Oxygen supply to the tissues during limited normovolemic hemodilution

Application of negative tissue interstitial pressure improves functional capillary density after hemorrhagic shock in the absence of volume resuscitation

Microvascular fluid exchange is primarily dependent on Starling forces and both the active and passive myogenic response of arterioles and post-capillary venules. Arterioles are classically considered resistance vessels, while venules are considered capacitance vessels with high distensibility and low tonic sympathetic stimulation at rest. However, few studies have investigated the effects of modulating interstitial hydrostatic pressure, particularly in the context of hemorrhagic shock. The objective of this study was to investigate the mechanics of arterioles and functional capillary density (FCD) during application of negative tissue interstitial pressure after 40% total blood volume hemorrhagic shock. In this study, we characterized systemic and microcirculatory hemodynamic parameters, including FCD, in hamsters instrumented with a dorsal window chamber and a custom-designed negative pressure application device via intravital microscopy. In large arterioles, application of negative pressure after hemorrhagic shock resulted in a 13 ± 11% decrease in flow compared with only a 7 ± 9% decrease in flow after hemorrhagic shock alone after 90 minutes. In post-capillary venules, however, application of negative pressure after hemorrhagic shock resulted in a 31 ± 4% decrease in flow compared with only an 8 ± 5% decrease in flow after hemorrhagic shock alone after 90 minutes. Normalized FCD was observed to significantly improve after application of negative pressure after hemorrhagic shock (0.66 ± 0.02) compared to hemorrhagic shock without application of negative pressure (0.50 ± 0.04). Our study demonstrates that application of negative pressure acutely improves FCD during hemorrhagic shock, though it does not normalize FCD. These results suggest that by increasing the hydrostatic pressure gradient between the microvasculature and interstitium, microvascular perfusion can be transiently restored in the absence of volume resuscitation. This study has significant clinical implications, particularly in negative pressure wound therapy, and offers an alternative mechanism to improve microvascular perfusion during hypovolemic shock.

Hypoxia Promotes Immune Evasion by Triggering β-Glucan Masking on the Candida albicans Cell Surface via Mitochondrial and cAMP-Protein Kinase A Signaling

Organisms must adapt to changes in oxygen tension if they are to exploit the energetic benefits of reducing oxygen while minimizing the potentially damaging effects of oxidation. Consequently, organisms in all eukaryotic kingdoms display robust adaptation to hypoxia (low oxygen levels). This is particularly important for fungal pathogens that colonize hypoxic niches in the host. We show that adaptation to hypoxia in the major fungal pathogen of humans Candida albicans includes changes in cell wall structure and reduced exposure, at the cell surface, of β-glucan, a key pathogen-associated molecular pattern (PAMP). This leads to reduced phagocytosis by murine bone marrow-derived macrophages and decreased production of IL-10, RANTES, and TNF-α by peripheral blood mononuclear cells, suggesting that hypoxia-induced β-glucan masking has a significant effect upon C. albicans-host interactions. We show that hypoxia-induced β-glucan masking is dependent upon both mitochondrial and cAMP-protein kinase A (PKA) signaling. The decrease in β-glucan exposure is blocked by mutations that affect mitochondrial functionality (goa1Δ and upc2Δ) or that decrease production of hydrogen peroxide in the inner membrane space (sod1Δ). Furthermore, β-glucan masking is enhanced by mutations that elevate mitochondrial reactive oxygen species (aox1Δ). The β-glucan masking defects displayed by goa1Δ and upc2Δ cells are suppressed by exogenous dibutyryl-cAMP. Also, mutations that inactivate cAMP synthesis (cyr1Δ) or PKA (tpk1Δ tpk2Δ) block the masking phenotype. Our data suggest that C. albicans responds to hypoxic niches by inducing β-glucan masking via a mitochondrial cAMP-PKA signaling pathway, thereby modulating local immune responses and promoting fungal colonization.IMPORTANCE Animal, plant, and fungal cells occupy environments that impose changes in oxygen tension. Consequently, many species have evolved mechanisms that permit robust adaptation to these changes. The fungal pathogen Candida albicans can colonize hypoxic (low oxygen) niches in its human host, such as the lower gastrointestinal tract and inflamed tissues, but to colonize its host, the fungus must also evade local immune defenses. We reveal, for the first time, a defined link between hypoxic adaptation and immune evasion in C. albicans As this pathogen adapts to hypoxia, it undergoes changes in cell wall structure that include masking of β-glucan at its cell surface, and it becomes better able to evade phagocytosis by innate immune cells. We also define the signaling mechanisms that mediate hypoxia-induced β-glucan masking, showing that they are dependent on mitochondrial signaling and the cAMP-protein kinase pathway. Therefore, hypoxia appears to trigger immune evasion in this fungal pathogen.

From Low Hematocrit Physiology to Isovolemic Hemodilution

The aim of this study, starting from some concepts of physiology, was to deal with the ever growing question of intentional anemia. Physiology suitably expresses the linear relationship between hematocrit and blood fluidity, and at the same time explains how viscosity and aggregability are important co-factors in the circulation, in disease and in thromboembolic complications. The need to understand how a low hematocrit level, compared to the volemia, is a fundamental requirement in surgery, and even more so in surgery with a high risk of embolism, like orthopedic traumatology surgery. Normovolemic hemodilution was analyzed both for the risks it involves and for how much it sticks to the changes induced by a similar situation on the various organs and apparatus. The authors, based on their experience in the field of anesthesiology, propose an operating schedule to safely perform intentional normovolemic hemodilution, describing the preoperative, intraoperative and finally the postoperative period. In conclusion, the authors advocate a wider use of this technique, confirming its safety in relation to volemia.

Influence of clonidine induced sympathicolysis on anaemia tolerance in anaesthetized pigs

Background

Clonidine effectively decreases perioperative mortality by reducing sympathetic tone. However, application of clonidine might also restrict anaemia tolerance due to impairment of compensatory mechanisms. Therefore, the influence of clonidine induced, short-term sympathicolysis on anaemia tolerance was assessed in anaesthetized pigs. We measured the effect of clonidine on anaemia tolerance and of the potential for macrohemodynamic alterations to constrain the acute anaemia compensatory mechanisms.

Methods

After governmental approval, 14 anaesthetized pigs of either gender (Deutsche Landrasse, weight (mean ± SD) 24.1 ± 2.4 kg) were randomly assigned to intravenous saline or clonidine treatment (bolus: 20 μg · kg⁻¹, continuous infusion: 15 μg · kg⁻¹ · h⁻¹). Thereafter, the animals were hemodiluted by exchange of whole blood for 6 % hydroxyethyl starch (MW 130.000/0.4) until the individual critical haemoglobin concentration (Hb_crit) was reached. Primary outcome parameters were Hb_crit and the exchangeable blood volume (EBV) until Hb_crit was reached.

Results

Hb_crit did not differ between both groups (values are median [interquartile range]: saline: 2.2 (2.0–2.5) g · dL⁻¹ vs. clonidine: 2.1 (2.1–2.4) g · dL⁻¹; n.s.). Furthermore, there was no difference in exchangeable blood volume (EBV) between both groups (saline: 88 (76–106) mL · kg⁻¹ vs. clonidine: 92 (85–95) mL · kg⁻¹; n.s.).

Conclusion

Anaemia tolerance was not affected by clonidine induced sympathicolysis. Consequently, perioperative clonidine administration probably has not to be omitted in view of acute anaemia.

Massive Transfusion in the Intensive Care Unit

The transfusion of blood components is a frequent oc currence in intensive care units. Transfusion therapy demands not only a greater ability to meet the needs of acutely ill patients with an enlarging variety of blood components, but also an increasing awareness of previ ously recognized and newly discovered hazards of blood transfusion. Use of the more commonly adminis tered blood components is reviewed, and problems per tinent to the large-volume massively or multiply trans fused patient are discussed.

Avoid Excessive Oxygen Levels in Experiments with Organisms, Tissues and Cells

O2 levels encountered in vivo in cells and tissues are almost always at least an order of magnitude less than atmospheric pO2 because of sensing, signalling and bioenergetic demand. Although deleterious reactions are minimized by protective mechanisms (residual toxic products scavenged and detoxified) ambient levels should be mimicked in experiments with whole organisms, their isolated organs, tissues or cells and also with cultures of cell lines. These are also important issues for microorganisms inhabiting low O2 niches within higher organisms and their cells. Here, we highlight the importance of optimization of micro-aerobic conditions for experimentation and the deleterious consequences of not doing so.

Replacing the Transfusion of 1-2 Units of Blood with Plasma Expanders that Increase Oxygen Delivery Capacity: Evidence from Experimental Studies

At least a third of the blood supply in the world is used to transfuse 1–2 units of packed red blood cells for each intervention and most clinical trials of blood substitutes have been carried out at this level of oxygen carrying capacity (OCC) restoration. However, the increase of oxygenation achieved is marginal or none at all for molecular hemoglobin (Hb) products, due to their lingering vasoactivity. This has provided the impetus for the development of “oxygen therapeutics” using Hb-based molecules that have high oxygen affinity and target delivery of oxygen to anoxic areas. However it is still unclear how these oxygen carriers counteract or mitigate the functional effects of anemia due to obstruction, vasoconstriction and under-perfusion. Indeed, they are administered as a low dosage/low volume therapeutic Hb (subsequently further diluted in the circulatory pool) and hence induce extremely small OCC changes. Hyperviscous plasma expanders provide an alternative to oxygen therapeutics by increasing the oxygen delivery capacity (ODC); in anemia they induce supra-perfusion and increase tissue perfusion (flow) by as much as 50%. Polyethylene glycol conjugate albumin (PEG-Alb) accomplishes this by enhancing the shear thinning behavior of diluted blood, which increases microvascular endothelial shear stress, causes vasodilation and lowering peripheral vascular resistance thus facilitating cardiac function. Induction of supra-perfusion takes advantage of the fact that ODC is the product of OCC and blood flow and hence can be maintained by increasing either or both. Animal studies suggest that this approach may save a considerable fraction of the blood supply. It has an additional benefit of enhancing tissue clearance of toxic metabolites.

Blood transfusion in patients having caesarean section: a prospective multicentre observational study of practice in three Pakistan hospitals

BACKGROUND

Increasing awareness of the risks of blood transfusion has prompted examination of red cell transfusion practice in obstetrics. A six-month prospective observational study was performed to examine blood transfusion practices in patients undergoing caesarean delivery at three hospitals in Pakistan.

METHODS

In the three hospitals (two private, one public) 3438 caesarean deliveries were performed in the study period. Data were collected on patient demographics, indications for transfusion, ordering physicians, consent, associations with obstetric factors, estimated allowable blood loss, calculated blood loss, pre- and post-transfusion haemoglobin and discharge haemoglobin.

RESULTS

A total number of 397 (11.5%) patients who underwent caesarean section received a blood transfusion. The highest transfusion rate of 16% was recorded in the public tertiary care hospital compared to 5% in the two private hospitals. Emergency caesarean delivery and multiparity were associated with blood transfusion (P<0.05). More emergency caesarean sections were performed in the public compared to the private hospitals (85.4% vs. 41.6%). More multiparous patients underwent caesarean section in the public hospital (57.8% vs. 40.4%). Attending physicians took the decision for transfusion in 98% of cases. In 343 (86%) patients, blood transfusion was given even when the haemoglobin was >7g/dL. The method for documenting the indication or consent for transfusion was not found in any of the three hospitals.

CONCLUSION

Blood transfusion was prescribed more readily in the public hospital. Identification of a transfusion trigger and the development of institutional guidelines to reduce unnecessary transfusion are required.

Responses to hypoxia in fungal pathogens

Hypoxia is typical for most battlefields of host-pathogen interactions in the human host. While adaptation of human cells to low levels of oxygen has been well established, little information exists on mechanisms of hypoxic adaptation in microbial pathogens. Importantly, the impact of hypoxia on microbial infection, virulence and pathogenesis is rarely investigated. Recent results on the human pathogens Candida albicans and Cryptococcus neoformans indicate that these fungi adapt to hypoxia specifically by altering several morphological phenotypes, metabolic and transcriptomal activities, as well as virulence traits. In this review, novel components and mechanisms involved in hypoxic adaptation of human fungals pathogens are summarized and discussed.

Regional blood flow during hyperoxic haemodilution

BACKGROUND

Ventilation with pure oxygen (hyperoxic ventilation, HV) increases arterial oxygen content (CaO(2)). However HV induces arteriolar constriction and thus potentially affects O(2) supply. We therefore investigated the effects of HV on regional blood flow (RBF) and O(2) supply of different vital organs during moderate normovolaemic anaemia.

METHODS

Twenty-two anaesthetized dogs were haemodiluted under normoxia (i.e. FiO(2) = 0.21) to a target haemoglobin concentration (Hb) of 7 g dl(-1) and were subsequently ventilated with pure O(2). RBF was determined by use of the radioactive microspheres method in the myocardium, kidney, skeletal muscle, liver, intestine, stomach, and pancreas at Hb = 7 g dl(-1) and after subsequent initiation of HV. RBF in proportion to cardiac output (RBF(relative)), the variation coefficient of RBF (VC) and regional O(2) supply (rDO(2)) were calculated.

RESULTS

Initiation of HV at Hb = 7.0 +/- 0.3 g dl(-1) reduced cardiac index (-17%) as well as RBF within the myocardium (-21%), pancreas (-25%), and skeletal muscle (-25%), whereas renal, hepatic, and intestinal RBF remained unchanged. Consequently RBF(relative) of the latter organs increased. Heterogeneity of RBF was marginally affected by HV.

CONCLUSION

The initiation of HV during moderate normovolaemic anaemia (Hb =7 g dl(-1)) was accompanied by RBF redistribution with preference for renal, hepatic and intestinal O(2) supply. Cardiac, pancreatic and muscular O(2) supply decreased, however without any critical restriction of organ function.

Hyperoxia in lethal methemoglobinemia: effects on oxygen transport, tissue oxygenation, and survival in pigs

BACKGROUND

Treatment of severe methemoglobinemia includes the avoidance of methemoglobin-inducing drugs, the application of methylene blue, and the administration of supplementary oxygen. However, the efficacy of the latter on oxygen transport, tissue oxygenation, and survival in the treatment of extreme methemoglobinemia is ambiguous. The objective was to assess whether using hyperoxic ventilation as the sole therapeutic intervention (i.e., ventilation with pure oxygen, Fio2 1.0) improves the short-term (6-hr) survival rate during otherwise lethal methemoglobinemia.

DESIGN

Prospective, randomized, controlled study.

SETTING

Experimental animal laboratory of a university hospital.

SUBJECTS

Fourteen anesthetized, mechanically ventilated pigs.

INTERVENTIONS

After induction of profound methemoglobinemia (60 +/- 2%) by the injection of 15 mg/kg 4-dimethylaminophenol, artificial ventilation either was continued with room air (G 0.21, n = 7) or was changed over to hyperoxic ventilation (G 1.0, n = 7). A constant level of methemoglobinemia was maintained by continuous infusion of 4-dimethylaminophenol throughout a 6-hr follow-up period.

MEASUREMENTS AND MAIN RESULTS

All animals died within the 6-hr follow-up period, but survival time was prolonged in animals ventilated with pure oxygen (G 0.21, 105 +/- 30 mins; G 1.0, 210 +/- 64 mins, p < .05). No differences were encountered between G 0.21 and G 1.0 with respect to the investigated variables of macrohemodynamics, oxygen transport, and tissue oxygenation.

CONCLUSIONS

Hyperoxic ventilation has negligible effects on oxygen transport and tissue oxygenation during lethal methemoglobinemia; nevertheless, survival was increased without severe adverse reactions provoked by hyperoxic ventilation.

Alginate plasma expander maintains perfusion and plasma viscosity during extreme hemodilution

Extreme hemodilution was performed in the hamster chamber window model using 6% Dextran 70, lowering systemic hematocrit by 60%. Animals were subsequently divided into three groups and hemodiluted to a hematocrit of 11% using 6% Dextran 70, 6% Dextran 500, and a 4% Dextran 70 + 0.7% alginate solution ( n = 6 each group). Final plasma viscosities were 1.4 ± 0.2, 2.2 ± 0.1, and 2.7 ± 0.2 cp, respectively, ( P < 0.05, high viscosity vs. low viscosity). Blood viscosities were 2.1 ± 0.2, 2.9 ± 0.4, and 3.9 ± 0.3 cp, respectively. The lowest blood and plasma viscosity group had a significantly lower functional capillary density, 37 ± 16%, whereas the two high-viscosity solutions were 71 ± 15% and 76 ± 12% ( P < 0.05, high viscosity vs. low viscosity), respectively. Arteriolar and venular flow in the Dextran 500 and alginate groups was higher than baseline (i.e., normal nontreated animals), whereas the low-viscosity group showed a reduction in flow. These microvascular changes were paralleled by changes in base excess, which was negative for the Dextran 70 group and positive for the other groups. However, tissue Po2 was uniformly low for all groups (average of 1.4 mmHg). Calculation of tissue oxygen consumption in the window chamber based on the microvascular data, flow, and intravascular Po2 showed that only the alginate + Dextran 70 solution-exchanged animals returned to baseline oxygen consumption, whereas the other groups were lower than baseline ( P < 0.05). These results show that hemodilution performed with high-viscosity plasma expanders yields systemic arterial pressures and functional capillary densities that are significantly higher ( P < 0.05) than those obtained with 6% Dextran 70, a fluid whose viscosity is similar to that of plasma. A condition for obtaining these results is that the oncotic pressure of the plasma expander be titrated to near normal, so that autotransfusion of fluid from the tissue into the vascular compartment does not reduce the effects of increasing plasma viscosity and increased shear stress on the microvascular wall.

Microvascular pressure and functional capillary density in extreme hemodilution with low- and high-viscosity dextran and a low-viscosity Hb-based O2 carrier

Blood losses are usually corrected initially by the restitution of volume with plasma expanders and subsequently by the restoration of oxygen-carrying capacity using either a blood transfusion or possibly, in the near future, oxygen-carrying plasma expanders. The present study was carried out to test the hypothesis that high-plasma viscosity hemodilution maintains perfused functional capillary density (FCD) by preserving capillary pressure. Microvascular pressure responses to extreme hemodilution with low- (LV) and high-viscosity (HV) plasma expanders and an exchange transfusion with a polymerized bovine cell-free Hb (PBH) solution were analyzed in the awake hamster window chamber model ( n = 26). Systemic hematocrit was reduced from 50% to 11%. PBH produced a greater mean arterial blood pressure than the nonoxygen carriers. FCD was higher after a HV plasma expander (70 ± 15%) vs. PBH (47 ± 12%). Microvascular pressure spanning the capillary network was higher after a HV plasma expander (16–19 mmHg) compared with PBH (12–16 mmHg) and a LV plasma expander (11–14 mmHg) but lower than control (22–26 mmHg). FCD was found to be directly proportional to capillary pressure. The use of a HV plasma expander in extreme hemodilution maintained the number of perfused capillaries and tissue perfusion by comparison with a LV plasma expander due to increased mean arterial blood pressure and capillary pressure. The use of PBH increased mean arterial pressure but reduced capillary pressure due to vasoconstriction and did not maintain FCD.

Mortality and morbidity in patients with very low postoperative Hb levels who decline blood transfusion

BACKGROUND

Guidelines for allogeneic transfusion emphasize minimizing use to avoid transmission of serious illness. However, there is little information on the risks associated from withholding transfusion.

STUDY DESIGN AND METHODS

A retrospective cohort study of patients who declined RBC transfusions for religious reasons was performed. This analysis was restricted to consecutive patients > or = 18 years old, who underwent surgery in the operating room from 1981 to 1994 and had a postoperative Hb count of 8 g per dL or less. The primary outcome was defined as any inhospital death occurring within 30 days of the surgery. Secondary outcome was 30-day mortality or in-hospital 30-day morbidity. Morbidity was defined as myocardial infarction, arrhythmia, congestive heart failure, or infection.

RESULTS

Of 2083 eligible patients, 300 had postoperative Hb counts of 8 g per dL or less. The study population was predominantly female (70.3%) with a mean age of 57 years (SD, +/- 17.7). In patients with a postoperative Hb level of 7.1 to 8.0, 0 died (upper 95% CI, 3.7%), and 9.4 percent (95% CI, 4.4-17.0%) had a morbid event. In patients with a postoperative Hb level of 4.1 to 5.0, 34.4 percent (95% CI, 18.6-53.2%) died and 57.7 percent (95% CI, 36.9-76.6%) had a morbid event or died. After adjusting for age, cardiovascular disease, and Acute Physiology and Chronic Health Evaluation II score, the odds of death in patients with a postoperative Hb level of < or = 8 g per dL increased 2.5 times (95% CI, 1.9-3.2) for each gram decrease in Hb level.

CONCLUSIONS

The risk of death was low in patients with postoperative Hb levels of 7.1 to 8.0 g per dL, although morbidity occurred in 9.4 percent. As postoperative blood counts fall the risk of mortality and/or morbidity rises and becomes extremely high below 5 to 6 g per dL.

Preservation of intestinal microvascular Po2 during normovolemic hemodilution in a rat model

The effect of hemodilution on the intestinal microcirculatory oxygenation is not clear. The aim of this study was to determine the effect of moderate normovolemic hemodilution on intestinal microvascular partial oxygen pressure (Po2) and its relation to the mesenteric venous Po2 (Pmvo2). Normovolemic hemodilution was performed in 13 anesthetized male Wistar rats. Systemic hemodynamic and intestinal oxygenation parameters were monitored. Intestinal microvascular Po2 was measured by using the oxygen-dependent quenching of palladium-porphyrin phosphorescence. Hemodilution decreased systemic hematocrit from 45.0% +/- 0.1% (average +/- SEM) to 24.6% +/- 1.6%. The mesenteric blood flow did not change from baseline values, resulting in a linear decrease in intestinal oxygen delivery (from 2.77 +/- 0.15 to 1.42 +/- 0.11 mLxkg(-1)xmin(-1)). The intestinal oxygen extraction ratio increased significantly from 24% +/- 1% to 42% +/- 4%. Pmvo2 decreased significantly (from 57 +/- 2 to 41 +/- 2 mm Hg), but intestinal oxygen consumption and microvascular Po2 remained unaffected. As a result, the difference between microvascular Po2 and Pmvo2 increased significantly during hemodilution. Intestinal microvascular Po2 and oxygen consumption were well preserved during moderate normovolemic hemodilution. These results might be explained by the notion of others that hemodilution induces recruitment of capillaries, resulting in redistribution of the intestinal blood flow in favor of the microcirculation, which allows a more efficient extraction of oxygen. These findings further indicate that the use of venous Po2 values as indicators of microvascular oxygenation may be misleading.

Hematologic management of gastrointestinal bleeding

The hematologic management of gastrointestinal (GI) bleeding requires evaluation of the underlying cause of bleeding, associated diseases that can exacerbate the bleeding, and identification of related and unrelated coagulation abnormalities. Erythrocyte transfusions are given to increase oxygen carrying capacity; however, there is limited information on the level of anemia that places a patient at increased risk of adverse events after a GI bleed and when patients should receive erythrocyte transfusion. Isolated thrombocytopenia is uncommon in patients with GI bleeding, and there is little evidence documenting the degree of thrombocytopenia associated with an increased risk of bleeding. Platelets are often administered when the count is 50,000 per cu/mL in a bleeding patient. The coagulopathy of liver disease is the most common abnormality seen in the setting of GI bleeding. Fresh-frozen plasma (FFP) should be given in a dose equivalent to the underlying abnormality and the common practice of administering 2 units of FFP is often insufficient in a bleeding patient.

Effect of acute normovolemic hemodilution on distribution of blood flow and tissue oxygenation in dog skeletal muscle

Acute normovolemic hemodilution (ANH) is efficient in reducing allogenic blood transfusion needs during elective surgery. Tissue oxygenation is maintained by increased cardiac output and oxygen extraction and, presumably, a more homogeneous tissue perfusion. The aim of this study was to investigate blood flow distribution and oxygenation of skeletal muscle. ANH from hematocrit of 36 +/- 3 to 20 +/- 1% was performed in 22 splenectomized, anesthetized beagles (17 analyzed) ventilated with room air. Normovolemia was confirmed by measurement of blood volume. Distribution of perfusion within skeletal muscle was determined by using radioactive microspheres. Tissue oxygen partial pressure was assessed with a polarographic platinum surface electrode. Cardiac index (3.69 +/- 0.79 vs. 4.79 +/- 0.73 l. min-1. m-2) and muscle perfusion (4.07 +/- 0.44 vs. 5.18 +/- 0.36 ml. 100 g-1. min-1) were increased at hematocrit of 20%. Oxygen delivery to skeletal muscle was reduced to 74% of baseline values (0.64 +/- 0.06 vs. 0.48 +/- 0.03 ml O2. 100 g-1. min-1). Nevertheless, tissue PO2 was preserved (27.4 +/- 1.3 vs. 29.9 +/- 1. 4 Torr). Heterogeneity of muscle perfusion (relative dispersion) was reduced after ANH (20.0 +/- 2.2 vs. 13.9 +/- 1.5%). We conclude that a more homogeneous distribution of perfusion is one mechanism for the preservation of tissue oxygenation after moderate ANH, despite reduced oxygen delivery.

Variation in red cell transfusion practice in the intensive care unit: a multicentre cohort study

OBJECTIVES: To determine the degree of interinstitutional transfusion practice variation and reasons why red cells are administered in critically ill patients. STUDY DESIGN: Multicentre cohort study combined with a cross-sectional survey of physicians requesting red cell transfusions for patients in the cohort. STUDY POPULATION: The cohort included 5298 consecutive patients admitted to six tertiary level intensive care units in addition to administering a survey to 223 physicians requesting red cell transfusions in these units. MEASUREMENTS: Haemoglobin concentrations were collected, along with the number and reasons for red cell transfusions plus demographic, diagnostic, disease severity (APACHE II score), intensive care unit (ICU) mortality and lengths of stay in the ICU. RESULTS: Twenty five per cent of the critically ill patients in the cohort study received red cell transfusions. The overall number of transfusions per patient-day in the ICU averaged 0.95 +/- 1.39 and ranged from 0.82 +/- 1.69 to 1.08 +/- 1.27 between institutions (P < 0.001). Independent predictors of transfusion thresholds (pre-transfusion haemoglobin concentrations) included patient age, admission APACHE II score and the institution (P < 0.0001). A very significant institution effect (P < 0.0001) persisted even after multivariate adjustments for age, APACHE II score and within four diagnostic categories (cardiovascular disease, respiratory failure, major surgery and trauma) (P < 0.0001). The evaluation of transfusion practice using the bedside survey documented that 35% (202 of 576) of pre-transfusion haemoglobin concentrations were in the range of 95-105 g/l and 80% of the orders were for two packed cell units. The most frequent reasons for administering red cells were acute bleeding (35%) and the augmentation of O2 delivery (25%). CONCLUSIONS: There is significant institutional variation in critical care transfusion practice, many intensivists adhering to a 100g/l threshold, and opting to administer multiple units despite published guidelines to the contrary. There is a need for prospective studies to define optimal practice in the critically ill.

A Canadian survey of transfusion practices in critically ill patients. Transfusion Requirements in Critical Care Investigators and the Canadian Critical Care Trials Group

OBJECTIVES

To characterize the contemporary red cell transfusion practice in the critically ill and to define clinical factors that influence these practices.

DESIGN

Scenario-based national survey.

STUDY POPULATION

Canadian critical care practitioners.

MEASUREMENTS AND MAIN RESULTS

We evaluated transfusion thresholds before transfusion and the number of red cell units ordered, under the given conditions. Of 254 Canadian critical care physicians, 193 (76%) responded to the survey. The primary specialty of most respondents was internal medicine (56%). Internal medicine respondents were in practice for an average of 8.4 +/- 5.7 (SD) yrs, and worked most often in combined medical/surgical intensive care units. Baseline hemoglobin transfusion thresholds averaged from 8.3 +/- 1.0 g/dL in a scenario involving a young stable trauma victim to 9.5 +/- 1.0 g/dL for an older patient after gastrointestinal bleeding. Transfusion thresholds differed significantly (p< .0001) between all four separate scenarios. With the exception of congestive heart failure (p> .05), all clinical factors (including age, Acute Physiology and Chronic Health Evaluation II score, preoperative status, hypoxemia, shock, lactic acidosis, coronary ischemia, and chronic anemia) significantly (p< .0001) modified the transfusion thresholds. A statistically significant (p< .01) difference in baseline transfusion thresholds was noted across four major regions (with a maximum of five academic centers per region) of the country. Low physician numbers in two of the regions did not allow for further investigation of regional variations.

CONCLUSIONS

There is significant variation in critical care transfusion practice, with many intensivists adhering to a 10.0-g/dL threshold, while other physicians described a much more restrictive approach to red cell transfusion. Also, many physicians opted to administer multiple units, despite published guidelines to the contrary. Additionally, the administration of red cells was strongly influenced by a number of clinical factors, many unique to intensive care unit patients. There is a need for prospective studies to define optimal practice in the critically ill.

Effects of diaspirin-cross-linked hemoglobin (DCLHb) on local tissue oxygen tension in striated skin muscle: an efficacy study in the hamster

Using the dorsal skin fold chamber model in the hamster, we analyzed local tissue partial oxygen pressure (PO2) in the striated skin muscle under nonischemic and postischemic conditions with a Clark-type multiwire oxygen surface electrode. Hypervolemic infusion (500 mg x kg(-1) I.V.) or isovolemic exchange transfusion (3.3 gm x kg(-1) I.V.; hematocrit 30%) with diaspirin-cross-linked hemoglobin (DCLHb) resulted in a slight decrease of the mean value of the local tissue PO2 (mm Hg) 1 hour after administration. Concomitantly, the frequency distribution curves of local tissue PO2 values were found to be more narrow (fewer values > 25 mm Hg and < 10 mm Hg). Resuscitation from severe hemorrhagic shock (bleeding of 33 ml x kg(-1) at 0.4 ml x min(-1)) with autologous blood (AuB), Dx-60, or DCLHb led to an increase of mean tissue PO2 values by 4.2-fold (p < 0.05 versus Dx-60), 1.9-fold, and 3.7-fold (p < 0.05 versus Dx-60), respectively, 2 hours after resuscitation. The reduction of tissue hypoxia (0-5 mm Hg) was significant only in the AuB- and DCLHb-treated animals. This study indicates that DCLHb effectively reverses tissue hypoxia after resuscitation from severe hemorrhagic shock by inducing a more homogeneous distribution of the local tissue PO2 levels.

Red cell transfusion therapy in the critical care setting

According to our own experience and published reports the frequency of red cell transfusion in intensive care units is in the range of 0.2 to 0.4 units per patient per day and is dependent upon the local strategy, the patients involved and the kind of surgery performed. The rationale for red cell transfusion is to maintain or restore the oxygen carrying capacity of the blood to avoid tissue hypoxia which occurs when oxygen delivery drops below a certain critical value. Besides bleeding, phlebotomy is also a significant source of blood loss in critically ill patients. According to several recent reviews and consensus articles there is no basis for a fixed indicator for transfusion, such as a haemoglobin concentration of < 100 gL-1. The decision to transfuse has to be made according to the patients individual status. The major adaptive mechanism in response to acute anaemia is an increase in cardiac output and hence blood flow to tissues. As a consequence even moderate degrees of acute anaemia may not be tolerated by patients with cardiac disease, whilst marked anaemia carries a considerable risk of ischaemia in patients with brain lesions or cerebral arterial stenoses. In critically ill patients it has been postulated that supply dependency of oxygen consumption occurs over a wide range of oxygen delivery, far above the critical values of oxygen delivery seen under normal conditions. Maximising oxygen delivery was therefore formulated as a goal in these patients. However, whether pathological supply dependency of oxygen delivery really exists in critically ill patients is still under discussion and recent studies found no benefit in maximising oxygen delivery to this patient group. However, individualised triggers for red blood cell transfusion are adequate for critically ill patients considering their co-morbidities and severity of disease. Finally, the decision to transfuse must also take into account the potential risks (infectious and non-infectious), as well as benefits for the individual patient. In the future, the level of transfusions may be reduced by using blood sparing techniques such as blood withdrawal in closed systems, bedside microchemistry, intravascular monitors, or autotransfusion of drainage blood in intensive care units.

The physiology of oxygen transport

Adequate organ function requires adequate provision of cells with oxygen (O2). The driving force for O2-diffusion from ambient air to its site of consumption in cell mitochondria is the oxygen partial pressure (pO2) gradient along this pathway. After uptake in the lungs, O2 transport in blood is achieved (1) through binding to haemoglobin and (2) through physical dissolution in plasma. While the sum of O2 in these two transport states defines total oxygen content of blood, the delivery of O2 to different organs is determined by cardiac output and arterial O2 content, being the product of both parameters. In the case of anaemia, intravascular volume and cardiac compensatory mechanisms determine the degree of O2 content reduction allowable prior tissue hypoxia and lactacidosis occur. When intravascular volume is preserved (e.g. normovolemic dilutional anaemia), reductions in O2 content are tolerated to a much higher degree than in hypovolemic anaemia (e.g. haemorrhagic shock).

Dopexamine improves liver oxygenation during crystalloid resuscitation from experimental hemorrhagic shock

OBJECTIVE

To evaluate the effects of dopexamine administration on hemodynamic variables and tissue oxygen tensions during crystalloid resuscitation from hemorrhagic shock.

DESIGN

Randomized, control trial.

SETTING

An animal laboratory at a university center.

SUBJECTS

Twelve piglets, mean weight 22 kg.

INTERVENTIONS

The animals were anesthetized and bled to a state of hemorrhagic shock and resuscitated, using a crystalloid solution infused at a rate of approximately 2.6 mL/min/kg (total amount 208 mL/kg). Cardiac output and mean arterial pressure (MAP were measured as indicators of volume filling during the 20- to 30-min resuscitation period and during the follow-up period until 80 mins from the start of resuscitation. Dopexamine was administered by infusion at 6 micrograms/kg-min from the start of volume replacement (dopexamine group, n = 6). The rest of the animals (control group, n = 6) were given volume replacement only.

MEASUREMENTS AND MAIN RESULTS

Systemic oxygen transport variables were calculated. Tissue oxygen tensions were continuously recorded from the liver, conjunctival layer, and via subcutaneous and transcutaneous electrodes in the abdominal region. MAP decreased from 119 +/- 2 (SEM) to 44 +/- 2 mm Hg and cardiac output decreased by 77% during the shock period. During resuscitation, cardiac output was restored in both groups. MAP increased close to the baseline during the early resuscitation period and decreased slowly during follow-up. Oxygen delivery remained at 46% of baseline, whereas systemic oxygen consumption was restored during resuscitation in both groups. Liver tissue oxygen tension increased well above baseline during resuscitation in the dopexamine group, and liver tissue oxygen tension was significantly higher than in the control group. After 60 mins of resuscitation, the liver oxygen tension decreased to control group values. None of the other tissue oxygen tensions showed any differences between groups.

CONCLUSIONS

Dopexamine administration during crystalloid resuscitation from hemorrhagic shock was well tolerated and resulted in significant and specific, although transient, improvement in liver oxygenation.

The effect of acute normovolemic hemodilution (ANH) on myocardial contractility in anesthetized dogs

The influence of severe acute normovolemic hemodilution (ANH) on myocardial contractility (MC) was investigated in 14 splenectomized, anesthetized dogs. MC was assessed by the maximum rate of left ventricular pressure increase (LVdp/dt(max)), end-systolic elastance (Ees), and preload recruitable stroke work (PRSW) (conductance catheter, left ventricular pressure-volume relationship). Measurements of myocardial perfusion and oxygenation (radioactive microsphere technique) assured comparability of the model to previously performed studies. Global and regional myocardial blood flow increased significantly upon hemodilution with preference to midmyocardium and subendocardium. This resulted in preservation of both myocardial oxygen delivery and consumption after ANH. Myocardial oxygen extraction as well as coronary venous Po2 were unaffected by ANH, while coronary venous lactate concentration decreased, indicating that myocardial oxygen need was met. LVdp/dt(max) decreased significantly after hemodilution (2278 +/- 577 vs 1884 +/- 381 mm Hg/s, P < 0.01), whereas Ees and PRSW increased significantly (1.76 +/- 0.54 vs 2.15 +/- 0.75 mm Hg/mL, P < 0.05, for Ees and 33 +/- 14 vs 45 +/- 14 mm Hg.mL, P < 0.05, for PRSW). While the decrease of LVdp/dt(max) most likely reflects ANH-induced changes of ventricular pre- and afterload, the increase of Ees and PRSW indicates a true increase of myocardial contractility during ANH in anesthetized dogs.

A cost comparison of allogeneic and preoperatively or intraoperatively donated autologous blood

We determined the cost of allogeneic packed red blood cells and autologous whole blood donated either preoperatively or in the operating room during hemodilution. Direct and indirect cost estimates were based on patients requiring simple transfusion and included procurement and preparation of the blood including testing performed, materials and time used, waste, and materials for administration. Data were derived from prospective blood bank time studies, material invoice records, and retrospective review of anesthesia times. Viral infection and transfusion reaction costs were accepted from previously published sources. Direct cost of purchasing and indirect costs of preparation resulted in an overall cost of $107.26 for the first unit of allogeneic packed red blood cells transfused. A second unit was slightly less costly ($100.89), as no type and screen was required and the same delivery set and filter can be used. The total cost of acquisition, processing, and transfusion of 1 U of preoperatively donated autologous blood was $97.83. The total cost of a 2-U transfusion of autologous whole blood donated in the operating room during acute normovolemic hemodilution was $83.10. These data suggest that autologous predonation of whole blood is somewhat less expensive than allogeneic packed red blood cells, and that hemodilution may be a cost effective alternative to autologous predonation in selected patients.

Pulmonary gas exchange capacity is reduced during normovolaemic haemodilution in healthy human subjects

PURPOSE

To test the hypothesis that a physiological compensatory mechanism maintains respiratory gas exchange during normovolaemic haemodilution.

METHODS

Pulmonary gas exchange capacity was evaluated in seven healthy subjects by measuring the lung diffusion of carbon monoxide (DLCO). During the measurement, various breath-holding times, inspiratory volumes, and sitting or supine positions, were randomly selected in an attempt to alter pulmonary capillary perfusion. KCO was calculated as the percentage of theoretical values of the ratio of DLCO by alveolar volume and normalized by sex, age, and height. Normovolaemic haemodilution (NH) was performed by bleeding an average blood volume of 1 L with simultaneous Dextran 60 replacement to obtain an haematocrit below 35%.

RESULTS

After NH, haemoblogin concentration [Hb] decreased from 14.94 +/- 0.96 to 12.5 +/- 0.98 g.dl-1 (P < 0.001). KCO decreased (P < 0.02) but remained closely correlated to [Hb] at every lung volume (P < 0.02). Breathholding time and body position had no effect.

CONCLUSION

Moderate NH impairs pulmonary gas exchange capacity in awake, resting healthy subjects. There is no evidence of any compensatory mechanism since the KCO vs [Hb] relationship is unchanged.

A cost comparison of allogeneic and preoperatively or intraoperatively donated autologous blood

We determined the cost of allogeneic packed red blood cells and autologous whole blood donated either preoperatively or in the operating room during hemodilution. Direct and indirect cost estimates were based on patients requiring simple transfusion and included procurement and preparation of the blood including testing performed, materials and time used, waste, and materials for administration. Data were derived from prospective blood bank time studies, material invoice records, and retrospective review of anesthesia times. Viral infection and transfusion reaction costs were accepted from previously published sources. Direct cost of purchasing and indirect costs of preparation resulted in an overall cost of $107.26 for the first unit of allogeneic packed red blood cells transfused. A second unit was slightly less costly ($100.89), as no type and screen was required and the same delivery set and filter can be used. The total cost of acquisition, processing, and transfusion of 1 U of preoperatively donated autologous blood was $97.83. The total cost of a 2-U transfusion of autologous whole blood donated in the operating room during acute normovolemic hemodilution was $83.10. These data suggest that autologous predonation of whole blood is somewhat less expensive than allogeneic packed red blood cells, and that hemodilution may be a cost effective alternative to autologous predonation in selected patients.

Chronic nonpulsatile blood flow. III. Effects of pump flow rate on oxygen transport and utilization in chronic nonpulsatile biventricular bypass

The relationship between blood flow and oxygen transport was studied in five calves with chronic nonpulsatile biventricular bypass. Seven days was allowed for recovery from the effects of anesthesia and operation; the natural heart was then fibrillated. Pump flows were maintained at nominal rates of 90, 100, or 120 ml.kg-1.min for 1 week each, with the sequence varied from experiment to experiment. Venous and arterial blood samples were taken at rest for blood gas analysis. Serum lactate analysis was done twice a week, on the third and seventh days after each pump flow change. Serum catecholamine levels were assayed on the seventh day of each flow rate. Progressive exercise tests were also conducted during each test segment. Basal oxygen consumption of a 4-month-old calf was 6.3 +/- 0.3 ml.kg-1.min-1. The mixed venous oxygen tension decreased when pump flow rate was reduced (29.6 +/- 1.0, 28.3 +/- 1.2, and 23.8 +/- 0.9 mm Hg at 120, 100, and 90 ml.kg-1.min-1 of pump flow, respectively), and oxygen extraction increased linearly when pump flow rate was reduced. Hemoglobin concentration significantly affected oxygen extraction rate. Serum lactate concentration increased significantly at a 90 ml.kg-1.min-1 perfusion compared with concentrations at other pump flow rates (7.81 +/- 2.42 mEq/L at 90 ml.kg-1.min-1 vs 0.71 +/- 0.19 and 0.73 +/- 0.81 mEq/L at 100 and 120 ml.kg-1.min-1, respectively; p < 0.01, analysis of variance, Scheffe F test). Maximum oxygen extraction during exercise was 78%. These results suggest that a critical flow level between 90 and 100 ml.kg-1.min-1 maintains oxidative metabolism in the calf with chronic nonpulsatile flow. The resulting oxygen delivery was slightly higher than that indicated in the literature. Maximal oxygen extraction was normal.

[Indications and role of albumin for vascular loading in the operating room]

Prospective clinical studies on albumin and non human colloids, administered peroperatively, were analysed. Only those with a level of evidence I or II were considered. On the basis of the cost-effectiveness ratio, albumin should not be the first choice treatment for peroperative plasma volume expansion. Low molecular weight hydroxyethylstarch is as efficient for restoration and maintenance of volaemia and colloid osmotic pressure. Albumin is administered when other colloids are contra-indicated or when their upper limit of volume has been reached.

Morbidity risk assessment in the surgically anemic patient

Clinicians have few data on which to base a decision to transfuse a surgical patient. We reviewed animal and human data to evaluate the effects that anemia and comorbidity have on surgical outcome. Experimental evidence consistently demonstrates increased cardiac output, decreased peripheral vascular resistance, and increased release of oxygen by red blood cells in response to anemia. Normal animals tolerate hemoglobin (Hb) levels down to approximately 5 g/dL. Below this level, cardiac ischemia and decreased ventricular function develop. In animals with experimental coronary artery stenosis, cardiac ischemia develops at Hb levels of 7-10 g/dL. Coexisting left ventricular hypertrophy, use of beta blockers, and hypoxemia also reduce animals' ability to tolerate anemia. The limited information on anemia tolerance of human surgical patients suggests that the presence of cardiac and pulmonary disease should influence transfusion decisions. A higher Hb threshold should be used in patients who have or are at risk of cardiac or pulmonary artery disease.

Tissue oxygenation with graded dissolved oxygen delivery during cardiopulmonary bypass

BACKGROUND

Intravascular perfluorochemical emulsions together with a high oxygen tension may increase the delivery of dissolved oxygen to useful levels. The hypothesis of this study is that increasing the dissolved oxygen content of blood with incremental doses of a perfluorochemical emulsion improves tissue oxygenation during cardiopulmonary bypass in a dose-related fashion.

METHODS AND RESULTS

Oxygen utilization was studied in a profoundly anemic canine model of hypothermic cardiopulmonary bypass. Forty-two dogs underwent normovolemic hemodilution to a hematocrit of 15.8% +/- 0.6% (mean +/- standard error of the mean). Cardiopulmonary bypass was begun and resulted in a hematocrit of 9.4% +/- 0.6%. A standard priming solution was used in the control group (n = 12), and the test groups received 1.35 gm perfluorochemical.kg-1 (n = 10 dogs), 2.7 gm perfluorochemical.kg-1 (n = 10 dogs), or 5.4 gm perfluorochemical.kg-1 (n = 10 dogs) through the venous return cannula. Each animal underwent a series of randomized pump flows (0.25, 0.5, 1.0, 1.5, 2.0, and 3.0 L.min-1.m-2) at 32 degrees C. After the randomized flows were completed at 32 degrees C, the temperature was raised to 38 degrees C and cardiopulmonary bypass was discontinued. Mortality from cardiac failure on separation from cardiopulmonary bypass was 42% in the control group and 20% in perfluorochemical-treated groups. The mean perfluorochemical dose was higher in survivors than in nonsurvivors (2.9 +/- 0.4 versus 1.3 +/- 0.5 gm perfluorochemical.kg-1; p < 0.05). No differences in oxygen consumption or transbody lactate gradient were found between groups during cardiopulmonary bypass. Analysis of mixed venous oxygen tension (a surrogate measure for tissue oxygenation) as a function of cardiopulmonary bypass flow normalized to body surface area showed that the control group had significantly lower mixed venous oxygen tension (p < 0.05) than the perfluorochemical emulsion-treated groups. Furthermore, the differences were related to the perfluorochemical emulsion dose. These differences in mixed venous oxygen tension continued after termination of cardiopulmonary bypass. The coronary sinus oxygen tension and cardiac arterial-venous oxygen content differences during and after cardiopulmonary bypass were similar among the control and perfluorochemical emulsion-treated animals. Dissolved oxygen consumption during and after cardiopulmonary bypass was calculated. Dissolved oxygen consumption increased in the perfluorochemical-treated animals in a perfluorochemical dose-related manner and was significantly higher in perfluorochemical-treated animals than in the control animals (p < 0.05).

CONCLUSIONS

Graded increases in mixed venous oxygen tension during cardiopulmonary bypass were observed in response to graded increases in the dissolved oxygen delivery. These data suggest that enhancing oxygenation with perfluorochemical-dissolved oxygen is an effective temporary substitute for the use of hemoglobin-bound oxygen during cardiopulmonary bypass. Perfluorochemical-dissolved oxygen may be particularly beneficial in the setting of multiple hypoxic stresses.

Is a hemoglobin of 10 g/dL required for surgery?

The "10/30" (hemoglobin/hematocrit) rule has long been recognized and accepted in the medical community as the threshold for transfusion in the perioperative setting. However, an increasing number of publications suggest there is no absolute threshold for transfusion, and that this decision should be based on an assessment of the overall clinical picture presented by the patient. This article reviews the risks associated with blood transfusions, and the data in humans and animals that describe the benefits of transfusion. Recommendations on the trigger for red cell transfusion are provided.

Effect of blood pressure on hemorrhage volume and survival in a near-fatal hemorrhage model incorporating a vascular injury

STUDY HYPOTHESIS

In a model of near-fatal hemorrhage that incorporates a vascular injury, stepwise increases in blood pressure associated with aggressive crystalloid resuscitation will result in increased hemorrhage volume and mortality.

DESIGN

This study used a swine model of potentially lethal hemorrhage in the presence of a vascular lesion to compare the effects of resuscitation with mean arterial pressures of 40, 60, and 80 mm Hg. Twenty-seven fully instrumented immature swine (14.8 to 20 kg), each with a surgical-steel aortotomy wire in place, were bled continuously from a femoral artery catheter to a mean arterial pressure of 30 mm Hg. At that point the aortotomy wire was pulled, producing a 4-mm aortic tear and uncontrolled intraperitoneal hemorrhage. When the animal's pulse pressure reached 5 mm Hg, the femoral artery hemorrhage was discontinued and resuscitation was begun.

INTERVENTIONS

Saline infusion was begun at 6 mL/kg/min and continued as needed to maintain the following desired endpoints: group 1 (nine) to a mean arterial pressure of 40 mm Hg, group 2 (nine) to a mean arterial pressure of 60 mm Hg, and group 3 (nine) to a mean arterial pressure of 80 mm Hg. After 30 minutes or a total saline infusion of 90 mL/kg, the resuscitation fluid was changed to shed blood infused at 2 mL/kg/min as needed to maintain the desired mean arterial pressure or to a maximum volume of 24 mL/kg. Animals were observed for 60 minutes or until death.

MEASUREMENTS AND MAIN RESULTS

Data were compared using repeated-measures analysis of variance with a post hoc Tukey-Kramer, Fisher's exact test, and Kruskal-Wallis. Mortality was significantly greater in group 3 (78%) compared with either group 1 (11%; P = .008) or group 2 (22%; P = .028). Mean survival times were significantly shorter in group 3 (44 +/- 12 minutes) compared with either group 1 (58 +/- 6 minutes; P = .007) or group 2 (59 +/- 3 minutes; P = .006). The average intraperitoneal hemorrhage volumes were 13 +/- 14 mL/kg, 20 +/- 25 mL/kg, and 46 +/- 11 mL/kg for groups 1, 2, and 3, respectively (group 1 versus 2, P = .425; group 1 versus 3, P < .001; group 2 versus 3, P = .014). Group 2 animals demonstrated significantly greater oxygen deliveries compared with groups 1 and 3.

CONCLUSION

In a model of near-fatal hemorrhage with a vascular injury, attempts to restore blood pressure with crystalloid result in increased hemorrhage volume and markedly higher mortality.