Physiologic Effects Of Hyperbaric Oxygen On Ischemiareperfusion Phenomenon

Rachmaninov's Third Piano Concerto in Hyperbaric Physiology Sven Van Poucke1, Philippe Jorens2 and Luc Beaucourt1

1 Unit of Hyperbaric Medicine, Department of Emergency Medicine, 2Department of Intensive Care Medicine, University Hospital Antwerp, Belgium

Abstract: Reperfusion injury is a complex but clinically well-defined entity although its complete mechanism is not entirely elucidated yet. The re-oxygenation phase is characterized by vasoconstriction, platelet and polymorphonuclear leukocytes activation, and release of mediators and the production of free radicals. The goals of the available therapeutic approaches are to oppose directly the consequences of re-oxygenation following the ischemic phase. Clinical and research evidence demonstrated that hyperbaric oxygen could be effective to reduce or stop ischemia-reperfusion related injury. Although many unanswered questions remain and the effects of hyperbaric oxygen seem paradoxical at first glance, the supportive literature is growing

Keywords: hyperbaric oxygen; ischemia-reperfusion; free radicals; hypoxia; no-reflow; oxygen paradox

1. INTRODUCTION

The third piano concerto of Rachmaninov rightfully retains the reputation as being the most daunting of all piano concertos making tremendous demands on the pianist... Similarly, the maintenance of adequate tissue oxygenation permitting a normal metabolism and essential for the physiologic functioning within different organs and tissues is equally challenging. It necessitates a preserved micro- and macro-vascular architecture. An ischemic accident develops when blood supply (and oxygen delivery), is insufficient or stopped. Consequently, the hypoxic tissue aggression may be

responsible for dramatic (cacophonous) consequences and irreversible tissue injury. Several organs are involved in this process (brain, heart, muscles, kidneys, intestinal tract) and the causes of the aggression are multiple (e.g., embolism; toxicity1,2; thrombosis ; surgical interventions and others3, 4). Recent studies indicate that an important mechanism by which cells adapt to changes in oxygenation involves modifications in endogenous nitric oxide (NO) and carbon monoxide (CO) signalling5-7. The moment the ischemic obstacle disappears and the perfusion and the tissue oxygenation are restored, there is a paradoxical aggravation of the initial state with a micro-circulatory ischemia and tissue necrosis is observed immediately on re-oxygenation. This paradoxical, harmful effect in which re-oxygenation induces tissue injury is called reperfusion injury or the oxygen paradox. The term ischemia-reperfusion reminds us of the fact that the tissue injury with an interruption or reduction of blood flow is the result of two phenomena: a direct effect of ischemic hypoxia and an indirect effect following reperfusion, i.e. re-oxygenation. In this context, and frequently observed in medicine, reperfusion syndromes may become the limiting factor in our clinical or surgical interventions or treatments. For example: cardiologists are confronted by ischemia-reperfusion with every successful balloon angioplasty or thrombolysis. Transplant surgeons face it with every successful organ replacement8. Plastic surgeons are challenged by ischemia-reperfusion with every flap. Orthopedic surgeons address it by decompression fasciotomies and reattachment of severed extremities. Finally, all physicians who have successfully resuscitated critically ill patients know the frustrating consequences of subsequent multi-organ failure syndrome.

Hyperbaric oxygen (HBO) is frequently associated with the nefarious production of free radicals9-13 but in certain conditions, perhaps within a narrow time frame14, hyperbaric oxygenation can be valuable in the treatment of ischemia-reperfusion, a condition also related with the production of free radicals. At the first glance it would seem that providing HBO would further fan the flames of oxygen mediated reperfusion injury. This chapter endeavours to navigate the extremely complex "concerto" of damage and defense, stability and derangement. This is indeed the tip of the iceberg. So what is under it?

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