Gregory M. T. Hare

What is really dangerous: anaemia or transfusion?

Summary While complex physiological mechanisms exist to regulate and optimize tissue oxygenation under various conditions, clinical and experimental evidence indicates that anaemia, unchecked, is associated with organ injury and unfavourable outcomes. More data (especially from human studies) are needed to answer questions regarding the optimal approaches to the treatment of acute and chronic anaemia. Meantime, allogeneic blood transfusions remain the most common treatment, particularly in surgical/trauma patients and those with moderate-to-severe anaemia. Clinical studies emphasize the paradox that both anaemia and transfusion are associated with organ injury and increased morbidity and mortality across a wide span of disease states and surgical interventions. Further characterization of the mechanisms of injury is needed to appropriately balance these risks and to develop novel treatment strategies that will improve patient outcomes. Here, we present the current understanding of the physiological mechanisms of tissue oxygen delivery, utilization, adaptation, and survival in the face of anaemia and current evidence on the independent (and often, synergistic) deleterious impact of anaemia and transfusion on patient outcomes. The risks of anaemia and transfusion in the light of substantial variations in transfusion practices, increasing costs, shrinking pool of donated resources, and ambiguity about actual clinical benefits of banked allogeneic blood demand better management strategies targeted at improving patient outcomes.

Walking Track Analysis: Utilization of Individual Footprint Parameters

Functional assessment of rat sciatic, tibial, and peroneal nerve injuries was performed using walking track analysis. Individual walking print length (PL), toe spread (TS), and intermediate toe spread (ITS) values were measured up to 24 weeks after specific nerve transection, with or without repair. Sciatic and tibial nerve manipulation initially affected all footprint measurements, consistent with loss of intrinsic and extrinsic motor function. After sciatic repair, TS demonstrated partial recovery without any substantial recovery in PL or ITS, compared with sciatic transection values. By contrast, after tibial repair, PL values recovered dramatically, between 16 and 24 weeks, to levels not significantly different from control subjects. This was not observed after tibial transection without repair. TS recovered partially, whereas ITS recovered to control levels by 20 weeks after tibial repair. Peroneal transection resulted in multiple contractures, rendering this group unmeasurable at 4 weeks. After peroneal repair, only the PL reflected significant loss of function at 2 weeks, recovering to control values by 8 weeks. Manual TS measurements in nonwalking rats did not reflect functional nerve regeneration. Thus, individual PL measurements alone can be used to characterize functional recovery after tibial and peroneal nerve injury, whereas TS reflected recovery after sciatic nerve injury.

Selective β1-antagonism with bisoprolol is associated with fewer postoperative strokes than atenolol or metoprolol: a single-center cohort study of 44,092 consecutive patients.

Background:Perioperative metoprolol increases postoperative stroke. Animal studies indicate that the mechanism may be related to attenuated &bgr;2-adrenoreceptor-mediated cerebral vasodilatation. The authors therefore conducted a cohort to study whether the highly &bgr;1-specific &bgr;-blocker (bisoprolol) was associated with a reduced risk of postoperative stroke compared with less selective &bgr;-blockers (metoprolol or atenolol). Methods:The authors conducted a single-center study on 44,092 consecutive patients with age 50 yr or more having noncardiac, nonneurologic surgery. The primary outcome was stroke within 7 days of surgery. The secondary outcome was a composite of all-cause mortality, postoperative myocardial injury, and stroke. A propensity score-matched cohort was created to assess the independent association between bisoprolol and less &bgr;1-selective agents metoprolol or atenolol. A secondary analysis using logistic regression, based on previously identified confounders, also compared selective &bgr;1-antagonism. Results:Twenty-four percent (10,756) of patients were exposed to in-hospital &bgr;-blockers. A total of 88 patients (0.2%) suffered a stroke within 7 days of surgery. The matched cohort consisted of 2,462 patients, and the pairs were well matched for all variables. Bisoprolol was associated with fewer postoperative strokes than the less selective agents (odds ratio = 0.20; 95% CI, 0.04–0.91). Multivariable risk-adjustment in the &bgr;-blockers-exposed patients comparing bisoprolol with the less selective agents was associated with a similarly reduced stroke rate. Conclusions:The use of metoprolol and atenolol is associated with increased risks of postoperative stroke, compared with bisoprolol. These findings warrant confirmation in a pragmatic randomized trial.

Priming of hypoxia-inducible factor by neuronal nitric oxide synthase is essential for adaptive responses to severe anemia

Cells sense and respond to changes in oxygen concentration through gene regulatory processes that are fundamental to survival. Surprisingly, little is known about how anemia affects hypoxia signaling. Because nitric oxide synthases (NOSs) figure prominently in the cellular responses to acute hypoxia, we defined the effects of NOS deficiency in acute anemia. In contrast to endothelial NOS or inducible NOS deficiency, neuronal NOS (nNOS)−/− mice demonstrated increased mortality during anemia. Unlike wild-type (WT) animals, anemia did not increase cardiac output (CO) or reduce systemic vascular resistance (SVR) in nNOS−/− mice. At the cellular level, anemia increased expression of HIF-1α protein and HIF-responsive mRNA levels (EPO, VEGF, GLUT1, PDK1) in the brain of WT, but not nNOS−/− mice, despite comparable reductions in tissue PO2. Paradoxically, nNOS−/− mice survived longer during hypoxia, retained the ability to regulate CO and SVR, and increased brain HIF-α protein levels and HIF-responsive mRNA transcripts. Real-time imaging of transgenic animals expressing a reporter HIF-α(ODD)-luciferase chimeric protein confirmed that nNOS was essential for anemia-mediated increases in HIF-α protein stability in vivo. S-nitrosylation effects the functional interaction between HIF and pVHL. We found that anemia led to nNOS-dependent S-nitrosylation of pVHL in vivo and, of interest, led to decreased expression of GSNO reductase. These findings identify nNOS effects on the HIF/pVHL signaling pathway as critically important in the physiological responses to anemia in vivo and provide essential mechanistic insight into the differences between anemia and hypoxia.

Regeneration across cold preserved peripheral nerve allografts

The feasibility of peripheral nerve allograft pretreatment utilizing cold storage (5°C in the University of Wisconsin Cold Storage Solution) or freeze‐thawing to prevent rejection was investigated. Regeneration across cold‐stored (3 or 5 weeks) or freeze‐thawed (FT), 3.0‐cm sciatic nerve allografts were compared to fresh auto‐ and allografts in an inbred rat model. At 16‐week post‐engraftment, only FT allografts appeared similar to autografts on gross inspection; FT grafts were neither shrunken nor adherent to the surrounding tissue as seen in the other allograft groups. Qualitatively, the pattern of regeneration in the graft segments of the fresh allograft and to a lesser extent of pretreated allografts was inferior to that of autografts as evidenced by a disruption in the perineurium, more extrafascicular axons, smaller and fewer myelinated axons, increased intrafascicular collagen deposition, and the persistence of perineurial cell compartmentation and perivascular infiltrates. Distal to these grafts, the regeneration became more homogenous between groups, although areas of ongoing Wallerian degeneration, new regeneration as well as compartmentation, were more prevalent in fresh and pretreated allografts. Although the number of myelinated fibres was equivalent to autografts, the fibre diameters, the number of large diameter fibres, and the G‐ratio were significantly decreased in the allograft groups, which, in part, accounted for the significant decrease in conduction velocity in the 3‐week stored and fresh allograft, and the slight decrease in the 5‐week stored and FT allograft groups. There was a small return in the Sciatic Function Index towards normal, but no consistent differences between groups were found. Prolonged cold storage and freeze‐thawing of nerve allografts resulted in regeneration that was better than fresh allografts, but inferior to autografts. With the concomitant use of host immunosuppression or other immunotherapies, these storage techniques can provide a means of transporting nerve allografts between medical centres and for converting urgent into elective procedures.

Differential HIF and NOS Responses to Acute Anemia: Defining Organ Specific Hemoglobin Thresholds for Tissue Hypoxia

Tissue hypoxia likely contributes to anemia-induced organ injury and mortality. Severe anemia activates hypoxia-inducible factor (HIF) signaling by hypoxic- and neuronal nitric oxide (NO) synthase- (nNOS) dependent mechanisms. However, organ-specific hemoglobin (Hb) thresholds for increased HIF expression have not been defined. To assess organ-specific Hb thresholds for tissue hypoxia, HIF-α (oxygen-dependent degradation domain, ODD) luciferase mice were hemodiluted to mild, moderate, or severe anemia corresponding to Hb levels of 90, 70, and 50 g/l, respectively. HIF luciferase reporter activity, HIF protein, and HIF-dependent RNA levels were assessed. In the brain, HIF-1α was paradoxically decreased at mild anemia, returned to baseline at moderate anemia, and then increased at severe anemia. Brain HIF-2α remained unchanged at all Hb levels. Both kidney HIF-1α and HIF-2α increased earlier (Hb ∼70-90 g/l) in response to anemia. Liver also exhibited an early HIF-α response. Carotid blood flow was increased early (Hb ∼70, g/l), but renal blood flow remained relatively constant, only increased at Hb of 50 g/l. Anemia increased nNOS (brain and kidney) and endothelia NOS (eNOS) (kidney) levels. Whereas anemia-induced increases in brain HIFα were nNOS-dependent, our current data demonstrate that increased renal HIFα was nNOS independent. HIF-dependent RNA levels increased linearly (∼10-fold) in the brain. However, renal HIF-RNA responses (MCT4, EPO) increased exponentially (∼100-fold). Plasma EPO levels increased near Hb threshold of 90 g/l, suggesting that the EPO response is sensitive. Collectively, these observations suggest that each organ expresses a different threshold for cellular HIF/NOS hypoxia responses. This knowledge may help define the mechanism(s) by which the brain and kidney maintain oxygen homeostasis during anemia.

Is methemoglobin an inert bystander, biomarker or a mediator of oxidative stress—The example of anemia?

Acute anemia increases the risk for perioperative morbidity and mortality in critically ill patients who experience blood loss and fluid resuscitation (hemodilution). Animal models of acute anemia suggest that neuronal nitric oxide synthase (nNOS)-derived nitric oxide (NO) is adaptive and protects against anemia-induced mortality. During acute anemia, we have observed a small but consistent increase in methemoglobin (MetHb) levels that is inversely proportional to the acute reduction in Hb observed during hemodilution in animals and humans. We hypothesize that this increase in MetHb may be a biomarker of anemia-induced tissue hypoxia. The increase in MetHb may occur by at least two mechanisms: (1) direct hemoglobin oxidation by increased nNOS-derived NO within the perivascular tissue and (2) by increased deoxyhemoglobin (DeoxyHb) nitrite reductase activity within the vascular compartment. Both mechanisms reflect a potential increase in NO signaling from the tissue and vascular compartments during anemia. These responses are thought to be adaptive; as deletion of nNOS results in increased mortality in a model of acute anemia. Finally, it is possible that prolonged activation of these mechanisms may lead to maladaptive changes in redox signaling. We hypothesize, increased MetHb in the vascular compartment during acute anemia may reflect activation of adaptive mechanisms which augment NO signaling. Understanding the link between anemia, MetHb and its treatments (transfusion of stored blood) may help us to develop novel treatment strategies to reduce the risk of anemia-induced morbidity and mortality.

Update on Blood Conservation for Cardiac Surgery

ATIENTS UNDERGOING CARDIAC SURGERY are atrisk of excessive bleeding, leading to increased usage ofallogeneic blood and hemostatic blood products. Although thispatient population represents a relatively small proportion ofsurgical patients, cardiac surgery consumes about 20% of theavailable blood supply in the United States,

Anaemia: can we define haemoglobin thresholds for impaired oxygen homeostasis and suggest new strategies for treatment?

Observational clinical studies in perioperative medicine have defined a progressive increase in mortality that is proportional to both chronic preoperative anaemia and acute interpretative reductions in haemoglobin concentration (Hb). However, this knowledge has not yet helped to define the critical Hb threshold for organ injury and mortality in specific patient populations or in individual patients. Nor has this knowledge enabled us to develop effective treatment strategies for anaemia, as evident from the lack of a demonstrable improvement in survival in patients randomised to higher Hb levels by various treatment strategies including allogeneic red blood cell transfusion, erythropoiesis-stimulating agents (ESAs) and haemoglobin-based oxygen carriers (HBOCs). These findings emphasise the need for a clearer understanding of the mechanism of anaemia-induced mortality. Towards achieving this goal, experimental studies have defined adaptive mechanism by which oxygen homeostasis is maintained during acute anaemia. The mechanisms include: (1) effective sensing of anaemia-induced tissue hypoxia; (2) adaptive cardiovascular responses to maintain adequate tissue oxygen delivery; (3) heterogeneity of organ-specific oxygen delivery to preferentially sustain vital organs which are essential for acute survival (heart and brain); (4) evidence of increased vital organ injury with interruption of cardiovascular responses to anaemia and (5) evidence of activation of adaptive cellular responses to maintain oxygen homeostasis and support survival during acute anaemia. Understanding these mechanisms may allow us to define treatment thresholds and novel treatment strategies for acute anaemia based on biological markers of tissue hypoxia. The overall goal of these approaches is to improve patient outcomes, including event-free perioperative survival.

Assessment and treatment of preoperative anemia: Continuing Professional Development.

PurposeThe purpose of this continuing professional development (CPD) module is to review the risk of anemia and transfusion in perioperative patients and to propose an approach for the diagnosis and treatment of preoperative anemia.Principal findingsPreoperative anemia has been associated with increased transfusion of red blood cells, organ injury, and mortality. Postoperative anemia has also been associated with impaired recovery from surgery. Transfusion also increases the risk of infection, organ injury, and mortality. Preoperatively, iron deficiency anemia can be corrected with oral or intravenous iron; certain types of patients might respond to administration of erythrocyte stimulating agents (ESAs). With ESAs, the increased risk of thrombosis should be balanced against the expected benefit.ConclusionsPreoperative diagnosis and treatment of anemia may reduce the risk of morbidity and mortality associated with both anemia and transfusion.RésuméObjectifL’objectif de ce module de développement professionnel continu (DPC) est de passer en revue le risque de l’anémie et de transfusion chez les patients en période périopératoire et de proposer une approche pour diagnostiquer et traiter l’anémie préopératoire.Constatations principalesL’anémie préopératoire a été associée à une augmentation des transfusions d’érythrocytes, des lésions organiques et de la mortalité. L’anémie postopératoire a également été associée à une moins bonne récupération après une chirurgie. Les transfusions augmentent également le risque d’infection, de lésions organiques et de mortalité. En période préopératoire, l’anémie ferriprive peut être corrigée à l’aide de fer administré par voie orale ou intraveineuse; certains types de patients pourraient répondre à l’administration d’agents de stimulation érythrocytaire (ESA). Avec les ESA, le risque accru de thrombose devrait être soupesé en regard des bienfaits escomptés.ConclusionLe diagnostic et le traitement préopératoires de l’anémie pourraient réduire le risque de morbidité et de mortalité associées à l’anémie et à la transfusion.