Critical haemoglobin concentration in anaesthetized dogs: comparison of two plasma substitutes

Blood acid-base, haematological and haemostatic effects of hydroxyethyl starch (130/0.4) compared to succinylated gelatin colloid infusions in normovolaemic dogs

Synthetic colloids are commonly administered to dogs to treat absolute or relative hypovolaemia. Voluven^® (tetrastarch 130/0.4) and Gelofusine^® (succinylated gelatin) are available to veterinarians in South Africa. In humans, use of these products has caused acid–base derangements, changes in haematology and impaired haemostasis. We aimed to investigate these effects in healthy normovolaemic dogs. Eight healthy adult beagle dogs underwent a cross-over study, receiving Voluven^® or Gelofusine^® (10 mL/kg/h for 120 min) once each with a 14-day washout between treatments. Dogs were premedicated with dexmedetomidine (10 µg/kg intramuscularly). Anaesthesia was induced with propofol and the dogs were maintained with isoflurane-in-oxygen. The anaesthetised dogs were connected to a multi-parameter monitor to monitor physiological parameters throughout. Catheters placed in a jugular vein and dorsal metatarsal artery allowed sampling of venous and arterial blood. Blood was collected immediately prior to commencement of colloid infusion, after 60 min infusion and at the end of infusion (120 min) to allow for arterial blood gas analysis, haematology and coagulation testing (activated partial thromboplastin time [aPTT], prothrombin time [PT] and thromboelastography [TEG]). There was no effect, between treatments or over time, on blood pH. The haemoglobin concentration, erythrocyte count and haematocrit decreased significantly over time (all p < 0.01), with no differences between treatments, and remained within normal clinical ranges. There were no differences between treatments or over time for the TEG, aPTT and PT tests of haemostasis. At the dose studied, Voluven^® and Gelofusine^® had comparably negligible effects on blood acid–base balance and coagulation in normovolaemic dogs.

Platelet closure time in anesthetized Greyhounds with hemorrhagic shock treated with hydroxyethyl starch 130/0.4 or 0.9% sodium chloride infusions

OBJECTIVE

To measure platelet closure time (PCT) in dogs during controlled hemorrhagic shock and after fluid resuscitation with hydroxyethyl starch (HES) 130/0.4 or 0.9% sodium chloride.

DESIGN

Experimental interventional study.

SETTING

University veterinary teaching hospital.

ANIMALS

Eleven healthy Greyhounds.

INTERVENTIONS

Dogs were anesthetized and had 48 mL/kg of blood removed to induce hemorrhagic shock. Dogs received 20 mL/kg of HES 130/0.4 (n = 6) or 80 mL/kg of 0.9% sodium chloride (NaCl; n = 5) intravenously over 20 minutes. PCT was measured using the Platelet Function Analyzer-100 with collagen and adenosine-diphosphate cartridges at: T0 = 60 minutes after induction of anesthesia prior to hemorrhage, T1 = during hemorrhagic shock, and T2 = 40 minutes after completion of fluid bolus. Packed cell volume and platelet count were concurrently measured.

MEASUREMENT AND MAIN RESULTS

Hemorrhagic shock did not significantly change PCT, with no difference between T0 and T1. Both the HES 130/0.4 and 0.9% NaCl group had a significantly increased mean PCT at T2 of 91.4 seconds (95% CI 69.3-113.4) and 95.5 seconds (95% CI 78.2-112.8), respectively, compared to T1. The magnitude of change was significantly greater for the 0.9% NaCl group than the HES 130/0.4 group. There was no difference in the magnitude of change in PCV and platelet count between the 2 groups. The PCV and platelet count were >25% and >100,000/μL, respectively, in all dogs, except for dogs in the HES 130/0.4 group at T2 where platelet counts were <100,000/μL.

CONCLUSION

Controlled hemorrhagic shock in Greyhounds under anesthesia did not cause a significant change in PCT. Both HES 130/0.4 and 0.9% NaCl administration after induction of shock increased PCT. These results do not support that HES 130/0.4 causes relevant platelet dysfunction beyond hemodilution.

Regional tolerance to acute normovolemic hemodilution: evidence that the kidney may be at greatest risk

OBJECTIVE

To evaluate the regional tolerance to acute normovolemic hemodilution (ANH).

DESIGN

Prospective animal study.

SETTING

University research laboratory.

PARTICIPANTS

Nine anesthetized (isoflurane) dogs.

INTERVENTIONS

Hematocrit reduced in 10% decrements using dextran-for-blood exchange until cardiac insufficiency observed.

MEASUREMENTS AND MAIN RESULTS

Cardiac index (CI) was measured using thermodilution and regional blood flow (RBF) in myocardium, brain, spinal cord, kidney, liver, duodenum, pancreas, spleen, skeletal muscle, and skin with radioactive microspheres. Oxygen delivery (DO2) was calculated from the product of respective blood flow and arterial oxygen content. Systemic oxygen extraction (EO2) and oxygen consumption (VO2) were calculated. Increases in CI during ANH were inadequate to prevent decreases in systemic DO2; however, an increased systemic EO2 maintained VO2 during graded ANH to hematocrit<10%. In the myocardium, brain, and spinal cord, increases in RBF were sufficient to maintain DO2 across the entire range of hematocrits, but this was not the case in the other organs studied. Of note, renal DO2 first decreased at a hematocrit of 30% and was only 25% of baseline at a hematocrit of 10%.

CONCLUSIONS

During graded ANH, increases in RBF were sufficient to maintain DO2 in only the heart, brain, and spinal cord. The especially marked decrease in DO2 in the kidney, combined with previous physiologic studies demonstrating its inability to augment EO2, suggest that this organ may be the most at risk of hypoxic damage during ANH.

Red cell transfusion management for patients undergoing cardiac surgery for congenital heart disease

BACKGROUND

Congenital heart disease is the most commonly diagnosed neonatal congenital condition. Without surgery, only 30% to 40% of patients affected will survive to 10 years old. Mortality has fallen since the 1990s with 2006 to 2007 figures showing surgical survival at one year of 95%. Patients with congenital heart disease are potentially exposed to red cell transfusion at many points in the surgical pathway. There are a number of risks associated with red cell transfusion that may be translated into increased patient morbidity and mortality.

OBJECTIVES

To evaluate the effects of red cell transfusion on mortality and morbidity on patients with congenital heart disease at the time of cardiac surgery.

SEARCH METHODS

We searched 11 bibliographic databases and three ongoing trials databases including the Cochrane Central Register of Controlled Trials (CENTRAL) (Issue 5, 2013), MEDLINE (Ovid, 1950 to 11 June 2013), EMBASE (Ovid, 1980 to 11 June 2013), ClinicalTrials.gov, World Health Organization (WHO) ICTRP and the ISRCTN Register (to June 2013). We also searched references of all identified trials, relevant review articles and abstracts from between 2006 and 2010 of the most relevant conferences. We did not limit the searches by language of publication.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) comparing red cell transfusion interventions in patients undergoing cardiac surgery for congenital heart disease. We included participants of any age (neonates, paediatrics and adults) and with any type of congenital heart disease (cyanotic or acyanotic). We excluded patients with congenital heart disease undergoing non-cardiac surgery. No co-morbidities were excluded.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trial quality and extracted data. We contacted study authors for additional information.

MAIN RESULTS

We identified 11 trials (862 participants). All trials were in neonatal or paediatric populations. The trials covered only three areas of interest: restrictive versus liberal transfusion triggers (two trials), leukoreduction versus non-leukoreduction (two trials) and standard versus non-standard cardiopulmonary bypass (CPB) prime (seven trials). Owing to the clinical diversity in the participant groups (cyanotic (three trials), acyanotic (four trials) or mixed (four trials)) and the intervention groups, it was not appropriate to pool data in a meta-analysis. No study reported data for all the outcomes of interest to this review. Risk of bias was mixed across the included trials, with only attrition bias being low across all trials. Blinding of study personnel and participants was not always possible, depending on the intervention being used.Five trials (628 participants) reported the primary outcome: 30-day mortality. In three trials (a trial evaluating restrictive and liberal transfusion (125 participants), a trial of cell salvage during CPB (309 participants) and a trial of washed red blood cells during CPB (128 participants)), there was no clear difference in mortality at 30 days between the intervention arms. In two trials comparing standard and non-standard CPB prime, there were no deaths in either randomised group. Long-term mortality was similar between randomised groups in one trial each comparing restrictive and liberal transfusion or standard and non-standard CPB prime.Four trials explored a range of adverse effects following red cell transfusion. Kidney failure was the only adverse event that was significantly different: patients receiving cell salvaged red blood cells during CPB were less likely to have renal failure than patients not exposed to cell salvage (risk ratio (RR) 0.26, 95% confidence interval (CI) 0.09 to 0.79, 1 study, 309 participants). There was insufficient evidence to determine whether there was a difference between transfusion strategies for any other severe adverse events.The duration of mechanical ventilation was measured in seven trials (768 participants). Overall, there was no consistent difference in the duration of mechanical ventilation between the intervention and control arms.The duration of intensive care unit (ICU) stay was measured in six trials (459 participants). There was no clear difference in the duration of ICU stay between the intervention arms in the transfusion trigger and leukoreduction trials. In the standard versus non-standard CPB prime trials, one trial examining the impact of washing transfused bypass prime red blood cells showed no clear difference in duration of ICU stay between the intervention arms, while the trial assessing ultrafiltration of the priming blood showed a shorter duration of ICU stay in the ultrafiltration group.

AUTHORS' CONCLUSIONS

There are only a small number of small and heterogeneous trials so there is insufficient evidence to assess the impact of red cell transfusion on patients with congenital heart disease undergoing cardiac surgery accurately. It is possible that the presence or absence of cyanosis impacts on trial outcomes, which would necessitate different clinical management of two groups. Further adequately powered, specific, high-quality trials are warranted to assess this fully.

The choice of the intravenous fluid influences the tolerance of acute normovolemic anemia in anesthetized domestic pigs

INTRODUCTION

The correction of hypovolemia with acellular fluids results in acute normovolemic anemia. Whether the choice of the infusion fluid has an impact on the maintenance of oxygen (O₂) supply during acute normovolemic anemia has not been investigated so far.

METHODS

Thirty-six anesthetized and mechanically ventilated pigs were hemodiluted to their physiological limit of anemia tolerance, reflected by the individual critical hemoglobin concentration (Hbcrit). Hbcrit was defined as the Hb-concentration corresponding with the onset of supply-dependency of total body O₂-consumption (VO₂). The hemodilution protocol was randomly performed with either tetrastarch (6% HES 130/0.4, TS-group, n = 9), gelatin (3.5% urea-crosslinked polygeline, GEL-group, n = 9), hetastarch (6% HES 450/0.7, HS-group, n = 9) or Ringer's solution (RS-group, n = 9). The primary endpoint was the dimension of Hbcrit, secondary endpoints were parameters of central hemodynamics, O₂ transport and tissue oxygenation.

RESULTS

In each animal, normovolemia was maintained throughout the protocol. Hbcrit was met at 3.7 ± 0.6 g/dl (RS), 3.0 ± 0.6 g/dl (HS P < 0.05 vs. RS), 2.7 ± 0.6 g/dl (GEL, P < 0.05 vs. RS) and 2.1 ± 0.4 g/dl (TS, P < 0.05 vs. GEL, HS and RS). Hemodilution with RS resulted in a significant increase of extravascular lung water index (EVLWI) and a decrease of arterial oxygen partial pressure (paO₂), and O₂ extraction ratio was increased, when animals of the TS-, GEL- and HS-groups met their individual Hbcrit.

CONCLUSIONS

The choice of the intravenous fluid has an impact on the tolerance of acute normovolemic anemia induced by acellular volume replacement. Third-generation tetrastarch preparations (e.g., HES 130/0.4) appear most advantageous regarding maintenance of tissue oxygenation during progressive anemia. The underlying mechanism includes a lower degree of extravasation and favourable effects on microcirculatory function.

Neuromuscular blockade with rocuronium bromide increases the tolerance of acute normovolemic anemia in anesthetized pigs

BACKGROUND

The patient's individual anemia tolerance is pivotal when blood transfusions become necessary, but are not feasible for some reason. To date, the effects of neuromuscular blockade (NMB) on anemia tolerance have not been investigated.

METHODS

14 anesthetized and mechanically ventilated pigs were randomly assigned to the Roc group (3.78 mg/kg rocuronium bromide followed by continuous infusion of 1 mg/kg/min, n = 7) or to the Sal group (administration of the corresponding volume of normal saline, n = 7). Subsequently, acute normovolemic anemia was induced by simultaneous exchange of whole blood for a 6% hydroxyethyl starch solution (130/0.4) until a sudden decrease of total body O(2) consumption (VO(2)) indicated a critical limitation of O(2) transport capacity. The Hb concentration quantified at this time point (Hb(crit)) was the primary endpoint of the protocol. Secondary endpoints were parameters of hemodynamics, O(2) transport and tissue oxygenation.

RESULTS

Hb(crit) was significantly lower in the Roc group (2.4 ± 0.5 vs. 3.2 ± 0.7 g/dl) reflecting increased anemia tolerance. NMB with rocuronium bromide reduced skeletal muscular VO(2) and total body O(2) extraction rate. As the cardiac index increased simultaneously, total body VO(2) only decreased marginally in the Roc group (change of VO(2) relative to baseline -1.7 ± 0.8 vs. 3.2 ± 1.9% in the Sal group, p < 0.05).

CONCLUSION

Deep NMB with rocuronium bromide increases the tolerance of acute normovolemic anemia. The underlying mechanism most likely involves a reduction of skeletal muscular VO(2). During acellular treatment of an acute blood loss, NMB might play an adjuvant role in situations where profound stages of normovolemic anemia have to be tolerated (e.g. bridging an unexpected blood loss until blood products become available for transfusion).

[Perioperative management of Jehovah's Witness patients. Special consideration of religiously motivated refusal of allogeneic blood transfusion]

The religious organization of Jehovah's Witnesses numbers more than 7 million members worldwide, including 165,000 members in Germany. Although Jehovah's Witnesses strictly refuse the transfusion of allogeneic red blood cells, platelets and plasma, Jehovah's Witness patients may nevertheless benefit from modern therapeutic concepts including major surgical procedures without facing an excessive risk of death. The present review describes the perioperative management of surgical Jehovah's Witness patients aiming to prevent fatal anemia and coagulopathy. The cornerstones of this concept are 1) education of the patient about blood conservation techniques generally accepted by Jehovah's Witnesses, 2) preoperative optimization of the cardiopulmonary status and correction of preoperative anemia and coagulopathy, 3) perioperative collection of autologous blood, 4) minimization of perioperative blood loss and 5) utilization of the organism's natural anemia tolerance and its acute accentuation in the case of life-threatening anemia.

Buffer capacity of 4% succinylated gelatin does not provide any advantages over acidic 6% hydroxyethyl starch 130/0.4 for acid-base balance during experimental mixed acidaemia in a porcine model

BACKGROUND AND OBJECTIVE

Four percent gelatine is an alkaline compound due to NH2 groups, whereas 6% hydroxyethyl starch 130/0.4 (HES130) has acidic features. We investigated whether these solutions lead to differences in acid-base balance in pigs during acidaemia and correction of pH.

METHODS

Anaesthetized pigs were randomized to HES130 or gelatine infusion (n = 5 per group). Animals received acid infusion (0.4 M solution of lactic acid and HCl diluted in normal saline) and low tidal volume ventilation (6-7 mL kg(-1), PaCO2 of 80-85 mmHg, pH 7.19-7.24). Measurements were made before and after induction of acidaemia, before and after correction of pH with haemofiltration (continuous venovenous haemofiltration) and tris-hydroxymethylaminomethane infusion. We measured parameters describing acid-base balance according to Stewart's approach, ketone body formation, oxygen delivery, haemodynamics, diuresis and urinary pH.

RESULTS

Acid-base balance did not differ significantly between the groups. In HES130-treated pigs, the haemodilution-based drop of haemoglobin (1.4 +/- 1.0 g dL(-1), median +/- SD) was paralleled by an increase in the cardiac output (0.5 +/- 0.4 L min(-1). Lacking increases in cardiac output, gelatine-treated pigs demonstrated a reduction in oxygen delivery (149.4 +/- 106.0 mL min(-1)). Tris-hydroxymethylaminomethane volumes required for pH titration to desired values were significantly higher in the gelatine group (0.7 +/- 0.1 mL kg(-1) h(-1) vs. HES130: 0.5 +/- 0.2 mL kg(-1) h(-1)).

CONCLUSION

The buffer capacity of gelatine did not lead to favourable differences in acid-base balance in comparison to HES130.

Heart, kidney, and intestine have different tolerances for anemia

Organ systems do not respond uniformly to changes in systemic oxygen delivery because of global and local redistributive mechanisms. We hypothesized that progressive hemodilution would evoke a different response in the microvascular oxygenation of the heart compared with kidney and gut. To evaluate this hypothesis, we studied the effect of stepwise isovolemic hemodilution on systemic hemodynamic and oxygenation parameters as well as the relation between systemic hematocrit (Ht) and microvascular PO(2) (microPO(2)) in heart, kidney, and intestines in an anesthetized and mechanically ventilated rat model. Baseline conditions were similar in the hemodilution group and in the control group. In the hemodilution group, Ht was diminished from 46.6 +/- 3.8% to 7.0 +/- 1.8% [mean +/- standard deviation (SD)]. This group had no effect on measured hemodynamics; only when Ht fell below 10% did blood pressure start to decrease. The microPO(2) values in heart, kidney, and intestines did not respond uniformly. Renal microPO(2) (56 +/- 10 mm Hg at baseline) started to decrease at a Ht of 38.5 +/- 8.6%, whereas intestinal microPO(2) (59 +/- 6 mm Hg at baseline) did not start to decrease until Ht reached 17.4 +/- 7.1%. Finally, cardiac microPO(2) (40 +/- 6 mm Hg at baseline) decreased only in the ultimate stage of the experiment at Ht of 8.7 +/- 3.5%. Based on these observations, we conclude that the regulation of microvascular oxygenation during progressive anemia is specific for each organ system. The relation between these observations and organ function and damage needs to be determined.

[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.

Red blood cell transfusion in clinical practice

Every year, about 75 million units of blood are collected worldwide. Red blood cell (RBC) transfusion is one of the few treatments that adequately restore tissue oxygenation when oxygen demand exceeds supply. Although the respiratory function of blood has been studied intensively, the trigger for RBC transfusion remains controversial, and doctors rely primarily on clinical experience. Laboratory assays that indicate failing tissue oxygenation would be ideal to guide the need for transfusion, but none has proved easy, reproducible, and sensitive to regional tissue hypoxia. The clinical importance of the RBCs storage lesion (ie, the time-dependent metabolic, biochemical, and molecular changes that stored blood cells undergo) is poorly understood. RBCs can be filtered, washed, frozen, or irradiated for specific indications. Donor screening and testing have dramatically reduced infectious risks in the developed world, but infection remains a major hazard in developing countries, where 13 million units of blood are not tested for HIV or hepatitis viruses. Pathogen inactivation techniques are in clinical trials for RBCs, but none is available for use. Despite serious immunological and non-immunological complications, RBC transfusion holds a therapeutic index that exceeds that of many common medications.

[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.

Critical oxygen delivery during cardiopulmonary bypass in dogs

BACKGROUND AND OBJECTIVE

To determine the minimal oxygen delivery and pump flow that can maintain systemic oxygen uptake during normothermic (37 degrees C) pulsatile and non-pulsatile cardiopulmonary bypass in dogs.

METHODS

Eighteen anaesthetized dogs were randomly assigned to receive either non-pulsatile (Group C; n = 9) or pulsatile bypass flow (Group P; n = 9). Oxygen delivery was reduced by a progressive decrease in pump flow, while arterial oxygen content was maintained constant. In each animal, critical oxygen delivery was determined from plots of oxygen uptake vs. oxygen delivery and from plots of blood lactate vs. oxygen delivery using a least sum of squares technique. Critical pump flow was determined from plots of lactate vs. pump flow.

RESULTS

At the critical point, oxygen delivery obtained from oxygen uptake was 7.7 +/- 1.1 mL min(-1) kg(-1) in Group C and 6.8 +/- 1.8 mL min(-1) kg(-1) in Group P (n.s.). These values were similar to those obtained from lactate measurements (Group C: 7.8 +/- 1.6 mL min(-1) kg(-1); Group P: 7.6 +/- 2.0 mL min(-1) kg(-1)). Critical pump flows determined from lactate measurements were 55.6 +/- 13.8 mL min(-1) kg(-1) in Group C and 60.8 +/- 13.9 mL min(-1) kg(-1) in Group P (n.s.).

CONCLUSIONS

Oxygen delivery values greater than 7-8 mL min(-1) kg(-1) were required to maintain oxygen uptake during normothermic cardiopulmonary bypass with either pulsatile or non-pulsatile blood flow. Elevation of blood lactate levels during bypass helps to identify inadequate tissue oxygen delivery related to insufficient pump flow.

EVIDENCE FOR MYOCARDIAL DEFECTS UNDER EXTREME ACUTE NORMOVOLEMIC HEMODILUTION WITH HYDROXYETHYL STARCH AND LACTATED RINGER'S SOLUTION

Acute normovolemic hemodilution (ANH) has been proposed to avoid the risks of allogenic transfusion. In reference to its cellular effects, ANH reports in the literature are scarce. Using electron microscopy (EM), we evaluated the effects of ANH on cardiac function and myocardial structure. Twenty-five dogs were prospectively randomized to a control group (n = 5) or to undergo ANH with 6% hydroxyethyl starch (HES; n = 10) or lactated Ringer's solution (LR; n = 10) administered, respectively, at a ratio of 1:1 or 1:3 to the volume of blood removed. Animals were gradually hemodiluted to a hematocrit of 10%, which was accomplished in 80 min. Pulmonary artery catheter and echocardiography were used to evaluate cardiac function. Myocardial samples were taken after the last time point for electron microscopy analysis. Data were obtained during five different stages of ANH, with a mean 20-min interval between each time point. Cardiac index increased significantly in both groups during ANH. A significant decrease in oxygen delivery and oxygen consumption, as well as an increase in oxygen extraction was verified in the LR group. Echocardiography demonstrated a decline in systolic function in the LR group at the end of the experiment. Electron microscopy analysis of the myocardium revealed slight lesions in cardiac cells in the HES group, and moderate-to-significant lesions in the LR group. In this animal species, ANH with HES resulted in better preservation of cardiac function, which was demonstrated by maintenance of systolic function and oxygenation parameters. Minor loss of cellular integrity with HES, in the presence of very low levels of hemoglobin, reinforces these findings.

A Comparative Evaluation of Inhaled Halothane, Isoflurane, and Sevoflurane During Acute Normovolemic Hemodilution in Dogs

The hemodynamic response to acute normovolemic hemodilution (ANH) can be affected by the anesthetics used. We randomized 18 mongrel dogs to undergo ANH with 3 different inhaled anesthetics: halothane, isoflurane, or sevoflurane. Hemodynamics, oxygen transport, and gastric pH were measured before blood withdrawal, at the end of hemodilution, and 30 and 60 min after the end of hemodilution. The baseline measurements of all hemodynamic variables were similar among groups, with the exception of heart rate, which was more rapid in the sevoflurane group. Thirty minutes after hemodilution, the cardiac index increased 88%, 86%, and 157% in the halothane, isoflurane, and sevoflurane groups, respectively, whereas arterial-venous oxygen differences and oxygen consumption were larger in the halothane group compared with the isoflurane and sevoflurane groups. Gastric pH obtained by tonometry did not change and was not different among groups. Because the hemodynamic response to ANH was not blunted, all three anesthetics may be safely used for the maintenance of anesthesia.

Experimental analysis of critical oxygen delivery

Systemic variables were evaluated with respect to O2 delivery to test the hypothesis that critical O2 delivery and critical Hb can be estimated by multiple variables collected simultaneously. Rats were subjected to transfusion with either fresh or stored blood and then subjected to stepwise isovolemic hemodilution. Critical levels were measured by the dual-regression method from plots of systemic variables against O2 delivery and Hb. Delivery was calculated from cardiac index and arterial O2 content. We found that 1) after hemodilution, O2 delivery changed in a nonlinear relationship with Hb; 2) critical delivery calculated using 30 different systemic variables was not statistically different from each other; 3) critical delivery and critical Hb were correlated but were not different between animals receiving fresh or stored blood; and 4) similar critical levels were found using a single variable from several animals and using several variables from the same subject. The best variables to estimate critical delivery were lactate, bicarbonate, base excess, O2 extraction ratio, expired CO2, pulse pressure, cardiac index, and systolic pressure. The data suggest that a multivariable analysis of critical delivery may help determine the physiological oxygenation boundary at the whole body level. This may assist in finding therapeutic triggers on an individual basis using systemic markers of the transition from aerobic to anaerobic metabolism.

Anaemia and red blood cell transfusion in the critically ill patient

Anaemia is a common finding in critically ill patients. There are often multiple causes. Obvious causes include surgical bleeding and gastrointestinal haemorrhage but many patients have no overt bleeding episodes. Phlebotomy can be a significant source of blood loss. In addition, critically ill patients have impaired erythropoiesis as a consequence of blunted erythropoietin production and direct inhibitory effects of inflammatory cytokines. The ability of a patient to tolerate anaemia depends on their clinical condition and the presence of any significant co-morbidity; maintenance of circulating volume is of paramount importance. There is no universal transfusion trigger. Current guidelines for critically ill and perioperative patients advise that at Hb values <70 g/L red blood cell transfusion is strongly indicated and at Hb values >100 g/L transfusion is unjustified. For patients with Hb values in the range 70 to 100 g/L the transfusion trigger should be based on clinical indicators. Most stable critically ill patients can probably be managed with a Hb concentration between 70 and 90 g/L. Uncertainties exist concerning the most appropriate Hb concentration for patients with significant cardio-respiratory disease.

Whole body oxygen consumption and critical oxygen delivery in response to prolonged and severe carbon monoxide poisoning

BACKGROUND

Carbon monoxide (CO) poisoning remains the leading cause of death by poisoning in the world. One of the major proposed mechanisms for CO toxicity is the binding of CO to cytochrome oxidase and interference with cellular oxygen utilization but evidence for this is inconclusive.

AIM OF STUDY

This study examined the effects of prolonged CO exposure on the dynamics of whole body oxygen consumption (VO(2)) and oxygen delivery (DO(2)) in an attempt to observe if CO exposure results in a defect of oxygen utilization defect as determined by a reduction in VO(2) during the course of poisoning prior to reaching the point where VO(2) is directly dependent on DO(2). This critical level of DO(2) (DO(2)crit) produced by CO poisoning was compared to historical values produced by other insults, which decrease global body DO(2).

METHODS

Five small dogs were ventilated for 2 h with 0.25% CO and room air followed by 0.5% CO until death. Cardiac index (Q), DO(2), VO(2), oxygen extraction ratio (OER), and systemic lactate were measured every 15 min until death.

RESULTS

Carboxyhemoglobin (COHb) levels increased linearly over 2.5 h to values above 80% until death. VO(2) remained constant and not significantly different from baseline below a COHb of 80%. At COHb levels above 80%, VO(2) precipitously dropped. Similarly lactate levels were not significantly elevated from baseline until VO(2) dropped. DO(2) decreased by 78% (from 23+/-6 ml/kg/min to 5+/-4 ml/kg/min) over time despite an increase in Q by 58% until levels of COHb were above 80%. OER increased from 19+/-5% to 50+/-11% until death. The calculated DO(2)crit was 10.7+/-4 ml/min/kg, which is not significantly different from values ranging from 7 to 13 ml/min/kg reported in the literature due to other insults, which reduce DO(2).

CONCLUSION

In this canine model of prolonged CO exposure, no gradual reduction in VO(2) or increase in systemic lactate prior to reaching DO(2)crit was noted. In addition, CO exposure does not appear to change the DO(2)crit. The combination of these findings does not support the theory that CO produces a whole body intracellular defect in oxygen utilization.

Comparative effects of red blood cell transfusion and increasing blood flow on tissue oxygenation in oxygen supply-dependent conditions

Red blood cell (RBC) transfusion is usually administered to improve oxygen delivery (DO(2)) in order to sustain tissue oxygen demand. However, this practice is not supported by firm clinical or experimental data. Using a randomized two-period crossover design, this study compared the efficacy of "fresh" RBC transfusion and increased blood flow to restore tissue oxygenation in oxygen supply-dependent conditions. In 12 ketamine-anesthetized mongrel dogs submitted to nonpulsatile normothermic cardiopulmonary bypass, DO(2) was reduced by a progressive decrease in pump flow. DO(2) dependency was defined as an O(2) uptake (V O(2)) decrease by more than 15% from baseline value. Then, intervention consisted of a 40% increase in DO(2) obtained either by transfusion of "fresh" dog's RBC (stored < 3 d) or by increase in pump flow. Animals received both interventions sequentially in a random order, while O(2) saturation was maintained constant. In O(2) supply-dependent conditions, rising pump flow from 1.6 +/- 0.4 to 2.7 +/- 0.7 L/ min increased DO(2) from 5.4 +/- 1.1 to 9.0 +/- 1.3 ml/kg/min (p < 0.01) and V O(2) from 3.5 +/- 0.4 to 4.1 +/- 0.5 ml/kg/min (p = 0.02). "Fresh" RBC transfusion, which increased the hemoglobin concentration from 6.4 +/- 0.9 to 11.1 +/- 1.3 g/dl, increased DO(2) from 5.4 +/- 1.2 to 9.0 +/- 1.4 ml/kg/min (p < 0.01) and V O(2) from 3.6 +/- 0.4 to 4.1 +/- 0.5 ml/kg/min (p = 0.02). There was no difference in V O(2) resulting from both interventions. In oxygen supply-dependent conditions, "fresh" RBC transfusion and increased blood flow are equally effective in restoring tissue oxygenation.

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.