Perioperative blood loss and its reduction: the role of the anaesthetist.

Anaesthesia may influence intraoperative bleeding in several ways, both physiological and pharmacological. Hydrostatic arterial pressure may be altered physiologically by changes in ventilatory pattern, positive end-expiratory pressure (PEEP) and posture, and also by pharmacologically induced alterations in myocardial contractility and peripheral vascular tone. Other factors, such as hypoxia and hypercapnia, may affect vascular diameter, while the use of regional sympathetic block induced by extradural or spinal anaesthesia is also very effective in the reduction of intraoperative blood loss. Elective hypotension is a specific anaesthetic technique which goes beyond the ability of a good, safe, non stress-inducing anaesthetic to reduce blood loss. Its use may be classified broadly as applicable to situations in which the particular operation would otherwise be impossible (for example cardiovascular or cerebrovascular surgery), situations in which excessive blood loss might be detrimental (for example orthopaedic, spinal and maxillo-facial operations) and situations in which blood loss interferes with surgical visibility or technique (such as middle ear and cosmetic surgery). The demands of elective hypotension in such situations are very different, ranging from acute, short-term "dial-apressure" hypotension, to sustained, moderate hypotension for reduction of blood loss. The use of significant hypotension for cosmetic surgery has always been surrounded by controversy, but with modern monitoring techniques, and in the hands of experienced anaesthetists, it is undeniably advantageous and contributes to remarkable results, particularly after injury and trauma. A further controversial topic is the use of hypotension to conserve blood loss in patients in whom the preoperative haemoglobin concentration is reduced or those in whom transfusion is undesirable for various reasons.