Survival of normothermic microvascular flaps after prolonged secondary ischemia: effects of hyperbaric oxygen

Although hyperbaric oxygen has been shown to improve the survival rate of ischemic grafts and flaps of many types, it has not been studied extensively in free tissue transfer. This study was designed to evaluate the effect of hyperbaric oxygen on flap survival when exposed to critical combinations of primary ischemia, reperfusion, and secondary ischemia times. Unilateral abdominal adipocutaneous island flaps based on the superficial inferior epigastric vessels were raised in 133 Sprague-Dawley rats. Primary normothermic ischemia was induced by applying a microvascular clamp to the vascular pedicle for 6 hours. The clamp was removed for 2 hours of reperfusion and then reapplied for a 6-, 10-, or 14-hour period of secondary ischemia. After completion of the secondary ischemia time, the clamp was removed, and the animals were randomly assigned to one of three treatment regimens. The control animals breathed normobaric air, and the others breathed normobaric 100% oxygen or hyperbaric oxygen (100% oxygen at the equivalent of 33 feet of seawater, 2.0 atmospheres absolute), respectively, for two periods of 90 minutes for 7 days. Flap survival was assessed at postoperative day 7 and was found to be an all-or-none phenomenon. Maximum likelihood-derived survival curves were fitted to the data and used to calculate the secondary ischemic time at which 50% of the flaps survived (D50). The D50 for the air and 100% oxygen groups was 6 hours, whereas that for the hyperbaric oxygen group was 10 hours. This difference in D50 was found to be statistically significant (analysis of variance, p < 0.05). Chi-squared statistical evaluation of pooled data reaffirmed a statistically significant increase in flap survival of the animals treated with hyperbaric oxygen vs. those treated with air or 100% oxygen (p < 0.03 and p < 0.01, respectively). Hyperbaric oxygen enhances the tolerance of normothermic, microvascular flaps to prolonged secondary ischemia. A similar effect was not noted in the 100% oxygen group; therefore the additional expense and technology of a hyperbaric chamber system is necessary to achieve this effect.