Pressure--volume relationships of tracheae in fetal newborn and adult rabbits

Microstructure and mechanics of the bovine trachea: Layer specific investigations through SHG imaging and biaxial testing

The trachea is a complex tissue made up of hyaline cartilage, fibrous tissue, and muscle fibers. Currently, the knowledge of microscopic structural organization of these components and their role in determining the tissue's mechanical response is very limited. The purpose of this study is to provide data on the microstructure of the tracheal components and its influence on tissue's mechanical response. Five bovine tracheae were used in this study. Adventitia, cartilage, mucosa/submucosa, and trachealis muscle layers were methodically cut out from the whole tissue. Second-harmonic generation(SHG) via multi-photon microscopy (MPM) enabled imaging of collagen fibers and muscle fibers. Simultaneously, a planar biaxial test rig was used to record the mechanical behavior of each layer. In total 60 samples were tested and analyzed. Fiber architecture in the adventitia and mucosa/submucosa layer showed high degree of anisotropy with the mean fiber angle varying from sample to sample. The trachealis muscle displayed neat layers of fibers organized in the longitudinal direction. The cartilage also displayed a structure of thick mesh-work of collagen type II organized predominantly towards the circumferential direction. Further, mechanical testing demonstrated the anisotropic nature of the tissue components. The cartilage was identified as the stiffest component for strain level < 20% and hence the primary load bearing component. The other three layers displayed a non-linear mechanical response which could be explained by the structure and organization of their fibers. This study is useful in enhancing the utilization of structurally motivated material models for predicting tracheal overall mechanical response.

Risk Factors for Tracheobronchomalacia in Preterm Infants With Bronchopulmonary Dysplasia

Aim: To identify the risk factors associated with the development of tracheobronchomalacia (TBM) in preterm infants with bronchopulmonary dysplasia (BPD). Methods: This was a retrospective cohort study using chart reviews of preterm infants born at ≤ 36 week's gestation who underwent flexible fiberoptic bronchoscopy in a tertiary pediatric referral center between January 2015 and January 2020. Indications for the bronchoscopy examination included lobar atelectasis on plain chest film, persistent CO₂ retention, recurrent extubation failure, or abnormal breathing sounds such as wheeze or stridor. Optimal cutoff values for each risk factor were also determined. Results: Fifty-eight preterm infants with BPD were enrolled, of whom 29 (50%) had TBM. There were no significant differences in gestational age and birth weight between those with and without TBM. Significantly more of the patients with TBM had severe BPD compared to those without TBM (68.9 vs. 20.6%, p < 0.001). Clinical parameters that were significantly different between the two groups were included in multivariate analysis. Among these factors, severe BPD was the most powerful risk factor for the development of TBM (odds ratio 5.57, 95% confidence interval 1.32-23.5, p = 0.019). The areas under the receiver operating characteristic curves for peak inspiratory pressure (PIP) and the duration of intubation were 0.788 and 0.75, respectively. The best predictive cutoff values of PIP and duration of intubation for TBM were 18.5 mmHg and 82 days, respectively. Conclusion: Preterm infants with severe BPD are at high risk for the development of TBM, and the risk is even higher in those who receive a higher PIP or are intubated for longer. Bronchoscopy examinations should be considered for the early diagnosis and management of TBM in infants with these risk factors.

Structural and functional development in airways throughout childhood: Children are not small adults

Children are not small adults and this fact is particularly true when we consider the respiratory tract. The anatomic peculiarities of the upper airway make infants preferential nasal breathers between 2 and 6 months of life. The pediatric larynx has a more complex shape than previously believed, with the narrowest point located anatomically at the subglottic level and functionally at the cricoid cartilage. Alveolarization of the distal airways starts conventionally at 36-37 weeks of gestation, but occurs mainly after birth, continuing until adolescence. The pediatric chest wall has unique features that are particularly pronounced in infants. Neonates, infants, and toddlers have a higher metabolic rate, and consequently, their oxygen consumption at rest is more than double that of adults. The main anatomical and functional differences between pediatric and adult airways contribute to the understanding of various respiratory symptoms and disease conditions in childhood. Knowing the peculiarities of pediatric airways is helpful in the prevention, management, and treatment of acute and chronic diseases of the respiratory tract. Developmental modifications in the structure of the respiratory tract, in addition to immunological and neurological maturation, should be taken into consideration during childhood.

Trachea Mechanics for Tissue Engineering Design

Trachea replacement for nonoperable defects remains an unsolved problem due to complications with stenosis and mechanical insufficiency. While native trachea has anisotropic mechanical properties, the vast majority of engineered constructs focus on uniform cartilaginous-like conduits. These conduits often lack quantitative mechanical analysis at the construct level, which limits analysis of functional outcomes in vivo, as well as comparisons across studies. This review aims to present a clear picture of native tracheal mechanics at the tissue and organ level, as well as loading conditions to establish design criteria for trachea replacements. We further explore the implications of failing to match native properties with regards to implant collapse, stenosis, and infection.

Laryngeal closure impedes non-invasive ventilation at birth

BACKGROUND

Non-invasive ventilation is sometimes unable to provide the respiratory needs of very premature infants in the delivery room. While airway obstruction is thought to be the main problem, the site of obstruction is unknown. We investigated whether closure of the larynx and epiglottis is a major site of airway obstruction.

METHODS

We used phase contrast X-ray imaging to visualise laryngeal function in spontaneously breathing premature rabbits immediately after birth and at approximately 1 hour after birth. Non-invasive respiratory support was applied via a facemask and images were analysed to determine the percentage of the time the glottis and the epiglottis were open.

HYPOTHESIS

Immediately after birth, the larynx is predominantly closed, only opening briefly during a breath, making non-invasive intermittent positive pressure ventilation (iPPV) ineffective, whereas after lung aeration, the larynx is predominantly open allowing non-invasive iPPV to ventilate the lung.

RESULTS

The larynx and epiglottis were predominantly closed (open 25.5%±1.1% and 17.1%±1.6% of the time, respectively) in pups with unaerated lungs and unstable breathing patterns immediately after birth. In contrast, the larynx and the epiglottis were mostly open (90.5%±1.9% and 72.3%±2.3% of the time, respectively) in pups with aerated lungs and stable breathing patterns irrespective of time after birth.

CONCLUSION

Laryngeal closure impedes non-invasive iPPV at birth and may reduce the effectiveness of non-invasive respiratory support in premature infants immediately after birth.

Tracheobronchomalacia Is Associated with Increased Morbidity in Bronchopulmonary Dysplasia

RATIONALE

Tracheobronchomalacia is a common comorbidity in neonates with bronchopulmonary dysplasia. However, the effect of tracheobronchomalacia on the clinical course of bronchopulmonary dysplasia is not well-understood.

OBJECTIVE

We sought to assess the impact of tracheobronchomalacia on outcomes in neonates with bronchopulmonary dysplasia in a large, multi-center cohort.

METHODS

We preformed a cohort study of 974 neonates with bronchopulmonary dysplasia admitted to 27 neonatal intensive care units participating in the Children's Hospital Neonatal Database who had undergone bronchoscopy. In hospital morbidity for neonates with bronchopulmonary dysplasia and tracheobronchomalacia (N=353, 36.2%) was compared to those without tracheobronchomalacia (N=621, 63.8%) using mixed-effects multivariate regression.

RESULTS

Neonates with tracheobronchomalacia and bronchopulmonary dysplasia had more comorbidities, such as gastroesophageal reflux (OR=1.65, 95%CI 1.23- 2.29, P=0.001) and pneumonia (OR=1.68, 95%CI 1.21-2.33, P=0.002) and more commonly required surgeries such as tracheostomy (OR=1.55, 95%CI 1.15-2.11, P=0.005) and gastrostomy (OR=1.38, 95%CI 1.03-1.85, P=0.03) compared with those without tracheobronchomalacia. Neonates with tracheobronchomalacia were hospitalitized (118 ± 93 vs 105 ± 83 days, P=0.02) and ventilated (83.1 ± 91.1 vs 67.2 ± 71.9 days, P=0.003) longer than those without tracheobronchomalacia. Upon discharge, neonates with tracheobronchomalacia and BPD were more likely to be mechanically ventilated (OR=1.37, 95CI 1.01-1.87 P=0.045) and possibly less likely to receive oral nutrition (OR=0.69, 95%CI 0.47-1.01, P=0.058).

CONCLUSIONS

Tracheobronchomalacia is common in neonates with bronchopulmonary dysplasia who undergo bronchoscopy and is associated with longer and more complicated hospitalizations.

Ventilation/perfusion ratio and right to left shunt in healthy newborn infants

Oxygenation impairment can be assessed non-invasively by determining the degree of right-to-left shunt and ventilation/perfusion (V_A/Q) inequality. These indices have been used in sick newborn infants, but normative values have not been reported which are essential to determine the magnitude of the abnormality. We, therefore, aimed to measure the shunt and V_A/Q in infants with no history of respiratory conditions and determine if there was any effect of supine or prone position and the reproducibility of the data. Data were analysed from infants who had undergone a hypoxic challenge and in a subset who had been assessed in the supine or prone position. Transcutaneous oxygen saturations (SpO₂) were recorded at fractions of inspired oxygen (F_IO₂) of 0.21 and 0.15. Two independent raters used a computer software algorithm which analysed and fitted paired data for F_IO₂ and SpO₂ and derived a curve which represented the best fit for each infant's data and calculated the shunt and V_A/Q. The raters ability to interpret the SpO₂ value which corresponded to a given F_IO₂ was compared. The downwards displacement of the F_IO₂ versus SpO₂ curve was used to estimate the degree of right-to-left shunt and the rightwards shift of the curve was used to calculate the V_A/Q ratio. The mean (SD) gestational age of the 145 infants was 39 (1.6) weeks, their birth weight was 2990 (578) gms and median (range) postnatal age at measurement 3 (1-8) days. The mean (SD) V_A/Q ratio was 0.95 (0.21). None of the infants had a right-to-left shunt. No significant differences were found in V_A/Q in the supine compared to the prone position. The intraclass correlation coefficient of V_A/Q between two independent raters was 0.968 (95% CI 0.947-0.980), p < 0.001. Right-to-left shunt and V_A/Q ratio in healthy newborn infants were similar in the prone compared to the supine position.

Paediatric Tracheomalacia

Intrathoracic tracheomalacia is characterized by increased compliance of the central airway within the thorax. This leads to excessive dynamic collapse during exhalation or periods of increased intrathoracic pressure such as crying. Extrathoracic tracheomalacia involves dynamic collapse of the airway between the glottis and sternal notch that occurs during inhalation rather than exhalation. The tone of the posterior membrane of the trachea increases throughout development and childhood, as does the rigidity of the tracheal cartilage. Abnormalities of airway maturation result in congenital tracheomalacia. Acquired tracheomalacia occurs in the normally developed trachea due to trauma, external compression, or airway inflammation. Although tracheomalacia can be suspected by history, physical examination, and supportive radiographic findings, flexible fiberoptic bronchoscopy remains the "gold standard" for diagnosis. Current treatment strategies involve pharmacotherapy with cholinergic agents, positive pressure ventilation, and surgical repair.

Mechanical ventilation injury and repair in extremely and very preterm lungs

Background

Extremely preterm infants often receive mechanical ventilation (MV), which can contribute to bronchopulmonary dysplasia (BPD). However, the effects of MV alone on the extremely preterm lung and the lung’s capacity for repair are poorly understood.

Aim

To characterise lung injury induced by MV alone, and mechanisms of injury and repair, in extremely preterm lungs and to compare them with very preterm lungs.

Methods

Extremely preterm lambs (0.75 of term) were transiently exposed by hysterotomy and underwent 2 h of injurious MV. Lungs were collected 24 h and at 15 d after MV. Immunohistochemistry and morphometry were used to characterise injury and repair processes. qRT-PCR was performed on extremely and very preterm (0.85 of term) lungs 24 h after MV to assess molecular injury and repair responses.

Results

24 h after MV at 0.75 of term, lung parenchyma and bronchioles were severely injured; tissue space and myofibroblast density were increased, collagen and elastin fibres were deformed and secondary crest density was reduced. Bronchioles contained debris and their epithelium was injured and thickened. 24 h after MV at 0.75 and 0.85 of term, mRNA expression of potential mediators of lung repair were significantly increased. By 15 days after MV, most lung injury had resolved without treatment.

Conclusions

Extremely immature lungs, particularly bronchioles, are severely injured by 2 h of MV. In the absence of continued ventilation these injured lungs are capable of repair. At 24 h after MV, genes associated with injurious MV are unaltered, while potential repair genes are activated in both extremely and very preterm lungs.

Brief mechanical ventilation impacts airway cartilage properties in neonatal lambs

Ultrasound imaging allows in vivo assessment of tracheal kinetics and cartilage structure. To date, the impact of mechanical ventilation (MV) on extracellular matrix (ECM) in airway cartilage is unclear, but an indication of its functional and structural change may support the development of protective therapies. The objective of this study was to characterize changes in mechanical properties of the neonatal airway during MV with alterations in cartilage ECM. Trachea segments were isolated in a neonatal lamb model; ultrasound dimensions and pressure-volume relationships were measured on sham (no MV; n = 6) and MV (n = 7) airways for 4 hr. Tracheal cross-sections were harvested at 4 hr, tissues were fixed and stained, and Fourier transform infrared imaging spectroscopy (FT-IRIS) was performed. Over 4 hr of MV, bulk modulus (28%) and elastic modulus (282%) increased. The MV tracheae showed higher collagen, proteoglycan content, and collagen integrity (new tissue formation); whereas no changes were seen in the controls. These data are clinically relevant in that airway properties can be correlated with MV and changes in cartilage ECM. MV increases the in vivo dimensions of the trachea and is associated with evidence of airway tissue remodeling. Injury to the neonatal airway from MV may have relevance for the development of tracheomalacia. We demonstrated active airway tissue remodeling during MV using an FT-IRIS technique which identifies changes in ECM.

Anisotropic material behaviours of soft tissues in human trachea: an experimental study

BACKGROUND

Human trachea is a multi-component structure composed of cartilage, trachealis muscle, mucosa and submucosa membrane and adventitial membrane. Its mechanical properties are essential for an accurate prediction of tracheal deformation, which has a significant clinic relevance. Efforts have been made in quantifying the material behaviour of tracheal cartilage and trachealis muscle. However, the material behaviours of other components have been least investigated.

METHODS

Three human cadaveric trachea specimens were used in this study. Trachealis muscle, mucosa and submucosa membrane and adventitia membrane were excised to perform the uniaxial test in axial and circumferential directions. In total, 72 tissue strips were prepared and tested. Tangent modulus was used to quantified the stiffness of each tissue strip at various stretch levels.

RESULTS

The obtained results indicated that all types of tracheal soft tissues were highly non-linear and anisotropic. Trachealis muscle in the circumferential direction had the most excellent extensibility; and the adventitial collagen membrane in the circumferential direction was the stiffest.

CONCLUSION

This study is helpful in understanding the material behaviour of trachea. Obtained results can be used for computational and analytic modelling to quantify the tracheal deformation.

Comparison of airway measurements during influenza-induced tachypnea in infant and adult cotton rats

Background

Increased respiratory rate (tachypnea) is frequently observed as a clinical sign of influenza pneumonia in pediatric patients admitted to the hospital. We previously demonstrated that influenza infection of adult cotton rats (Sigmodon hispidus) also results in tachypnea and wanted to establish whether this clinical sign was observed in infected infant cotton rats. We hypothesized that age-dependent differences in lung mechanics result in differences in ventilatory characteristics following influenza infection.

Methods

Lung tidal volume, dynamic elastance, resistance, and pleural pressure were measured in a resistance and compliance system on mechanically-ventilated anesthestized young (14–28 day old) and adult (6–12 week old) cotton rats. Animals at the same age were infected with influenza virus, and breathing rates and other respiratory measurements were recorded using a whole body flow plethysmograph.

Results

Adult cotton rats had significantly greater tidal volume (TV), and lower resistance and elastance than young animals. To evaluate the impact of this increased lung capacity and stiffening on respiratory disease, young and adult animals were infected intra-nasally with influenza A/Wuhan/359/95. Both age groups had increased respiratory rate and enhanced pause (Penh) during infection, suggesting lower airway obstruction. However, in spite of significant tachypnea, the infant (unlike the adult) cotton rats maintained the same tidal volume, resulting in an increased minute volume. In addition, the parameters that contribute to Penh were different: while relaxation time between breaths and time of expiration was decreased in both age groups, a disproportionate increase in peak inspiratory and expiratory flow contributed to the increase in Penh in infant animals.

Conclusion

While respiratory rate is increased in both adult and infant influenza-infected cotton rats, the volume of air exchanged per minute (minute volume) is increased in the infant animals only. This is likely to be a consequence of greater lung elastance in the very young animals. This model replicates many respiratory features of humans and consequently may be a useful tool to investigate new strategies to treat respiratory disease in influenza-infected infants.

Correlation of tracheal smooth muscle function with structure and protein expression during early development

With increased survival of premature infants, understanding the impact of development on airway function and structure is imperative. Airway smooth muscle plays a primary role in the modulation of airway function. The purpose of this study is to correlate the functional maturation of airway smooth muscle during the perinatal period with structural alterations at the cellular, ultrastructural, and molecular levels. Length-tension and dose-response analyses were performed on tracheal rings acquired from preterm and term newborn lambs. Subsequent structural analyses included isolated airway smooth muscle cell length, electron microscopy, and myosin heavy chain isoform expression measurements. Functionally the compliance, contractility, and agonist sensitivity of the tracheal rings matured during preterm to term development. Structurally, isolated cell lengths and electron microscopic ultrastructure were not significantly altered during perinatal development. However, expression of myosin heavy chain isoforms increased significantly across the age range analyzed, correlating with the maturational increase in smooth muscle contractility. In conclusion, the developmental alterations in tracheal function appear due, in part, to enhanced smooth muscle myosin heavy chain expression.

Sequential alterations of tracheal mechanical properties in the neonatal lamb: effect of mechanical ventilation

Alterations in neonatal airway mechanics resulting from ventilatory therapies are implicated in airway collapse and chronic disease. Quantifying the functional impact of mechanical ventilation (MV) on the neonatal airway and elucidating the time course of these changes will support development of protective therapies. The objective of this study was to test the hypothesis that conventional MV would result in decreased static and dynamic elastance of an isolated tracheal segment and thinning of the muscle (trachealis) region of the tracheal wall in a time dependent manner. Tracheal segments were isolated in newborn lambs spontaneously breathing through the distal trachea; segments were MV (n = 7; PIP/PEEP = 35/5 cmH2O; 40 breaths/min) or instrumented, non-ventilated (SHAM; n = 7; PIP/PEEP = 0/0 cmH2O) for 4 hr. At baseline and hourly, tracheal segments were filled with saline, and static pressure-volume curves were constructed as the pressure response to stepwise volume infusions. Then, cross-sectional ultrasound images were captured at 0 cmH2O on SHAM, and at 0 cmH2O, peak inspiratory pressure (PIP) and positive end expiratory pressure (PEEP), on MV tracheae for subsequent dimensional analysis. Tracheal elasticity indices were derived from static pressure-volume data, and during dynamic ventilation using ultrasound images to calculate the stress-strain relationships. Over 4 hr of MV, tracheal internal diameter (ID) increased (14%; P < 0.05). Markers of tracheal mechanical properties indicated a decrease in elasticity under both static (bulk modulus; 28%; P < 0.05) and dynamic (elastic modulus; 282 %; P < 0.05) conditions, indicating a significant alteration in elastic components. No time dependent changes were identified in dimensions or mechanical properties in the SHAM group.

CONCLUSIONS

MV results in dimensional alterations that increased anatomical dead space and reduced static and dynamic elastance of the neonatal trachea.

An ultrasound imaging method for in vivo tracheal bulk and Young's moduli of elasticity

Alterations in neonatal airway mechanical properties resulting from ventilatory therapies such as mechanical ventilation have been implicated in airway collapse and chronic disease. Advances in ultrasound (US) technology allow for real-time imaging and accurate measurement of tracheal dimensions in vivo; thus, changes in mechanical properties can be tracked longitudinally. In this report we introduce an adaptation of engineering concepts using US imaging data to study airway mechanics in vivo. In this protocol, tracheal segments are isolated in a spontaneously breathing newborn lamb model and the segments are exposed to time-cycled, pressure-limited mechanical ventilation. Serially, tracheal segments are filled with saline and pressure-volume relationships are recorded with stepwise volume infusions. US dimensional measurements of the segments are made while static (no distending pressure) and at pressure limits during dynamic ventilator cycling. US measurements are used to normalize pressure-volume data for resting volume, calculation of bulk modulus, stress-strain relationships and the adapted Young's modulus associated with tangential wall stress. Temporal changes in bulk and Young's moduli demonstrate the time dependence of alterations in conducting airway mechanical properties in vivo during the course of mechanical ventilation. This methodology will provide a means to evaluate respiratory therapies with respect to airway mechanics.

Ventilator-induced airway injury: a critical consideration during mechanical ventilation of the infant

The clinical management of respiratory failure in the newborn often focuses on lung parenchymal stiffness due to immaturity, surfactant deficiency, infiltrates, and other causes. However, health care personnel should also consider the airway, which plays an important role in gas exchange and lung mechanics. The airway can be easily injured, and an injured airway can significantly alter both the acute and chronic course of lung disease in infants. Further, there are developmental changes that affect the susceptibility of the neonatal airway to injury. Recognizing and preventing causes of airway injury can help to ensure optimal outcomes for the critically ill neonate.

Expression of Matrix Metalloproteinases 2, 7 and 9, and Their Tissue Inhibitors 1 and 2, in Developing Rabbit Tracheae

BACKGROUND

Structural changes in the developing conducting airway impact the rigidity of the airway, altering the airway's ability to sustain its shape during ventilation. The developmental changes in airway compliance oppose the changes in compliance of the developing lung; thus the expression profiles of matrix modeling proteins likely are also opposite in these developing organs.

OBJECTIVES

To determine the profiles of matrix metalloproteinases (MMPs) -2, -7, and -9 and tissue inhibitors (TIMPs) -1 and -2 in the developing trachea and test the hypothesis these profiles would contrast those previously reported for the lung.

METHODS

Rabbits tracheae were harvested at 21 days of gestation, 3 and 17 days postgestation and at adulthood. Tissue homogenates were analyzed by substrate zymography for the activity of MMPs, and reverse zymography for TIMPs. Immunostainings on neonatal lamb tracheal rings were used to localize MMP-2 and 9.

RESULTS

Analysis revealed an age-dependent decrease in total MMP-2 quantity and the ratio of active to latent forms. TIMP-2 shows a time-dependent increase throughout airway development. Total MMP-9 and TIMP-1 quantities were unchanged across these ages, although MMP-9 protein was found predominantly in its latent form during development and predominantly in its active form during adulthood. Respiratory epithelial cells reacted positive for both MMP-2 and 9 and trachealis muscle fibers were positive for MMP-2. No MMP-7 expression was identified in the rabbit airway.

CONCLUSIONS

The opposing developmental patterns in MMP-2 expression between the airway and lung lead to speculation regarding the role of MMP-2 activity on changes in organ compliance.

Airway structure, function and development in health and disease

Until they are fully mature, the airways are highly susceptible to damage. Factors that may contribute to vulnerability of immature airways and the occurrence of bronchopulmonary dysplasia (BPD) in preterm neonates include decreased contractility of smooth muscles of the airway, which leads to generation of lower forces, and immaturity of airway cartilage, leading to increased compressibility of developing airways. Mechanical ventilation has little effect on adult airways, but affects the dimensions and mechanical properties of preterm and newborn airways. Techniques for clinical evaluation of airway function include: (i). measurements of airway function during tidal breathing (airway resistance and reactivity are significantly elevated in infants with BPD); (ii). forced expiratory flow measurements [small-airway obstruction in infants with BPD is indicated by markedly reduced maximal volume measurements (Vmax)]; (iii). radiography procedures (plain radiographs, fluoroscopy, computed tomography and virtual bronchoscopy); and (iv). endoscopy procedures (rigid or flexible bronchoscopy, with or without measurement of oesophageal pressure). Imaging has demonstrated an excessively decreased airway cross-sectional area during exhalation in infants with BPD and acquired tracheomegaly in very preterm infants who had received mechanical ventilatory support. To further advance our understanding of how the airways develop, and to design less damaging protocols for mechanical ventilation in preterm neonates, basic laboratory studies of airway ultrastructure need to be performed and the results correlated with clinical pulmonary function studies.

Comparison of elastic properties and contractile responses of isolated airway segments from mature and immature rabbits

Immature rabbits have greater maximal airway narrowing with bronchoconstriction in vivo compared with mature animals. As isolated immature lungs have a lower shear modulus, it is unclear whether the greater airway narrowing in the immature lung is secondary to less tethering between the airways and the lung parenchyma or to differences in the mechanical properties of the mature and immature airways. In the present study, we compared the mechanical properties of fluid-filled, isolated, intraparenchymal airway segments of the same generation from mature and immature rabbits. Stimulation with ACh resulted in greater airway narrowing in immature than mature bronchi. The immature bronchi were more compliant, had a lower resting airway volume, and were more collapsible compared with the mature bronchi. When the airways were contracted with ACh under isovolume conditions, the immature bronchi generated greater active pressure, and they were more sensitive to ACh than were mature bronchi. Our results suggest that maturational differences in the structure and function of the airways in the absence of the lung parenchyma can account for the greater maximal narrowing of immature than mature airways in vivo.

Influence of intrauterine growth restriction on airway development in fetal and postnatal sheep

Epidemiologic studies suggest that intrauterine growth restriction (IUGR) can lead to impaired lung function, yet little information exists on the effects of IUGR on airway development. Our objectives were to characterize morphometrically effects of IUGR on airway structure in the fetus and to determine whether alterations persist into postnatal life. We used two groups of sheep, each with appropriate controls; a fetal group was subjected to IUGR by restriction of placental function from 120 to 140 d (term approximately 147 d), and a postnatal group, killed 8 wk after birth, was subjected to IUGR from 120 d to birth at term. In both fetuses and postnatal lambs, IUGR did not alter lung weight relative to body weight. In IUGR fetuses, the luminal areas and basement membrane perimeters of the trachea and larger bronchi (generations 0-8, trachea = 0) were smaller than in controls. Airway wall areas, relative to basement membrane perimeters, were reduced in IUGR fetuses compared with controls, largely due to reduced areas of cartilage and epithelium. At 8 wk after birth, there were no significant differences in airway dimensions between IUGR and control lambs. However, the number of profiles of bronchial submucosal glands, relative to basement membrane perimeters, was lower in IUGR lambs than in controls and the area of epithelial mucin was increased. We conclude that restriction of fetal growth during late gestation impairs the growth of bronchial walls that could affect airway compliance in the immediate postnatal period. Although airway growth deficits are reversed by 8 wk, alterations in mucus elements persist.

Effect of transpulmonary pressure on airway diameter and responsiveness of immature and mature rabbits

We previously demonstrated that airway responsiveness is greater in immature than in mature rabbits; however, it is not known whether there are maturational differences in the effect of transpulmonary pressure (Ptp) on airway size and airway responsiveness. The relationship between Ptp and airway diameter was assessed in excised lungs insufflated with tantalum powder. Diameters of comparable intraparenchymal airway segments were measured from radiographs obtained at Ptp between 0 and 20 cmH(2)O. At Ptp > 8 cmH(2)O, the diameters were near maximal in both groups. With diameter normalized to its maximal value, changing Ptp between 8 and 0 cmH(2)O resulted in a greater decline of airway caliber in immature than mature airways. The increases in lung resistance (RL) in vivo at Ptp of 8, 5, and 2 cmH(2)O were measured during challenge with intravenous methacholine (MCh: 0.001-0.5 mg/kg). At Ptp of 8 cmH(2)O, both groups had very small responses to MCh and the maximal fold increases in RL did not differ (1.93 +/- 0.29 vs. 2.23 +/- 0.19). At Ptp of 5 and 2 cmH(2)O, the fold increases in RL were greater for immature than mature animals (13.19 +/- 1.81 vs. 3.89 +/- 0.37) and (17.74 +/- 2.15 vs. 4.6 +/- 0.52), respectively. We conclude that immature rabbits have greater airway distensibility and this difference may contribute to greater airway narrowing in immature compared with mature rabbits.

Differences in airway structure in immature and mature rabbits

Our laboratory has previously demonstrated that maximal bronchoconstriction produces a greater degree of airway narrowing in immature than in mature rabbit lungs (33). To determine whether these maturational differences could be related to airway structure, we compared the fraction of the airway wall occupied by airway smooth muscle (ASM) and cartilage, the proportion of wall area internal to ASM, and the number of alveolar attachments to the airways, from mature and immature (6-mo- and 4-wk-old, respectively) rabbit lungs that were formalin fixed at total lung capacity. The results demonstrate that the airway walls of immature rabbits had a greater percentage of smooth muscle, a lower percentage of cartilage, and fewer alveolar attachments compared with mature rabbit airways; however, we did not find maturational differences in the airway wall thickness relative to airway size. We conclude that structural differences in the airway wall may contribute to the greater airway narrowing observed in immature rabbits during bronchoconstriction.

Rate constant for forced expiration decreases with lung growth during infancy

Airway caliber and lung volume (VL) increase many fold between infancy and adulthood; however, these two components of the lung may not increase proportionately during lung growth and development. We evaluated in infants the rate of emptying during forced expiration from near total lung capacity to residual volume. From the flow-volume curves we calculated (1) a rate constant (k) as the change in flow divided by the change in volume between 50% and 75% of expired forced vital capacity (FVC), and (2) the fraction of the FVC expired in 0.5 s (FEV(0.5)/FVC). Seventeen normal healthy infants were evaluated twice; mean ages (ranges) at first and second tests were 30 (5 to 76) and 58 (28 to 98) wk. Analysis of cross-sectional and longitudinal data indicated that the rate of emptying during forced expiration measured by both parameters was greatest in the youngest infants and decreased during infancy. Our findings are consistent with the concept that younger infants have large airways relative to their VL and that VL increases more rapidly than airway caliber early in life.

Assessment of the dynamic relationship between external diameter and lumen flow in isolated bronchi

Histologic studies of large airways suggest that outer airway wall area increases during bronchoconstriction, which could influence the relationship between external diameter and lumen flow. Using isolated bronchi from pigs we simultaneously recorded external diameter using sonomicrometry, and lumen flow of liquid using an electromagnetic flowmeter to determine the relationship between the two. External diameter fell from 4.13+/-0.19 to 3.65+/-0.19 mm (11+/-3%, n = 5) during maximal electrical field stimulation (EFS), and flow decreased by 67+/-9%. External diameter plotted against lumen flow showed hysteresis between contraction and relaxation. External narrowing ceased towards the end of a stimulation period, but flow continued to decrease. This differed from predictions based on an assumption that the wall area does not change during contraction. Histologic sections of bronchi fixed after acetylcholine (ACh) challenge showed an increase in total wall area. These results illustrate dynamic wall movement during bronchoconstriction induced by EFS or acetylcholine.

Compliance and stability of the bronchial wall in a model of allergen-induced lung inflammation

Airway wall remodeling in response to inflammation might alter load on airway smooth muscle and/or change airway wall stability. We therefore determined airway wall compliance and closing pressures in an animal model. Weanling pigs were sensitized to ovalbumin (OVA; ip and sc, n = 6) and were subsequently challenged three times with OVA aerosol. Control pigs received 0.9% NaCl (n = 4) in place of OVA aerosol. Bronchoconstriction in vivo was assessed from lung resistance and dynamic compliance. Semistatic airway compliance was recorded ex vivo in isolated segments of bronchus, after the final OVA aerosol or 0.9% NaCl challenge. Internally or externally applied pressure needed to close bronchial segments was determined in the absence or presence of carbachol (1 microM). Sensitized pig lungs exhibited immediate bronchoconstriction to OVA aerosol and also peribronchial accumulations of monocytes and granulocytes. Compliance was reduced in sensitized bronchi in vitro (P < 0.01), and closing pressures were increased (P < 0.05). In the presence of carbachol, closing pressures of control and sensitized bronchi were not different. We conclude that sensitization and/or inflammation increases airway load and airway stability.

Quantitative bronchoscopic assessment of airway collapsibility in newborn lamb tracheae

To date, quantitative studies of the inherent characteristics of the developing airway wall have required excision of an airway segment or surgical creation of an isolated segment. We hypothesized that airway wall characteristics, at various collapsing pressures, and attendant changes in stiffness after smooth muscle stimulation could be quantitated bronchoscopically from airway pressure-area relationships. Neonatal lamb tracheal segments (n = 12) were suspended over hollow mounts, in a buffer-filled chamber, and subjected to a range (0 to -4.0 kPa) of pressures to determine wall stiffness under collapsing forces before and after stimulation of the trachealis with methacholine. Luminal images were recorded through a 3.6-mm flexible bronchoscope under the same conditions, subsequently corrected for distortion, and a cross-sectional area was quantitated. Both pressure-volume and pressure-area relationships detected significant changes in airway wall stiffness after methacholine administration (p < 0.002), and the magnitude of change was similar between methods. These data suggest that quantitative flexible bronchoscopy can be used clinically in the intact airway to assess wall stiffness.

Failure of aged rats to accumulate eosinophils in allergic inflammation of the airway

To investigate the effect of aging on the allergic airway response, we examined the bronchoconstrictive responses and cellular inflammatory changes in a rat model of bronchial asthma by evaluating young and old animals. Two different age groups of Brown-Norway rats, actively sensitized by injection of ovalbumin into the foot pads, were used: 7 to 8 weeks old (young group) and 100 to 120 weeks old (aged group). Both the aged and young rats produced on ovalbumin-specific IgE antibody and exhibited an immediate asthmatic response after exposure to ovalbumin, but the degree of specific IgE antibody was significantly higher in young rats. The young group showed a marked increase in the number of eosinophils and neutrophils in bronchoalveolar lavage fluid 2 days after exposure to ovalbumin, whereas no eosinophilia was seen in the aged group. To evaluate the mechanism of the decreased accumulation of eosinophils in aged rats, cells from popliteal lymph nodes from ovalbumin-sensitized rats were incubated with ovalbumin for 48 hours. Although eosinophil chemotactic activity, determined by a modified Boyden chamber method, was present in the supernatant of cultured lymph node cells from young rats, it was absent from those of aged rats. In vivo administration of anti-IL-5 monoclonal antibody revealed that one of the factors of eosinophil chemotactic activity was IL-5. Lymph node cells from aged rats tended to produce greater amounts of interferon-gamma than did those from young animals. Findings indicate that aged rats have a defect in eosinophil accumulation in sites exposed to antigen, probably because of an age-dependent alteration in T cells.

Mechanical ventilation of the newborn. An overview

Mechanical ventilation of the newborn infant is an ever-changing area. Its evolution has been hampered and stimulated by problems of small size, inadequate technology, unexpected complications, and changing expectations. With synchronized ventilation, a new technique in the neonatal ICU, clinicians again are reassessing their assumptions. HFV, a "new" technique for 15 years, has found a niche in the treatment of infants failing CV. Its use as an initial therapy for RDS, advocated by some, remains controversial. Monitoring gas flow patterns, tidal and minute volumes, and lung mechanics has become a part of the CV, but complications still occur. The only thing certain is that change will continue.

Maturation affects the maximal pulmonary response to methacholine in rabbits

Maximal bronchoconstriction in normal adults produces only small reductions in pulmonary function; however, in normal infants severe airway obstruction limits testing to low agonist concentrations. In this study, maximal pulmonary response to methacholine was evaluated in 5 immature (1 month old) and 5 mature (6 months old) rabbits. Animals were anesthetized, paralyzed, and mechanically ventilated via a tracheostomy tube. Changes in pulmonary function were assessed from maximal deflation flow volume curves following inhalation of doubling concentrations of methacholine between 0.6 and 320 mg/mL. Following 320 mg/mL methacholine, the immature rabbits had a greater percent decline in forced vital capacity (FVC) than the mature animals (55 +/- 15% vs. 36 +/- 10%; P < 0.05). At 50% FVC, isovolume flows were measurable in the 5 mature rabbits, and 4 of 5 had plateaus in their dose-response curves. At the higher methacholine doses, only 1 of 5 immature animals had measurable isovolume flows because of the decrease in FVC. There was no significant difference between immature and mature animals in the methacholine dose required to decrease baseline flows by 50%. We conclude that in rabbits maturation affects maximal pulmonary response but not the sensitivity to methacholine.

Early maturation of force production in pig tracheal smooth muscle during fetal development

The contractility of airway smooth muscle is fully established at late term at birth but its responsiveness during fetal life has not been defined. In this study, the contractile force of airway smooth muscle to acetylcholine (ACh), K+ depolarizing solution, and electrical field stimulation (EFS) was measured in tracheas from small fetal pigs. Contraction to either agonist and to EFS was detectable in fetuses of as low as 9 g body weight, which corresponds to approximately 36 days of gestation. Isometric force increased progressively with age, reaching 4.1 +/- 0.4 mN for K+ and 5.8 +/- 0.5 mN for ACh (10(-4) M) at 600 g fetal weight (90 days). However, when normalized for cross sectional area of smooth muscle, the stress was essentially the same from 17- to 600-g fetuses. (K+: 17 g = 74.4 +/- 10.6 mN/mm2, 600 g = 89.3 +/- 13.0 mN/mm2; ACh [10(-4) M]: 17 g = 76.3 +/- 16.0 mN/mm2, 600 g = 127.0 +/- 13.0 mN/mm2). The sensitivities of the various groups to ACh were not significantly different (e.g., EC50: 30 g = 4.0 +/- 0.2 x 10(-6) M, 600 g = 3.7 +/- 1.1 x 10(-6) M). EFS produced frequency-dependent contractile responses in all groups. With increasing fetal size, there was a corresponding increase in force. When this force was normalized to a maximum ACh response (10(-4) M), there was no significant difference between groups of fetuses. Histologic examination showed the major tissue components of the trachea were present in fetuses above 7 g. Immunocytochemistry detected myosin, caldesmon, and filamin in the smooth muscle from fetuses of 7 g and above, showing that contractile and actin-binding proteins were present from a very early age. It is concluded that smooth muscle contractile function is well developed very early in fetal life in pigs.

Pathogenesis of respiratory failure in the preterm infant

Respiratory failure in the preterm results from not only surfactant deficiency but also the immaturity of a number of other elements that have a structural basis. Airway, alveolar, fluid clearance, and epithelial and endothelial barrier functions also are important to lung function. Immaturities in these lung elements have identifiable adverse consequences for lung function such as pulmonary interstitial emphysema and pulmonary edema. The maturation of each of these elements appears to be achievable by agents such as corticosteroids, and maturation will result in an improved response to surfactant treatments. While surfactant treatments can improve respiratory failure by minimizing lung injury, other aspects of lung immaturity continue to contribute to respiratory compromise in the preterm. A thorough understanding of respiratory failure in the newborn depends on a better appreciation of the contribution of immaturity of the different structural elements of the lung on lung function.

Mechanical properties of the isolated equine trachea

In order to study the in vitro mechanical properties of the equine trachea submitted to the compressive pressures observed in vivo, the pressure-volume relationship was determined in intra- and extra-thoracic tracheal segments taken post mortem from 29 healthy horses (one to 15 years old; 352 to 651 kg). At the same time, the cross-sectional lumen area (X-SA) at the mid-point of the segment was measured using a slit-lamp transillumination and photographic measurement by endoscopy. The tracheal specific compliance (Cs) as well as the relative changes in X-SA and in the sagittal and transverse diameters, for intraluminal pressures from 5 to - 5 kPa, were calculated. The extrathoracic tracheal Cs was 0.060 +/- 0.002 kPa-1 and, at an intraluminal pressure of - 5 kPa, X-SA was reduced to about 73 per cent of its resting value. The intrathoracic segments were more compliant and, at similar compressive pressure, their X-SA was more reduced. These data show that the equine tracheal compliance is high and suggest that the increase in pulmonary resistance observed during strenuous exercise may be partly explained by a partial tracheal collapse.

Combined conventional ventilation with high frequency oscillation in neonates

Combined high frequency oscillation (HFO) with conventional ventilation was used on a group of 12 ventilator dependent neonates in order to investigate the frequency response of the respiratory system to HFO. Pressure oscillations were measured at the airway opening with a transducer and volume oscillations at the chest wall using a calibrated respiratory jacket. Pressure oscillations increased with increasing frequency but the oscillating volumes showed a variable pattern with maximum oscillating volumes of up to 4 ml/kg. Three babies showed no change in volume between 2-25 Hz, 3 babies showed increasing volumes up to 25 Hz and the remainder showed a maximum volume (or resonant frequency) at 15-20 Hz. Oscillating volumes were higher when the conventional ventilator was in the expiratory phase. The differences in frequency response were unrelated to severity of lung disease, birth weight, age at testing, or size of the endotracheal (ET) tube. Local factors in the ET tube and large airways may account for some of the observed differences.

The effect of shape, age and extension on the compliance of equine tracheal segments

The influence of the age and weight of the animal as well as that of the extension and the shape of the trachea on the mechanical properties of the cervical trachea was studied in 33 isolated tracheal segments obtained from freshly slaughtered horses. The relationship between intraluminal pressure and volume was determined in the cervical tracheal segments positioned firstly under normal longitudinal tension and secondly in hyperextension. At the same time, changes in the area of the cross-section of the lumen (X-SA) at the midpoint of the segment were measured using photographs obtained by slit-lamp transillumination and endoscopy. The sagittal (DS) and transverse (DT) diameters were determined for each segment and the DT/DS ratio was calculated to give an estimate of the shape of the X-SA. The results showed that (i) neither the age nor the weight of the horses had any influence on the mechanical properties of the trachea; (ii) extension decreases the compressibility of the tracheal segment in vitro; (iii) there is a wide variation in the extrathoracic tracheal X-SA shape in horses; and (iv) the shape of the X-SA has a major influence on the mechanical properties of the trachea. It was concluded that (i) hyperextension of the neck will partly facilitate respiration at high levels of ventilation by elongating the trachea and by decreasing its collapsibility; and (ii) the tracheal collapse which may occur during high levels of ventilation will be more or less important depending on the individual X-SA shape.

Use of a touch sensitive screen and computer assisted image analysis for quantitation of developmental changes in pulmonary structure

The extensive changes in pulmonary function occurring during early development may reflect variations in the anatomic structure of the respiratory apparatus during this period. Accurate definition of these alterations could yield important information concerning the structure-function correlations of the respiratory system. To facilitate the acquisition of morphometric data from histologic sections of pulmonary tissues, we propose the use of a computer assisted image analysis system with a touch sensitive screen as an interactive peripheral. This allows planimetric measurements and computation of the dimensions of areas of selected light intensities within an image. We present the description, design, and applications of such an image analysis system and report representative results regarding developmental changes in pulmonary structure. In addition, we correlate these results with previously published information regarding pulmonary mechanics during early development to help clarify the maturational changes in pulmonary structure-function relationships.

Influence of smooth muscle tone and longitudinal tension on the collapsibility of immature airways

Mechanical properties and pressure-flow relationships of tracheae excised from very premature lambs were studied in a plethysmograph. Control (Group I) data revealed the tracheae to be extremely compliant, collapsible airways, with an inflation compliance (Si) of 0.033 (+/- 0.004 SE) mmHg-1, collapsing compliance (Sc) of 0.026 (+/- 0.001 SE) mmHg-1, and pressure-flow relationships similar to those of a Starling resistor. Acetylcholine administration (Group II) lowered both Si, 0.026 (+/- 0.003 SE) mmHg-1 and Sc, 0.022 (+/- SE) mmHg-1, as did longitudinal stretch (Group III): Si, 0.021 (+/- .003 SE) mmHg-1, and Sc, 0.017 (+/- 0.002 SE) mmHg-1. Alterations in tracheal collapsibility were also evidenced by significant reductions in resistance to airflow when the tracheae were subjected to compressive forces. Altering both smooth muscle tone (acetylcholine administration) and longitudinal length simultaneously (Group IV) produced results similar to those obtained for Group III. These data help to define the functional characteristics of immature airways and may provide insight for more effective clinical management of the premature infant.

Effect of high-frequency jet ventilation on preterm and rabbit tracheal mechanics

The effect of high-frequency jet ventilation (HFJV) on both tracheal dimensions and mechanics was evaluated in preterm and term rabbit airways. Seven tracheal segments were studied at 27 days (group I) and 31 days (group II) of gestation, respectively. Tracheal dimensions and segmental pressure-volume relationships were determined before and after 60 minutes of HFJV (peak pressure 20 cm H2O; mean airway pressure 6.7 to 6.8 cm H2O at 10 Hz). Both tracheal lengths and diameters increased significantly (p less than 0.01) in each group and resulted in increased tracheal volumes: 109% in group I (p less than 0.01) and 60% in group II (p less than 0.01). The mean specific tracheal compliance decreased in group I, from 0.036 cm H2O-1 to 0.015 cm H2O-1 (p less than 0.01), and in group II from 0.029 cm H2O-1 to 0.021 cm H2O-1 (p less than 0.01). Furthermore, the collapsing transmural pressure (the pressure required for total collapse of tracheal segments) decreased significantly (p less than 0.01) in both groups. These data demonstrate significant dimensional and mechanical deformation of tracheal segments after HFJV. An increased propensity toward collapsibility is also observed following HFJV. These changes are similar to those with tracheomalacia. The influence of such deformation on tracheal gas flow during HFJV needs to be further investigated.

Lung mechanics in growing guinea pigs

Lung mechanics were studied in four groups of guinea pigs: neonatal (1), prepubertal (II), postpubertal (III) and adult (IV). The animals were anesthetized and tracheotomized, and an esophageal catheter was inserted. Functional residual capacity (FRC) was measured by body plethysmography. After curarization, the lungs were inflated to a transpulmonary pressure of 30 cm H2O. The resulting change in volume was added to FRC to obtain the total lung capacity (TLC). Lung compliance (CL) normalized for dry lung weight (CL/LW) was calculated. An exponential function was fitted to the experimental P-V curve and the shape constant k was considered as an index of lung distensibility. During growth TCL (ml) increased with BW (g): TLC = 0.3 BW0.69 (r = 0.96). Two significantly different equations (P less than 0.001) were found between TLC (ml) and LW (g): between I and II, TLC = 32.4 LW1.24 (r = 0.88) and between III and IV, TLC = 22.9 LW0.53 (r = 0.73). An increase in lung distensibility as reflected by a significant increase in both CL/LW and the shape constant k was observed. In neonates and in prepubertal animals CL/LW was equal to 1.53 +/- 0.64 and to 2.38 +/- 0.51 ml/cm H2O/g (mean +/- 1 SD), respectively (P less than 0.01) and k to 0.146 +/- 0.026 and 0.219 +/- 0.035 cm H2O-1 (P less than 0.001). From prepuberty to adulthood no further significant changes were observed. These results showed that even in a species with an advanced maturation at birth, lung mechanics changed with growth.

Time-dependency and static mechanics of immature airways and saccules

Immature rabbit lungs were inflated, then deflated from the fetal pulmonary fluid (FPF)-filled state. Stereomicroscopic observation and measurement of volume change (delta V) during each pressure step and after 15 and 120 sec at each pressure revealed the following: (1) Only airways inflate from atmospheric pressure (P0) to P25. Significant time-dependency here is due to FPF flow through the narrowest airways, airways dilation and recruitment as functions of tissue and surface forces, and, perhaps, interfacial adsorption of surfactants. (2) Saccular recruitment and distention are the principal transformations from P30 to P35. Time-dependency here is the result of FPF flow and labile bubble production. (3) Time-dependency during deflation from P25 to P10 is due to diminishing influence of inflation processes and to decreasing radii of curvature at air/liquid interfaces as FPF refills the saccular air-spaces. Redistribution of air and hypophase liquid probably also play a role. (4) Deflation from P10 to P0 is determined by FPF flow through the smallest airways, interfacial forces, and recoil of previously distended airways as liquid locks are formed. Some implications are that FPF flow through the smallest airways is a gate to saccular ventilation; time-dependent processes place airspaces at risk to rupture; and different time constants of saccules and airways renders 120 sec pressure steps adequate for evaluation of the latter but not the former.