Comparison of ICG Microangiography and Conventional Angiography in Severe Frostbite

Overview

Severe frostbite injury is a significant cause of morbidity in northern climates. Minnesota has some of the highest numbers of severe frostbite injuries in North America. As a result, Regions Hospital has the best opportunity to study this disease process and improve outcomes for frostbite patients. The diagnostic methods for severe frostbite injury vary from institution to institution and there is no standard practice. Commonly utilized methods include conventional angiography, Technetium 99 triple phase bone scans, SPECT studies, Indocyanine Green microangiography, and doppler studies. The proposed pilot study aims to directly compare conventional angiography imaging to ICG microangiography in adult patients with severe frostbite. Severe frostbite is defined as 4th degree: frostbite resulting in vascular occlusion and tissue ischemia. Both imaging modalities have been used for the diagnosis and monitoring of severe frostbite injury but there has never been a study directly comparing these two imaging modalities.

Frostbite injury has been a significant cause of morbidity and mortality for as long as humans have experienced cold winters. When frostbite occurs, there are two primary mechanisms through which tissue damage occurs, though the main mechanism focused on here is systemic cooling. Frostbite is a multiphase process that starts with systemic cooling. As the body cools, blood is shunted from the extremities to the core to maintain heat in the vital organs. During this shunting, there is extensive vasospasm in the extremities and the tissue becomes ischemic. Upon further cooling, tissue ice crystals form within the cells and interstitial spaces causing desiccation and cell destruction. This occurs in the soft tissues and also affects the endothelium of the micro- and macro-vasculature. Clots form in the microvasculature, worsening the ischemia. All ischemic tissues release pro-inflammatory factors, which increase inflammation and worsen clotting. Upon rewarming of the frozen tissues, the reperfusion can worsen the inflammatory state as these factors released by the ischemic tissue are now able to circulate. Rapid rewarming is used as a tool to mitigate this reperfusion injury, but it does not eliminate the damage. After rapid rewarming, the tissue is assessed for the extent of damage to determine next treatment steps. Diagnosis varies by institution, but the main goal of all diagnostic modalities is to determine if there is a vascular cutoff causing tissue ischemia. At Regions Hospital, the main diagnostic method that has been used for over 20 years is conventional angiography. This clearly demonstrates the microvasculature of the tissues and allows the intra-arterial catheter directed sheaths to be placed to start thrombolytics, heparin, and vasodilators to treat the tissue ischemia. Other diagnostic modalities used elsewhere include Technetium 99 triple phase bone scans, SPECT imaging, and indocyanine green (ICG) microangiography. All of these imaging modalities demonstrate the perfusion of the affected tissue but do not allow for directed delivery of thrombolytics. In institutions that use these imaging techniques, thrombolytics are typically delivered in an intravenous fashion rather than intra-arterial. ICG microangiography is a technology that has had rapidly expanding applications in recent decades. These have included retinal imaging, determining perfusion of colorectal anastomoses, biliary imaging, and assessing the blood supply of tissue flaps. It has recently been used in diagnosis of frostbite with good correlation with severe final amputation levels. It is easy to use, non-radiating, can be performed at the bedside, and cheaper than other imaging modalities. Given that every hospital has different diagnostic and treatment algorithms, these different imaging modalities have rarely been compared in the same patient. The current best metric is comparing the imaging modality with the amount of tissue that is amputated as demonstrated in prior studies cited here. This research application provides a unique opportunity to compare two imaging modalities head-to-head in the same patient and determine their concordance or discordance.