Detection of acute smoke-induced airway injury in a New Zealand white rabbit model using optical coherence tomography

Automatic proximal airway volume segmentation using optical coherence tomography for assessment of inhalation injury

BACKGROUND

Acute respiratory distress syndrome (ARDS) is a severe form of acute lung injury with a mortality rate of up to 40%. Early management of ARDS has been difficult due to the lack of sensitive imaging tools and robust analysis software. We previously designed an optical coherence tomography (OCT) system to evaluate mucosa thickness (MT) after smoke inhalation, but the analysis relied on manual segmentation. The aim of this study is to assess in vivo proximal airway volume (PAV) after inhalation injury using automated OCT segmentation and correlate the PAV to lung function for rapid indication of ARDS.

METHODS

Anesthetized female Yorkshire pigs (n = 14) received smoke inhalation injury (SII) and 40% total body surface area thermal burns. Measurements of PaO2-to-FiO2 ratio (PFR), peak inspiratory pressure (PIP), dynamic compliance, airway resistance, and OCT bronchoscopy were performed at baseline, postinjury, 24 hours, 48 hours, 72 hours after injury. A tissue segmentation algorithm based on graph theory was used to reconstruct a three-dimensional (3D) model of lower respiratory tract and estimate PAV. Proximal airway volume was correlated with PFR, PIP, compliance, resistance, and MT measurement using a linear regression model.

RESULTS

Proximal airway volume decreased after the SII: the group mean of proximal airway volume at baseline, postinjury, 24 hours, 48 hours, 72 hours were 20.86 cm (±1.39 cm), 17.61 cm (±0.99 cm), 14.83 cm (±1.20 cm), 14.88 cm (±1.21 cm), and 13.11 cm (±1.59 cm), respectively. The decrease in the PAV was more prominent in the animals that developed ARDS after 24 hours after the injury. PAV was significantly correlated with PIP (r = 0.48, p < 0.001), compliance (r = 0.55, p < 0.001), resistance (r = 0.35, p < 0.01), MT (r = 0.60, p < 0.001), and PFR (r = 0.34, p < 0.01).

CONCLUSION

Optical coherence tomography is a useful tool to quantify changes in MT and PAV after SII and burns, which can be used as predictors of developing ARDS at an early stage.

LEVEL OF EVIDENCE

Prognostic, level III.

Point-of-care endoscopic optical coherence tomography detects changes in mucosal thickness in ARDS due to smoke inhalation and burns

BACKGROUND

The prevalence of acute respiratory distress syndrome (ARDS) in mechanically ventilated burn patients is 33%, with mortality varying from 11-46% depending on ARDS severity. Despite the new Berlin definition for ARDS, prompt bedside diagnosis is lacking. We developed and tested a bedside technique of fiberoptic-bronchoscopy-based optical coherence tomography (OCT) measurement of airway mucosal thickness (MT) for diagnosis of ARDS following smoke inhalation injury (SII) and burns.

METHODS

16 female Yorkshire pigs received SII and 40% thermal burns. OCT MT and PaO₂-to-FiO₂ ratio (PFR) measurements were taken at baseline, after injury, and at 24, 48, and 72h after injury.

RESULTS

Injury led to thickening of MT which was sustained in animals that developed ARDS. Significant correlations were found between MT, PFR, peak inspiratory pressure (PIP), and total infused fluid volume.

CONCLUSIONS

OCT is a useful tool to quantify MT changes in the airway following SII and burns. OCT may be effective as a diagnostic tool in the early stages of SII-induced ARDS and should be tested in humans.

Spatiotemporal correlation of optical coherence tomography in-vivo images of rabbit airway for the diagnosis of edema

Detection of an early stage of subglottic edema is vital for airway management and prevention of stenosis, a life-threatening condition in critically ill neonates. As an observer for the task of diagnosing edema in vivo, we investigated spatiotemporal correlation (STC) of full-range optical coherence tomography (OCT) images acquired in the rabbit airway with experimentally simulated edema. Operating the STC observer on OCT images generates STC coefficients as test statistics for the statistical decision task. Resulting from this, the receiver operating characteristic (ROC) curves for the diagnosis of airway edema with full-range OCT in-vivo images were extracted and areas under ROC curves were calculated. These statistically quantified results demonstrated the potential clinical feasibility of the STC method as a means to identify early airway edema.

Optical imaging of subacute airway remodeling and adipose stem cell engraftment after airway injury

Acquired airway injury is frequently caused by endotracheal intubations, long-term tracheostomies, trauma, airway burns, and some systemic diseases. An effective and less invasive technique for both the early assessment and the early interventional treatment of acquired airway stenosis is therefore needed. Optical coherence tomography (OCT) has been proposed to have unique potential for early monitoring from the proliferative epithelium to the cartilage in acute airway injury. Additionally, stem cell therapy using adipose stem cells is being investigated as an option for early interventional treatment in airway and lung injury. Over the past decade, it has become possible to monitor the level of injury using OCT and to track the engraftment of stem cells using stem cell imaging in regenerative tissue. The purpose of this study was to assess the engraftment of exogenous adipose stem cells in injured tracheal epithelium with fluorescent microscopy and to detect and monitor the degree of airway injury in the same tracheal epithelium with OCT. OCT detected thickening of both the epithelium and basement membrane after tracheal scraping. The engraftment of adipose stem cells was successfully detected by fluorescent staining in the regenerative epithelium of injured tracheas. OCT has the potential to be a high-resolution imaging modality capable of detecting airway injury in combination with stem cell imaging in the same tracheal mucosa.

Recent advances in optical coherence tomography for the diagnoses of lung disorders

There have been many advances in the field of diagnostic and therapeutic pulmonary medicine in the past several years, with major progress in the field of imaging. Optical coherence tomography (OCT) is a high-resolution (micron level) imaging modality currently being advanced with the potential to image airway wall structures in real time and at higher resolution than previously possible. OCT has the potential to increase the sensitivity and specificity of biopsies, create 3D images of the airway to guide diagnostics, and may have a future role in diverse areas such as the evaluation and treatment of patients with obstructive sleep apnea, tracheal stenosis, airway remodeling and inhalation injury. OCT has recently been investigated to monitor airway compliance in chronic obstructive pulmonary disease and asthma patients as well as differentiate causes of pulmonary hypertension. In future clinical and research applications, OCT will likely be combined with other endoscopic based modalities such as ultrasound, spectroscopy, confocal, and/or photoacoustic tomography to determine functional and biomolecular properties. This article discusses the current uses of OCT, its potential applications, as it relates to specific pulmonary diseases, and the future directions for OCT.

Investigating in vivo airway wall mechanics during tidal breathing with optical coherence tomography

Optical coherence tomography (OCT) is a nondestructive imaging technique offering high temporal and spatial resolution, which makes it a natural choice for assessing tissue mechanical properties. We have developed methods to mechanically analyze the compliance of the rabbit trachea in vivo using tissue deformations induced by tidal breathing, offering a unique tool to assess the behavior of the airways during their normal function. Four-hundred images were acquired during tidal breathing with a custom-built endoscopic OCT system. The surface of the tissue was extracted from a set of these images via image processing algorithms, filtered with a bandpass filter set at respiration frequency to remove cardiac and probe motion, and compared to ventilatory pressure to calculate wall compliance. These algorithms were tested on elastic phantoms to establish reliability and reproducibility. The mean tracheal wall compliance (in five animals) was 1.3±0.3×10(-5) (mm Pa)(-1). Unlike previous work evaluating airway mechanics, this new method is applicable in vivo, noncontact, and loads the trachea in a physiological manner. The technique may have applications in assessing airway mechanics in diseases such as asthma that are characterized by significant airway remodeling.

Quantification of airway thickness changes in smoke-inhalation injury using in-vivo 3-D endoscopic frequency-domain optical coherence tomography

Smoke inhalation injury is frequently accompanied by cyanide poisoning that may result in substantial morbidity and mortality, and methods are needed to quantitatively determine extent of airway injury. We utilized a 3-D endoscopic frequency-domain optical coherence tomography (FD-OCT) constructed with a swept-source laser to investigate morphological airway changes following smoke and cyanide exposure in rabbits. The thickness of the mucosal area between the epithelium and cartilage in the airway was measured and quantified. 3-D endoscopic FD-OCT was able to detect significant increases in the thickness of the tracheal walls of the rabbit beginning almost immediately after smoke inhalation injuries which were similar to those with combined smoke and cyanide poisoning.

Linking optics and mechanics in an in vivo model of airway fibrosis and epithelial injury

Chronic mucosal and submucosal injury can lead to persistent inflammation and tissue remodeling. We hypothesized that microstructural and mechanical properties of the airway wall could be derived from multiphoton images. New Zealand White rabbits were intubated, and the tracheal epithelium gently denuded every other day for five days (three injuries). Three days following the last injury, the tracheas were excised for multiphoton imaging, mechanical compression testing, and histological analysis. Multiphoton imaging and histology confirm epithelial denudation, mucosal ulceration, subepithelial thickening, collagen deposition, immune cell infiltration, and a disrupted elastin network. Elastase removes the elastin network and relaxes the collagen network. Purified collagenase removes epithelium with subtle subepithelial changes. Young's modulus [(E) measured in kiloPascal] was significantly elevated for the scrape injured (9.0+/-3.2) trachea, and both collagenase (2.6+/-0.4) and elastase (0.8+/-0.3) treatment significantly reduced E relative to control (4.1+/-0.7). E correlates strongly with second harmonic generation (SHG) signal depth decay for enzyme-treated and control tracheas (R(2)=0.77), but not with scrape-injured tracheas. We conclude that E of subepithelial connective tissue increases on repeated epithelial wounding, due in part to changes in elastin and collagen microstructure and concentration. SHG depth decay is sensitive to changes in extracellular matrix content and correlates with bulk Young's modulus.

Detection and monitoring of early airway injury effects of half-mustard (2-chloroethylethylsulfide) exposure using high-resolution optical coherence tomography

Optical coherence tomography (OCT) is a noninvasive, high-resolution imaging technology capable of delivering real-time, near-histologic images of tissues. Mustard gas is a vesicant-blistering agent that can cause severe and lethal damage to airway and lungs. The ability to detect and assess airway injury in the clinical setting of mustard exposure is currently limited. The purpose of this study is to assess the ability to detect and monitor progression of half-mustard [2-chloroethylethylsulfide (CEES)] airway injuries with OCT techniques. A ventilated rabbit mustard exposure airway injury model is developed. A flexible fiber optic OCT probe is introduced into the distal trachea to image airway epithelium and mucosa in vivo. Progression of airway injury is observed over eight hours with OCT using a prototype time-domain superluminescent diode OCT system. OCT tracheal images from CEES exposed animals are compared to control rabbits for airway mucosal thickening and other changes. OCT detects the early occurrence and progression of dramatic changes in the experimental group after exposure to CEES. Histology and immunofluorescence staining confirms this finding. OCT has the potential to be a high resolution imaging modality capable of detecting, assessing, and monitoring treatment for airway injury following mustard vesicant agent exposures.

Airway epithelium stimulates smooth muscle proliferation

Communication between the airway epithelium and stroma is evident during embryogenesis, and both epithelial shedding and increased smooth muscle proliferation are features of airway remodeling. Hence, we hypothesized that after injury the airway epithelium could modulate airway smooth muscle proliferation. Fully differentiated primary normal human bronchial epithelial (NHBE) cells at an air-liquid interface were co-cultured with serum-deprived normal primary human airway smooth muscle cells (HASM) using commercially available Transwells. In some co-cultures, the NHBE were repeatedly (x4) scrape-injured. An in vivo model of tracheal injury consisted of gently denuding the tracheal epithelium (x3) of a rabbit over 5 days and then examining the trachea by histology 3 days after the last injury. Our results show that HASM cell number increases 2.5-fold in the presence of NHBE, and 4.3-fold in the presence of injured NHBE compared with HASM alone after 8 days of in vitro co-culture. In addition, IL-6, IL-8, monocyte chemotactic protein (MCP)-1 and, more markedly, matrix metalloproteinase (MMP)-9 concentration increased in co-culture correlating with enhanced HASM growth. Inhibiting MMP-9 release significantly attenuated the NHBE-dependent HASM proliferation in co-culture. In vivo, the injured rabbit trachea demonstrated proliferation in the smooth muscle (trachealis) region and significant MMP-9 staining, which was absent in the uninjured control. The airway epithelium modulates smooth muscle cell proliferation via a mechanism that involves secretion of soluble mediators including potential smooth muscle mitogens such as IL-6, IL-8, and MCP-1, but also through a novel MMP-9-dependent mechanism.

In vivo optical coherence tomography detection of differences in regional large airway smoke inhalation induced injury in a rabbit model

Smoke inhalation injury causes acute airway injury that may result in airway compromise with significant morbidity and mortality. We investigate the ability of high resolution endobronchial optical coherence tomography (OCT) to obtain real-time images for quantitatively assessing regional differences between upper tracheal versus lower tracheal and bronchial airway injury responses to smoke inhalation in vivo using a prototype spectral domain (SLD)-OCT system we constructed, and flexible fiber optic probes. 33 New Zealand White rabbits are intubated and mechanically ventilated. The treatment groups are exposed to inhaled smoke. The OCT probe is introduced through the endotracheal tube and maintained in place for 5 to 6 h. Images of airway mucosa and submucosa are obtained at baseline and at specified intervals postexposure. Starting within less than 15 min after smoke inhalation, there is significant airway thickening in the smoke-exposed animals. This is maintained over 5 h of imaging studies. The lower tracheal airway changes, correlating closely with carboxyhemoglobin levels, are much greater than upper tracheal changes. Significant differences are seen in lower trachea and bronchi after acute smoke inhalation compared to upper trachea as measured in vivo by minimally invasive OCT. OCT is capable of quantitatively detecting regional changes in airway swelling following inhalation injury.