Frostbite represents a spectrum of injury ranging from irreversible cellular destruction to reversible changes seen after
rewarming. These changes include increases in tissue
edema, circulatory stasis, and progressive
thrombosis leading to further tissue
necrosis. For this reason, it is often difficult at the time of surgical
debridement to determine the extent of
frostbite injury. This delayed tissue injury is similar to that seen in muscle during
ischemia/reperfusion injury. Muscle that initially appears viable on reperfusion may subsequently necrose due to collapse of the microcirculation. Adherent neutrophils have been specifically cited as important components in
ischemia/reperfusion injury and have also been suggested to play a role in
frostbite injury. We have used an intravital microscopic muscle preparation to study microcirculatory changes carefully in
frostbite injury during
rewarming. The right gracilis muscle of male Wistar rats is dissected free from its primary vascular pedicle and the rat is positioned on a specially constructed microsurgical stage. Temperature changes of the muscle are recorded. The prepared axial pattern flap is transilluminated with a microscope and projected on a video screen, allowing measurement of arteriolar diameters and changes in the numbers of stuck and rolling neutrophils before
frostbite, during
rewarming, and for several hours later. Cold
silicone oil is used to freeze the muscle to -5+/-2 degrees C in 2 to 3 minutes and to hold this temperature for 5 minutes. The muscle is rewarmed with 42 degrees C
normal saline placed directly on the muscle surface. Baseline vessel diameter and leukocyte counts in 100-mm segments of the microvasculature are recorded as well as at 5, 15, and 30 minutes, and at 1, 2, and 3 hours postrewarming of frozen muscle. Observations from our initial 11 animals show that reperfusion of the muscle following freezing is varied temporally and spatially, with circulation to most vascular segments restored 5 to 10 minutes after
rewarming. In 9 of 11 animals we observed the shedding of "white clots" in small arterioles and venules occurring as soon as 5 minutes after thawing. In some instances shedding continued for as long as 1 hour after
rewarming. Microvascular
hemorrhage was widespread 1 hour following the thaw, but there was no significant increase in neutrophil adherence observed until 3 hours following
rewarming. The exact nature of the
vascular injury and the composition of the "white clots" are now being determined from ultrastructural studies. Blood flow in microcirculation stops during freezing, but small-vessel perfusion returns immediately on thawing. This suggests that the vascular architecture is maintained during the freezing and thawing. Unlike
ischemia/reperfusion injury, neutrophil adhesion plays a smaller role in the early response to
frostbite injury. The early microcirculatory observations seen after
rewarming suggest progressive and severe perturbations in platelet function and
fibrin formation that are significantly different from
ischemia/reperfusion injury.