Blood substitutes and the cardiovascular system

Ferrrous hemoglobin scavenging of endothelium derived nitric oxide is a principal mechanism for hemoglobin mediated vasoactivities in isolated rat thoracic aorta

The exact mechanism of hemoglobin (Hb) associated vasoconstriction has not been elucidated. We investigated this problem using isolated superfused rat aortic rings with intact endothelium. Human stroma-free hemoglobin solution (SFH) at 2uM reversed vaso relaxation induced by 33uM acetylcholine (Ach). Further, pretreatment with 4uM SFH inhibited Ach(333uM) induced dilation. The SFH induced contraction was reversible by glyceryltrinitrate (GTN), a nitric oxide (NO) donor. Preincubation with a NO synthase inhibitor nitro-L-arginine-methyl ester (NAME, 0.4nM) caused almost complete inhibition of the Hb vasoactivity. Unlike SFH (ferrous oxyHb), prenitrosylated SFH (HbNO) or ferric Hb derivatives (e.g., metHb, HbCN) did not elicit vasoconstriction. The presence of 2uM SFH did not significantly reduce the vasodilatory effectiveness of endothelium independent vasodilators isoproteranol (ISO) and papaverine (PPV). These results suggest that a primary mechanism for Hb vasoconstrictor activity is ferrous Hb scavenging of endothelium derived NO, a signal for guanylate cyclase-cGMP mediated smooth muscle relaxation. Additionally, it appears that the Hb induced vasoactivities may be modulated with NO independent vasodilators.

Blood substitutes: fluids, drugs, or miracle solutions?

Oxygen-carrying volume-expanding solutions that can sustain life in the absence of red blood cells have been developed. Concerns about side effects, sources of hemoglobin, and the ultimate demonstration of efficacy will have to be satisfactorily addressed before anesthesiologists routinely administer such solutions in place of red cells during surgery.

Tissue oxygenation with graded dissolved oxygen delivery during cardiopulmonary bypass

BACKGROUND

Intravascular perfluorochemical emulsions together with a high oxygen tension may increase the delivery of dissolved oxygen to useful levels. The hypothesis of this study is that increasing the dissolved oxygen content of blood with incremental doses of a perfluorochemical emulsion improves tissue oxygenation during cardiopulmonary bypass in a dose-related fashion.

METHODS AND RESULTS

Oxygen utilization was studied in a profoundly anemic canine model of hypothermic cardiopulmonary bypass. Forty-two dogs underwent normovolemic hemodilution to a hematocrit of 15.8% +/- 0.6% (mean +/- standard error of the mean). Cardiopulmonary bypass was begun and resulted in a hematocrit of 9.4% +/- 0.6%. A standard priming solution was used in the control group (n = 12), and the test groups received 1.35 gm perfluorochemical.kg-1 (n = 10 dogs), 2.7 gm perfluorochemical.kg-1 (n = 10 dogs), or 5.4 gm perfluorochemical.kg-1 (n = 10 dogs) through the venous return cannula. Each animal underwent a series of randomized pump flows (0.25, 0.5, 1.0, 1.5, 2.0, and 3.0 L.min-1.m-2) at 32 degrees C. After the randomized flows were completed at 32 degrees C, the temperature was raised to 38 degrees C and cardiopulmonary bypass was discontinued. Mortality from cardiac failure on separation from cardiopulmonary bypass was 42% in the control group and 20% in perfluorochemical-treated groups. The mean perfluorochemical dose was higher in survivors than in nonsurvivors (2.9 +/- 0.4 versus 1.3 +/- 0.5 gm perfluorochemical.kg-1; p < 0.05). No differences in oxygen consumption or transbody lactate gradient were found between groups during cardiopulmonary bypass. Analysis of mixed venous oxygen tension (a surrogate measure for tissue oxygenation) as a function of cardiopulmonary bypass flow normalized to body surface area showed that the control group had significantly lower mixed venous oxygen tension (p < 0.05) than the perfluorochemical emulsion-treated groups. Furthermore, the differences were related to the perfluorochemical emulsion dose. These differences in mixed venous oxygen tension continued after termination of cardiopulmonary bypass. The coronary sinus oxygen tension and cardiac arterial-venous oxygen content differences during and after cardiopulmonary bypass were similar among the control and perfluorochemical emulsion-treated animals. Dissolved oxygen consumption during and after cardiopulmonary bypass was calculated. Dissolved oxygen consumption increased in the perfluorochemical-treated animals in a perfluorochemical dose-related manner and was significantly higher in perfluorochemical-treated animals than in the control animals (p < 0.05).

CONCLUSIONS

Graded increases in mixed venous oxygen tension during cardiopulmonary bypass were observed in response to graded increases in the dissolved oxygen delivery. These data suggest that enhancing oxygenation with perfluorochemical-dissolved oxygen is an effective temporary substitute for the use of hemoglobin-bound oxygen during cardiopulmonary bypass. Perfluorochemical-dissolved oxygen may be particularly beneficial in the setting of multiple hypoxic stresses.

Current status of injectable oxygen carriers

In this review the current status of what commonly are termed "blood substitutes" is discussed. The term blood substitute is a misnomer because the formulations under development at this time transport respiratory gases but do not perform the metabolic, regulatory, and protective functions of blood. Either hemoglobin or a perfluorochemical form the base to transport oxygen; the advantages and disadvantages of each base are discussed. The availability of a blood substitute in the U.S. will require approval by the Food and Drug Administration (FDA) and, by law, both its efficacy and safety must be demonstrated prior to approval. Showing efficacy of any blood substitute is complicated by the oxygen reserve and the compensatory mechanisms to acute blood loss in man. The challenge is to prove that the administration of these formulations offer clinical advantages compared with replacement of volume alone. Several efficacy models, the most attractive among them being perioperative hemodilution, should provide data that would bring these formulations into clinical practice. When hemoglobin is not within the favorable environment of the red cell, whether the hemoglobin is derived from expression vectors developed through recombinant biotechnology or from lysed human red cells, it acquires a left-shifted oxygen disassociation curve. Further, because the tetramer disassociates when injected intravenously and the resulting dimers are cleared rapidly from the circulation by the kidneys, intravascular dwell time is brief. Hemoglobins have been modified chemically and linked intramolecularly, intermolecularly, and to macromolecules to correct these problems. While these manipulations have normalized the p50 and extended the dwell time significantly, some toxicity problems remain unresolved. The binding of nitric oxide to hemoglobin preparations and the presumably resultant systemic and pulmonary hypertension observed in animals may be the most difficult to overcome, although the implications of these reactions in man is poorly understood. Perfluorochemicals (PFC) provide a fundamentally different and simpler approach to oxygen transport than hemoglobin formulations. Typically, the PFCs used are liquids composed of 8 to 10 carbon atoms that dissolve oxygen and obey Henry's law. Thus, the recipient's inspired oxygen and cardiac output assume importance. Because they are insoluble in water, PFCs are administered as emulsions, that is, as small droplets about 0.1 to 0.2 microns in diameter. In this respect, they are very similar to the lipid emulsions widely used for parenteral nutrition. Egg yolk phospholipid and poloxamers are most commonly used as emulsifiers. PFCs are not metabolized and are excreted unchanged by the lungs, following temporary storage by the monocyte-macrophage system (MMS).(ABSTRACT TRUNCATED AT 400 WORDS)

Perfluorocarbon-based red blood cell substitutes

Since our review 5 years ago, a new generation of PFC emulsion has been developed and is undergoing extensive testing. This new generation is the result of the application of physicochemical principles, applied to both the choice of the PFC itself and the emulsifier, as well as advances in emulsion-producing technology. The efficacy of PFCs in general for oxygen transporting capability has been fully recognized, as exemplified by the limited license issued to Fluosol. The latter also represents the recognition of the relative absence of major toxicity of PFCs in general. The development of new products owes much to the lessons learned during the past 20 years and to advances made in the physical chemistry of PFCs. These advances now permit the rational selection or design of the most appropriate PFC and the design of emulsifiers best suited for the purpose. Perflubron represents a clear advance over the Fluosol-DA-type formulation. It is only one but the most advanced of the second-generation products. At least three other commercial entities (Hema-Gen/PFC, Green Cross, Adamantech) are also developing products based on the above principles. Five years ago we concluded that, in spite of the enormous complexity of PFC emulsions as large volume parenterals, they have shown remarkable biocompatibility. The advances in the past 5 years have confirmed this conclusion. The advances occurring during the past 5 years show that the application of the proper technology can lead to product improvement, and that PFC preparations with significant transfusional and nontransfusional potential are, in fact, feasible. It remains to be seen whether high PFC-content emulsion can be successfully deployed in initial, prehospital resuscitation situations. The high PFC content will reduce the absolute requirement for the maintenance of FIO2 > 0.8 in the case of Fluosol-DA for optimal efficacy. The second-generation products also seem to lend themselves to intraoperative use, because they can be removed from the blood postoperatively by plasmapheresislike methods. They are also suitable in combination with autologous blood donation/transfusion. All of these potential applications are in various stages of exploration and, if found to be efficacious, will likely conserve the supply of whole blood and blood components. The nontransfusional applications, particularly those in diagnostic imaging, seem to show substantial promise. Because they involve smaller doses than transfusional applications, they may enter clinical use earlier. The applications in radiation and chemotherapy of malignant diseases represent an intermediate position between the transfusional and nontransfusional uses.(ABSTRACT TRUNCATED AT 400 WORDS)

Preservation of regional and global left ventricular function by intracoronary infusion with oxygenated fluorocarbon emulsion Therox in dogs

We tested the oxygen transport and delivery capacity of the novel perfluorocarbon emulsion, Therox (F44E, 1,2-bis-perfluorobutyl-ethylene) by comparing left ventricular regional and global function in dogs during perfusion of the left anterior descending coronary artery (LAD) with oxygenated Krebs buffer and oxygenated Therox emulsion (20% w/v) at 20 ml/min for two separate 3 min periods. During LAD perfusion with oxygenated Krebs buffer, complete loss of systolic wall thickening in the LAD perfusion area was observed, dP/dt was significantly reduced and left ventricular end-diastolic pressure (LVEDP) was increased. In contrast, LAD perfusion with oxygenated Therox maintained regional wall thickening at 60-70% of control and completely preserved global function as measured by dP/dt and LVEDP. Thus, Therox is an effective oxygen carrier in this animal model.

The effects of alpha-alpha cross-linked hemoglobin on the feeding and locomotor activity of rats

The feeding and locomotor activities of rats were used as an assay for the potentially toxic effects of an oxygen-carrying blood substitute. Rats lived in individual cages where they could feed ad lib by pressing a lever once for each small food pellet, drink water, or run in a wheel; a 12-h light/dark cycle was continuously in effect. After being anesthetized and hemorrhaged one-third of their total blood volume, individual rats were resuscitated with one of the following fluids: their own shed blood (OB), bis(3,5-dibromosalicylfumarate) alpha-alpha cross-linked hemoglobin (HbXL), human serum albumin (HSA), or Ringer's lactate (RL). Rats in a fifth group were not resuscitated (NR). During the dark period on the day of hemorrhage, the food intake and running activity of rats in all groups decreased. Food intake and locomotor activity of rats in the HbXL, NR and OB groups were more suppressed than the HSA or RL groups. The food intake of rats in the HbXL and NR groups remained significantly more suppressed during the dark period of the first recovery day; running continued to be suppressed in the HbXL group on the first recovery day, but not the second recovery day. In an effort to determine the extent to which the rats in the HbXL group were impaired, an increasing number of lever presses was required for each food pellet beginning with recovery day number 3 for all treatment groups. As the ratio of presses per pellet was increased, food intake decreased and running increased for all groups; no differences between groups were significant.(ABSTRACT TRUNCATED AT 250 WORDS)

The cardiovascular effects of the surfactant pluronic F68 in anesthetized dogs

We tested the cardiovascular actions of the surfactant Pluronic F68 by infusing it into anesthetized dogs, in doses approximating those which would be received when Fluosol-DA were used to resuscitate moderate or severe hemorrhage (0.66 and 1.11 g/Kg). In order to alleviate the reactions to Pluronic, the surfactant was purified by treatment with activated charcoal and the dogs were pretreated with corticosterone. Pluronic F68 caused dose-dependent increments in cardiac filling pressures and in systemic and pulmonary arterial blood pressure. Heart rate and contractility remained unchanged. There was an increase in the cardiac output which was dose-dependent and unrelated to the filling pressures. Regional blood flow, as determined by radionuclide-labelled microspheres, tended to increase, but only that to the heart, kidney cortex and lung (bronchial arterial) increased to a statistically significant extent. It would appear that Pluronic F68, when purified with charcoal and after steroid prophylaxis, possesses significant hemorheologic and cardiovascular effects, indicating the need for further investigation of various purification methods and the effects of purified preparations.

Chronotropic effects of in vitro perfusion with albumin, stroma-free hemoglobin, and polyhemoglobin solutions

A negative chronotropic effect of bovine stroma-free hemoglobin was identified in spontaneously contracting myocardiocytes derived from neonatal rats. This model allowed for the evaluation of direct effects of hemoglobin solutions without the influence of hemodynamic reflexes. Significant slowing of myocardiocyte beating rates were observed from 0.8 to 11.6 g.dl-1 stroma-free hemoglobin solution. Polymerization of hemoglobin into soluble crosslinked polyhemoglobin markedly reduced its negative chronotrophic effects by more than 50% at most concentrations assayed. Chronotropic potency (% change in beating rate from baseline per g.dl-1 was significantly higher for stroma-free hemoglobin when compared to polyhemoglobin and albumin solutions. Colloid osmotic potency (% change in beating rate from baseline per mmHg) however, was similar for stroma-free hemoglobin and polyhemoglobin but significantly lower for albumin solutions. This negative chronotropic effect may in part contribute to the transient bradycardia observed following hemoglobin infusion.