Blood and Blood Substitutes by Abigail Kraft
From the age of fourteen, I have worked in emergency medicine. Few people can say that; most of them are from the same town where I grew up: Darien, Connecticut. There, the town’s emergency medical services, Darien EMS – Post 53, are run almost entirely by teenagers. To paraphrase Charles Dickens, it was the best of ideas, it was the worst of ideas – on the one hand, I have had the kind of experiences in medicine that most people my age can only dream of; on the other hand, well, I’ve had the kind of experiences in medicine that most people my age can only dream of.
A lot of these experiences center around blood, which, to someone with the background I have in medieval history, only makes sense. Blood, as Aryeh Shander points out when discussing transfusion, is one of the four humors that medieval doctors believed governed the body, along with black bile, yellow bile, and phlegm.
Bleeding was the solution to nearly every medical problem because they believed that opening a vein released the buildup of the other humors. Blood itself was the sign of a happy, hard-working temperament; people called “sanguine” were generally believed to be healthy, good parents, and close to God. Zoe Washburne, in the canceled television show Firefly, mentions that sanguine means bloody – and hopeful. Blood can mean something tragic, but it can also mean something wonderful.
I was fifteen years old when I responded to a fatal motorcycle accident. There are a lot of images from that night that will never leave me, but one of the most important is the amount of blood the patient lost. The average adult male has about six liters of blood – three big bottles of soda, for purposes of visualization. This patient, between hitting the wall and reaching the emergency room, probably lost five and a half of those.
As our protocol advised, the EMT-Intermediate on duty had put in two large-bore IVs, pouring normal saline solution into the patient’s veins as quickly as possible. It was visible even to me, though, that he was bleeding much faster than the IVs could replace the fluid. The floor of the ambulance was covered in blood, but – interestingly, I thought, as I performed chest compressions for the fifteen minutes it took to reach the hospital – the blood coming out of the patient was visibly lighter than the blood already under our feet. The saline was diluting his blood so quickly that it was apparent to the naked eye.
Years went by. I moved up the pyramid from gofer (“Go for this! Go for that!”) to EMT-Basic just after my sixteenth birthday. I got my CPR and Basic Life Support Instructor certification a year and a half later, just days before I got my driver’s license. I moved up even further, becoming a crew chief, in charge of most of the scene at emergencies and allowed to mentor EMTs who were still wet behind the ears; then I graduated from high school and went off to the University of Chicago.
My first year of college, I tried on “not wanting to be a doctor” for size – I felt like that was what everyone expected of me, so I should do something different, in a version of teenage rebellion that felt a lot like cutting off my nose to spite my face. What finally made me “come out of the medical supply closet,” as I like to put it, was a combination of teaching two football players how to lift my injured friend after an ill-timed fire alarm and the stoner movie Harold and Kumar Go To White Castle.
At the end of the movie, Kumar, who had insisted that he had no interest in going to medical school in spite of his excellent test scores, admits that he has always wanted to be a doctor – that he’s not doing it because his father expects it of him, but because he really loves the practice of medicine. I left the movie theater almost dizzy with relief, the blood rushing to my head, and immediately signed up for the school’s pre-health advisory group.
Unfortunately, just admitting my real ambition didn’t make my life go smoothly. A few months later, I developed a duodenal ulcer, and the dean of students sent me home on what she called “medical leave” and what I called “exile.” Practically as soon as I reached my parents’ house, my mother had me employed at the local independent bookstore and signed up for the EMT-Intermediate class back at Post. I had always wanted to be an EMT-I, but the class had not been offered between my eighteenth birthday, when I became eligible, and my college matriculation – maybe gastrointestinal bleeding wasn’t such a curse! I learned a lot in that class, from complications of birth to airway adjuncts, but most importantly, I learned how to gain intravenous access. That skill meant that I could pick up volunteer shifts at the emergency departments locally, I could ride along with paramedics, and I could run the entire scene on calls.
I learned even more once I had my certification, as usually seems to happen. I had the theory under my belt, but it wasn’t until I tried that I discovered just how hard it is to get a good stick on an elderly woman taking blood thinners, or to keep my mouth shut when a brand-new paramedic manages to miss a teenager’s antecubital vein twice in a row. I also learned that even the most laid-back, friendly person can lose his temper, when my EMT-I instructor (a long-time paramedic) was so infuriated by a doctor’s conflicting orders that he threw a chair through the sliding glass doors and parked his ambulance horizontally across three parking spaces reserved for doctors on call.
This instructor had just given us a lecture on how important it is to bring hemorrhaging patients to the emergency department as quickly as possible, because the protocol of replacing lost blood with saline was only of limited use; if we didn’t move fast enough, the patient would end up with what he called “Kool-aid blood.” He mentioned that people were working on inventing blood substitutes that we could carry on the ambulance, and I was hooked. How many lives could be saved with an innovation like that? It couldn’t be easy, but it would be incredibly rewarding – which, to tie this section up, is also true of a career in medicine: not easy, but incredibly rewarding.
Blood transfusion is a procedure that is both extremely common and fraught with complications. Traumatic injuries are considered a significant public health concern; injury is the most frequent cause of premature death in the United States, and among the top 15 killers of people between 5 and 44 years of age worldwide – clearly, research into reducing the mortality rate from trauma is very important. Blood transfusions are an integral part of treating victims of trauma: hemorrhage causes about a third of deaths soon after traumatic injury. (Shaz)
The annual demand for blood globally, according to the World Health Organization, is about 100 million units, and the United States uses about 12 million of these. Unfortunately, even without considering the possibility of natural disasters and wars that would increase the need for blood, the WHO has predicted that by 2020, there will be a shortage of 3-4 million units per year. (Simoni) Napolitano notes that a “report on blood donations found that during 2001, 12.7% of hospitals reported cancellations of surgeries due to donor blood shortages and 18.9% reported shortages of blood for nonsurgical purposes.” Already, we have a problem: patients need more blood than donors are giving.
The chronic shortage of blood is far from the only problem with the current standard of blood transfusion. For one thing, allogeneic transfusion (transfusion of blood from someone other than the patient) is expensive: every new screening and testing procedure to protect patients from bloodborne illnesses adds to the cost, and now the cost of each quality-adjusted life year from more sensitive HIV testing is over $1.5 million U.S. (Shander). Whether this is “worth it” can be debated, but it is a significant expense. The Yoshiba study also found significant costs in terms of effort; out of current procedures in place in Japan, they listed as “urgent, stressful, or wasteful” such services as “blood-typing and screening for irregular antibodies (T/S), cross-matching, storage, delivery, administration, and disposal,” all vital parts of an allogeneic transfusion.
Storing the blood appropriately is another source of problems. Although the shelf life of refrigerated, packed red blood cells is 42 days, after only half that time, the capacity of hemoglobin to release oxygen can be depleted to the point where the transfusion may actually do more harm than good (Simoni); Eastman calls this the “RBC storage defect” and mentions that because of it, trauma patients given blood more than two weeks old may be more likely to suffer multiple organ failures or major infectious complications.
Even when the red blood cells are fresh enough, massive transfusion (the replacement of more than 50% of a patient’s blood volume in less than 24 hours) can cause coagulopathy (Shaz), immunosuppressive effects, allergic reactions, or hemolysis (Simoni) – the increase in mortality seems to be linearly related to the number of units transfused (Cardone). Coagulopathy, which occurs as a result of dilution and consumption of coagulation factors, is part of the “blood vicious cycle” or “triad of death” along with hypothermia and acidosis; it can kill a trauma victim, and it can be caused by blood transfusion (Shaz).
Getting blood from a donor, into a blood bank, and then into the patient is, as we have already seen, not nearly as simple as it sounds on the surface. It is, in fact, frequently impossible – the idea of typing and cross-matching blood at an accident scene or, worse, a battlefield, is laughable (Simoni). Shander points out that the “delicate chain of supply” is likely to break down during disasters – exactly the times when it needs to work the best. There are several reasons, beyond the intrinsic dangers and potential unavailability, why a patient may not even be a candidate for allogeneic transfusion. Some patients, often Jehovah’s witnesses, refuse blood products for religious reasons; patients with autoimmune hemolytic anemia cannot tolerate transfusion because the immune system will destroy the transfused cells along with the body’s own cells (Shander). For these people, something besides stored human red blood cells is necessary.
Shander suggests that, considering the evident lack of safety of blood transfusion, it should be minimized. Cardone lists several techniques for minimizing blood loss during surgery – antifibrinolytic drugs used at the time of surgery, recombinant FVIIa (unpopular for its price, about 35 times as much as a unit of red blood cells), fibrin sealants, preoperative autologous donation, acute normovolemic hemodilution (in which blood is removed from the patient and replaced with ordinary plasma expanders like normal saline or Ringer’s lactate, then reinfused during surgery), and cell salvage.
The problem with these methods is that, while they are very useful for planned surgeries, they are impossible to use in emergency situations. A patient who has just been in a serious motor vehicle accident is unlikely to have a stash of pre-donated blood in the trunk, and EMTs don’t carry fibrin sealants or human blood factors; this necessitates other solutions. The usual solution on an ambulance en route to a hospital, since the late 1960s (Zhao) is crystalloid or colloid solution, which quickly increases the blood pressure so enough blood can perfuse the tissues (Shaz); however, in a situation involving hemorrhage, blood loss and IV fluids combine to form what EMS instructors colorfully refer to as “Kool-aid blood” – too dilute to carry enough oxygen. This is more likely to happen the further from a hospital the patient is, and as Moore points out, 50 million Americans live more than an hour away from the possibility of a blood transfusion.
This is where artificial blood substitutes come into play. Sarani wishes for “a widely available and stable oxygen-carrying agent,” which could go places stored human red blood cells cannot, such as with deployed medics (Eastman) or EMTs expecting very long transport times (Moore), and could eliminate many of the problems associated with allogeneic transfusion (Shander). Simoni imagines “an effective, nontoxic, safe, and economical blood substitute” that could maintain blood pressure and oxygen transportation as “the “holy grail” of biotechnology,” even if those were the only functions of blood it could perform. Greenburg, with his unprecedented background both in the chemistry of acellular hemoglobin and in clinical trials of blood substitutes, agrees, saying that “When blood is not available or otherwise not an option, the therapeutic armamentarium would be significantly enhanced” by an ideal blood substitute. The question, now, is how to make a blood substitute that is as close as possible to the ideal these scientists imagine.
The Zhao study tested the rheology of several types of fluids used for resuscitation in hemorrhagic shock, on the basis of the idea that hemorheological characteristics (hematocrit, elongation and aggregation indices of erythrocytes, and the viscosities of whole blood and plasma) change when the patient goes into shock, and a fluid that does the best job of mimicking the rheology of healthy blood will also do the best job of resuscitating the patient. They studied normal saline, hypertonic saline, and a mixture of hypertonic saline and Dextran; the latter did the best job, returning “the decreased plasma viscosity in hemorrhagic shock toward nearly baseline conditions.” This backs up the assertion that the right kind of fluid can make a great difference in patient survival.
The primary problem with first-generation blood substitutes was toxicity. Acute phase studies showed that perfluorocarbon emulsions were effective oxygen carriers, but follow-up studies discovered that within two years, chronic tissue reactions caused various ill effects, and inevitably killed the patient with chemical pneumonitis (Nosé). Consequently, this avenue of research has been closed. Hemoglobin is an excellent carrier of oxygen in cells, but when it is outside the cell, it is toxic (Kawaguchi), because it dissociates from the tetramer into a dimer and is subsequently filtered out by the kidneys, where the toxin stays (Zhang).
Simoni lists several pathophysiological reactions to free hemoglobin: “vasoconstriction, aggravation of oxidative stress, and amplification of systemic inflammatory reactions.” These are due to the fact that hemoglobin scavenges nitric oxide and generates free radicals, which can injure the endothelium, potentially leading to heart attacks, strokes, hypertension, and death (Sarani). Free hemoglobin is not the panacea we might have dreamed, but researchers are seeking improvements in various directions.
There are several possible ways to make safer hemoglobin-based oxygen carriers. Koder describes developing “maquettes–simple models that are progressively altered to test and determine the ultimate characteristics of their constructions” to better understand the important properties of hemoglobin; this study determined that keeping water away from the haem-binding site and coupling helices are vital for carrying oxygen. Zhang suggests surrounding the hemoglobin with “biodegradable polymers, simulating the structure of RBCs”; Yamaguchi agrees, bringing up the concept of hemoglobin vesicles in phospholipid bilayer membranes. Hess, in his report, mentions Olson’s work in reducing the nitric oxide affinity of hemoglobin, hitting the problem from the opposite end. Simoni calls these developments “an important milestone in the development of an effective oxygen carrier.” Eliminating the problem of dissociation-related toxicity is, at the very least, a good start.
Researchers have different ideas on what constitutes an ideal blood substitute. Fronticelli’s idea requires the following characteristics: “(1) absence of renal filtration, (2) absence of NO scavenging effects, (3) stability toward auto-oxidation and heme loss, and (4) calibrated oxygen delivery.” Greenburg, on the other hand, wants a blood substitute that “meets the needs of the situation or disease for which it is being used, primarily as a bridge until red cells are available or when blood is not an option or is not available.”
Napolitano, dreaming big, imagines blood substitutes with “enhanced intravascular retention, reduced osmotic activity, and attenuated hemodynamic derangements such as vasoconstriction. Although not without substantial morbidity and mortality, the current safety of allogeneic blood transfusion demands that comparative studies show minimal adverse effects as well as efficacy and potential for novel applications.” Rather than confusing the issue, this actually broadens the potential uses of blood substitutes.
One interesting angle research has shed light on is the idea that specific blood substitutes could be used for specific problems. Creteur calls this “fascinating,” foreseeing the ability to choose a product by “oxygen affinities, oxygen-carrying capacities, viscosities, and oncotic pressures.” Natanson describes specific alterations manufacturers can perform, such as increasing the affinity of the blood substitute for oxygen, or polymerizing the hemoglobin to enlarge and stabilize the molecules so they will not break down and destroy the kidneys. Nosé points out that oxygen-carrying macromolecules may perform so differently from artificial red cells that they could be used together in hypoxic situations. Kawaguchi mentions potential improvements in microcirculation due to particularly small blood substitute molecules, as well as adjustable oxygen affinity that could specifically target areas of ischemia; Yoshiba agrees, suggesting use with focal ischemias in the brain or healing wounds.
Even though none of these blood substitutes has been approved for widespread clinical use, there are a few reports from doctors who chose to use them for compassionate reasons. Mackenzie describes the use of Hemopure on a Jehovah’s Witness patient struck by a car: the man suffered from rib fractures, a collapsed lung, severe pelvic fractures, a lacerated liver, and lacerations on several arteries; his blood pressure plummeted to 84/44 and his hemoglobin to 4.5 g/dL; and he and his family absolutely refused to allow a blood transfusion.
The doctors suggested the FDA compassionate use protocol, and the family agreed. Hemopure brought his hemoglobin up to 7.4 g/dL within 24 hours; he left the hospital after only thirteen days, and his followup has been uneventful. Blood loss was simultaneously minimized by repairing his pelvic fractures with screws. Moore describes a patient who survived being shot in the abdomen with a hunting rifle without organ failure, and attributes this to the infusion of PolyHeme within 15 minutes of his arrival at the hospital, followed by another 40 units of packed human red blood cells. This case illustrates the potential of artificial oxygen carriers to be used in addition to, rather than in place of, stored red blood cells.
Clinical use of artificial blood substitutes will be predicated at least as much on their ability to make it through bureaucratic hoops as on their actual usefulness in emergency situations. Along these lines, Sarani states that “a discussion of outcomes of clinical trials assessing HBOC administration is needed to place in the proper perspective the known risks and potential benefit of these agents.” The best bet seems to be to sell these products not precisely as blood substitutes, but as bridges between nothing and arrival at a hospital where a transfusion can be performed.
Moore expects a “noninferiority outcome,” meaning that PolyHeme could be used in situations where red blood cells might be preferable, but are unavailable; similarly, Cohn points out that the risks involved in blood substitutes need only be comparable to the risks of ordinary allogeneic blood transfusions, as well as the risks of “inaction or use of plasma expanders”.
Greenburg believes that artificial oxygen carriers will make it through trials because what seem at first to be adverse events related to their use are in fact “the result of underlying pre-existing renal disease or inadequate treatment of hypovolemia.” Yoshiba, in the conclusion of the study of Japanese blood transfusion, plans on using artificial oxygen carriers to fill the gap between sending blood for typing and acquiring the necessary blood, and dreams of a day when there might be “ubiquitous installation in ambulance, helicopter, private medical office, and in the public health facilities in a similar manner with an automated external defibrillator for immediate use” – perhaps a distant day, but one when a severely injured patient’s distance from a hospital might be much less of a death sentence.
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