Bailliere's clinical anaesthesiology最新文献

Emergency and massive transfusion represents a particular challenge for clinician and transfusion medicine specialist. The impressive improvements of blood component preparation and quality over the last 10 years have rendered most of the specific risks of massive transfusion negligible. Today specific risks caused by massive transfusion are only to be expected in case of very fast transfusion (replacement of one blood volume in 3–4 hours) and/or large volume replacement (two blood volumes in 24 hours). Furthermore, the individual situation of the patient is the main decisive factor in the outcome. On the other hand, emergency transfusion includes several particular risks which can only be sufficiently managed by appropriate organizational measures and defined replacement schemes. These are different in situations related to complications of elective surgery or to emergency admission. This article describes how to manage emergency transfusion under these different circumstances in order that the highest safety and rapid blood supply can be achieved.

Infectious agents, especially viruses, can be transmitted by human blood products to recipients. Of major importance are viruses such as human immunodeficiency virus 1 and 2 (HIV-1/2), hepatitis B (HBV) and hepatitis C virus (HBC), and human T-cell leukaemia virus type I and II. Also, other viruses such as cytomegalovirus, Epstein-Barr virus, human parvovirus B19, and hepatitis A and G virus can be transmitted by infected blood products. Various methods are applied to prevent the transmission of blood-borne agents to recipients, for example donor selection, testing for various infectious agents of all blood donations and viral inactivation of plasma derivatives. With all these precautionary measures, the estimated risk for infection by screened blood products in Europe and the USA is approximately 1 in 50 000 to 1 in 600 000 (for HBV, HCV and HIV-1/2) per transfused blood product. In the future, the safety of blood products will probably be increased by testing all blood donations with nucleic acid amplification techniques and by (photo)chemical decontamination of cellular blood components.

Although current blood supply is safer than ever, allogeneic blood transfusion still involves immunological and infectious risks. The use of allogeneic blood in surgery can be reduced by the introduction of autologous blood (AB) transfusion programmes. Pre-operative blood donation is potentially the most widely used method to obtain AB in elective surgery patients. However, its success is restricted by the patient's ability to donate the required amount of blood that depends on the total red blood cells (RBCs) mass and the capacity of the patient to reconstitute the RBCs collected at each donation. The difficulty in recovering the RBCs collected depends on an inadequate stimulation of endogenous erythropoeitin (EPO) production induced by blood donations. It was suggested that recombinant human EPO (rHuEPO) could be used to stimulate erythropoiesis in pre-depositing patients with the aim of increasing initial Hct levels or accelerating the reconstitution of RBCs lost during collection.

The clinical studies carried out so far in surgical patients showed rHuEPO to be effective in stimulating erythropoiesis, with a consequent increase in the volume of red cells produced during the course of treatment and in the number of units pre-deposited. It was also effective in correcting anaemia induced by collection of blood units. However it emerged that these patients are more prone to develop a ‘functional’ iron-deficiency, because the erythropoiesis increased by rHuEPO, requires abundant iron for Hb synthesis, and storage iron is shifted to Hb. If iron reserves are inadequate or insufficient, the response to rHuEPO is blunted and higher doses of the drug are necessary. Orally administered iron is not sufficient to deliver appropriate amounts of iron for rHuEPO-stimulated erythropoiesis and intravenous supplementation should be adopted to optimize the erythropoietic response to rHuEPO therapy.

It can be concluded that rHuEPO therapy may be safe and effective, in selected patients, in stimulating peri-operative erythropolesis and, consequently, in reducing the exposure to homologous blood. Given the considerable cost of rHuEPO is mandatory to ensure the optimal conditions necessary for the stimulation of erythropoiesis and intravenous iron therapy should be given together with rHuEPO.

The basic aspects of the delivery, consumption and deficits of oxygen are briefly recalled. As well as haemoglobin (Hb) or haematocrit (Hct) levels, several ‘non-Hb’ variables (notably O₂ demand, cardiac output and the arterial saturation of the available Hb) are important for adequate whole-body oxygenation. Their interaction with Hb can be analysed by computer simulation, which shows that the ‘critical’ level of Hb or Hct, sometimes called the ‘transfusion trigger’, is an individual and not a generally valid figure. This conclusion is borne out by clinical experience with Hb or Hct levels ranging approximately from 11 to <8 g/dl or from 33% to <24%, respectively. For the myocardium, whose performance is decisive for the compensation of low Hb or Hct levels, 7–8 g/dl for Hb or 21–24% Hct may be the limit in otherwise ideal circumstances, but in patients with overt or silent episodes of myocardial ischaemia, a level of less than 10 g/dl (30%) carries risks that should be avoided.

Early treatment of haemorrhagic shock offers few theoretical, but many practical problems. Ideal or minimum volume replacement, and O₂ carrying capacity, are essential but logistics of pre-hospital management impose severe restrictions on what may, or may not, be done. Scoop-and-run, withheld fluid replacement and small volume resuscitation are alternative strategies under discussion at present. This review covers historical aspects of the introduction of small volume resuscitation, general properties required for its application, toxicological studies, available clinical and experimental data, physical, pharmacological and immune effects, mechanisms of action, prospects for clinical use. The controversial case of its interaction with uncontrolled bleeding is covered. It is concluded that the multiple physical, physiological and immune effects of hypertonic saline resuscitation, many of which require further research suggest potential clinical applications, in the primary treatment of hypovolemic shock, in cardiac surgery with cardiopulmonary bypass and in myocardial infarct. The interaction of hypertonic solutions with pro-inflammatory mediators has barely been scratched, and may induce a critical review of many concepts.

Cardiac surgery involves major perturbations of normal physiology and organ perfusion including haemorrhage, an extracorporeal circuit, non-pulsatile blood flow, hypothermia and the resulting initiation of a systemic inflammatory response. Varying degrees of volume support are essential pre, during and post cardiopulmonary bypass (CPB), as avoiding hypovolaemia improves both organ perfusion and outcome.

A target haematocrit determines whether or not blood is used; this review concentrates on available artificial solutions.

The colloid versus crystalloid controversy smoulders on particularly regarding pump primes and practice differs between centres, with cost and concerns of the safety of various colloids remaining the major contentions. Whilst cost is an issue there is no convincing evidence linking adverse outcomes to modern colloid solutions. Using crystalloid solutions, an expansion of the interstitial space and reduced colloid osmotic pressure (COP) seem to be inevitable consequences of CPB.

This may be important, because maintaining COP using colloid primes (often with hypertonic saline) has been associated with improved postoperative oxygenation and reduced ICU stay.

Colloids are indispensible for volume support, but all of them, even human serum albumin, have side-effects. These include unspecific effects such as fluid overload, impairment of renal function and dilution of plasma coagulation factors, as well as specific effects on certain plasma components and cellular elements. Some of these secondary effects are regularly used therapeutically, for example thromboprophylaxis from dextran. Allergic reactions are seen with all colloids but most frequently with gelatin, which also has the poorest volume effect. Allergic reactions to dextran have been successfully prevented by hapten inhibition. Severe and persistent itching has been described after hydroxyethyl starch and is associated with tissue storage of undegradable hydroxyethyl starch residues. With human serum albumin, the main problems are limited availability and price. When choosing a colloid, it is important to weigh the therapeutic value against the risk for all types of adverse effects and not only allergic reactions.

Water binding colloids (albumin, dextrans, synthetically modified starches and gelatins) which are large enough to remain within the intravascular space play a key role in rational fluid therapy, generating sufficient colloid osmotic pressure gradient against the extra-vascular space to restore and/or maintain normal plasma volume. Apart from their value as plasma volume expanders (10% solutions of dextran or hydroxyethyl starch (HES)) or plasma substitutes (3–6% solutions of albumin, dextran, HES or, to a lesser extent, gelatin), some colloids (dextran and, to a lesser extent, HES) specifically improve microcirculatory perfusion and prevent or attenuate potentially pathological sequelae of cascade activation after surgery, trauma and shock, particularly thromboembolism and ischaemia-reperfusion injury arising from leukocyte-endothelial interaction.

Although all the above colloids are generally well tolerated, high doses of dextran or HES (exceeding 1.5 g/kg) may interfere with haemostasis whilst gelatins may compromise immunodefence (fibronectin opsonizing function). Some protracted storage of persistent residues occurs after HES and rare renal complications have been reported after very high doses of 10% dextran, HES or albumin in dehydrated medical patients. Anaphylactic reactions also occasionally occur with all colloids, particularly after gelatins and dextrans, although hapten inhibition has now virtually eliminated the risk with dextran.

The ‘crystalloid-colloid debate’ has a history clouded by practical issues of availability of colloids. The two kinds of solutions have different physical properties and different roles. While crystalloids can replete the circulation, they do so at the expense of a larger fluid load to the body which may have drawbacks.

Studies comparing mortality between the use of crystalloids and colloids have shown differing results. A definitive answer is unlikely from such studies. Early vigorous restoration of the circulation is more important.