A defined period of sensitivity of an experimental burn wound to a second injury

We sought to determine whether the zone of stasis of an experimental burn exhibits a defined period of sensitivity to a sublethal exposure to ischemia. A standard burn injury was inflicted on the abdominal skin of rats consisting of two burn areas separated by a 5-mm strip corresponding to the zone of stasis. Either immediately or after a delay of 24, 48, or 72 hours, the area of the burn and interspace was elevated as a flap based on the inferior epigastric vessels and subjected to a 5-hour period of ischemia that in itself is insufficient to cause necrosis. Forty-eight hours later, the flap was harvested to assess the viability of the panniculus carnosus. Necrosis of the panniculus in the interspace was not seen in any animal where burn was not subjected to ischemia (n = 6) or in any flap subjected to a 5-hour period of ischemia immediately (less than 30 minutes) after the burn (n = 7). In contrast, when the period of ischemia was delayed 24 hours after the burn, necrosis of the muscle in the interspace was seen in 5 out of 6 flaps (P = .0047), as it was in rats in which the start of ischemia was delayed 48 hours (n = 2). However, this effect was abolished with a delay of 72 hours (n = 2). These findings show that the zone of stasis displays increased sensitivity to ischemia 24 to 48 hours after burning. Thus, sequential subnecrotic injuries can have a cumulative effect, but a delay between the two is required.