodiesterase (PDE) V inhibitor, sildenafil, would improve DI-induced BP in individuals with AHR.

METHODS

Thirty-two individuals were screened and 15 met all the inclusion/exclusion criteria (7 subjects with AHR and 8 healthy subjects). A single-dose Mch challenge inducing a 20% reduction in FEV1 in the absence of DIs was first identified. Thereafter, every study participant had 4 pairs of visits, each pair testing DI-induced BP and BD against the single-dose Mch, with no drug, or pretreatment with 25, 50 and 100 mg of sildenafil, respectively, in consecutive order.

RESULTS

Sildenafil did not influence baseline lung function. However, in the absence of DIs, the drug caused a dose-dependent attenuation of the Mch-induced decrease in FEV1 by 17% (median value; 25th percentile: 1, 75th percentile: 16), 35% (-3, 61) and 37% (13, 79) for the 25-, 50- and 100-mg doses, respectively (p = 0.0004). No differences between the two participant groups were found. There were no effects of sildenafil on DI-induced BP or BD.

CONCLUSION

We infer from these results that the mechanism responsible for the defective ability of DIs to protect the airways from bronchoconstriction is unlikely to be due to dysregulation of cGMP. Of importance, a potential role for PDE V inhibition as a bronchoprotector treatment needs to be explored.

Chronic continuous positive airway pressure (CPAP) reduces airway reactivity in vivo in an allergen-induced rabbit model of asthma

Previous studies have demonstrated that chronic mechanical strain produced by continuous positive airway pressure (CPAP) reduces in vivo airway reactivity in rabbits and ferrets. For CPAP to potentially have a therapeutic benefit for asthmatic subjects, the reduction in airway responsiveness would need to persist for 12-24 h after its discontinuation, require application for only part of the day, and be effective in the presence of atopic airway inflammation. In the present study, airway responsiveness to acetylcholine or methacholine was measured during mechanical ventilation following three different protocols in which active, nonanesthetized, tracheotomized rabbits were treated with High vs. Low CPAP (6 vs. 0 cmH(2)O). 1) High CPAP was applied continuously for 4 days followed by 1 day of Low CPAP; 2) High CPAP was applied at night and Low CPAP during the daytime for 4 days, and 3) High CPAP was applied for 4 days in animals following ovalbumin (Ova) sensitization and challenge. For all three protocols, treatment with High CPAP resulted in significantly reduced airway responsiveness compared with treatment with Low CPAP. Cumulatively, our in vivo results in rabbits suggest that high CPAP, even when applied only at night, produces a persistent reduction of airway responsiveness. In addition, CPAP reduces airway responsiveness even in the presence of atopic airway inflammation.

Individual canine airway response variability to a deep inspiration

In healthy individuals, a DI can reverse (bronchodilation) or prevent (bronchoprotection) induced airway constriction. For individuals with asthma or COPD, these effects may be attenuated or absent. Previous work showed that the size and duration of a DI affected the subsequent response of the airways. Also, increased airway tone lead to increased airway size variability. The present study examined how a DI affected the temporal variability in individual airway baseline size and after methacholine challenge in dogs using High-Resolution Computed Tomography. Dogs were anesthetized and ventilated, and on 4 separate days, HRCT scans were acquired before and after a DI at baseline and during a continuous intravenous infusion of methacholine (Mch) at 3 dose rates (17, 67, and 200 μg/min). The Coefficient of Variation was used as an index of temporal variability in airway size.We found that at baseline and the lowest dose of Mch, variability decreased immediately and 5 minutes after the DI (P < 0.0001). In contrast, with higher doses of Mch, the DI caused a variable response. At a rate of 67 μg/min of Mch, the temporal variability increased after 5 minutes, while at a rate of 200 μg/min of Mch, the temporal variability increased immediately after the DI. Increased airway temporal variability has been shown to be associated with asthma. Although the mechanisms underlying this temporal variability are poorly understood, the beneficial effects of a DI to decrease airway temporal variability was eliminated when airway tone was increased. If this effect is absent in asthmatics, this may suggest a possible mechanism for the loss of bronchoprotective and bronchodilatory effects after a DI in asthma.

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.

Maintenance of airway caliber in isolated airways by deep inspiration and tidal strains

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.

Temporal variability in the responses of individual canine airways to methacholine

Previous work showed that individual airway size, before any spasmogen, varied widely in the same animals on different days. The effect of this variable baseline size on the airway response to a subsequent challenge is unknown. The present study examined how the variability in individual airway baseline size in dogs was related to that after methacholine challenge on 4 different days using high-resolution computed tomography scans. Dogs were anesthetized and ventilated, and on 4 separate days randomly varying between 1 and 8 wk apart, baseline scans were acquired, followed by a continuous intravenous infusion of methacholine at three rates in increasing order (17, 67, and 200 microg/min). As the measure of variability, we used the coefficient of variation (CV) of the four airway luminal measurements of each airway at baseline and at each dose of methacholine. For most airways, there was wide variability both between and within dogs in the response to a given dose of methacholine (CV = 33-38%). Airways with any level of methacholine stimulation had greater variability than those at baseline. The airway variability was greatest at the lowest dose of methacholine administered but was elevated at all the doses. In conclusion, there was substantial day-to-day variability in baseline airway size. Most importantly, the same dose of methacholine to the same individual airway showed even greater variability than that at baseline. If we consider that increased heterogeneity may potentiate clinical symptoms, then airway response variability may play an important role in the manifestation of airway disease.

Chronic inflation of ferret lungs with CPAP reduces airway smooth muscle contractility in vivo and in vitro

The mechanical stress imposed on the lungs during breathing is an important modulator of airway responsiveness in vivo. Our recent study demonstrated that continuous positive airway pressure applied to the lungs of nonanesthetized, tracheotomized rabbits for 4 days decreased lower respiratory system responsiveness to challenge with ACh (Xue Z, Zhang L, Ramchandani R, Liu Y, Antony VB, Gunst SJ, Tepper RS. J. Appl Physiol 99: 677-682, 2005). In addition, airway segments excised from the lungs of these animals and studied in vitro exhibited reduced contractility. However, the mechanism for this reduction in contractility was not determined. The stress-induced decrease in airway responsiveness could have resulted from alterations in the excitation-contraction coupling mechanisms of the smooth muscle cells, or it might reflect changes in the structure and/or composition of the airway wall tissues. In the present study, we assessed the effect of prolonged chronic stress of the lungs in vivo on airway smooth muscle force generation, myosin light chain phosphorylation, and airway wall structure. To enhance the potential development of stress-induced structural changes, we applied mechanical stress for a prolonged period of time of 2-3 wk. Our results demonstrate a direct connection between the decreased airway responsiveness caused by chronic mechanical stress of the lungs in vivo and a persistent decrease in contractile protein activation in the airway smooth muscle isolated from those lungs. The chronic stress also caused an increase in airway size but no detectable changes in the composition of the airway wall.

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.

Effects of graded exercise on bronchial blood flow and airway dimensions in sheep

Exercise stimulus-response relationships for airway blood supply and dimensions have not been described in mammalian species. These relationships are vital for postulates concerning integrated reflex factors normally controlling the airways and which may underlie the asthma syndromes of exercise. This study defines airways stimulus-response relationships in exercising sheep. Ewes between 35 and 40kg were instrumented at left thoracotomy under thiopentone/isoflurane general anaesthesia. Pulsed Doppler ultrasonic transducers were mounted on the bronchial artery, and transit-time plus single-crystal sonomicrometers on the left main bronchus. These recorded simultaneously and continuously bronchial blood flow (Q(br)) and conductance (C(br)), bronchial circumference (Circ(br)) and wall thickness (Th(br)). In Protocol 1 (P1), four sheep ran duplicate 5min protocols on a horizontal treadmill at continuous step-up-and-down speeds of 1min duration, namely, 0.8, 1.6, 2.2, 1.6 and 0.8mph (moderate exercise), followed by 10min recovery. In P2, four sheep ran duplicate 2min protocols at constant 4mph (strenuous exercise), and in P3, one sheep ran duplicate protocols each of 3min at 2.2, 4.4 and 6mph (severe exercise). Regression analysis and repeated measures ANOVA were used to assess differences between times, runs and exercise intensity. In P1, airway effects were directly related to graded exercise effort sustained over 5min. Peak effects occurred at 2.2mph, except for Th(br). Heart rate and P(a) rose (to 156% and 111% of resting, respectively), and Q(br) and C(br) fell (to 83% and 75%; both P<0.001). Circ(br) fell to 96% (P=0.02), and Th(br) rose at low speeds early and late, and thinned at the highest speed. In P2 and P3 for all variables the steady-state effects were systematically greater than for P1 (4.4mph: C(br) to 43%, Circ(br) to 93%; 6.6mph: C(br) to 25%, Circ(br) to 82%). There was no significant recovery hyperaemia, but there was residual post-exercise bronchoconstriction. The exercise stimulus-response relationships from rest to a maximal 6mph for sheep airway circumference and its bronchial circulation are inverse and functionally constrictor.

Functional imaging of airway narrowing

The report will focus on studies that illustrate how high resolution computed tomography can be used to provide new insights into airway and lung function, that cannot be obtained with any other methodology in humans or animal models. In one series of experiments, we have clearly demonstrated that even large cartilaginous airways are capable of complete closure in vivo. These unequivocal in vivo results invalidate the ubiquitous concept that there is a limit to airway narrowing in normal subjects. In another series of experiments, we have investigated potential reasons why asthmatic subjects might show airway constriction following deep inspiration instead of the normal dilation. Experimental results show that a constrictor response to deep inspiration can be generated in normal airways simply by minimizing tidal stresses. The absence of these normal rhythmic stresses alters the smooth muscle throughout the airway tree, such that subsequent large stresses lead to a further constriction. These results also offer a possible mechanism by which the response to deep inspiration is altered in asthmatic subjects. By allowing accurate measurement of the size of individual airways, computed tomography with modern commercially available scanners thus provides a unique opportunity to evaluate specific hypotheses regarding mechanisms underlying lung disease.

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.