Near Infrared Spectroscopy & Predicting the Likelihood of Future Wound Healing

Even for the most experienced wound care clinicians, achieving closure of a chronic wound is never easy. Wound management is a coordinated, complex process in which success is dependent on everything from nutritional requirements to bacterial load and mechanical forces. However, despite the complexities of the wound environment, one universal fact is that the perfusion of oxygenated blood to the wound is always required. Fortunately, there has been a continual evolution of techniques that help to gauge perfusion. The ankle-brachial index is a simple test that can measure macroscopic blood flow to a limb, but is highly dependent on the compressibility of the patient’s blood vessels. Similarly, angiograms can provide a detailed image of blood flow to a potentially ischemic limb, but necessitate a trip to the operating room (OR) and can cause damage to renal function. Transcutaneous oxygen perfusion (TcPO₂) was a major advance in that it offers the opportunity to place a probe against the skin to determine the level of tissue oxygenation in a specific location. This finally gave the clinician a simple tool to measure perfusion of periwound areas, provided the probe could be attached to the surface. Unfortunately, these types of measurements are limited due to a lack of reproducibility and difficulties determining exactly what a “tissue perfusion unit” actually means. More recently, scientists have used measurements of reflected light to calculate perfusion by taking advantage of subtle color changes that occur in hemoglobin when it is oxygenated, as compared to when it is not. By recording the ratio of oxygenated to deoxygenated hemoglobin, one can accurately measure the amount of blood reaching the tissues, as well as the level of oxygenation of that blood. This is a true measure of tissue perfusion.

Near infrared spectroscopy (NIRS) capitalizes on this concept of transmitting specific wavelengths of light (760nm and 830nm) and measuring the amount of reflected light to determine the ratio of deoxygenated to oxygenated hemoglobin. Unlike systems that utilize visible (ie, hyperspectral) light, the near infrared (NIR) spectrum can penetrate deeper into the skin while eliminating other potentially confounding wavelengths. Deeper light penetration is more accurate and reproducible, and diminishes the effect of skin pigmentation color on the data. This article will discuss the use of NIRS as it relates to the measurement of perfusion and the management of chronic wounds. 

NIRS & SKIN PERFUSION

There are some clear advantages to using a NIRS light-based system for measuring skin perfusion. NIRS is noncontact, meaning that no part of the device comes in contact with the patient. There are also no disposable elements or items that have to be sterilized between patient measurements, unlike as with TcPO₂. The data can be collected very quickly – as fast as five seconds for capture and processing – and is highly reproducible. Images can be taken before and after a treatment is performed in a single clinic visit, and can be compared to prior and future office visits with great consistency. No dyes and no trips to the OR are necessary, unlike as with the widely used fluorescein-based systems. For all of the aforementioned reasons, NIRS is highly cost effective and very safe.

One of the newest products to reach the chronic wound care market that utilizes NIRS is the KD203 imaging device (Kent Imaging, Calgary, AB). The newest version of the device features a small, handheld unit that weighs about the same as a standard laptop computer (Figure 1). The device uses light in the NIR spectrum that harmlessly passes through the skin and is reflected off the blood supplying the tissue to determine tissue oxygen saturation. The NIR light has two key features that make it useful for measuring the viability of living tissue: Firstly, NIR light is not absorbed by tissue as much as visible or ultraviolet light. Secondly, NIR light is mainly absorbed by hemoglobin and water. Most importantly, the wavelength-dependent light absorption of hemoglobin differs if it is carrying oxygen from when it is not. This makes NIR light very useful in detecting oxygenated and deoxygenated blood, which conveys a comprehensive picture of tissue health and the healing capacity of wounds or tissue transplants. Captured data from the KD203 can be saved in a variety of formats and can be stored on the device itself, or can be downloaded to an electronic health record through a variety of formats. 

LED (light-emitting diode) light is used for focusing the camera lens and guarantees that a distance from the wound surface is maintained each time the camera is used. Consequently, the perspective remains unchanged and the images can be compared from visit to visit. The captured images can be standard color images or spectroscopic images that show the relative levels of oxygenated and deoxygenated hemoglobin. The color images and spectroscopic images can be observed side by side in order to assess the level of perfusion in the wound and periwound areas. One advantage of this system as compared to TcPO₂ is that the entire skin surface can be examined simultaneously, rather than under a single spot. This enables the clinician to appreciate the perfusion of the wound bed itself and the surrounding tissues.  

CLINICAL APPLICATIONS

One of the most rewarding aspects of NIRS, from the author’s standpoint, is the accuracy by which it can assess subtle changes in perfusion. One clinical example involves a patient who underwent a hallux amputation following gangrene of the distal portion of the toe (Figures 2A-B). The resultant stump area appears to be clinically healthy, but the NIRS image clearly points to a drop in perfusion along the wound margin. Approximately one week later, the surgical site went on to a full wound dehiscence with ischemic changes to the skin. (NIRS predicted what would happen a full week in advance of what the clinical picture would suggest.) In hindsight, if defensive actions were taken, such as loosening suture tension or implementing hyperbaric oxygen therapy (HBOT), it is possible that the ischemic dehiscence might have been avoided. Another example involves the use of NIRS to assess wound bed perfusion as a method for predicting the ability to achieve closure (Figures 3A-B). 

A study on multispectral oximetry imaging readings with associated healing trajectory¹ found that wounds should have at least 40% oxygenated to deoxygenated hemoglobin in order to achieve closure.  Although wounds having lower levels of oxygenated hemoglobin did close on occasion, 40% appears to be a good benchmark for wound evaluation. NIRS is also effective for monitoring the quality of the debridement of a wound. Using the KD203 device, wounds can be examined before and after debridement to verify that necrotic or poorly vascularized tissue on the wound surface has been completely removed (Figures 4A-D). 

Following surgery, imaging of the wound surface can be conducted to predict where dehiscence may occur. In one case, a patient underwent a lipoma excision from her foot. Although the clinical appearance was satisfactory, the NIRS image indicated there was a large area of devitalized tissue along the wound margin. Approximately one week later, the wound dehisced and the devitalized tissue sloughed (Figures 5A-B). In another case, a chronic wound suspected of having an ischemic etiology was examined and found to be well perfused. The clinician was able to rule out a vascular etiology and focus on other causes for the lack of progression of the wound (Figures 6A-B). 

SUMMARY

The KD203 NIRS device is an example of the rapidly emerging digital imaging assessment tools available to wound care providers for the management of chronic wounds. By establishing the condition of the wound bed and surrounding tissues, clinicians can actually predict whether or not a vascular component is influencing wound closure. Using this device, one can predict, up to one week in advance, that necrosis or dehiscence of a flap is probable. This is not a trivial matter and allows the clinician to take evasive actions, such as suture removal to relieve tension and HBOT, in order to save poorly oxygenated skin before it dies. Numerous examples can be sited in which the clinical appearance of the wound is normal, but the NIRS image shows a completely different picture.

The ability to measure perfusion in the wound bed itself is a unique attribute of this technology. Preliminary data indicate that wounds with < 40% oxygenated hemoglobin are likely to worsen, or at least show little potential for healing. In these cases, vascular intervention or even more aggressive debridement in the surgical setting may be essential for stimulating a wound to heal. When compared to other modalities, the advantages of NIRS and the KD203 device become apparent. The device's form factor makes it easy to use in both the clinic and the OR. NIRS has the clear advantage of deeper tissue penetration, noncontact measurement, rapid image acquisition, and no need for hazardous intravenous dyes. Most importantly, it is emerging as a device for predicting the future of wounds to heal. 

Adam Landsman is assistant professor of surgery at Harvard Medical School. He may be reached via email at adamlandsman@aol.com. 

Reference

1.  Livingston M. Multispectral Oximetry Imagery Readings with Associated Healing Trajectory. Kent Imaging Inc. Accessed online: www.kentimaging.com/wp-content/uploads/2017/10/Kent_Wounds_6-Page_LD_final.pdf