34th Annual Scientific Meeting proceedings

Fluorescence-guided surgery has emerged in both human and veterinary surgery over the past two decades as a useful intraoperative tool for various applications.  Near-infrared fluorescence-guided (NIRF) surgery is the use of infrared light and a fluorescent contrast agent to illuminate tissues of interest intraoperatively. Advantages of NIRF include real-time visual enhancement of the surgical field, relatively low-cost fluorescence imaging probes, and the avoidance of ionizing radiation.  Multiple studies have shown that NIRF-guided surgery improves surgical precision. 

Fluorescence is the ability of a molecule to absorb light at one wavelength and then emit light at a longer wavelength which is detected by a near-infrared camera. As a reminder, the wavelength of visible light ranges from 400 to 700nm. Near-infrared light ranges from 700 to 1700 nm.  The most commonly used fluorescent imaging probe is indocyanine green (ICG) with an excitation peak of 780 nm and an emission peak of 820 nm.  It has a short half-life of 150 seconds and is cleared from the blood exclusively by the liver within 15 minutes of intravenous injection.

In human medicine, NIRF-guided surgery has been predominantly utilized in oncologic and gastrointestinal procedures to help delineate margins and evaluate tissue perfusion, respectively.  Interest in various pulmonary applications has expanded in recent years in human medicine.  A clinical study evaluating the use of IV ICG for video-assisted thoracoscopic segmentectomy compared to the traditional inflation/deflation method for patients with chronic lung disease found that use of ICG was associated with shorter operative time and a reduction in the incidence of unclear intersegmental planes in these patients. Injection of ICG into pulmonary parenchyma has also been evaluated as a tool for pre-operative marking of small pulmonary nodules undergoing wedge resection; a clinical study found that both CT-guided and bronchoscopic injection were safe and effective, with the bronchocsopic approach facilitating the marking of multiple lesions with less risk of causing pneumothorax.  A case report also described the use of IV ICG to assist in the identification of bullous lesions in two patients with spontaneous pneumothorax in which the bullae were poorly identified intraoperatively using traditional means. This report found that the fluorescent signal was detected in normal lung tissue 10.5 seconds after injection and lasted up to 525 seconds.  The bullae showed obviously decreased fluorescence compared to the surrounding parenchyma, enhancing their identification and demonstrating a precise border of the bullae.

In addition to the intravenous and intra-parenchymal injection of ICG described above, inhalational ICG is novel application receiving recent attention for evaluation of lung tumors.  A study evaluating the use of inhaled ICG for the intraoperative identification of tumor margins in rodent models found that inhaled ICG was found throughout healthy lung tissue but rarely within tumor tissue due to both mechanical airway obstruction from the tumor and alveolar macrophage uptake of inhaled ICG in healthy tissues.  Compared to IV ICG administration, inhalational ICG had a higher efficiency for tumor margin detection. 

Finally, ICG may also prove useful for resection of pulmonary neoplasia through its ability to identify draining lymph nodes and leakage from resection site.  Studies are currently underway to evaluate the efficacy of lymph node mapping for pulmonary tumors in dogs via intra-tumor injection of ICG.  Additionally, inhaled ICG has been described to look for post-operative air leaks in beagles.  Administration of 5 mL of 2.5 mg/mL aerosolized ICG via a pediatric jet nebulizer facilitated identification of 24 of 25 leaking pleural defects with time for lesion identification averaging 14 seconds. This method may facilitate improved post-resection leak testing in patients undergoing video-assisted thoracic resection of pulmonary lesions.