Annals of Burns and Fire Disasters - vol. XIII - n° 1 - March 2000

EFFECT OF ASPIRIN ON INTERMEDIATE BURN SURVIVAL IN RATS

Shalom k Westreich M.

Department of Plastic Surgery, Assaf Harofeh Medical Centre,
Tel-Aviv University, Sackler School of Medicine, Zerifin, Israel

SUMMARY. Aspirin does not improve intermediate burn zone survival. It is possible that the active metabolites of cyclo-oxygenase are not important in intermediate burn zone survival, but other possibilities exist. It may be that administration post-burn does not reverse the noxious tissue events that cause cell death, or that aspirin must be given for a longer period of time. This suggests that aspirin is probably not beneficial in the clinical setting for the improvement of burn wound survival and healing.

Introduction

Thermal burn injuries are a major health care problem and any therapeutic mode that can decrease inherent morbidity and mortality would benefit burn patients and society in general. The aim of all burn treatment regimens is to increase survival of the intermediate burn zone, thus decreasing morbidity, mortality, and the need for operations and hospitalization.
Complex local and systemic events are responsible for intermediate zone viability. Among these are inflammation, ischaernia, coagulation, and oxygen free radical (OFR) formation. These pathophysiological changes are similar to those seen in the ischaernic skin flap. Aspirin, with its ability to neutralize the enzyme cyclo-oxygenase, has been shown to be effective in increasing ischaemic flap survival and burn wound perfusion.` Because of the similarities between unsuccessful flaps and intermediate zone burns, we conducted a study to evaluate if aspirin, which increases the survival of ischaemic flaps, could also increase intermediate zone burn survival.

Material and method

Twenty male Sprague-Dawley rats (average weight 250 ± 20 9) were randomly divided into two experimental groups:

• aspirin treatment group (N° 10)
• control group (N° 10)

Each rat was anaesthetized using an intraperitoneal injection of pentobarbital (40 mg/kg). The back of each rat was shaved, and a brass comb measuring 20 x 20 x 55 min was used to inflict the bums. The comb had four rectangular contact sectors (10 x 20 trim) cut at 5 mm intervals (Fig. 1).

Fig. 1 - Brass comb with four contact sectors of 10 x 20 min set at intervals of 5 min.

The comb was equilibrated in boiling water, briefly blotted, and then placed for 30 see without pressure on one side of the rat's back. After re-equilibration of the brass comb in boiling water, a second identical comb burn was made on the other side. This created a standard injury with four areas of full-thickness bum, corresponding to the four teeth of the brass comb. Each area measured 10 x 20 mmi, and created intermediate burns' between and around the full-thickness burn zones, the area studied in the experiment.
To simulate clinical use of the drug (i.e. post-injury), the aspirin treatment group received intramuscular injections of 200 mg/kg of aspirin immediately after burn infliction and 24 and 48 h later. The control group received sham injections of saline.
The total burn area was evaluated on days 1, 3, and 7 using fluorescein fluorescence (15 mg/kg). The area of the burn was copied onto a clear plastic sheet. Burn areas were determined by cutting out the marked area of the burn and then weighing the cut-outs on an analytical scale.
The Student t test was used to determine if there was a statistical difference between the groups.

Results

No statistical difference was observed between the test and control groups.

Discussion

Bums of human skin can be divided into three distinct zones:

1. coagulative zone - immediate tissue necrosis due to the thermal injury
2. intermediate zone - damaged tissue, with no potential for survival
3. erythematous zone - minor degree of thermal injury that nearly always heals uneventfully

Since the area of burn coagulation will invariably slough, burn treatment protocols aim at increasing survival of the intermediate burn area. Complex events effect the viability of the intermediate burn zone. These include inflammation, ischaernia, coagulation, and OFR formation. These changes are commonly seen in unsuccessful cutaneous flaps. It seemed to us logical that any therapeutic mode that could increase ischaemic flap survival would be of benefit also to the intermediate burn zone.
The pathophysiological reactions determining random pattern flap survival, which are similar to those seen in intermediate burn area survival, include:

• decrease in perfusion and ischaemia
• specific and nonspecific inflammatory reaction
• OFR formation

Ischaemia - Tissue ischaemia is a combination of systemic and local reactions. The systemic reaction causes an increase in vascular permeability. This "third spacing" decreases effective vascular volume and decreases perfusion. Systemic cardiac depressant factors could further decrease cardiac output, thus decreasing perfusion of nonvital organs such as the skin. Local reactions such as vasoconstriction, coagulation (due to low flow and tissue damage), oedema (due to increased local vascular permeability), and vascular plugging (due to inflammatory infiltrate) could accentuate local ischaemia.
Inflammatory reaction - The extensive damage caused by the burn stimulates the immune system, with potentially devastating effects. Polymorphonuelears (PMN) and macrophages could damage potentially viable tissue by OFR formation. The inflammatory infiltrate could cause the plugging of blood vessels, causing ischaemia. Other cytokines and echisonoid products such as thrombaxane A2 and prostaglandin F_2a could stimulate vasoconstriction and coagulation and further increase ischaemia.
OFR formation - OFRs have the ability to peroxidize cell membranes, inactivate enzymes containing sulphydryl groups, depolymerize carbohydrates, and hydroxylate nucleic acids.
All these reactions alone or in combination can cause cell death.
Aspirin is one of the most widely used drugs today, as an antipyretic, anti-inflammatory and analgesic. Its anti-aggregation effect has made it an effective prophylactic drug against atherosclerotic heart disease and cerebrovascular accident. Aspirin is an irreversible inhibitor of the enzyme cyclo-oxygenase, which is the key enzyme in the metabolism of arachidonic acid and its products: thrombaxane, the leukotrienes, and the prostaglandins.
In a previous work we showed that aspirin at this high dose had a beneficial effect on failing random pattern flaps in rats.
Aspirin has a theoretical potential to improve intermediate burn zone survival by its ability to:

1. modulate the inflammatory reaction by decreasing the formation of thrombaxane A₂, leukotriene C, and prostaglandins by permanently blocking the enzyme cyclo-oxygenase in arachidonic acid metabolism
2. increase perfusion by a direct relaxing effect on vascular smooth muscle
3. decrease reperfusion injury by increasing initial perfusion
4. reduce OFR formation by decreasing the inflammatory reaction and a direct effect on PMNs
5. release adrenocorticosteroids via an indirect effect on the hypothalamus
6. stabilize the endothelial leak

Robson showed that a dose of 325 mg/kg aspirin can increase burn wound perfusion when given pre-bum. Although burn viability studies using this drug have never been performed, the increased perfusion should be reflected in increased survival of the intermediate burn area, Aspirin at the dose used in our experimental animals (200 mg/kg) would obviously not be suitable for clinical application, but if aspirin did benefit burn survival, other safer drugs with similar properties could be used.
In this study, with aspirin given post-burn as in a clinical situation, the drug failed to improve intermediate burn area survival. The reasons for this could be:

• the timing of administration - post-burn administration of aspirin may not have the ability to reverse the noxious inflammatory, thrombotic, and systemic effects of the burn
• dosage - the drug may not achieve effective levels at the burn site owing to haernodynarnic compromise or local tissue events
• the length of time when the medication was administered was shorter than that required
• other major factors affecting intermediate burn survival that are not affected by aspirin

RESUME. L'aspirine n'est pas capable d'améliorer la survie de la zone intermédiaire des brûlures. La raison pourrait être que les métabolites actifs de la cyclo-oxygénase ne sont pas importants pour la survie de la zone intermédiaire des brûlures, mais d'autres possibilités existent. Il est possible que l'administration après la brûlure n'est pas capable d'inverser les évènements nuisibles pour les tissus qui causent la mort cellulaire, ou bien que l'aspirine doit être administrée pour une période de temps plus longue. Cela indiquerait que l'aspirine probablement ne serait pas utile dans les cas cliniques pour améliorer la survie des patients brûlés et pour leur guérison.