Blood, an essential component of life that delivers nutrients and oxygen throughout the body, constitutes about 7.5% of a human’s body weight (average adult blood volume = 5.5 L). According to Taber’s Cyclopedic Medical Dictionary, blood is composed of about 43% cells (erythrocytes, leukocytes, thrombocytes) and 57% plasma (ions, proteins, hormones, lipids, and water). Blood is a Casson fluid (due to its particulate suspensions), therefore as its viscosity increases (as in the aorta), more pressure is needed to maintain constant aortic blood flow (30 cm/sec) in order to deliver blood throughout the entire circulatory system in 60 seconds.

Artificial blood is a saline-based blood substitute designed for trauma victims that suffer from massive blood loss or low Hb levels. Research in attempt to create the perfect blood substitute has been going on for over 50 years. Most artificial bloods today are cell free to avoid blood type cross-matching, immunosuppressive effects, and viral or bacterial contamination. Artificial blood is superior to natural blood because it can be readily available (due to a longer shelf life), can transport more Hb (due to cell-free solution), and it can act like a Newtonian fluid at high shear rates due to its lower viscosity. The majority of artificial blood substitutes created have side effects of hypertension, short biological half-life, and increased bilirubin levels. Although there are side effects, several companies are in Phase II and III trails and there is hope for a successful artificial blood substitute in the near future. (Cohn 2003).

Blood pressure (BP) is defined by Taber’s Cyclopedic Medical Dictionary as the amount of tension applied to the walls of arteries due to the strength of the heart’s contraction. Average BP is 120/80 mmHg (systolic/diastolic) and hypertension (high BP) is considered BP over 140/90 mmHg, which can be caused by many factors including rapid heart beat and vasoconstriction. Some things that affect BP are heart contraction, blood volume, blood velocity, diameter of blood vessels, and change in elastance of blood vessels. MAP is the mean (average) arterial pressure of blood traveling through all the arteries during one cardiac cycle and can be defined as: MAP = [(2xdiastolic) + systolic] / 3. The minimum MAP required to perfuse coronary arteries, brain, and kidneys is 60 mmHg and the average MAP is about 93.3 mmHg (McAuley).

Nitric Oxide (NO), also known as endothelium-derived relaxing factor, plays a key role in blood flow regulation and oxygen delivery to body parts. It is manufactured in endothelial cells (embedded in blood vessels) in response to increased shear stress and it’s biosynthesized from oxygen, L-arginine, and nitric oxide synthase. The function of NO is to signal the surrounding smooth muscle to relax, dilate, and create more rapid blood flow, some NO is binds to iron sites on Hb to avoid hyperdilation (Allen 2006, Downer 2001).

Baxter Healthcare created an artificial blood in the 1990s and ran phase III trials in 1998. Their artificial blood was a diaspirin cross-linked hemoglobin (DCLHb) cell-free fluid intended to treat severe traumatic hemorrhage shock. Baxter designed the substitute to be cell-free to allow quicker transport of O₂ throughout the circulatory system and to avoid blood-type matching. Extracellular Hb is proven to be two to three times more efficient in delivering O₂ than RBC-bound Hb because the naked molecules are closer to the vessel walls. Baxter’s diaspirin cross-linked design is mandatory because Hb doesn’t have enough pressure on its own to release O₂ from its iron binding sites. Allosteric mutations were preformed to alter the active site structure of the human Hb tetramere to raise the pressure on the Hb (to p50 of RBCs) and decrease it’s affinity for O₂ (Bloomfeild et al. 2004).

Baxter’s phase III testing involved randomized patients which demonstrated hemorrhaging and tissue hypoxia, of which half received 1 L of 10% DCLHb and half received a control of 1 L of normal saline intravenously. The patients were all monitored for up to 28 days following infusion. Logrank analysis after the testing period demonstrated 46% mortality in the DCLHb group, which was significantly higher than the 17% mortality of the saline group. Although the results were largely inconclusive, the majority of the individuals in the DCLHb group died due to respiratory distress and/or multiple organ failure (Baxter Healthcare Corporation 1998). Baxter’s DCLHb failed because NO had an increased affinity for naked Hb due to the absence of membrane protection by an RBC. This lead to increased rates of NO scavenging and an insufficient amount of NO available for smooth muscle, causing blood vessels to hyperactively contract without dilation. Contraction causes decreased blood flow through vessels and the body tries to counteract that by increasing blood pressure, but this eventually leads to hypertension (Bloomfield et al. 2004, Downer 2001).

Over 20 million transfusion patients worldwide receive a major disease from a contaminated blood donation supply, this increases the need for a safe blood substitute (WHO 2000). Many potential solutions to fix Baxter’s NO scavenging problem are being brainstormed today, such as polyheme, sodium nitrate supplements, and artificial RBCs. The proposed solution in this paper focuses on polyheme to reduce NO scavenging rates, thereby preventing hypertension. Polyheme is chemically altered human Hb that is universally compatible, can be stored for up to 12 months, has extremely low contamination chances due to purification, and rapidly restores lost blood volume and Hb levels. Polyheme is created by purifying human blood, extracting Hb, and polymerizing Hb monomers into polymers that have lower NO affinity than free Hb. These Hb polymers are mixed into a saline solution and called Polyheme blood substitute. Polyheme has about the same P50 (about 32 mmHg) as DCLHb because the crosslinkage in the Hb was altered the same way. The largest difference in Polyheme vs. DCLHb is the decrease in NO affinity due to polymerization. A normal Hb molecule has a diameter of 7 nm, while a Polyheme molecule has a diameter of 9.8 nm, therefore this doesn’t change the viscosity of DCLHb a significant amount (USDHHS 1999). The viscosity of Polyheme is about 1.3 cP, when compared to natural blood (m_bld = 3 cP) one observes a definite increase in volumetric blow flow through the calculations in figure 2. This is a desired effect so that Polyheme can bypass vascular disruptions to provide oxygen to ischemic tissues (Cohn 2003). One 500 mL unit of Polyheme contains 50g of Hb, this is about the same as a normal blood transfusion (Hb concentration in blood = 0.166 m³Hb/m³blood), therefore Polyheme provides a sufficient concentration of Hb to the patient.

-Amy Shah