Visualization of Wound Healing Progression With Near Infrared Spectroscopy

A Retrospective Study

Adam Landsman, DPM, PhD, FACFAS

Disclosures

Wounds. 2020;32(10):265-271. 

In This Article

Abstract and Introduction

Abstract

Objective: The aim of this retrospective study is to determine if near infrared spectroscopy (NIRS) can be used to evaluate wounds and adjacent soft tissues to identify patterns involved in tissue oxygenation and wound healing as well as predict which wounds may or may not heal.

Materials and Methods: In this study, 25 patients with either diabetic foot ulcers or venous leg ulcers were examined retrospectively to determine if NIRS could be used to predict which wounds may or may not close. All patients had either diabetic or venous ulcers and were being actively treated in the clinic. Regardless of the treatment rendered, all wounds were tracked with NIRS at regular intervals. Retrospectively, the de-identified images were reviewed to determine any patterns that might exist. Wound bed and periwound oxygenation patterns were observed and classified, including correlation with both the clinical appearance and the NIRS images. Images of wounds that closed and those that did not were compared.

Results: Four distinct patterns of tissue oxygenation that appeared to have some value for predicting which wounds would heal, and which would not, were identified among the 25 patients. A mechanism has also been proposed to try to explain the patterns of healing observed; Hyperperfusion, Imbibition, Neovascularization, and Trailing (HINT) describes various aspects of these patterns.

Conclusions: As with any imaging technology, both qualitative and quantitative data are used to determine what is happening clinically. This study represents an early attempt to understand the role of NIRS and percent oxygenated hemoglobin in the wound healing process. It also lays the groundwork for identifying patterns associated with wound closure.

Introduction

The closure of chronic wounds can be a complex and difficult process that is highly dependent on the optimization of conditions to facilitate healing. In order to understand the conditions that favor wound closure, the factors that are considered detrimental have to be identified and eliminated. This list would usually include reduction of bioburden, control of mechanical forces, and maintenance of adequate concentrations of blood-borne growth factors and cellular materials to drive the healing process.

Adequate tissue oxygenation is a critical step in achieving wound closure. The classic description of wound healing involves a 3-stage process in which debridement is followed by inflammation, proliferation, and maturation.[1] Inflammation is the initial reaction to debridement in which bleeding occurs, followed by platelet infiltration, degranulation, and release of growth factors. Subsequently, the cells are attracted to the wound site and begin to proliferate to close the wound. Finally, the wound matures as the skin structure remodels to become well aligned and strengthened.

In its simplest form, oxygenation of the wound bed and periwound areas can be a strong reflection of the condition of a wound and its potential for healing. Segmental pressures and the ankle-brachial index (ABI) are simple tests that have been widely used to gauge the level of blood flow to the leg and foot.[2] Angiography and some ultrasonic devices can also demonstrate the arterial blood flow. However, when it comes to wounds, a system is also needed to allow one to measure oxygenation to the skin at the capillary level.

Radioisotope-labeled blood cells have been used to trace the flow in the soft tissues and are very helpful at identifying areas of inflammation or infection, but they have limited value for assessing the finer details of wound bed perfusion. Scanning devices using indocyanine green (ICG) dye have been used in the operating room to assess blood flow to areas where critical skin ischemia may be present, such as during plastic surgical procedures, but this requires expensive equipment, and the test itself can only be conducted a very limited number of times during a case due to the persistence of the dye.[3,4]

Transcutaneous oxygen pressure (TcPO2) can be used to assess tissue oxygenation as a measure of perfusion over small regions without the need for dyes. However, this technology is very limited to small spots, typically on the order of 5 mm in diameter or less. Although this can be useful for assessing a specific location, it is difficult to determine the overall blood flow to a larger region of the surrounding skin.[5]

Near infrared spectroscopy (NIRS) is one of the newer options for evaluating oxygen delivery and usage in the microvasculature. This technology uses reflected light to calculate perfusion by taking advantage of subtle color changes that occur in hemoglobin when it is oxygenated. By recording the ratio of oxygenated to deoxygenated hemoglobin, one can accurately measure the percentage of oxygenated blood reaching the skin.[6] Because the light can be generated by an array of diodes and detected over a larger area, it is possible to capture large regions of anatomy simultaneously, such as the entire plantar surface of the foot.

The NIRS transmits very specific wavelengths of light between 600 nm and 1000 nm, and it measures the amount of reflected light to determine the ratio of deoxygenated to oxygenated hemoglobin. Unlike systems that utilize visible light, the near infrared spectrum can penetrate deeper into the skin while eliminating other potentially confounding wavelengths. Deeper light penetration is more accurate and reproducible, and it diminishes the effect of skin pigmentation color on the data.

There are some clear advantages to using this type of light-based system for measuring skin perfusion. With NIRS, it is a noncontact system. Unlike with TcPO2, there are no disposable elements or items that have to be sterilized between patient measurements. The data can be captured and processed in as fast as 5 seconds and is highly reproducible. Images can include much larger surfaces (eg, the entire plantar surface of the foot) and can be taken before and after a treatment is performed in a single clinical or surgical visit. It also can be compared with prior visits with great consistency. Unlike the ICG-based systems, there are no injectable dyes used with NIRS.

Over the last decade, there has been a great deal of speculation about how to interpret images taken using NIRS. The earliest studies found that 40% oxygenated hemoglobin in the wound bed was needed for healing.[7] Similarly, the 40% number has been useful for predicting the viability of skin flaps and grafts during surgery. Nonetheless, many flaps that have achieved 40% or more oxygenated hemoglobin have failed,[8] while others have survived, so this number is not a hard and fast cutoff. A 2011 study demonstrated that the postoperative differences in tissue oxygen saturation within a skin flap correlated with the development of skin necrosis.[9] It is important to consider that wound healing is not wholly dependent on vascular perfusion but rather oxygenation in the context of numerous other biologic influences, including mechanical forces to the wound bed, disease state, general health, infection, and the presence of bioactive materials (eg, collagen and growth factors).[10]

The purpose of the current study was to review a series of cases in which wounds were observed using NIRS to ascertain patterns of oxygen saturation. By examining patterns within the wound bed and the surrounding soft tissues, the author hoped better identify signs that might help clinicians to understand when a wound might reasonably be expected to close. It was the author's hypothesis that there was a pattern of changes observed within the wound and the periwound regions that could be useful in predicting when wounds will progress toward healing. In this series, data from 25 wounds were observed to determine if a pattern emerged that might help one gauge where a wound is functioning in the collective healing process. Data were collected from wounds that closed and did not close. The vast majority of the wounds were diabetic foot ulcers (DFUs) and venous leg ulcers (VLUs), but a portion of wounds associated with other etiologies was also examined.

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