a number of oxygen - carrying alternatives to blood have been in 		   clinical evaluation since amberson   administered 		   haemoglobin - saline solutions in clinical trials . of these , five patients with 		   secondary anaemia due to haemorrhage or infection received multiple injections , 		   and three of these patients showed clinical improvement , consisting of 		   increased haemoglobin and haematocrit values and reticulocytosis . the use of 		   these early haemoglobin solutions had a number of drawbacks , however , including 		   signs of renal impairment , resulting from short intravascular persistence and 		   impurities of the haemoglobin introduced during preparation . the first 		   generation of haemoglobin therapeutics has addressed the infectious disease 		   risk , storage , stability and typing issues . these solutions have the potential 		   to augment the oxygen - carrying capacity of patients ' red blood cells 		   temporarily and help avoid allogenic transfusion . early approaches to the development of oxygen carriers involved the 		   use of stroma - free haemoglobin solutions . these 		   solutions did not require blood typing or cross - matching and could be stored 		   for long periods . it was discovered , however , that removal of haemoglobin from 		   red cells leads to the loss of 2,3-diphosphoglycerate and diminished 		   oxygen - delivery characteristics . furthermore , haemoglobin tetramers tend to 		   dissociate into dimers , which have a short intravascular persistence ( due to 		   rapid renal excretion ) and a nephrotoxic action secondary to dimer 		   precipitation in the proximal tubule . therefore , in 		   recent years , a variety of methods have been developed to prevent these 		   problems by chemically modifying and stabilizing the haemoglobin molecule . several haemoglobin therapeutics are now in clinical trials as temporary 		   alternatives to blood or as therapeutic agents for ischaemia . the various 		   haemoglobin products under development are derived from three principal 		   sources : human , bovine and genetically engineered haemoglobin . the rationale for using human haemoglobin is related to the reduced 		   risk of causing allergic or other immune reactions in the recipient . the viral 		   risks associated with blood transfusions , already reduced by extensive donation 		   screening tests , are greatly minimized during the haemoglobin therapeutic 		   manufacturing process by rigorous viral - inactivation procedures . some 		   chemically stabilized haemoglobin solutions , such as diaspirin cross - linked 		   haemoglobin ( dclhb or hemassist , baxter health - care 		   corporation , chicago , illinois , usa ) are subjected to ultrafiltration and heat 		   treatment during the manufacturing process to inactivate viruses . other human haemoglobin - based products are currently being 		   developed , including pyridoxylated and glutaraldehyde polymerised haemoglobin 		   ( polyheme , northfield laboratories inc , evanston , illinois , 		   usa ) , o - raffinose cross - linked and polymerised haemoglobin 		   ( hemolink , hemosol inc , toronto , canada ) and pyridoxylate 		   haemoglobin polyoxyethylene ( apex corporation , plainfield , illinois , usa ) . bovine haemoglobin is readily available and , potentially , has a lower 		   cost as a raw material than human haemoglobin . like human haemoglobin furthermore , infectious 		   agents present in animal blood , such as the bovine spongiform encephalopathy 		   prion , are not as well characterized and therefore are more difficult to detect 		   than those in human blood . animal - sourced products must also be subjected to 		   rigorous purification procedures , because contamination with animal - derived 		   proteins may cause allergic reactions in humans . bovine haemoglobin is usually 		   derived from protected herds in countries without bovine spongiform 		   encephalopathy incidence , however . in addition , the specific purification 		   techniques used may be able to eliminate the protein responsible for bovine 		   spongiform encephalopathy transmission , even though the nature of the protein 		   is unknown . a bovine haemoglobin product was recently approved for veterinary 		   use in canines ( oxyglobin , biopure corp , cambridge , 		   massachusetts , usa ) . a second bovine haemoglobin , glutaraldehyde polymerized 		   bovine haemoglobin ( hemopure , biopure corp ) , is in a phase 		   iii human clinical trial in europe . this product was also tested in patients in 		   sickle cell crisis without producing side effects . a 		   third bovine - derived product , polyethylene - glycol conjugated bovine haemoglobin 		   ( enzon inc , piscataway , new jersey , usa ) is also under investigation . these techniques allow flexibility in product 		   design , enabling haemoglobin molecules to be modified to address specific 		   clinical needs . in addition , many of the potential side effects of 		   bovine - sourced products , or indeed the sourcing complications associated with 		   human red blood cells , may be avoided by using recombinant technology . purification requirements for recombinant products are quite high , but the 		   source material is theoretically unlimited . the only recombinant haemoglobin 		   subjected to clinical trials is obtained by genetic engineering from 		   escherichia coli and is a modified human haemoglobin tetramer 		   cross - linked with a glycine bridge between the  -subunits 		   ( optro , somatogen inc , boulder , colorado , usa ) . pilot studies of tolerance were carried out on 24 		   volunteers . no renal , hepatic , or pulmonary toxicity 		   was noted . there was no renal excretion , and no significant variation in either 		   systolic or diastolic arterial blood pressure . fever , higher than 38c , 		   occurred 3 - 8 h after infusion in 12 individuals , with headache , myalgia and 		   chills . no further episodes of fever were noted after improving the 		   purification process . further clinical development of this product has been 		   stopped , however , pending new generations of recombinant haemoglobin solutions . the most extensively studied of the modified haemoglobins , is 		   diaspirin cross - linked haemoglobin ( dclhb ) or hemassist 		   ( baxter corp . ) . dclhb is produced using 3,5-dibromosalicylate to form a 		   four - carbon fumarate bridge between the two -subunits of the haemoglobin 		   molecule . the stabilization of dclhb during manufacture not only permits heat 		   treatment , but also induces the haemoglobin to assume a low - affinity 		   conformation similar to that in the human red blood cell in the presence of 		   2,3-diphosphoglycerate . consequently , dclhb has similar oxygen - carrying 		   properties to those of fresh blood . however , dclhb has a slightly lower 		   affinity for oxygen than fresh blood ( right shifted oxygen dissociation curve ) , 		   which allows it to release oxygen more readily to tissues compared with red 		   blood cells . dclhb has been extensively studied in preclinical studies with 		   excellent results . in 		   animal models of haemorrhagic shock , dclhb was more effective than infusion of 		   large volumes of lactated ringer 's solution in restoring arterial blood 		   pressure and tissue oxygenation . in preclinical 		   studies , the safety profile was also excellent without undesirable effects such 		   as antigenicity , complement or white blood cell activation or renal toxicity . an early and sustained increase in mean arterial pressure associated with a 		   decreased heart rate was frequently reported . this 		   pressure increase is dose dependent and is easily controlled using 		   antihypertensive agents . three mechanisms may contribute to the vasopressive 		   effect of dclhb : nitric oxide scavenging , 		   stimulation of endothelin release , and 		   sensitization of -adrenergic receptors . dclhb , administered at doses ranging from approximately 20 to 1000 ml , 		   has been investigated in a number of clinical trials in patients undergoing 		   orthopaedic surgery , abdominal aortic repair and major abdominal surgery , and in critically ill patients with septic shock . in these completed studies , involving more than 700 		   patients , safety assessments indicated that dclhb was well tolerated . in a 		   small study , dclhb ( 750 ml ) effectively eliminated 		   the need for any transfusions of blood for a period of 7 days in one - third ( four 		   out of 12 ) of patients undergoing elective hip arthroplasty with high blood 		   loss . a larger trial in post bypass cardiac surgery patients requiring 		   transfusion was undertaken to determine the efficacy of dclhb in reducing or 		   preventing the postoperative use of blood transfusions . in that multicenter study , 209 patients were enrolled 		   when it was determined that they required transfusion after bypass . the 		   patients randomly received up to three doses ( total 750 ml ) of dclhb or up to 		   three units of packed red blood cells within 24 h after removal from bypass . dclhb was effective in avoiding the transfusion of packed red blood cells in 		   59% of recipients on the day of surgery , when all patients in the control group 		   received allogeneic blood . on the day after surgery , 39% of patients receiving 		   dclhb had still avoided receiving blood . at 7 days after surgery , 19% of the 		   dclhb - treated patients had completely avoided a transfusion of packed red blood 		   cells . the most common adverse reactions were transient and nonserious , and 		   consisted of hypertension , yellowing of the skin ( reported as jaundice ) , 		   haemoglobinuria and hyperamylasaernia . standl et al   compared the effects of 		   stored red cells , freshly donated blood and ultrapurified polymerized bovine 		   haemoglobin [ haemoglobin - based oxygen carrier ( hboc ) ] on haemodynamic 		   variables , oxygen - transport capacity and muscular tissue oxygenation after 		   acute and almost complete isovolaemic haemodilution in a canine model . the 		   results showed a higher oxygenation potential of hboc than with autologous 		   stored red cells because of more pronounced oxygen extraction . horn et 		   al   investigated the effect of hboc on skeletal 		   muscle tissue oxygenation in comparison with hetastarch during nearly complete 		   arterial stenosis . after isovolaemic haemodilution with ringer 's lactate 		   solution to a haematocrit of 25% , a 95% artificial stenosis of the popliteal 		   artery was established . in both groups oxygen delivery and oxygen consumption 		   of the muscle in contrast , oxygen extraction ratio increased after 		   infusion of hboc and was higher after the second application when compared with 		   hetastarch - treated animals . during stenosis tissue oxygenation was decreased 		   and remained low after administration of hetastarch , but increased to nearly 		   baseline values after hboc treatment , suggesting that increased oxygen 		   extraction in the hboc group is associated with improved skeletal muscle tissue 		   oxygenation during severe arterial stenosis . in a whole - animal sheep model , vlahakes et al   showed in awake sheep that extreme haemodilution to a 		   haematocrit of 2.4  0.5% was only tolerated when a polymerized bovine 		   haemoglobin solution was used , but not in animals treated with hydroxyethyl 		   starch devoid of oxygen - carrying capacity . all of the animals that survived 		   acute haemodilution also survived the following 25 days , without evidence of 		   renal or hepatic dysfunction . similar results were achieved recently when the 		   blood of anesthetized dogs was haemodiluted to a haematocrit of 2.0  1.8% 		   using a polymerized bovine haemoglobin solution . 		   even at this extremely low haematocrit , the dogs were haemodynamically stable , 		   there was no evidence of lactic acidosis and there were no histologic signs of 		   ultrastructural destruction in liver and kidney . several phase ii and iii trials have been performed with this product 		   in haemorrhagic surgery , cardiac surgery and vascular surgery , but data have 		   not yet been published . the properties of haemoglobin therapeutics include effective transport 		   and delivery of oxygen , much like red cells . unlike the red cell , however , the 		   haemoglobin molecule in solution appears to have positive effects on the 		   microcirculation of key tissues and organs , with easier penetration into 		   capillaries because of low molecular size and low solution viscosity . these 		   properties and the vasopressor effects suggest a number of potential 		   therapeutic applications . considering the emergency use setting , haemoglobin therapeutics may 		   allow emergency department personnel to avoid time - consuming and expensive 		   cross - matching . haemoglobin therapeutics may allow more effective on - scene or 		   emergency department resuscitation of trauma patients and reduced risk of organ 		   failure secondary to the hypoxic insult . furthermore , haemoglobin therapeutics can be formulated to exert a 		   colloid osmotic pressure which , combined with the vasopressor activity , allows 		   a rise in blood pressure without any detrimental effects on regional perfusion . 		   as a result , haemoglobin therapeutics may be capable of treating the 		   hypotension , while maintaining regional perfusion , in patients with septic 		   shock . other potential indications for haemoglobin therapeutics include 		   stroke and myocardial infarction , perfusion of tumours to enhance cancer 		   therapy , and sickle cell anaemia . haemoglobin therapeutics under investigation are relatively 		   short - lived compared with transfused red cells , placing potential limits on 		   their future applications . beyond these approaches , a range of other solutions 		   that may have longer intravascular persistence and could , therefore , be closer 		   to the concept of blood substitutes are being developed . such initiatives 		   include liposome - encapsulated haemoglobin , synthetic heme - embedded lipid 		   microspheres , and other oxygen - carrying artificial red cells . preclinical studies have shown haemoglobin therapeutics to be as 		   effective as blood and more effective than standard colloid or crystalloid 		   solutions for resuscitation from haemorrhagic and septic shock . in the early 		   phases of clinical trials , haemoglobin therapeutics could provide an important new 		   option for the treatment and prevention of ischaemia resulting from 		   hypoperfusion . furthermore , they may be able to provide an immediate on - site 		   replacement for traumatic blood loss , prevent global ischaemia and organ 		   failure , treat focal ischaemia , and provide effective haemodynamic support for 		   septic shock - induced hypotension .