The Acute Chest Syndrome in Sickle Cell Disease
Possible Role of Nitric Oxide in Its Pathophysiology and Treatment

MARK T. GLADWIN, ALAN N. SCHECHTER, JAMES H. SHELHAMER, and FREDERICK P. OGNIBENE

The Critical Care Medicine Department of the Warren G. Magnuson Clinical Center and the Laboratory of Chemical Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland

	INTRODUCTION

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Sickle cell anemia is one of the most prevalent genetic diseases worldwide. Pulmonary disease, manifested as the acute chest syndrome (ACS), is a common complication of sickle cell anemia, accounting for 25% of premature deaths (1). The last decade has witnessed a convergence of research pathways that are leading toward a better understanding of ACS pathophysiology and possible new therapies. A fascinating picture is emerging from one of these pathways, the study of nitric oxide (NO) biology, suggesting that hemoglobin binds NO and regulates local pulmonary NO concentration and delivery of NO to the microvascular system. This mechanism raises the possibility of new therapies for sickle cell anemia based on NO.

This pulmonary perspective will summarize the current understanding of the pathogenesis and clinical characteristics of sickle cell anemia and the acute chest syndrome. New data on the effects of NO on sickle cell hemoglobin (hemoglobin S) and the microvasculature will be detailed. In addition, a description of the interactions of NO with hemoglobin to regulate hypoxic pulmonary vasoconstriction and ventilation/ perfusion matching will be presented. In conclusion, this discussion will provide conjecture on the putative role of NO and hemoglobin in the pathogenesis of sickle cell anemia and the acute chest syndrome and its therapeutic implications.

	PATHOGENESIS AND CLINICAL CHARACTERISTICS OF SICKLE CELL ANEMIA

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Sickle cell anemia is the most common genetic disease affecting African Americans. Approximately 0.15% of African American children are homozygous for the sickle cell gene, and 8% have sickle cell trait. This autosomal recessive disorder is characterized by a single amino acid substitution (glutamic acid to valine) in the beta subunits of the hemoglobin tetrameric protein. Upon deoxygenation, hemoglobin S undergoes conformational changes that expose a hydrophobic region surrounding the valine moiety in the beta-subunit. Polymerization with other hemoglobin tetramers occurs with the formation of long polymer chains that ultimately distort the erythrocyte membrane (2).

Hemoglobin S molecules aggregate upon deoxygenation to form polymer nuclei that become the seeds for further polymerization (2). In sickle red cells, hemoglobin S polymerizes as the cells traverse the microvasculature. Factors that increase the intracellular concentration of hemoglobin (red blood cell dehydration), that increase time spent in the microcirculation (increased expression of adhesion molecules), or deoxygenation of hemoglobin, all contribute to increased polymerization. Increased levels of non-S hemoglobins such as fetal hemoglobin (hemoglobin F) or hemoglobin A₂ slow the rate of polymerization and reduce intracellular polymer content at any oxygen saturation. As polymer content increases, the red cell becomes increasingly rigid, retarding transit through the microvasculature. The severity of sickle cell disease has been described by models that emphasize either the intracellular kinetics of polymer formation (and cell sickling) or the extent of polymerization (and cell rigidity) at the reduced oxygen saturation values of the various tissues and organs (3).

The pathogenesis of sickle cell anemia is also thought to be affected by adhesion of the sickle erythrocytes to the microvascular endothelium. Increased expression of adhesion molecules on erythrocytes and endothelial cells, interaction with leukocytes, increased levels of circulating inflammatory cytokines, enhanced microvascular thrombosis, and endothelial damage are all thought to contribute to obstruction of the arterioles by polymer-containing sickle erythrocytes (Table 1).

Perfusion abnormalities have been documented by a number of techniques in patients with sickle cell anemia. Cutaneous flow, as measured by laser Doppler velocimetry, is characterized by local, large amplitude 8.3-s oscillations. Compared with controls, graded periods of proximal ischemia result in a delayed time-to-peak flow and longer payback ratio (time for flow to return to baseline) (21, 22). Studies of patients with sickle cell anemia with leg ulcers using ³¹P-nuclear magnetic resonance spectroscopy (³¹P-MRS) of the lower extremity document reduced muscle phosphocreatine and elevated inorganic phosphate (23). These findings are compatible with a depletion of high energy phosphates secondary to limited blood flow and oxygen supply (24, 25). However, these values were normal at rest in patients with sickle cell anemia without leg ulcers (23). Vascular perfusion during exercise has been determined by clearance of technetium-99m (^99mTC) from the lower limbs of patients with sickle cell disease (26). Perfusion was increased at rest, but with exercise both clearance and peripheral vascular reserve were reduced. These studies and others describe a vascular system with altered perfusion, particularly during stress, compatible with obstruction to blood flow from the abnormal sickle erythrocytes under conditions of lowered oxygen saturation. It seems very likely that modifying this perfusion abnormality may be clinically beneficial.

The abnormal flow properties (rheology) of sickle red cells, caused by their rigidity and abnormal interactions with endothelial and other blood cells, produce well-known consequences. A profound anemia occurs in all patients (hematocrit, 20 to 30%), but it is usually well tolerated without transfusion. Microvascular occlusions result in acute and chronic ischemic injury to the lungs, kidneys, liver, spleen, skeleton, skin, and central nervous system. Sickle cell disease is characterized by periods of stability, punctuated by episodes of severe pain involving the back, chest, abdomen, and joints. This syndrome is referred to as the acute painful crisis or vaso-occlusive crisis (VOC). Although VOC may sometimes be triggered by infection, dehydration, cold, stress, menses, and alcohol, its physiologic basis is largely unknown. Vaso-occlusive crises typically occur after 2 yr of age (when hemoglobin F has been replaced by hemoglobin S) and peak in early adulthood, after sexual maturity. Approximately one-third of persons have only rare VOC, one-third have two to six episodes a year, and one-third have greater than six episodes a year. VOC are the most common cause of hospitalization, especially for a large subset of patients with sickle cell anemia, and are a major burden both to the sick patients and to society. An increased risk of early death is associated with pulmonary disease (the acute chest syndrome), renal failure, seizures, an elevated baseline white blood cell count greater than 15,000 cells/mm³, and a low level of fetal hemoglobin (1). The median age at death for men is 42 yr and for women it is 48 yr (1).

	PATHOGENESIS AND CLINICAL PRESENTATION OF THE ACUTE CHEST SYNDROME

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Pulmonary disease, manifested as the acute chest syndrome (ACS) is a common complication of sickle cell anemia. It is the second most common cause of hospitalization in persons with sickle cell anemia and accounts for 25% of premature deaths (1). The prospective Cooperative Study of Sickle Cell Disease observed in 3,751 subjects with sickle cell anemia a 29% incidence of ACS over a 2-yr period (27). This translates into an attack rate of 12.8 episodes/100 patient years for homozygous sickle cell disease. ACS rates vary directly with steady-state leukocyte counts, hemoglobin concentration, and, inversely, with age and hemoglobin F levels (27).

Current understanding of the pathophysiology of adult ACS is limited but suggests that ACS may be a specific form of acute lung injury that can progress to the acute respiratory distress syndrome (ARDS). This injury is caused by multiple exogenous insults or triggers superimposed upon the genetic-based pathophysiology. These insults are believed to involve "sludging of blood" in the pulmonary microvasculature secondary to erythrocytes with high polymer content, and resulting infarction of the pulmonary parenchyma, as well as possible bone marrow fatty embolization from infarcted bone, microvasculature in situ thrombosis, macrovascular pulmonary embolism, and infection (Table 2). The reader is referred to recent reviews for further discussion of this hypothesized pathophysiology (32, 33, 40). It should be noted that pulmonary involvement may contribute to further hypoxemia and subsequent intracellular hemoglobin S polymerization, thus causing a "vicious cycle."

The Cooperative Study of Sickle Cell Disease collected data on 1,722 ACS episodes (11). Patients with the ACS presented with fever (80%), cough (74%), chest pain (57%), dyspnea (28%), productive cough (24%), hypoxia (mean Pa_O2 of 71 mm Hg), leukocytosis, and infiltrates on chest radiographs, and they often progressed to multilobar pulmonary disease indistinguishable from the acute respiratory distress syndrome (11). Interestingly, in contrast to adults, children are more likely to have fever, cough, upper and middle lobe infiltrates on chest radiograph, and to present in the winter months. Adults more commonly experience chest pain and dyspnea, with fever and cough only occurring in approximately 60% of cases. Indeed, 50% of adults experience the pain of VOC prior to developing ACS, whereas only 11% of children have an antecedent crisis. Adults present with lower lobe disease and more frequently develop multilobe involvement (36 versus 24% of children) and pleural effusion (21 versus 3%) (11). Although bacteremia is more common in children (3.5 versus 1.8%), the severity of illness is greater for adults. Adults receive more frequent transfusions (39 versus 22%), are hospitalized longer (9 versus 5.4 d), and suffer a higher mortality (4.3 versus 1.1%).

These differences may be explained by age-dependent etiologies for ACS. The seasonal peak, increased rate of bacteremia, and milder course observed in children may reflect an infectious etiology, whereas the lower lobe predominance, frequent pain, lower rate of bacteremia, and severe course observed in adults may be attributable to bony infarction and atelectasis of the lower lobes, pulmonary thrombosis and infarction, and/or fat embolism as the precipitating causes in this group (11).

	NITRIC OXIDE, HEMOGLOBIN, AND SICKLE CELL ANEMIA

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Recent work by Stamler and colleagues (41) suggests that NO binding to hemoglobin may play a role in the regulation of vascular tone, with NO binding and release tied to oxygen- induced allosteric structural transitions. In the lungs, hemoglobin is highly saturated with oxygen, and NO produced in the lungs is thought to bind to cysteine-93 on the beta-chain. This S-nitrosohemoglobin is carried by the red cells to the microvascular system, where oxygen tensions are reduced. After deoxygenation, allosteric structural changes in the hemoglobin molecule favor the release of NO, which diffuses to the arterial wall and causes vasodilation. The deoxygenated (T form) hemoglobin heme is then capable of binding NO, favoring the formation of alpha-nitrosylhemoglobin (Figure 1). The classic model of allosteric modification of oxygen affinity by protons, and carbon dioxide binding, known as the Bohr and Haldane effects, respectively, may have to be extended to NO. These observations suggest that therapeutic delivery of NO may be beneficial to patients with sickle cell anemia who have impaired microvascular perfusion because of a direct vasodilatory effect of NO in the periphery.

View larger version (59K): [in this window] [in a new window]   Figure 1.   Model for nitric oxide interaction with hemoglobin. Nitric oxide (NO) bound to hemoglobin regulates vascular tone, with NO binding and release tied to oxygen-induced allosteric structural transitions: In the lungs, hemoglobin is highly saturated with oxygen, and NO produced in the lungs is thought to bind to some of the cysteine-93 residues found on each of the beta-chains. This S-nitrosohemoglobin is carried by the red cells to the microvascular system where oxygen tensions are reduced. Accompanying deoxygenation, allosteric structural changes in the hemoglobin molecule favor the release of NO, which diffuses to the arterial wall and causes vasodilation. The deoxygenated hemoglobin heme is then capable of binding NO, favoring the formation of nitrosylhemoglobin, primarily on the alpha chain.

Other recent studies, however, suggest that binding of NO to hemoglobin may also modify the hemoglobin protein structure, increase oxygen avidity, and potentially interfere directly or indirectly with the hydrophobic interactions between beta-globin moieties that lead to polymerization. Head and colleagues (44) examined the effect of NO on red blood cells in vitro and the effect of inhaled NO in a pool of nine volunteers with sickle cell anemia and three control volunteers. None of these patients was in sickle cell VOC. Intracellular hemoglobin S has an elevated partial pressure of oxygen at 50% hemoglobin saturation (P₅₀) compared with hemoglobin A, because of competition of polymerization with oxygen binding. The P₅₀ value is inversely related to the hemoglobin affinity for oxygen. In the study of Head and colleagues, the P₅₀ of the in vitro sickle erythrocytes was reduced by an average of 4.8 ± 1.7 mm Hg after a 15-min incubation with 80 parts per million (ppm) NO gas. The control RBCs did not change significantly from their normal oxygen affinity. They subsequently treated nine patients with 80 ppm inhaled NO for 45 min via a nonrebreathing circuit. In these in vivo experiments P₅₀ was reduced by an average of 4.6 ± 2.0 mm Hg in the patients with sickle cell anemia but did not change in control subjects breathing 80 ppm NO. This change was sustained for 1 h in five of seven patients retested and was not associated with marked increases in methemoglobin (44). A possible explanation for these observed effects is that binding of NO to the beta-cysteine residue 93 modified the hemoglobin protein structure and increased oxygen avidity (45). This same effect could potentially reduce hemoglobin S polymerization. It is now known that the 93rd amino acid on the beta-globin chain is highly conserved across species, and modification of this residue with thiol reagents effectively inhibits polymerization and sickling (46, 47).

Four further observations suggest a possible beneficial effect of NO in sickle cell anemia. First of all, an inverse relationship between subjective pain scores and NO metabolite levels has been reported in patients with sickle cell anemia presenting to an emergency room in acute pain crisis (48). Second, in an animal model used to study the perfusion characteristics of sickle red blood cells (49), the rat cerebral circulation was visualized by craniotomy window or laser-Doppler flowmetry while fluorescently labeled human sickle erythrocytes were infused. These human sickle red blood cells had increased adherence to rat cerebral microvessels compared with those in control rats. After NO synthase inhibition with N-omega-nitro-L-arginine methyl ester (L-NAME), five of the nine treated rats had a significant reduction in flow and died within 30 min, whereas no mortality was observed in the control rats. Thirdly, it has recently been discovered that hydroxyurea, a chemotherapeutic agent known to reduce morbidity in patients with sickle cell anemia, can react with heme proteins to generate NO (50). Although the primary beneficial mechanism of action of hydroxyurea almost certainly derives from stimulation of marrow hemoglobin F production, and possibly a reduction in neutrophil numbers, the ability to dissociate into NO is intriguing and might also have contributed to clinical benefit shown in the multicenter study of hydroxyurea therapy. Finally, in a recent abstract, McDade and colleagues (51) described the effects of increasing molar concentrations of NO gas on purified hemoglobin S polymerization and erythrocyte sickling under various oxygen tensions. In the presence of NO, hemoglobin polymerization was delayed, hemoglobin solubility increased, and the percentage of erythrocytes of sickled morphology at a Pa_O2 of 30 mm Hg decreased.

	THE ROLE OF NITRIC OXIDE AND HEMOGLOBIN IN HYPOXIC PULMONARY VASOCONSTRICTION: IMPLICATIONS FOR THE ACUTE CHEST SYNDROME

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While the 93rd amino acid (cysteine) on the beta-hemoglobin chain has recently been identified as a putative binding site for NO, it has long been established that NO binds avidly to the heme moiety of hemoglobin. In fact, heme has an affinity for NO that is 3,000 times that of oxygen (52). Heme iron forms a planar structure encircled by four nitrogen atoms in the center of a protoporphyrin ring. A histidine residue binds from below, leaving a sixth site available for binding oxygen, carbon monoxide, NO, and other ligands. NO is rapidly taken out of solution by oxyhemoglobin and converted to nitrate after the oxidation of ferrous iron to ferric (met-hemoglobin) (42, 52- 55). In vitro and in vivo hemoglobin serves as a biologic sink for NO. Recent data suggest that this NO-binding property of hemoglobin may play a vital role in physiologic NO balance in the lungs. Indeed, hypoxic pulmonary vasoconstriction depends on the rapid clearance of airway and alveolar-derived NO by hemoglobin (56). We will now summarize recent studies describing the role of NO in hypoxic vasoconstriction, the role of hemoglobin in this process, and conclude with a hypothesis about how anemia, NO, and a failure of hypoxic vasoconstriction may conspire to produce the acute chest syndrome.

Nitric Oxide and Hypoxic Vasoconstriction

The maintenance of stable blood oxygen and carbon dioxide tension requires an equal balance of regional lung ventilation and perfusion. Oxygen levels in the alveoli mediate local vascular tone to achieve this balance. A fascinating picture is emerging that NO affects these oxygen-mediated changes in perfusion (57). The lung contains all three nitric oxide synthase enzymes (NOSs): type I or neuronal NOS, type II or inducible NOS, and type III or endothelial NOS. Types I and III NOS are expressed constitutively, regulated by intracellular calcium concentrations and produce picomolar levels of NO, whereas type II NOS is induced and produces nanomolar levels of NO, independent of calcium concentration (64, 65). The production of NO by these enzyme systems regulates hypoxic vasoconstriction as well as the drop in pulmonary vascular resistance observed at birth. In fact, adenovirus-mediated transfer of the type III NOS gene into rat lungs reduces acute hypoxic vasoconstriction (66), and in fetal animal lungs, type III NOS expression is induced by oxygenation and ventilation, reducing pulmonary vascular resistance (57). Selective inhibition of type II NOS has also been shown to increase pulmonary vascular resistance by 70 to 80% (67).

Dweik and colleagues (68) have recently demonstrated in humans that type II NOS activity is regulated by oxygen concentrations in the physiologic range. NO levels measured at the mouth of 11 subjects during oral breathing (to diminish nasal NOS contribution) correlated directly with FI_O2 in the hypoxic range. Subjects breathing 0.15 FI_O2 exhaled approximately 10 to 20 parts per billion (ppb) NO, whereas subjects breathing 0.05 FI_O2 exhaled 1 to 5 ppb NO. The investigators then examined NOS II enzyme kinetics in vitro under varying oxygen concentrations and in the presence of excess substrate (L-arginine, NADPH, FAD, FMN, and tetrahydrobiopterin). Again, in vitro NOS II production of NO directly correlated with oxygen concentration. These findings suggest that oxygen, in the physiologic range, is the rate-limiting substrate for NO synthesis, and this may be the mechanism coupling oxygen, NO, and vascular tone to maintain an equal ventilation/ perfusion ratio.

Hemoglobin, Nitric Oxide, and Hypoxic Vasoconstriction

Studies in isolated, perfused animal lungs have demonstrated that red blood cells are necessary for maintaining hypoxic pulmonary vasoconstriction (69). Deem and colleagues (56) have demonstrated that this interdependence is hematocrit "dose-dependent" and is likely mediated by the NO binding (biologic sink) properties of hemoglobin discussed earlier. In their experiments, mechanically ventilated whole-lung perfused rabbits were exposed to 30, 10, and 0% hematocrits. Exhaled NO, NOx levels, and pulmonary artery pressures were monitored during normoxic ventilation (21% O₂, 5% CO₂) and hypoxic ventilation (5% O₂, 5% CO₂). Consistent with hypoxic pulmonary vasoconstriction, exhaled NO concentrations decreased during hypoxic ventilation, and pulmonary artery pressures increased. However, baseline exhaled NO concentrations were significantly higher and pulmonary artery pressures were lower in the anemic animals, and the anemic animals had higher exhaled NO concentrations and lower pulmonary artery pressures during hypoxic ventilation. Nonspecific NOS inhibition with N-omega-nitro-L-arginine methyl ester (L-NAME) normalized exhaled NO levels in the anemic animals and increased hypoxic pulmonary vasoconstriction in the 0% hematocrit group, whereas there was no change in the 30% hematocrit group. This suggests that the anemia-induced impairment in hypoxic pulmonary vasoconstriction is NO mediated.

That this process occurs in humans is supported by the studies of Dweik and colleagues (68) of human in vivo airway NO physiology. NO sampled endobronchially during fiberoptic bronchoscopy is elevated initially (5 to 10 ppb) and during exhalation drops to 0 to 1 ppb. Low midexpiratory to endexpiratory NO levels represent alveolar gas, suggesting that alveolar NO production is in equilibrium with uptake by hemoglobin and local metabolism.

The clinical implications of this physiologic mechanism become evident: significant anemia impairs NO steady-state consumption and hypoxic vasoconstriction, leading to ventilation/ perfusion (/Q) mismatch and shunt physiology in disease states such as pneumonia, ARDS, and ACS of sickle cell anemia. In earlier work, Deem and coworkers (74) demonstrated that acute isovolemic anemia impairs gas exchange in a perfused atelectatic rabbit lung model. Anemia increased intrapulmonary shunt (S/T), measured by blood gas analysis, and increased blood flow to the collapsed lung, measured by fluorescent microspheres (Figure 2).

View larger version (20K): [in this window] [in a new window]   Figure 2.   Pulmonary NO production is regulated by oxygen concentration in the physiologic range. Reductions in oxygen concentration result in reductions in NO production and hypoxic pulmonary vasoconstriction. However, reductions in NO are dependent on active scavenging of NO by hemoglobin. Severe reductions in red blood cells (RBC) and hemoglobin result in increased local NO levels, despite reduced NO production, and loss of hypoxic pulmonary vasoconstriction.

Hemoglobin, Nitric Oxide, Hypoxic Vasoconstriction, and the Acute Chest Syndrome

These mechanisms may play a role in ACS of sickle cell anemia. Virtually all patients with ACS are anemic (mean hemoglobin, 7.8 g/dl; range, 2.7 to 10.9 g/dl) (75), and more than half continue to hemolyze, with mean reported hemoglobin decreases of 1.6 g/dl during their acute illness (76). Even mild impairments in hypoxic pulmonary vasoconstriction would lead to lower hemoglobin saturations and increased hemoglobin S polymerization. A vicious cycle of red cell sickling, hemolysis, anemia, further hypoxia secondary to loss of NO scavenging and hypoxic pulmonary vasoconstriction, and further red cell sickling might lead to end-organ microvascular occlusion and injury. Reports of several case series suggest that transfusion rapidly improves oxygenation and the clinical course of ACS (77). One study (80) prospectively documented arterial Pa_O2 after transfusion in 27 patients with ACS. The mean pretransfusion oxygen tension was 65 ± 15 mm Hg and 12 to 24 h after transfusion increased to 86 ± 19 mm Hg. Restoration of hypoxic pulmonary vasoconstriction may have contributed to this observed effect of transfusion.

A number of studies have reported a direct association between hemoglobin level and oxygen saturation (81, 82). Rackoff and colleagues (81) observed a parallel reduction in pulse-oximetry-measured hemoglobin saturation and hemoglobin concentration; however, this was explained by a rightward shift in the measured O₂ dissociation curve (and P₅₀) rather than a reduction in Pa_O2 (which was not measured invasively). Studies that have measured Pa_O2 invasively have revealed hypoxemia with mean values of 85 mm Hg (83, 84). Interestingly, the AaPO₂ is inversely proportional to hemoglobin concentration, with hemoglobin levels of 6 to 7 mg/dl associated with differences of 25 to 40 mm Hg and hemoglobin levels of 11 g/dL associated with AaPO₂ differences of 5 to 20 mm Hg (84). Calculated shunt values after breathing 100% oxygen for 20 min range from 4 to 19% (84). These observations may be explained by an increased cardiac output secondary to anemia and resultant increased pulmonary shunt; alternatively, these observations support the theory that anemia leads to increased local pulmonary NO accumulation stimulating vasodilation and shunt.

	INHALED NITRIC OXIDE AND THE ACUTE CHEST SYNDROME

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Even though inhaled NO has recently been considered as a possible therapy for ACS, there are virtually no data on the effects of inhaled NO on the clinical course of ACS. To date there exists only anecdotal descriptions that NO may have an impact on the hypoxemia and duration of ACS (40, 85). Atz and Wessel (85) described the effects of inhaled NO on the clinical course of two mechanically ventilated pediatric patients with ACS. After 15 min of 80 ppm inhaled NO, the two patient's Pa_O2 values increased from 69 and 107 mm Hg to 176 and 185 mm Hg, respectively. This was accompanied by decreases in right ventricular systolic pressure (measured by echodardiography) from 60 to 40 mm Hg in the first patient. The second patient had a pulmonary artery catheter in place and showed a drop in mean pulmonary artery pressure from 41 to 26 mm Hg, with an increase in cardiac index and no change in the pulmonary artery occlusion pressure. These changes were sustained, and NO was weaned to off after 92 and 47 h, respectively. Although there was a clear initial improvement in oxygenation and a decrease in pulmonary artery pressures, it is not known whether the sustained improvement that followed was the result of the NO therapy or of the aggressive hydration, exchange transfusions, and oxygen therapy that both patients received. Recent clinical studies (86, 87) of inhaled NO in patients with ARDS attest to the fact that controlled prospective studies with detailed physiologic monitoring are urgently needed. Even though inhaled NO has been clearly demonstrated to improve the Pa_O2/FI_O2 ratio in patients with ARDS, the control patients' Pa_O2/FI_O2 ratios rise to match the treatment group within 1 to 4 d. Furthermore, to date neither differences in time receiving ventilation nor mortality can be demonstrated.

The mechanism of the observed effect of NO on oxygenation is thought to be secondary to improvements in regional / matching. Even transient improvements in / would potentially improve hemoglobin saturation and reduce erythrocyte sickling in the pulmonary vasculature as well as in distal organs. The acute chest syndrome of sickle cell anemia may represent a unique pulmonary disorder in which brief improvements in oxygenation may have profound effects on outcome. A number of caveats should be appreciated. Some patients who receive inhaled NO for the treatment of ARDS may develop rebound hypoxemia and pulmonary hypertension upon drug withdrawal, suggesting that endogenous NO synthesis may be down-regulated. It is possible that patients treated acutely may improve, only to relapse after the treatment is terminated. At this time, preventive and treatment options for ACS are based on a number of standard therapies such as hydration, oxygen therapy, antibiotics (11, 33, 37, 38), exchange transfusion (77), and incentive spirometry (30). The use of inhaled NO remains experimental and cannot be recommended until clinical trials provide supporting data.

	CONCLUSIONS

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Research centered around NO biology has led to an expanded understanding of the critical interdependence of NO, hemoglobin, and the microvasculature, with possible therapeutic implications for sickle cell anemia and the acute chest syndrome. We propose an amended pathogenic model of the ACS, as shown in Figure 3, that incorporates a new understanding of the role of NO and hemoglobin in the regulation of hypoxic pulmonary vasoconstriction and possibly delivery of NO to the microvasculature. The anemic patient with ACS suffers a two-pronged attack: loss of pulmonary NO scavenging and hypoxic pulmonary vasoconstriction, and loss of peripheral NO delivery. Interruption of this cycle by transfusing normal (hemoglobin A containing) erythrocytes might improve hypoxic pulmonary vasoconstriction, restore peripheral NO delivery, as well as replace sickled erythrocytes.

View larger version (42K): [in this window] [in a new window]   Figure 3.   The "vicious cycle" of vaso-occlusive crisis an the acute chest syndrome in sickle cell disease. Regional tissue hypoxia and other factors lead to hemoglobin S polymerization (*) and formation of polymer fibers (**) in sickle erythrocytes. Erythrocyte rigidity and sickling results in microvasculature occlusion, tissue infarction, and erythrocyte hemolysis. Anemia leads to a reduction in NO binding by red cells. Local pulmonary NO accumulation impairs hypoxic pulmonary vasoconstriction such that insults that lead to lung injury (the acute chest syndrome) are manifested by worsened shunt physiology and hemoglobin desaturation. The anemic patient with acute chest syndrome suffers a two-pronged attack: loss of pulmonary NO scavenging (with loss of hypoxic pulmonary vasoconstriction) and loss of peripheral NO delivery (with loss of peripheral vasodilation).

These developments lend themselves to a number of important research questions. (1) To what degree does anemia and loss of hypoxic pulmonary vasoconstriction contribute to the hypoxia of ACS? (2) Will sickle erythrocytes bind NO to the same extent as normal (hemoglobin A containing) erythrocytes, i.e., is there an impairment in NO scavenging in sickle cell anemia even in the absence of anemia? (3) Is there an "ideal" hematocrit that balances blood viscosity, hypoxic pulmonary vasoconstriction, / matching, NO delivery, and pulmonary artery pressures? (4) Will supraphysiologic NO delivery improve in vivo hemoglobin function (oxygen avidity and tendency to polymerization), erythrocyte function (tendency to sickle and adherence to endothelium), and microvascular flow and oxygen delivery? (5) Will exogenous delivery of NO down-regulate endogenous production, leading to rebound crisis or other problems?

The answers to these questions may lead to new therapeutic strategies for sickle cell anemia and the acute chest syndrome and provide insight into other pulmonary disorders such as ARDS and pulmonary hypertension.

	Footnotes

Correspondence and requests for reprints should be addressed to Mark T. Gladwin, M.D., and Frederick P. Ognibene, M.D., Critical Care Medicine Department, National Institutes of Health, Building 10, Room 7D43, Bethesda, MD 20892-1662.

(Received in original form October 26, 1998 and in revised form December 17, 1998).

Acknowledgments: Supported by intramural funds from the National Institutes of Health.

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