ASSESSMENT OF ISCHEMIA-INDUCED REPERFUSION INJURY IN THE PIG LATISSIMUS-DORSI MYOCUTANEOUS FLAP MODEL

Experiments were conducted to assess ischemia-induced reperfusion inju ry in the pig latissimus dorsi myocutaneous flap model. Forty Yorkshir e pigs (19.5 +/- 0.6 kg) were assigned to groups A, B, C, and D (n = 1 0 pigs). Bilateral 8 x 13 cm latissimus dorsi myocutaneous flaps were constructed in each pig, and one flap was assigned to ischemic treatme nt and the contralateral flap served as a nonischemic control. The tre atment flaps in groups A, B, C, and D were subjected to 2, 4, 6, and 8 hours of warm global ischemia, respectively. Pigs in groups A, B, C, and D were divided into two subgroups (n = 5 pigs), and extents of ski n and muscle necrosis in control and treatment flaps were assessed wit h the fluorescein and nitroblue tetrazolium dye stain tests, respectiv ely, after 2 and 7 days of reperfusion. Significantly (p < 0.01) great er extents of skin and muscle necrosis were observed in latissimus dor si myocutaneous flaps subjected to 4, 6, or 8 hours of ischemia compar ed with their contralateral controls. Extents of skin and muscle necro sis also increased significantly (p < 0.01) with increases in ischemia time in treatment flaps. Of particular importance was the observation that there was no significant difference in the extent of skin or mus cle necrosis between 2 and 7 days of reperfusion in all control and tr eatment groups. This observation indicates that 2 days of reperfusion time is adequate to assess the maximum extent of skin and muscle ische mia-induced reperfusion injury in pig latissimus dorsi myocutaneous fl aps. Furthermore, it was observed that 1-cm segments of latissimus dor si muscle were not too thick to allow the use of the nitroblue tetrazo lium dye stain test for assessment of muscle viability, as judged by t he highly correlated (r = 0.98, n = 40) linear relationship between as sessment of muscle viability from one transverse cut surface of muscle segments and by weighing total viable and nonviable muscles dissected from the flaps according to the nitroblue tetrazolium dye stain on bo th transverse cut surfaces. It is important to note that the maximum l ength of the latissimus dorsi myocutaneous flap model for ischemia-ind uced reperfusion injury research should not exceed the maximum length of skin viability in the nonischemic control in order to avoid the com plication of skin necrosis due to excessive length of skin. The patter n of muscle necrosis revealed by nitroblue tetrazolium dye staining in dicated that muscle necrosis occurred mainly and consistently in the p roximal dorsal portion of the latissimus dorsi myocutaneous flap, wher e the muscle was the thickest, thus permitting easy access for multipl e muscle biopsies without significant damage to the flap. Histologic s tudy revealed that the pig latissimus dorsi muscle was composed of typ es I (19 percent) and II (81 percent) muscle fibers. There was no obse rvable indication of difference in tolerance of ischemia-induced reper fusion injury between these two types of fibers because the nitroblue tetrazolium dye stain was quite uniform in the viable and nonviable ar eas of the latissimus dorsi muscle. The technical information obtained from the present experiments is important when using the pig latissim us dorsi myocutaneous flap model to study the pathophysiology and phar macology of ischemia-induced reperfusion injury in myocutaneous flaps.