1.

Introduction

Inhalation airway injury is a major cause of morbidity and mortality. Inhalation exposure risks include thermal, chemical, and toxic injuries, as well as secondary infectious complications. In inhalation burn victims, airway hyperemia, edema, sloughing, and necrosis are contributing pathophysiologic alterations that lead to critical airway compromise.^{1, 2} Management of inhalation burn injury patients remains difficult because respiratory symptoms and airway compromise may sometimes be delayed up to 2 to 5 days after exposure.^{3, 4} At the present time, there are no reliable methods to accurately determine which patients will proceed to develop life-threatening airway compromise following inhalation burn exposure. Assessment of hypoxemia and carboxyhemoglobin are indicators of more distal alveolar inhalation exposures and injury and may not correlate with proximal airway damage and impending airway risks. Therefore, physicians must rely on bronchoscopy and relatively rudimentary clinical findings to make critical decisions regarding prophylactic intubation and ventilatory support in the early period following burn and smoke exposure. ^{5, 6, 7, 8} Development of a relatively noninvasive method for detecting and monitoring changes occurring in airway injury patients is a much needed clinical advance.

Advances in rapid acquisition high-resolution flexible fiberoptic optical coherence tomography (OCT) now offer the theoretical potential for obtaining repeated minimally invasive, real-time near histologic level optical imaging of airway epithelial, mucosal, and submucosal tissues for inhalation injury evaluation. The objective of this research was to develop methods and assess the feasibility of real-time, in vivo flexible fiberoptic OCT imaging in acute cold smoke-inhalation airway injury evaluation.

2.

Materials and Methods

2.4.

Administration of Smoke

To administer smoke to the animals, modifications of a standard rabbit smoke-inhalation animal model was used.⁹ Seventy grams of unbleached cotton was burned in a modified bee smoker (Smoke Stack Smoker #644, Brushy Mountain Bee Farm, Moravian Falls, North Carolina) for approximately $20\min$ . The bee smoker was then connected to the inlet port of the mechanical ventilator with the tidal volume and ventilation rate set at $700\mathrm{ml}$ and 25 breaths per minute, respectively. A Mylar Douglas bag (model 6060, Hans Rudolph, Kansas City, Missouri) was connected to a second “smoke exposure” ventilator [ventilator 2 (also a dual phase control respirator, model 613, Harvard Apparatus, Chicago, Illinois)] via the multiport valve to the output port to actively fill the bag with smoke from the bee smoker and over $25\mathrm{L}$ of smoke was collected. The Mylar bag was then connected to the inlet port of the smoke exposure ventilator and set to a tidal volume of $60\mathrm{ml}$ and a ventilation rate of 18 breaths per minute to deliver the (now cool) smoke-filled contents of the Douglas bag to the rabbit in a controlled manner. The rabbit was disconnected from the regular ventilator (1) circuit and connected to the smoke ventilator (2) for exposure. Exposure was achieved by giving the animal repetitive alternate increments of 18 breaths of smoke, followed by 100% oxygen (accomplished by switching between the traditional ventilator setup and the smoke ventilator). The animals were administered either 0 (controls) or 90 breaths (high dose) of smoke. Measurements of blood and concurrent OCT were obtained at preexposure, and 15, 30, 60, 90, 120, 180, 240, 300, and $360\min$ postexposure (though the OCT catheter was left in position in the airway throughout the period and was capable of taking continuous OCT images if desired (Fig. 1 ).

Fig. 1

Experimental setup of the intubated subject. The OCT probe has been inserted though the endometrial tube into the distal trachea or proximal bronchi and is held in position throughout the study using a constricting gasket adapter. The arrows show areas to be imaged.

2.5.

OCT System and Probes

The OCT system used in this study contains a superluminescent diode source that delivers an output power of $10\mathrm{mW}$ at a central wavelength of $1310\mathrm{nm}$ with a full width at half maximum (FWHM) of $80\mathrm{nm}$ , resulting in approximately $10\text{-}\mathrm{\mu }\mathrm{m}$ axial resolution.

In the reference arm, a rapid-scanning optical delay line is used that employs a grating to control the phase and group delays separately so that no phase modulation is generated when the group delay is scanned. The phase modulation is generated through an electrooptic phase modulator that produces a carrier frequency. The axial line scanning rate is $500\mathrm{Hz}$ , and the modulation frequency of the phase modulator is $500\mathrm{kHz}$ . Reflected beams from the two arms are recombined in the interferometer and detected on a photodetector. The detected optical interference fringe intensity signals are bandpass filtered at the carrier frequency. Resultant signals are then digitized with an analog-digital converter that performs 12-bit-at-5-MHz signal conversion and transferred to a computer where the structural image is generated.^{10, 11}

Flexible fiber optic OCT probes were constructed from single-mode fiber (ThorLabs, Newton, New Jersey). The bare ended fiber was attached to a 0.7-mm diameter gradient index (GRIN) lens (NSG America, Irvine, California), using optical adhesive (Dymax Co., Torrington, Connecticut) under a microscope. A right angle light path was achieved using a 0.7-mm prism. The probe was placed in fluorinated ethylene propylene tubing (17-gauge thin wall, Zeus, Orangeburg, South Carolina) for added fiber support (Fig. 2 ).

Fig. 2

Translational flexible fiberoptic probes used in OCT smoke-induced airway injury measurements. (a) Top figure shows a schematic of the flexible fiberoptic probe with external diameter 1.8-mm outer sheath containing a single-mode optical fiber encased in protective coating, connected to 0.7-mm GRIN lens and reflecting prism. The longitudinal direction of imaging is obtained using a linear motor to move the internal components as shown, resulting in cross-sectional images of $16\mathrm{mm}$ . (b) Photograph of an actual probe constructed for use in this study revealing the components described above. Centimeter ruler included for size scale.

A linear motor (Newport Instruments, Irvine, California) was used to drive the coated flexible fiberoptic distally and proximally along the length of the probe within the sheath, moving the GRIN lens and prism imaging components within the sheath to obtain linear images along the long axis of the trachea and bronchi. Axial scans were obtained every $10\mathrm{\mu }\mathrm{m}$ along the length of the probe during 16-mm long scan sweeps. A 16-mm scan is obtained in $3.2\sec$ . Translational imaging has a number of distinct theoretical advantages over rotational scanning methods for initial investigations in relatively large lumen sites such as trachea and proximal bronchi.

2.6.

Airway Thickness Measurements

Airway thickness was measured as the distance between the epithelial surface and the surface of landmark submucosal cartilaginous rings in the trachea and bronchi and includes epithelial, mucosal, submucosal, and glandular tissues above the cartilage. All measurements were obtained by two independent researchers (JJ, LT) who were blinded to all clinical information. Airway thickness measurements were taken at as many landmark cartilaginous rings as could be seen in an individual animal (generally three to five cartilage ring sites per animal) as shown in Fig. 3 . Measurements of the airway thickness at the landmark rings were obtained at baseline and at each specified period following inhalation injury. The percentage of change from baseline was calculated for each airway thickness measurement at every time point. Values were then averaged for all airway thickness measurements within a given animal at each specified point.

Fig. 3

Example of longitudinal OCT image of rabbit trachea (a) compared to histologic image shown (b). Epithelium $\left(e\right)$ , glands $\left(g\right)$ , cartilage $\left(c\right)$ , submucosal layers $\left(sm\right)$ , lamina propria $\left(lp\right)$ , and muscularis $\left(\mathrm{L}m\right)$ are clearly seen (Ref. 21). In vivo longitudinal images of rabbit trachea at baseline [(c) and (e)] and $6\mathrm{h}$ following inhalational smoke exposure [(d) and (f)]. Marked increase in the thickness of the epithelial, mucosal, and submucosal layers are evident, particularly visible above the cartilage rings. (e) and (f) images demonstrate the measurement methods for determining airway thickness above each tracheal ring for the images obtained in (c) and (d), respectively.

3.

Results

To ensure that the rabbits received adequate smoke, we verified blood $\mathrm{COHgb}$ levels using co-oximetry and observed values of 1.6 to 18.3%. Translational (longitudinal) images 16-mm long were obtained from the probe in the lower trachea and proximal bronchi from preexposure and over a period of $6\mathrm{h}$ following smoke exposure. The 16-mm longitudinal sweep distance provided images with easily recognizable landmarks that could be followed continuously throughout the experimental course. Depth of penetration was approximately $1.6\mathrm{mm}$ , which allowed visualization of the full depth of the airways in the rabbit to well below the surface of the cartilaginous tracheal and bronchial rings (Fig. 3).

In some animals, dramatic increases in thickness of the airway could be seen beginning at intervals as short as $5\min$ following smoke exposure. However, there was marked variability in the degree and timing of changes across the group of smoke-exposed animals. In some animals no significant changes were seen at all, while in others, an increase of average thickness of as much as 260% developed. In contrast, in the control animals, minimal changes were seen in the majority of animals; though as a group there was a tendency toward some increase in overall airway layer thickness over time in these intubated, ventilated animals (Fig. 4 ).

Fig. 4

Percentage of change in baseline of airway thickness determined by OCT versus time (minutes) following smoke-inhalation exposure compared to controls. Statistically significant increases in airway thickness are seen that begin very early following smoke exposure. No significant changes from baseline are detected in the control animals. Average change in airway thickness was greater than 120% compared to baseline by $5\mathrm{h}$ following smoke exposure.

Table 1

Change in airway thickness following smoke exposure.

Overall, there was a significant increase in the thickness of mucosa of those animals that received smoke, when compared to those animals that did not (Table 1 ). On average, airway layer thickness increased from baseline to $78.5\pm 38\%$ [standard error of mean (SEM)] above baseline at $30\min$ , peaking at a mean of $120\pm 33\%$ (SEM) above baseline at $5\mathrm{h}$ postexposure in the smoke-treated group ( $p<0.05$ compared to baseline and compared to controls). In comparison, the control group showed an $11.4\pm 8.7\%$ (SEM) increase at $30\min$ , with the peak of $39.2\pm 24\%$ (SEM) increase at $6\mathrm{h}$ (Fig. 4).

Animals tolerated the $6\mathrm{h}$ of F-OCT monitoring well, and no significant complications were seen from the OCT catheter.

4.

Discussion

This study demonstrates that OCT was able to detect significant changes in airway thickening occurring following smoke-inhalation injury in vivo in a clinically relevant animal model. The ability of flexible fiberoptic OCT to be performed repeatedly and reliably in a noncontact minimally invasive fashion attests to the potential value in this important clinical application. The feasibility of the methodology as well as the capabilities of the technology were demonstrated.

We utilize a translationally scanning 1.8-mm diameter probe with a long sweep distance setting $\left(16\mathrm{mm}\right)$ to obtain easily recognizable, measurable images of the longitudinal cross section of the proximal lower airways in this rabbit model. The airway thickness could be readily assessed quantitatively and easily identifiable cartilaginous landmarks could be repeatedly and reproducibility located. Statistically and clinically significant changes in the thickening of the airway were seen in those animals that received smoke when compared to control animals that were treated in a similar manner but did not receive smoke exposure. Most importantly, immediately after sacrifice when airway observation by OCT was performed and when subsequent histologic evaluation was performed, the swelling that had been seen with OCT was no longer evident. This caused us to believe that the swelling we observed was due to primarily to hyperemia rather than edema, inflammation, or necrosis. Previous studies ^{1, 2, 12, 13} have demonstrated that the very early changes examined in these studies consist primarily of hyperemia and edema—changes that may be lost during preparation of histologic specimens. These findings are consistent with earlier reports of the events associated with acute cold smoke-inhalation injury in the Bidani cotton smoke animal model⁹ and in early findings in patients following acute inhalation smoke injury. Future studies will be needed to determine whether the very early changes that we are able to detect on OCT correlate with the extent of later injury and are predictive of development of life-threatening airway complications that occur at later times following exposure.

Despite its frequent clinical use, early standard bronchoscopy with or without invasive biopsy may not accurately reveal the extent of impending airway edema that often develops, including vessel hyperemia, edema, and sloughing of tracheal ciliated columnar epithelium.^{2, 13, 14} From within the lumen of the airway during bronchoscopy, it is difficult to determine the presence or extent of airway mucosal swelling, particularly in the early phases following inhalation burn injury, and it is not possible to make any quantitative assessment of airway edema. The ability to obtain this information using OCT (possibly with optical Doppler tomography) without invasive biopsies (on a repeated basis) could lead to improved patient care, allow more complete monitoring of inhalation injury, provide a method for assessing response to therapy, and could potentially be used as a sensitive tool for determining the effectiveness of proposed therapeutic interventions. In future studies, the ability to assess collagen disruption with polarization sensitive OCT in the airways of thermally injured burn patients might be important areas for investigation as well.¹⁵ Thus, OCT may provide much more direct information on proximal airway thermal and toxic injury than can be obtained by other methods available at this time.

There are a number of limitations to the conclusions that can be drawn from this study. First, there was substantial variability in the overall injury response and airway changes seen in the smoke-exposed animals. The reasons for this variability are uncertain at this time. Possible factors may include regional differences in the airflow and deposition patterns of the smoke as applied in this animal model, variable biological responses inherent within the animals, or other mechanical factors that we have not identified. Because no significant changes were seen in any of the control animals, we do not believe that the variability seen is due to experimental conditions other than smoke exposure or technical limitations of the OCT techniques and measurement methods, but rather due to true variability in airway pathophysiologic changes, as a result of smoke exposure. The degree of $\mathrm{COHgb}$ elevation seen in our animal models was also variable (1.6 to 18.3%), and considerably lower than those levels reported by Bidani,⁹ whose group administered the smoke directly from the smoking chamber into the ventilator (rather than through a Douglas bag). This may indicate a lower-level exposure in our animals that may account for some of these findings. Second, because there are no postmortem OCT or histopathologic changes seen to correlate with these acute in vivo OCT injury findings, the accuracy of the OCT imaging and fidelity of the images to underlying in vivo pathologic changes that occurred cannot be confirmed with certainty. Furthermore, we examined very acute early injury changes, up to $6\mathrm{h}$ following exposure. Many of the most significant changes manifest at $24\mathrm{h}$ and later.⁹ Longer term studies and higher dose exposures will clearly be needed to confirm and extend the current findings. Most importantly, the relationship between the early changes seen in these studies and the ultimate extent of airway injury developing over time needs to be determined. OCT should allow us to study this question.

The animals tolerated the presence of the flexible fiberoptic probe without difficulties. Care was taken to ensure that the probes were not in direct contact with the mucosal surface where they could potentially block the inhaled smoke from contacting the airway wall during administration, could potentially injure airway mucosa, or affect development of edema or hyperemia. Because the animals were anesthetized and mechanically ventilated, probe position remained stable throughout most of the studies. A number of methods could be developed in the future for ensuring stability of the probe location in longer-term animal studies or patients while avoiding the potential problems listed above. From a technical standpoint, OCT equipment, acquisition methods, functional OCT capabilities, and OCT probe technology are advancing rapidly. ^{16, 17, 18, 19, 20} Ultrahigh-resolution, three-dimensional, ultrafast acquisition may further enhance the capabilities of OCT for inhalation airway injury assessment.¹⁶

5.

Conclusions

This study demonstrates that minimally invasive flexible fiberoptic OCT assessment of aspects of acute smoke-induced airway injury is feasible. Changes in epithelial, mucosal, and submucosal airway structures were evident extremely early following exposure to cold smoke. Future studies will be needed to confirm and extend these very promising preliminary in vivo OCT inhalation airway injury findings and to determine causes for potential regional and individual variability in response to smoke exposure. Most importantly, longer-term studies assessing the relationship between acute airway changes and subsequent development of later airway compromise will be very important in animal models as well as patients with inhalation burn injury.