Experimental analysis of critical oxygen delivery

The Limits of Acute Anemia

For many years, physicians’ approach to the transfusion of allogeneic red blood cells (RBC) was not individualized. It was accepted that a hemoglobin concentration (Hb) of less than 10 g/dL was a general transfusion threshold and the majority of patients were transfused immediately. In recent years, there has been increasing evidence that even significantly lower hemoglobin concentrations can be survived in the short term without sequelae. This somehow contradicts the observation that moderate or mild anemia is associated with relevant long-term morbidity and mortality. To resolve this apparent contradiction, it must be recognized that we have to avoid acute anemia or treat it by alternative methods. The aim of this article is to describe the physiological limits of acute anemia, match these considerations with clinical realities, and then present “patient blood management” (PBM) as the therapeutic concept that can prevent both anemia and unnecessary transfusion of RBC concentrates in a clinical context, especially in Intensive Care Units (ICU). This treatment concept may prove to be the key to high-quality patient care in the ICU setting in the future.

Monitoring the tissue perfusion during hemorrhagic shock and resuscitation: tissue-to-arterial carbon dioxide partial pressure gradient in a pig model

Background

Despite much evidence supporting the monitoring of the divergence of transcutaneous partial pressure of carbon dioxide (tcPCO₂) from arterial partial pressure carbon dioxide (artPCO₂) as an indicator of the shock status, data are limited on the relationships of the gradient between tcPCO₂ and artPCO₂ (tc-artPCO₂) with the systemic oxygen metabolism and hemodynamic parameters. Our study aimed to test the hypothesis that tc-artPCO₂ can detect inadequate tissue perfusion during hemorrhagic shock and resuscitation.

Methods

This prospective animal study was performed using female pigs at a university-based experimental laboratory. Progressive massive hemorrhagic shock was induced in mechanically ventilated pigs by stepwise blood withdrawal. All animals were then resuscitated by transfusing the stored blood in stages. A transcutaneous monitor was attached to their ears to measure tcPCO₂. A pulmonary artery catheter (PAC) and pulse index continuous cardiac output (PiCCO) were used to monitor cardiac output (CO) and several hemodynamic parameters. The relationships of tc-artPCO₂ with the study parameters and systemic oxygen delivery (DO₂) were analyzed.

Results

Hemorrhage and blood transfusion precisely impacted hemodynamic and laboratory data as expected. The tc-artPCO₂ level markedly increased as CO decreased. There were significant correlations of tc-artPCO₂ with DO₂ and COs (DO₂: r = − 0.83, CO by PAC: r = − 0.79; CO by PiCCO: r = − 0.74; all P < 0.0001). The critical level of oxygen delivery (DO_2crit) was 11.72 mL/kg/min according to transcutaneous partial pressure of oxygen (threshold of 30 mmHg). Receiver operating characteristic curve analyses revealed that the value of tc-artPCO₂ for discrimination of DO_2crit was highest with an area under the curve (AUC) of 0.94, followed by shock index (AUC = 0.78; P < 0.04 vs tc-artPCO₂), and lactate (AUC = 0.65; P < 0.001 vs tc-artPCO₂).

Conclusions

Our observations suggest the less-invasive tc-artPCO₂ monitoring can sensitively detect inadequate systemic oxygen supply during hemorrhagic shock. Further evaluations are required in different forms of shock in other large animal models and in humans to assess its usefulness, safety, and ability to predict outcomes in critical illnesses.

Management of Perioperative Iron Deficiency in Cardiac Surgery: A Modified RAND Delphi Study

BACKGROUND

Over the last decade, preoperative anemia has become recognized as a clinical condition in need of management. Although the etiology of preoperative anemia can be multifactorial, two-thirds of anemic elective surgical patients have iron deficiency anemia. At the same time, one-third of non-anemic elective surgical patients are also iron deficient.

METHODS

Modified-RAND Delphi methodology was employed to identify areas of consensus among an expert panel regarding the management of iron deficiency in patients undergoing cardiac surgery. A list of statements was sent to panel members to respond to using a five-point Likert Scale. All panel members subsequently attended a face-to-face meeting. The initial survey was presented and discussed, and panel members responded to each statement on the Likert scale again. Based on the second survey, the panel came to a consensus on recommendations.

RESULTS

The panel recommended all patients undergoing cardiac surgery be evaluated for iron deficiency, whether or not anemia is present. Evaluation should include iron studies and reticulocyte hemoglobin content. If iron deficiency is present, with or without anemia, patients should receive parenteral iron. Erythropoietin stimulating agents may be appropriate for some patients.

CONCLUSIONS

Consensus of an expert panel resulted in a standardized approach to diagnosing and managing iron deficiency in patients undergoing cardiac surgery.

RBC Transfusion Triggers: Is There Anything New?

For many years, in daily clinical practice, the traditional 10/30 rule (hemoglobin 10 g/dL - hematocrit 30%) has been the most commonly used trigger for blood transfusions. Over the years, this approach is believed to have contributed to a countless number of unnecessary transfusions and an unknown number of overtransfusion-related deaths. Recent studies have shown that lower hemoglobin levels can safely be accepted, even in critically ill patients. However, even these new transfusion thresholds are far beyond the theoretical limits of individual anemia tolerance. For this reason, almost all publications addressing the limits of acute anemia recommend physiological transfusion triggers to indicate the transfusion of erythrocyte concentrates as an alternative. Although this concept appears intuitive at first glance, no solid scientific evidence supports the safety and benefit of physiological transfusion triggers to indicate the optimal time point for transfusion of allogeneic blood. It is therefore imperative to continue searching for the most sensitive and specific parameters that can guide the clinician when to transfuse in order to avoid anemia-induced organ dysfunction while avoiding overtransfusion-related adverse effects. This narrative review discusses the concept of anemia tolerance and critically compares hemoglobin-based triggers with physiological transfusion for various clinical indications.

Oxygenation during general anesthesia and thoracic surgery in a patient with Titusville low-oxygen affinity hemoglobin

The purpose of this case study is to describe the physiological characteristics of a patient with the low-oxygen affinity Titusville hemoglobin variant. A 46-yr-old man with exertional dyspnea was diagnosed with a mediastinal lymphadenopathy of unknown origin and, to obtain definitive diagnosis by biopsy, underwent endobronchial ultrasound-guided transbronchial needle aspirate under sedation and video-assisted thoracoscopy under general anesthesia. High inspired fraction of oxygen ( FIO2 ) was used to guarantee adequate oxygenation even during the one-lung ventilation needed for thoracoscopy. Following radial and pulmonary arterial catheterization, continuous mixed-venous oxygen saturation ( SVO2 ), cardiac output, oxygen delivery (DO₂), oxygen consumption (V̇o₂), and oxygen extraction ratio (ERO₂) were measured. Serial blood gas analyses were obtained at different FIO2 . Anesthesia and surgery were carried out safely. Data obtained during the clinical case were utilized to 1) construct an in vivo Titusville hemoglobin dissociation curve and 2) describe oxygen delivery and consumption of a human with Titusville mutation. Titusville hemoglobin showed relatively high P50 (i.e., 30 vs. normal of 27) and very low cooperativity (Hill coefficient of 1.45 vs. normal 2.27), which was compensated in our patient by increases in cardiac output, rather than by augmenting oxygen extraction.

Management of patients who refuse blood transfusion

A small group of people belonging to a certain religion, called Jehovah's witness do not accept blood transfusion or blood products, based on biblical readings. When such group of people are in need of health care, their faith and belief is an obstacle for their proper treatment, and poses legal, ethical and medical challenges for attending health care provider. Due to the rapid growth in the membership of this group worldwide, physicians attending hospitals should be prepared to manage such patients. Appropriate management of such patients entails understanding of ethical and legal issues involved, providing meticulous medical management, use of prohaemostatic agents, essential interventions and techniques to reduce blood loss and hence, reduce the risk of subsequent need for blood transfusion. An extensive literature search was performed using search engines such as Google scholar, PubMed, MEDLINE, science journals and textbooks using keywords like ‘Jehovah's witness’, ‘blood haemodilution’, ‘blood salvage’ and ‘blood substitutes’.

Hypometabolism and hypothermia in the rat model of endotoxic shock: independence of circulatory hypoxia

We tested the hypothesis that development of hypothermia instead of fever in endotoxic shock is consequential to hypoxia. Endotoxic shock was induced by bacterial lipopolysaccharide (LPS, 500 μg kg(-1) i.v.) in rats at an ambient temperature of 22 °C. A β3-adrenergic agonist known to activate metabolic heat production, CL316,243, was employed to evaluate whether thermogenic capacity could be impaired by the fall in oxygen delivery (ḊO2) during endotoxic shock. This possibility was rejected as CL316,243 (0.15 mg kg(-1) i.v.) evoked similar rises in oxygen consumption (V̇O2) in the presence and absence of endotoxic shock. Next, to investigate whether a less severe form of circulatory hypoxia could be triggering hypothermia, the circulating volume of LPS-injected rats was expanded using 6% hetastarch with the intention of improving tissue perfusion and alleviating hypoxia. This intervention attenuated not only the fall in arterial pressure induced by LPS, but also the associated falls in V̇O2 and body temperature. These effects, however, occurred independently of hypoxia, as they were not accompanied by any detectable changes in NAD(+)/NADH ratios. Further experimentation revealed that even the earliest drops in cardiac output and ḊO2 during endotoxic shock did not precede the reduction in V̇O2 that brings about hypothermia. In fact, ḊO2 and V̇O2 fell in such a synchrony that the ḊO2/V̇O2 ratio remained unaffected. Only when hypothermia was prevented by exposure to a warm environment (30 °C) did an imbalance in the ḊO2/V̇O2 ratio become evident, and such an imbalance was associated with reductions in the renal and hypothalamic NAD(+)/NADH ratios. In conclusion, hypometabolism and hypothermia in endotoxic shock are not consequential to hypoxia but serve as a pre-emptive strategy to avoid hypoxia in this model.

Oxygen transport in the microcirculation and its regulation

OBJECTIVE

Cells require energy to carry out their functions and they typically use oxidative phosphorylation to generate the needed ATP. Thus, cells have a continuous need for oxygen, which they receive by diffusion from the blood through the interstitial fluid. The circulatory system pumps oxygen-rich blood through a network of increasingly minute vessels, the microcirculation. The structure of the microcirculation is such that all cells have at least one nearby capillary for diffusive exchange of oxygen and red blood cells release the oxygen bound to hemoglobin as they traverse capillaries.

METHODS

This review focuses first on the historical development of techniques to measure oxygen at various sites in the microcirculation, including the blood, interstitium, and cells.

RESULTS

Next, approaches are described as to how these techniques have been employed to make discoveries about different aspects of oxygen transport. Finally, ways in which oxygen might participate in the regulation of blood flow toward matching oxygen supply to oxygen demand is discussed.

CONCLUSIONS

Overall, the transport of oxygen to the cells of the body is one of the most critical functions of the cardiovascular system and it is in the microcirculation where the final local determinants of oxygen supply, oxygen demand, and their regulation are decided.

Hyperoxemic reperfusion after prolonged cardiac arrest in a rat cardiopulmonary bypass resuscitation model

BACKGROUND

The effect of hyperoxygenation at reperfusion, particularly in the setting of cardiac arrest, remains unclear. This issue was studied in a prolonged cardiac arrest model consisting of 25 min cardiac arrest in a rat resuscitated with cardiopulmonary bypass (CPB). The objective of this study was to determine the effect of hyperoxygenation following prolonged cardiac arrest resuscitation on mitochondrial and cardiac function.

METHODS

Male Sprague-Dawley rats (400-450 g) were anesthetized with ketamine and xylazine and instrumented for closed chest cardiopulmonary bypass (CPB). Following a 25-min KCl-induced cardiac arrest, the animals were resuscitated by CPB with 100% oxygen. Three minutes after successful return of spontaneous circulation (ROSC), the animals received either normoxemic reperfusion (CPB with 40-50% oxygen) or hyperoxemic reperfusion (CPB with 100% oxygen) for 1 h. Post-resuscitation hemodynamics, cardiac function, mitochondrial function and immunostaining of 3-nitrotyrosine were compared between the two different treatment groups.

RESULTS

At 1 h after ROSC, the hyperoxemic reperfusion group had a significant higher mean arterial pressure, less metabolic acidosis and better diastolic function than the normoxemic reperfusion group. Cardiac mitochondria from the hyperoxemic reperfusion group had a higher respiratory control ratio (RCR) and cardiac tissue showed less nitroxidative stress compared to the normoxemic reperfusion group.

CONCLUSIONS

One hour of hyperoxemic reperfusion after 25 min of cardiac arrest in an in vivo CPB model resulted in significant short-term improvement in myocardial and mitochondrial function compared with 1h of normoxemic reperfusion. This myocardial response may differ from previously reported post-arrest hyperoxia mediated effects following shorter arrest times.

Perioperative Red Blood Cell Transfusion: Harmful or Beneficial to the Patient?

Although the transfusion of red blood cells (RBCs) is safer than ever regarding infections, it is still associated with several adverse reactions and therefore should only be used on the basis of evidence-based triggers. However, prevention of RBC transfusion and subsequent substitution of blood losses with acellular solutions will inevitably result in dilutional anemia. Acute dilutional anemia can be compensated by the body over a wide range of hemoglobin concentrations without a critical restriction of tissue oxygenation. On the other hand, chronic anemia is known to be a potent cause of morbidity and mortality. As a consequence, the impact of perioperative anemia on mortality is difficult to describe, because anemia, as well as the transfusion of RBCs, can influence the clinical outcome. The resulting 'Gordian knot' cannot be cut easily, and this circumstance forces clinical physicians to make a daily trade-off between transfusion-associated and anemia-associated risks. This review focuses on the physiology of oxygen transport, the hazards of acute anemia, the hazards of RBC transfusion, and the literature putting these problems into perspective.

Pathophysiological alterations in oxygen delivery to the tissues

This paper reviews co-factors that impact on oxygen delivery and uptake, in the attempt to unravel the mechanisms underlying the correlation between the decrease in oxygen delivery and oxygen consumption. In sequence, the following factors are analyzed that, besides a decrease in haemoglobin concentration, impair tissue metabolism: (1) lung diffusion and perfusion limitation in oxygen transport, (2) decrease in cardiac output, (3) impairment of peripheral microvascular perfusion and (4) reduced ability of cells to extract oxygen. The contribution of the various factors is modeled aiming to present a decisional flow chart for the functional evaluation of the efficiency of the oxygen transport system.

Effects of a combination hemoglobin based oxygen carrier-hypertonic saline solution on oxygen transport in the treatment of traumatic shock

BACKGROUND

Logistics complicate fluid resuscitation of traumatic shock on the battlefield. Traumatic shock can result in oxygen debt (O(2)D) accumulation that is fatal. However, the ability of fluid strategies to repay O(2)D are not commonly reported. This pilot study examined various resuscitation fluids, including a combination of PEGylated bovine hemoglobin and hypertonic saline (AfterShock™) on their ability to repay O(2)D in traumatic shock.

METHODS

41 anesthetized swine underwent hemorrhage to an O(2)D of 80 mL/kg. Animals received one of the following: 500 mL whole blood, 500 mL AfterShock™, 500 mL hypertonic (7.2%) saline, 250 mL hypertonic (7.2%) saline, 500 mL Hetastarch (6%), or 500 mL lactated Ringer's. Oxygen transport variables (O(2)D, oxygen consumption, oxygen delivery, central venous hemoglobin oxygen saturation, oxygen extraction ratios), lactate clearance, and survival were monitored for 3h after treatment. Data were analyzed using mixed-model ANOVA and comparisons were made to the performance of whole blood.

RESULTS

Only animals receiving AfterShock™, 500 mL hypertonic saline, and 500 mL Hetastarch survived to 180 min. While not statistically significant AfterShock™ demonstrated trends in improving the repayment of O(2)D and in improving oxygen transport variables despite having lower levels of global oxygen delivery compared to whole blood, Hetastarch and 500 mL hypertonic saline groups.

CONCLUSION

Use of 500 mL AfterShock™, 500 mL of 7.2% saline or 500 mL of Hetastarch resulted in improved short-term survival. While not statistically significant, AfterShock™ demonstrated trends in improving O(2)D. These findings may have implications for designing resuscitation fluids for combat casualty care.

The optimal hematocrit

Nearly 15 million units of packed red blood cells and whole blood are transfused annually in the United States alone. Until recently, the major risks from blood transfusion were thought to be transmission of viral infections, and overall, blood transfusion was believed by most providers to be safe. A safe hemoglobin threshold above which red cell transfusion is clearly unnecessary has not been established. This article addresses the numerous problems that surround the use and consequences of blood transfusion, such as hemoglobin and hematocrit levels, oxygenation, storage time, immunomodulation, infection, and anemia. The relevant literature is comprehensively reviewed.

Current issues relating to the transfusion of stored red blood cells

The development of blood storage systems allowed donation and transfusion to be separated in time and space. This separation has permitted the regionalization of donor services with subsequent economies of scale and improvements in the quality and availability of blood products. However, the availability of storage raises the question of how long blood products can and should be stored and how long they are safe and effective. The efficacy of red blood cells was originally measured as the increment in haematocrit and safety began with typing and the effort to reduce the risk of bacterial contamination. Appreciation of a growing list of storage lesions of red blood cells has developed with our increasing understanding of red blood cell physiology and our experience with red blood cell transfusion. However, other than frank haemolysis, rare episodes of bacterial contamination and overgrowth, the reduction of oxygen-carrying capacity associated with the failure of some transfused cells to circulate, and the toxicity of lysophospholipids released from membrane breakdown, storage-induced lesions have not had obvious correlations with safety or efficacy. The safety of red blood cell storage has also been approached in retrospective epidemiologic studies of transfused patients, but the results are frequently biased by the fact that sicker patients are transfused more often and blood banks do not issue blood products in a random order. Several large prospective studies of the safety of stored red blood cells are planned.

Cardiovascular responses to peripheral chemoreflex activation and comparison of different methods to evaluate baroreflex gain in conscious mice using telemetry

Peripheral chemoreceptors located in the carotid bodies are the primary sensors of systemic hypoxia. Although the pattern of responses elicited by peripheral chemoreceptor activation is well established in rats, lambs, and rabbits, the cardiovascular responses to peripheral chemoreflex activation in conscious mice have not been delineated. Here we report that stimulation of peripheral chemoreceptors by potassium cyanide (KCN) in conscious mice elicits a unique biphasic response in blood pressure that is characterized by an initial and robust rise followed by a decrease in blood pressure, which is accompanied by a marked reduction in heart rate. The depressor and bradycardic responses to KCN were abolished by muscarinic receptor blockade with atropine, and the pressor response was abolished by alpha-adrenergic receptor blockade with prazosin, suggesting that vagal and sympathetic drive to the heart and sympathetic drive to the vasculature mediate these cardiovascular responses. These studies characterized the chemoreflex in conscious mice and established the reliability of using them for studying hypoxia-related diseases such as obstructive sleep apnea. In another series of experiments, two methods for analyzing baroreflex sensitivity were compared: the classical pharmacological approach using phenylephrine and sodium nitroprusside (i.e., the Oxford technique) or the sequence method for analyzing spontaneous baroreflex activity. Our findings indicate that both methods are reliable, and the sequence method certainly has its benefits as a predictive tool in the context of long-term noninvasive studies using telemetry. However, for absolute determination of baroreflex function, analysis of spontaneous baroreflex activity should be complemented by the classical pharmacological method.

Vagal tone dominates autonomic control of mouse heart rate at thermoneutrality

It is generally accepted that cardiac sympathetic tone dominates the control of heart rate (HR) in mice. However, we have recently challenged this notion given that HR in the mouse is responsive to ambient temperature (Ta) and that the housing Tais typically 21–23°C, well below the thermoneutral zone (∼30°C) of this species. To specifically test the hypothesis that cardiac sympathetic tone is the primary mediator of HR control in the mouse, we first examined the metabolic and cardiovascular responses to rapid changes in Tato demonstrate the sensitivity of the mouse cardiovascular system to Ta. We then determined HR in 1) mice deficient in cardiac sympathetic tone (“β-less” mice), 2) mice deficient in cardiac vagal tone [muscarinic M2receptor ( M2R−/−) mice], and 3) littermate controls. At a Taof 30°C, the HR of β-less mice was identical to that of wild-type mice (351 ± 11 and 363 ± 10 beats/min, respectively). However, the HR of M2R−/−mice was significantly greater (416 ± 7 beats/min), demonstrating that vagal tone predominates over HR control at this Ta. When these mice were calorically restricted to 70% of normal intake, HR fell equally in wild-type, β-less, and M2R−/−mice (ΔHR = 73 ± 9, 76 ± 3, and 73 ± 7 beats/min, respectively), suggesting that the fall in intrinsic HR governs bradycardia of calorically restricted mice. Only when the Tawas relatively cool, at 23°C, did β-less mice exhibit a HR (442 ± 14 beats/min) that was different from that of littermate controls (604 ± 10 beats/min) and M2R−/−mice (602 ± 5 beats/min). These experiments conclusively demonstrate that in the absence of cold stress, regulation of vagal tone and modulation of intrinsic rate are important determinants of HR control in the mouse.

Increased mortality, postoperative morbidity, and cost after red blood cell transfusion in patients having cardiac surgery

BACKGROUND

Red blood cell transfusion can both benefit and harm. To inform decisions about transfusion, we aimed to quantify associations of transfusion with clinical outcomes and cost in patients having cardiac surgery.

METHODS AND RESULTS

Clinical, hematology, and blood transfusion databases were linked with the UK population register. Additional hematocrit information was obtained from intensive care unit charts. Composite infection (respiratory or wound infection or septicemia) and ischemic outcomes (myocardial infarction, stroke, renal impairment, or failure) were prespecified as coprimary end points. Secondary outcomes were resource use, cost, and survival. Associations were estimated by regression modeling with adjustment for potential confounding. All adult patients having cardiac surgery between April 1, 1996, and December 31, 2003, with key exposure and outcome data were included (98%). Adjusted odds ratios for composite infection (737 of 8516) and ischemic outcomes (832 of 8518) for transfused versus nontransfused patients were 3.38 (95% confidence interval [CI], 2.60 to 4.40) and 3.35 (95% CI, 2.68 to 4.35), respectively. Transfusion was associated with increased relative cost of admission (any transfusion, 1.42 times [95% CI, 1.37 to 1.46], varying from 1.11 for 1 U to 3.35 for >9 U). At any time after their operations, transfused patients were less likely to have been discharged from hospital (hazard ratio [HR], 0.63; 95% CI, 0.60 to 0.67) and were more likely to have died (0 to 30 days: HR, 6.69; 95% CI, 3.66 to 15.1; 31 days to 1 year: HR, 2.59; 95% CI, 1.68 to 4.17; >1 year: HR, 1.32; 95% CI, 1.08 to 1.64).

CONCLUSIONS

Red blood cell transfusion in patients having cardiac surgery is strongly associated with both infection and ischemic postoperative morbidity, hospital stay, increased early and late mortality, and hospital costs.

Applicability of recent methods used to estimate spontaneous baroreflex sensitivity to resting mice

Short-term blood pressure (BP) variability is limited by the arterial baroreflex. Methods for measuring the spontaneous baroreflex sensitivity (BRS) aim to quantify the gain of the transfer function between BP and pulse interval (PI) or the slope of the linear relationship between parallel BP and PI changes. These frequency-domain (spectral) and time-domain (sequence) techniques were tested in conscious mice equipped with telemetric devices. The autonomic relevance of these indexes was evaluated using pharmacological blockades. The significant changes of the spectral bandwidths resulting from the autonomic blockades were used to identify the low-frequency (LF) and high-frequency (HF) zones of interest. The LF gain was 1.45 +/- 0.14 ms/mmHg, with a PI delay of 0.5 s. For the HF gain, the average values were 2.0 +/- 0.19 ms/mmHg, with a null phase. LF and HF bands were markedly affected by atropine. On the same 51.2-s segments used for cross-spectral analysis, an average number of 26.4 +/- 2.2 slopes were detected, and the average slope in resting mice was 4.4 +/- 0.5 ms/mmHg. Atropine significantly reduced the slopes of the sequence method. BRS measurements obtained using the sequence technique were highly correlated to the spectral estimates. This study demonstrates the applicability of the recent methods used to estimate spontaneous BRS in mice. There was a vagal predominance in the baroreflex control of heart rate in conscious mice in the present conditions.

[Tolerance to perioperative anemia. Mechanisms, influencing factors and limits]

Die zu erwartende Kostensteigerung im Transfusionswesen (steigender Fremdblutbedarf bei gleichzeitig rückläufiger Spendebereitschaft, Behandlungspflicht transfusionsassoziierter Folgeerkrankungen) erhöht den sozioökonomischen Stellenwert der Entwicklung institutionsspezifischer Transfusionsprogramme. Ein wesentlicher Bestandteil hierbei ist – neben einer schonenden Operationstechnik und der konsequenten perioperativen Anwendung fremdblutsparender Maßnahmen – die Ausschöpfung der natürlicherweise vorhandenen „Anämietoleranz“ des menschlichen Organismus (Toleranz größerer Blutverluste durch Verlust von „verdünntem“ Blut, Hinauszögern des Transfusionsbeginns bis nach chirurgischer Blutstillung, Gewinnung von autologem Blut). In der vorliegenden Übersicht werden die Mechanismen, Einflussgrößen und Grenzen dieser natürlichen Anämietoleranz für den Gesamtorganismus und für einzelne Organsysteme zusammengefasst und die sich daraus ergebende Indikation zur Erythrozytentransfusion abgeleitet. Unter kontrollierten Bedingungen (Narkose, strikte Aufrechterhaltung von Normovolämie, komplette Muskelrelaxierung, Hyperoxämie, Hypothermie) werden von kardiopulmonal gesunden Individuen kurzzeitig auch extreme Grade der Verdünnungsanämie [Hämoglobin- (Hb-)Wert <3 g/dl (<1,86 mmol/l)] ohne Transfusion toleriert. In der klinischen Routine bleibt diese Situation – nicht zuletzt in Ermangelung eines adäquaten Monitorings – jedoch auf spezielle Sonderfälle beschränkt (z. B. unerwartete große Blutverluste bei Zeugen Jehovahs, unerwarteter Engpass bei der Bereitstellung von Fremdblut). Die derzeit geltenden Empfehlungen verschiedener Expertenkommissionen decken sich dahingehend, dass perioperativ (1) bis zu einer Hb-Konzentration von 10 g/dl (6,21 mmol/l) auch bei alten Patienten und Patienten mit kardiopulmonalen Begleiterkrankungen eine Transfusion von Erythrozyten in der Regel nicht notwendig ist und (2) eine Transfusion bei jungen, gesunden Patienten ohne kardiopulmonale Vorerkrankungen (einschließlich Schwangeren und Kindern) erst ab einer Hb-Konzentration von <6 g/dl (<3,72 mmol/l) notwendig wird. Auch beatmete Intensivpatienten mit Polytrauma und Sepsis scheinen nicht von einer Transfusion auf Hb-Konzentration >9 g/dl (>5,59 mmol/l) zu profitieren. Bei massiven Blutverlusten und diffuser Blutungsneigung scheint ein Hb von 10 g/dl (6,21 mmol/l) zur Stabilisierung der Blutgerinnung beizutragen.

Perioperative blood transfusion and blood conservation in cardiac surgery: the Society of Thoracic Surgeons and The Society of Cardiovascular Anesthesiologists clinical practice guideline

BACKGROUND

A minority of patients having cardiac procedures (15% to 20%) consume more than 80% of the blood products transfused at operation. Blood must be viewed as a scarce resource that carries risks and benefits. A careful review of available evidence can provide guidelines to allocate this valuable resource and improve patient outcomes.

METHODS

We reviewed all available published evidence related to blood conservation during cardiac operations, including randomized controlled trials, published observational information, and case reports. Conventional methods identified the level of evidence available for each of the blood conservation interventions. After considering the level of evidence, recommendations were made regarding each intervention using the American Heart Association/American College of Cardiology classification scheme.

RESULTS

Review of published reports identified a high-risk profile associated with increased postoperative blood transfusion. Six variables stand out as important indicators of risk: (1) advanced age, (2) low preoperative red blood cell volume (preoperative anemia or small body size), (3) preoperative antiplatelet or antithrombotic drugs, (4) reoperative or complex procedures, (5) emergency operations, and (6) noncardiac patient comorbidities. Careful review revealed preoperative and perioperative interventions that are likely to reduce bleeding and postoperative blood transfusion. Preoperative interventions that are likely to reduce blood transfusion include identification of high-risk patients who should receive all available preoperative and perioperative blood conservation interventions and limitation of antithrombotic drugs. Perioperative blood conservation interventions include use of antifibrinolytic drugs, selective use of off-pump coronary artery bypass graft surgery, routine use of a cell-saving device, and implementation of appropriate transfusion indications. An important intervention is application of a multimodality blood conservation program that is institution based, accepted by all health care providers, and that involves well thought out transfusion algorithms to guide transfusion decisions.

CONCLUSIONS

Based on available evidence, institution-specific protocols should screen for high-risk patients, as blood conservation interventions are likely to be most productive for this high-risk subset. Available evidence-based blood conservation techniques include (1) drugs that increase preoperative blood volume (eg, erythropoietin) or decrease postoperative bleeding (eg, antifibrinolytics), (2) devices that conserve blood (eg, intraoperative blood salvage and blood sparing interventions), (3) interventions that protect the patient's own blood from the stress of operation (eg, autologous predonation and normovolemic hemodilution), (4) consensus, institution-specific blood transfusion algorithms supplemented with point-of-care testing, and most importantly, (5) a multimodality approach to blood conservation combining all of the above.

Genetic and dietary interactions: role of angiotensin AT1a receptors in response to a high-fructose diet

The renin-angiotensin system (RAS) has been implicated in the cardiovascular complications of diabetes. We showed that a high-fructose diet increases blood pressure and plasma angiotensin and impairs glucose tolerance. We investigated the role of angiotensin AT(1a) receptors in the development of fructose-induced cardiovascular and metabolic dysfunction. Male angiotensin AT(1a) knockout (AT1aKO) and wild-type (AT1aWT) mice with arterial telemetric catheters were fed a standard diet or one containing 60% fructose. Fructose increased mean arterial pressure (MAP) in AT1aWT but only during the dark phase (8% increase). In AT1aKO mice, fructose unexpectedly decreased MAP, during both light and dark periods (24 and 13% decrease, respectively). Analytical methods were used to measure systolic arterial pressure (SAP) and pulse interval (PI) variability in time and frequency domains. In fructose-fed AT1aWT mice, there was an increase in SAP variance and its low-frequency (LF) domain (11 +/- 3 vs. 23 +/- 4 mmHg(2), variance, and 7 +/- 2 vs. 17 +/- 3 mmHg(2), LF, control vs. fructose, P < 0.004). There were no changes in SAP variance in AT1aKO mice. Depressor responses to alpha(1)-adrenergic blockade were augmented in fructose-fed AT1a WT compared with AT1aKO mice. Fructose inhibited glucose tolerance with a greater effect in AT1aWT mice. Fructose increased plasma cholesterol in both groups (P < 0.01) and reduced ANG II in AT1aKO mice. Results document prominent interactions between genetics and diet with data showing that in the absence of angiotensin AT(1a) receptors, a fructose diet decreased blood pressure.

[Tolerance to perioperative anemia. Mechanisms, influencing factors and limits]

Die zu erwartende Kostensteigerung im Transfusionswesen erhöht den sozioökonomischen Stellenwert der Entwicklung institutionsspezifischer Transfusionsprogramme. Ein wesentlicher Bestandteil hierbei ist – neben einer schonenden Operationstechnik und der konsequenten perioperativen Anwendung fremdblutsparender Maßnahmen – die Ausschöpfung der natürlichen „Anämietoleranz“ des menschlichen Organismus. Im vorliegenden Beitrag werden die Mechanismen, Einflussgrößen und Grenzen dieser Anämietoleranz für den Gesamtorganismus und für einzelne Organsysteme zusammengefasst und die sich daraus ergebende Indikation zur Erythrozytentransfusion abgeleitet. Die derzeit geltenden Empfehlungen decken sich dahingehend, dass bis zu einer Hämoglobinkonzentration von 10 g/dl (6,21 mmol/l) auch bei alten Patienten oder kardiopulmonalen Begleiterkrankungen eine perioperative Transfusion in der Regel nicht notwendig ist und bei jungen, gesunden Patienten ohne kardiopulmonale Vorerkrankungen (einschließlich Schwangeren und Kindern) erst ab <6 g/dl (<3,72 mmol/l) notwendig wird. Auch beatmete Intensivpatienten mit Polytrauma und Sepsis scheinen nicht von einer Transfusion auf eine Hämoglobinkonzentration >9 g/dl (>5,59 mmol/l) zu profitieren. Bei massiven Blutverlusten und diffuser Blutungsneigung scheint ein Wert von 10 g/dl (6,21 mmol/l) zur Stabilisierung der Blutgerinnung beizutragen.

Red cell transfusions and guidelines: a work in progress

Blood transfusion utilization continues to rise, yet it has never undergone prospective safety and efficacy testing. Recent data regarding oxygen delivery, microcirculation, and inflammation all point toward potential problems with allogeneic transfusion. Outcome data from retrospective data bases are sobering, calling to question the present practices of red cell transfusion.

Deficiency in angiotensin AT1a receptors prevents diabetes-induced hypertension

The renin-angiotensin system has been implicated in the etiology of the cardiovascular complications of diabetes. Our studies extend these findings to show a specific role for angiotensin AT1a receptors in mediating diabetes-induced hypertension. Male angiotensin AT1a knockout (AT1aKO) and wild-type (AT1aWT) mice with arterial telemetric catheters were injected with streptozotocin (STZ; 150 mg/kg ip). The STZ dose was selected on the basis of a dose-response experiment in C57/BL mice. Blood glucose, water intake, body weight, blood pressure (BP), and heart rate (HR) were measured over a 2-wk period. Estimates of BP and HR variance (BPV and HRV) and their low- and high-frequency domains were also determined. STZ induced similar levels of hyperglycemia and polydypsia in the groups. Mean arterial pressure (MAP) was increased from 100 ± 6 to 124 ± 6 mmHg in diabetic AT1aWT. MAP was unchanged in AT1aKO (80 ± 4 vs. 85 ± 5 mmHg, basal vs. STZ). Treatment with an ACE inhibitor, captopril, produced a greater reduction in MAP (−18%) in diabetic AT1aWT than in AT1aKO (−3.4%). BPV was lower in AT1aKO (19 ± 0.5 vs. 9 ± 2 mmHg2, AT1aWT vs. AT1aKO). Diabetes reduced BPV but only in AT1aWT (19 ± 0.5 vs. 8 ± 1 mmHg2, basal vs. STZ). There were no changes in HR in either group. In AT1aKO, STZ increased HRV and its high-frequency domain with no changes seen in AT1aWT. Results document that ANG AT1a receptors are critical in diabetes-induced hypertension and in cardiac autonomic responses.

Hydroxyethyl starch is superior to lactated Ringer as a replacement fluid in a pig model of acute normovolaemic haemodilution

BACKGROUND

Tissue tolerance to oxygen privation during acute normovolaemic haemodilution with different fluids remains unclear. We tested the hypothesis that hydroxyethyl starch (HES) is superior to lactated Ringer's solution in pigs for preserving tissue perfusion during acute normovolaemic haemodilution.

METHODS

Twenty-four animals were randomized into control, lactated Ringer's solution and HES groups. All groups, except the control, underwent acute normovolaemic haemodilution. Haemodynamics, oxygen parameter indices, global anaerobic metabolic markers, echocardiographic parameters, gastric tonometry and serum osmolarity were monitored at baseline, immediately after (0 min) and 60 and 120 min after the end of haemodilution. Myocardial, liver, stomach and intestine samples were collected for further evaluation.

RESULTS

Cardiac and oxygen parameter index responses to acute normovolaemic haemodilution were comparable. However, the increment in cardiac index, stroke volume index, and left ventricular stroke work index were more sustained in the starch group. In the lactated Ringer's group, gastric pH decreased significantly and was accompanied by a significant increase in lactate. Myocardial ultrastructure was better preserved in the starch group. The other tissue samples presented no change.

CONCLUSIONS

In this model of ANH, the starch group had a superior haemodynamic response. Minor loss of myocardial cellular integrity and preserved gastric pHi reinforce these findings.

Indications for Blood Transfusion in Cardiac Surgery

In addition to its life-saving effect in hemorrhagic shock, transfusion of allogenic packed red blood cells can be beneficial in situations where a critically low hematocrit is contributing to a state of oxygen-supply dependency. These benefits are countered by the risks of transfusion-associated lung injury, transfusion-associated immunomodulation, and cellular hypoxia after RBC transfusion. The critical hematocrit is patient and organ specific, and varies intraoperatively according to the duration and temperature of bypass, as well as for a variable postoperative period. Future randomized studies must prospectively evaluate regional indicators of tissue oxygenation in transfusion algorithms.

Optimal frequency ranges for extracting information on cardiovascular autonomic control from the blood pressure and pulse interval spectrograms in mice

The analysis of blood pressure (BP) and heart rate (HR) variability by spectral methods has proven a useful tool in many animal species for the assessment of the vagal and sympathetic contributions to oscillations of BP and HR. Continuous BP measurements obtained in mice by telemetry were used to characterize the spectral bandwidths of autonomic relevance by using an approach with no a priori. The paradigm was based on the autonomic blockades obtained with conventional drugs (atropine, prazosin, atenolol). The spectral changes were estimated in all of the combinations of spectral bandwidths. The effect of hydralazine was also tested using the same systematic analysis, to detect the zones of sympathetic activation resulting reflexly from the vasodilatory action of the drug. Two zones of interest in the study of the autonomic control of BP and HR were observed. The first zone covered the 0.15-0.60 Hz range of the systolic BP spectrum and corresponds to the low-frequency zone (or Mayer waves). This zone reflects sympathetic control since the power spectral density of this zone was significantly reduced with alpha1-adrenoceptor blockade (prazosin), while it was significantly amplified as a result of a reflex sympathetic activation (hydralazine). The second zone covered the 2.5-5.0 Hz range of the pulse interval spectrum and corresponded to the high-frequency zone (respiratory sinus arrhythmia) under vagal control (blocked by atropine). These zones are recommended for testing the autonomic control of circulation in mice.

Decreased supply-dependent oxygen consumption in the skeletal muscle of the spontaneously hypertensive rat during acute hypoxia

The purpose of this study was to correlate microvascular oxygen delivery (DO2) and consumption (VO2) in the skeletal muscle of spontaneously hypertensive rats (SHRs) and Wistar Kyoto rats (WKY) with hemodynamics during acute hypoxia. We expected greater abnormalities in central and microvascular hemodynamics during hypoxic induced shock in the SHR compared with the WKY due to microvascular rarefaction. The inspired oxygen fraction (FiO2) was lowered from 0.21 to 0.15, 0.1, 0.08, and 0.05 in anesthetized, mechanically ventilated rats. Lactate and base deficit values were similar for both groups at 0.21 and 0.15 FiO2, but higher in SHR at lower FiO2. Baseline aortic blood flow (SHR, 56.2+/-4.0 mL min; WKY, 61.8+/-5.3 mL min) and systemic DO2 (SHR, 9.02+/-0.82 mL min; WKY, 9.32+/-0.54 mL min) increased similarly when FiO2 was lowered to 0.15. Further reductions in FiO2 caused lower aortic flow and systemic DO2 in the SHR than WKY at 0.08 and 0.05 FiO2. Spinotrapezius blood flow increased from baseline (SHR, 24.8+/-1.8 nL s; WKY, 22.7+/-2.1 nL s) in both groups when FiO2 was reduced to 0.15; further reductions in FiO2 decreased blood flow in both groups, with lower values in the SHR group at 0.1 and 0.08 FiO2. The SHR group demonstrated higher venous oxygen saturation at low values of FiO2 compared with WKY. This reduced oxygen extraction in SHR resulted in a lower supply-dependent VO2 at low values of spinotrapezius DO2, perhaps attributed to arteriolar thickening and rarefaction seen in chronic hypertension.

Anemia and perioperative red blood cell transfusion: a matter of tolerance

In the past, anemia in the perioperative period has been treated by red blood cell (RBC) transfusions relatively uncritically. RBC transfusions were believed to increase oxygen delivery by increasing hemoglobin concentration. Arbitrary transfusion triggers such as the "10/30 rule" (i.e., RBC transfusion indicated below a hemoglobin concentration of 10 g/dL or a hematocrit of 30%) were applied. However, there is now increasing evidence that RBC transfusions are associated with adverse outcomes and should be avoided whenever possible. Restraining from RBC transfusions and maintaining normovolemia in patients suffering from surgical blood loss results in acute anemia. Therefore, knowing the compensatory mechanisms during acute anemia is crucial. This review focuses on acute anemia tolerance, its limits, and physiologic transfusion triggers in the perioperative period.

Adenovirus-mediated small-interference RNA for in vivo silencing of angiotensin AT1a receptors in mouse brain

Because of the lack of pharmacological approaches, molecular genetic methods have been required to differentiate between angiotensin type 1(AT1) receptor subtypes AT1a and AT1b. RNA interference is a new tool for the study of gene function, producing specific downregulation of protein expression. In this study, we used the small hairpin RNA (shRNA) cassette method to screen target sites for selectively silencing AT1a or AT1b receptor subtypes in cultured Neuro-2a cells using real-time RT-PCR. For in vivo functional studies, we used C57BL mice with arterial telemetric probes and computerized licking monitors to test the effect of adenovirus carrying the DNA sequence coding AT1a shRNA (Ad-AT1a-shRNA). Ad-AT1a-shRNA was injected into the lateral ventricle (intracerebroventricular) or the brain stem nucleus tractus solitaries/dorsal vagal nucleus (NTS/DVN) with measurement of water intake, blood pressure (BP), and heart rate (HR) for up to 20 days after injection. Tissue culture studies verified the specificity and the efficiency of the constructs. In animal studies, beta-galactosidase staining and Ang receptor binding assays showed expression of shRNA and downregulation of Ang AT1 receptors in the subfornical organ and NTS/DVN by >70%. Intracerebroventricular injection of Ad-AT1a-shRNA increased water intake with no effect on BP or HR. In contrast, microinjection of Ad-AT1a-shRNA into NTS/DVN caused a decrease in BP with no effect on HR or water intake. Results demonstrate the use of the RNA interference method in site-directed silencing of gene expression and provide a method for the in vivo study of Ang AT1 receptor function.