Microdialysis in reconstructive surgery : a clinical and experimental study focusing on monitoring flap metabolism and viability

Abstract

When flap procedures are used to repair complex tissue wounds adequate flap circulation is of supreme importance. If the circulation is disturbed by either vascular occlusion, hemorrhage, or infection, delayed healing and flap necrosis may ensue. Early recognition of flap ischemia followed by appropriate surgical and/or pharmacotherapeutical measures may be crucial for flap survival. In the present thesis the microdialysis (MD) technique was used for the first time in reconstructive surgery to study the metabolism in newly raised flaps. The technique permits repeated measurements of various substances in the interstitial tissue. This makes it reasonable to assume that it might be useful in clinical practice not only to detect acute metabolic disturbances during flap construction, but also to signal metabolic changes in the postoperative period indicating tissue ischemia and threatening flap necrosis. Some specific questions were raised, which will be addressed in this research project:

・Are interstitial concentrations of glucose, lactate, and glycerol affected by surgery in pedicled flaps?

・Is the metabolism of free flaps influenced by transient ischemia?

・Can postoperative complications, leading to flap ischemia, be detected at an early stage by MD monitoring.

・Does transient ischemia induce different metabolic patterns in flaps of different tissue composition?

・Do arterial and venous occlusions cause identical metabolic responses in skin flaps?

・Can MD be used for early prediction of where necrosis will later on appear in flap tissue?

A method was first developed to secure sterile conditions during MD and surgery. Then studies were performed in patients and pigs, which generated responses to the above mentioned questions.

When pedicled musculocutaneous flaps were raised in patients the interstitial glucose, lactate, and glycerol concentrations increased. The glucose increase was transient, whereas the lactate and glycerol levels remained elevated longer. This metabolic pattern may thus be typical of flaps with preserved vascular supply.

During the construction of free flaps, the flap tissue was exposed to transient, global ischemia. The ischemia induced rapid interstitial glucose and pyruvate decreases, whereas the lactate and glycerol levels increased. This metabolic pattern reappeared if postoperative complications in the form of vascular occlusion or hemorrhage struck the flap. When measures were taken to reestablish adequate circulation, the metabolic changes returned to normal. Decreasing glucose and pyruvate concentrations in combination with increasing lactate and glycerol levels is thus a metabolic pattern which may be typical of complete flap ischemia in man.

Human free flaps of different tissue composition (muscle, fat, skin) responded slightly different to transient ischemia. Substances known to be involved in glycolysis (glucose, pyruvate, lactate) changed faster in muscle flaps than in flaps composed of adipose tissue or skin, whereas the interstitial glycerol level increased more in flaps predominantly composed of fat than in the two other flap types.

In pigs, provided with skin flaps, a similar metabolic pattern was found during ischemia as previously observed in man. During transient arterial ischemia - induced by clamping the flap artery - the interstitial lactate and glycerol levels rose more than during venous ischemia of similar length (2 hours). If these metabolic changes reflect tissue damage, arterial ischemia for 2 hours may be more harmful to a newly raised skin flap than venous ischemia. Interstitial tissue samples were collected from various sites along the length of pedicled skin flaps of pigs. They were analyzed for glucose, lactate, and glycerol concentrations. Fluorescein penetration tests were also performed after flap elevation. The fluorescein test underestimated the area of future flap viability considerably. By contrast, the lactate and glycerol levels showed marked increases on passing the necrotic border. They reached much higher levels on the distal side of this border than on the proximal side. "High" lactate and glycerol levels in a certain part of a flap may thus suggest low tissue viability and a high risk of future tissue necrosis in that particular area.

Conclusion: Metabolic processes in both pedicled and free flaps can be studied by use of MD. Flap ischemia induces a specific metabolic pattern. MD surveillance in the postoperative period can detect this pattern, which may signal vascular occlusion and threatening flap necrosis. MD can also be used to separate arterial ischemia from venous, and to predict, at an early postoperative stage, where tissue necrosis will appear later on in a newly raised flap.