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.     

ANH自体输血在完全性前置胎盘合并胎盘植入剖宫产术中的应用

目的:研究急性等容血液稀释(ANH)自体输血在完全性前置胎盘合并胎盘植入剖宫产术中的应用价值。方法:选择2012年1月~2015年12月在我院进行诊治的完全性前置胎盘合并胎盘植入患者80例,随机分为三组,对照组(n=26)进行常规处理,ANH组(n=27)进行急性等容血液稀释,AHH(急性高容血液稀释)组(n=27)进行急性高容血液稀释。比较三组产妇的体重、年龄、手术时间、术中出血量、输注异体血例数,ANH组和AHH组血液稀释前后和三组产妇术后的动脉血气分析值、术后2 h血常规,三组新生儿娩出后1min、5min Apgar评分及脐动脉血血气分析值。结果:血液稀释后,两组的血红蛋白、红细胞比容和碱剩余均较血液稀释前明显降低(P0.05),两组血气分析无明显差异(P0.05);ANH组输注异体血的比例明显低于AHH组和对照组(P0.05),剖宫产术后2h,ANH组的血红蛋白、红细胞比容和血小板均明显高于AHH组和对照组(P0.05);三组新生儿的血红蛋白、红细胞比容、碱剩余、血乳酸和Apgar评分均无明显差异(P0.05)。结论:ANH自体输血能减少异体血的输注和产妇剖宫术中红细胞的丢失,节约临床用血,对产妇和新生儿的影响较小。     

Hemodynamic effects after partial exchange transfusion with pyridoxylated polyhemoglobin in chimpanzees.

Partial exchange transfusion with 8.5% pyridoxylated polyhemoglobin solution [PolyHb-PPa] was performed in five anesthetized spontaneously-breathing male chimpanzees weighing 22-30 kg. Mean exchange volume was 42.5 +/- 10.7 ml/kg BW (26.8-54.6 ml/kg), mean exchange rate 56.7 +/- 7.1% (48.2-67.4%). All animals survived long-term. The chimpanzee's hemodynamics remained stable for the 5 h observation period. Right and left ventricular filling pressures remained constant, mean arterial pressure and mean pulmonary arterial pressure increased by up to 40% after the exchange. Cardiac output remained unaffected by the partial exchange and stroke volume did not change substantially although oxygen capacity and oxygen transport capacity decreased by about a third. The failure of cardiac output to rise after partial exchange transfusion with PolyHb-PPa contrasts with results after isovolemic hemodilution using non-oxygen-carrying blood substitutes and is not adequately explained by the oxygen capacity of 8.5% PolyHb-PPa (9.3 ml O2/dl).     

Efficacy and Safety of Antifibrinolytic Agents in Reducing Perioperative Blood Loss and Transfusion Requirements in Scoliosis Surgery: A Systematic Review and Meta-Analysis

Background

Routine use of antifibrinolytic agents in spine surgery is still an issue of debate.

Objective

To gather scientific evidence for the efficacy and safety of antifibrinolytic agents including aprotinin, tranexamic acid (TXA) and epsilon aminocaproic acid (EACA, traditionally known as Amicar) in reducing perioperative blood loss and transfusion requirements in scoliosis surgery.

Methods

We conducted a systematic review and meta-analysis for randomized controlled trials (RCTs), retrospective case-control studies, and retrospective cohort studies on the use of antifibrinolytic agents in scoliosis surgery by searching in the MEDLINE and EMBASE databases and the Cochrane Database of Systematic Reviews and Controlled Trials of papers published from January 1980 through July 2014. Safety of the antifibrinolytic agents was evaluated in all included studies, while efficacy was evaluated in RCTs.

Results

Eighteen papers with a total of 1,158 patients were eligible for inclusion in this study. Among them, 8 RCTs with 450 patients were included for evaluation of pharmacologic efficacy (1 RCT was excluded because of a lack of standard deviation data). Mean blood loss was reduced in patients with perioperative use of antifibrinolytic agents by 409.25 ml intraoperatively (95% confidence interval [CI], 196.57–621.94 ml), 250.30 ml postoperatively (95% CI, 35.31–465.30), and 601.40 ml overall (95% CI, 306.64–896.16 ml). The mean volume of blood transfusion was reduced by 474.98 ml (95% CI, 195.30–754.67 ml). The transfusion rate was 44.6% (108/242) in the patients with antifibrinolytic agents and 68.3% (142/208) in the patients with placebo. (OR 0.38; 95% CI; 0.25–0.58; P<0.00001, I² = 9%). All studies were included for evaluation of safety, with a total of 8 adverse events reported overall (4 in the experimental group and 4 in the control group).

Conclusion

The systematic review and meta-analysis indicated that aprotinin, TXA, and EACA all significantly reduced perioperative blood loss and transfusion requirements in scoliosis surgery. There was no evidence that the use of antifibrinolytic agents was a risk factor for adverse events, especially thromboembolism, in scoliosis surgery.     

The use of intraoperative autotransfusion during cranial vault remodeling for craniosynostosis.

Intraoperative autotransfusion salvages blood shed during surgery for use in immediate resuscitation of the patient. The purpose of this study was to determine whether such autotransfusion decreases the volume of homologous blood transfused in patients undergoing primary cranial vault remodeling for craniosynostosis. The Cobe-Bret 2 autologous blood recovery system (Hemo Concepts, Union, N.J.) was used in 11 cases, and an equal number of consecutive cases did not receive intraoperative autotransfusion. There were no significant differences between the groups with respect to age, sex, and weight. Mean estimated blood loss was 43.2 ml/kg (range, 20.3 to 65.0 ml/kg) in the intraoperative autotransfusion group and 40.2 ml/kg (range, 6.8 to 72.3 ml/kg) in the control group (not statistically significant; p < 0.05). There was no significant difference in volume of homologous blood transfusion between the two groups. The autotransfusion group received 34.1 ml/kg of homologous blood (range, 0 to 60.7 ml/kg), and the control group received a mean of 32.7 ml/kg (range, 14.5 to 60.2 ml/kg). The autotransfusion group received a mean of 10.4 ml/kg of recovered autologous blood (range, 0 to 21.4 ml/kg). In four of the 11 autotransfusion patients, insufficient autologous blood was recovered intraoperatively to warrant transfusion. Results of this study suggest little benefit for the use of intraoperative autotransfusion in primary cranial vault remodeling for craniosynostosis in the young patient. It was hypothesized that this finding was a result of the following: (1) intraoperative autotransfusion blood was usually available only toward the end of the procedure, after homologous blood had already been administered, and (2) the volume of recovered intraoperative autotransfusion blood is minimal, compared with the homologous transfusion volume requirements during an extensive cranial vault remodeling and fronto-orbital advancement procedure. In the context of unproven cost benefit and increasing similar evidence from other comparative studies, emphasis should be directed to other medical and surgical strategies to minimize the need for perioperative blood transfusion.     

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.     

Subcutaneous PO2 as an index of the physiological limits for hemodilution in the rat.

This study was designed to test the hypothesis that changes in subcutaneous PO2 (PscO2) during progressive hemodilution will reliably predict a "critical point" at which tissue O2 consumption (VO2) becomes dependent on O2 delivery (QO2). Twelve pentobarbital-anesthetized male Sprague-Dawley rats (315-375 g) underwent stepwise exchange of plasma for blood (1.5 ml of plasma for each 1 ml of blood lost). The initial exchange was equal to 25% of the estimated circulatory blood volume, and each subsequent exchange was equal to 10% of the estimated circulatory blood volume. After nine exchanges, the hematocrit (Hct) fell from 42 +/- 1 to 6 +/- 1%. Cardiac output and O2 extraction rose significantly. PscO2 became significantly reduced (P < 0.05) after exchange of 45% of the blood volume (Hct = 16 +/- 1%). VO2 became delivery dependent when QO2 fell below 21 ml x min(-1) x kg body wt(-1) (mean Hct = 13 +/- 1%). Eight control rats undergoing 1:1 blood-blood exchange showed no change in PscO2, pH, HCO3(-), or hemodynamics. Measurement of PscO2 may be a useful guide to monitor the adequacy of QO2 during hemodilution.     

Evaluation of Neuronal Apoptosis Precursors in an Experimental Model of Acute Normovolemic Hemodilution

Background

The effects of acute anemia on neuronal cells and the safe limits of hematocrit are not well established. The objective of this study was to evaluate neuronal pro- and anti-apoptotic Bax and Bcl-x proteins, caspase-3 and -9 activity, and DNA fragmentation after acute normovolemic hemodilution (ANH).

Methods

Twenty-four pigs were anesthetized and randomized into 4 groups: Sham, ANH to 15% hematocrit (ANH15%), ANH to 10% hematocrit (ANH10%) and hypoxia (Hx). ANH was achieved by simultaneous blood withdrawal and hydroxyethyl starch infusion. Hx consisted of ventilation with a 6% inspired oxygen fraction for 60 minutes. Bax and Bcl-x proteins as well as DNA fragmentation were evaluated in cortical nuclear and mitochondrial fractions. Caspase-3 and -9 activity was evaluated in the cortical mitochondrial and hippocampal cytosolic fractions. The data were compared using analysis of variance followed by Tukey’s test (P<0.05).

Results

No changes were observed in Bax protein expression after hemodilution in the ANH15% and ANH10% groups compared to the Sham group. Bax expression in the Hx group was increased in the nuclear and mitochondrial fractions compared to all other groups. No significant difference was observed in Bcl-x expression. Caspase-3 and -9 activity in the cytosolic and mitochondrial fractions was different in the Hx group compared to all other groups. No statistical significance in DNA fragmentation was found among the Sham, ANH15% or ANH10% groups.

Conclusion

ANH to 10 and 15% hematocrit did not induce alterations in apoptosis precursors, suggesting that cerebral oxygenation was preserved during these anemic states.     

贺斯-平衡液为稀释剂的急性等容血液稀释对家兔IL-1、TNF-α等细胞因子的影响

目的：评价贺斯-平衡液为稀释剂的急性等容血液稀释（ANH）对家兔血清IL-1、IL-2、IL-6和TNF-α等细胞因子含量的影响,为临床应用提供理论依据。方法：选择20只健康成年家兔,随机分为两组（n=10）：C组为对照组、H组为贺斯组;实验家兔麻醉后行气管切开、高频喷射通气,游离股动脉、股静脉;C组不进行血液稀释;H组在股动脉放血的同时经股静脉输入2倍放血量的稀释液：6％贺斯＋复方乳酸钠溶液,晶／胶为2：1,放血量V=TBV×（HoHf）／Hav,所放血液于放血后60～120min回输。分别在放血前（T0）,放血后30min（T1）、60min（t2）、120min（T3）和24h（T4）取静脉血检测Hb、Hct和血清IL-1、IL-2、IL-6和TNF-α的浓度。结果：H组在ANH后从一时点开始血清IL-1、IL-2、IL-6和TNF-α均有增加,T3达高峰,T4开始回落;T1、T2、T3和T4时点与C组间比较差别显著（P〈0．01）;与ANH前自身对照差别显著（P〈0．01）;C组在各时间点的血清IL-1、IL-2、IL-6和TNF-α含量变化差别不显著。结论：贺斯-平衡液为稀释剂的急性等容血液稀释（ANH）对家兔血清IL-1、IL-2、IL-6和TNF-α等免疫性细胞因子浓度有上调作用;可引起机体强度不大、作用时间较短的良性应激反应,对机体免疫功能有增强作用。     

Effects of erythrocyte flexibility on microvascular perfusion and oxygenation during acute anemia

Responses to exchange transfusion using red blood cells (RBCs) with normal and reduced flexibility were studied in the hamster window chamber model during acute moderate isovolemic hemodilution to determine the role of RBC membrane stiffness in microvascular perfusion and tissue oxygenation. Erythrocyte stiffness was increased by 30-min incubation in 0.02% glutaraldehyde solution, and unreacted glutaraldehyde was completely removed. Filtration pressure through 5-microm pore size filters was used to quantify stiffness of the RBCs. Anemic conditions were induced by two isovolemic hemodilution steps using 6% 70-kDa dextran to a hematocrit (Hct) of 18% (moderate hemodilution). The protocol continued with an exchange transfusion to reduce native RBCs to 75% of baseline (11% Hct) with either fresh RBCs (RBC group) or reduced-flexibility RBCs (GRBC group) suspended in 5% albumin at 18% Hct; a plasma expander (6% 70-kDa dextran; Dex70 group) was used as control. Systemic parameters, microvascular perfusion, capillary perfusion [functional capillary density (FCD)], and oxygen levels across the microvascular network were measured by noninvasive methods. RBC deformability for GRBCs was significantly decreased compared with RBCs and moderate hemodilution conditions. The GRBC group had a greater mean arterial blood pressure (MAP) than the RBC and Dex70 groups. FCD was substantially higher for RBC (0.81 +/- 0.07 of baseline) vs. GRBC (0.32 +/- 0.10 of baseline) and Dex70 (0.38 +/- 0.10 of baseline) groups. Microvascular tissue Po(2) was significantly lower for Dex70 and GRBC vs. RBC groups and the moderate hemodilution condition. Results were attributed to decreased oxygen uploading in the lungs and obstruction of tissue capillaries by rigidified RBCs, indicating that the effects impairing RBC flexibility are magnified at the microvascular level, where perfusion and oxygenation may define transfusion outcome.     

Improved oxygenation in ischemic hamster flap tissue is correlated with increasing hemodilution with Hb vesicles and their O2 affinity

The aim of this study was to test the influence of oxygen affinity of Hb vesicles (HbVs) and level of blood exchange on the oxygenation in collateralized, ischemic, and hypoxic hamster flap tissue during normovolemic hemodilution. Microhemodynamics were investigated with intravital microscopy. Tissue Po2 was measured with Clark-type microprobes. HbVs with a P50 of 15 mmHg (HbV15) and 30 mmHg (HbV30) were suspended in 6% Dextran 70 (Dx70). The Hb concentration of the solutions was 7.5 g/dl. A stepwise replacement of 15%, 30%, and 50% of total blood volume was performed, which resulted in a gradual decrease in total Hb concentration. In the ischemic tissue, hemodilution led to an increase in microvascular blood flow to maximally 141-166% of baseline in all groups (median; P < 0.01 vs. baseline, not significant between groups). Oxygen tension was transiently raised to 121 +/- 17% after the 30% blood exchange with Dx70 (P < 0.05), whereas it was increased after each step of hemodilution with HbV15-Dx70 and HbV30-Dx70, reaching 217 +/- 67% (P < 0.01) and 164 +/- 33% (P < 0.01 vs. baseline and other groups), respectively, after the 50% blood exchange. We conclude that despite a decrease in total Hb concentration, the oxygenation in the ischemic, hypoxic tissue could be improved with increasing blood exchange with HbV solutions. Furthermore, better oxygenation was obtained with the left-shifted HbVs.     

Microvascular PO2 during extreme hemodilution with hemoglobin site specifically PEGylated at Cys-93(beta) in hamster window chamber

The oxygen transport capacity of nonhypertensive polyethylene glycol (PEG)-conjugated hemoglobin solutions were investigated in the hamster chamber window model. Microvascular measurements were made to determine oxygen delivery in conditions of extreme hemodilution [hematocrit (Hct) 11%]. Two isovolemic hemodilution steps were performed with a 6% Dextran 70 (70-kDa molecular mass) plasma expander until Hct was 35% of control. Isovolemic blood volume exchange was continued using two surface-modified PEGylated hemoglobins (P5K2, P(50) = 8.6, and P10K2, P(50) = 8.3; P(50) is the hemoglobin Po(2) corresponding to its 50% oxygen saturation) until Hct was 11%. P5K2 and P10K2 are PEG-conjugated hemoglobins that maintain most of the hemoglobin allosteric properties and have a cooperativity index of n = 2.2. The effects of these molecular solutions were compared with those obtained in a previous study using MP4, a PEG-modified hemoglobin whose P(50) was 5.4 and cooperativity was 1.2 (Tsai et al., Am J Physiol Heart Circ Physiol 285: H1411-H1419, 2003). Tissue oxygen levels were higher after P5K2 (7.0 +/- 2.5 mmHg) and P10K2 (6.3 +/- 2.3 mmHg) versus MP4 (1.7 +/- 0.5 mmHg) or the nonoxygen carrier Dextran 70 (1.3 +/- 1.2 mmHg). Microvascular oxygen delivery was higher after P5K2 and P10K2 (2.22 and 2.34 ml O(2)/dl blood) compared with MP4 (1.41 ml O(2)/dl blood) or Dextran 70 (0.90 ml O(2)/dl blood); however, all these values were lower than control (7.42 ml O(2)/dl blood). The total hemoglobin in blood was similar in all cases; therefore, the improvement in tissue Po(2) and oxygen delivery appears to be due to the increased cooperativity of the new molecules.     

Blood Loss During Pediatric Operations

The gravimetric method of measuring blood loss was used during all types of pediatric operations at The Montreal Children''s Hospital. Results of 1787 such measurements indicated that the method is of value in the management of transfusion. Grading blood loss as minor, moderate or severe, in terms of its relation to total blood volume or body weight is a procedure of practical value. Operations were grouped into those that usually led to a loss of less than 10% of the blood volume (7.5 ml./kg.), those with a loss usually between 10% and 14% and those with blood loss usually over 14% (10.5 ml./kg.), in order that appropriate plans for transfusion could be made to reduce the incidence of serious hypovolemia without fear of cardiovascular overloading. Blood loss at operation (adenotonsillectomy) was noted to vary considerably among surgeons but was fairly constant for each surgeon and seemed to be independent of the surgeon''s experience. Two alleged hemostatic agents, adrenochrome carbazone (Statimo) and estrogenic substances (Premarin), were not effective in reducing the amount of blood lost during adenotonsillectomy. The importance of calculation of approximate equivalent amounts of blood at various ages of childhood is emphasized.     

A comparative study of plasma renin activity, aldosterone and atrial natriuretic hormone between normotensive and hypertensive people living at below sea level altitude (Jordan Valley) and those residing at moderate altitude above

A cross-sectional study was designed to investigate the association of the levels of plasma renin activity (PRA) and aldosterone (ALDO) and atrial natriuretic hormone (ANH) with the rates of hypertension prevalence in people living at 340 meters below sea level, the Jordan Valley (JV) and those residing at 620 meters above sea level, Irbid City. 1072 natives from the JV and 1402 natives from Irbid City were covered by a questionnaire to estimate hypertension prevalence in the JV and Irbid city (population age: 35-65 years). Male subjects were selected from the JV (24 hypertensives, 46+/-15 years old, and 93 normotensives, 33+/-13 years old) and from Irbid City (31 hypertensives, 47+/-12 years old, and 89 normotensives, 40+/-13 years old) to evaluate the levels of PRA, ALDO and ANH. Hypertension was less common in the JV than in Irbid City (9.9% vs. 13.6%). The levels of PRA in the hypertensive subjects compared to the normotensive subjects were lower in the JV (1.7+/-1.0 vs. 2.6+/-1.4 ng/ml/hr) but were similar in Irbid City (2.9+/-2.7 vs. 3.2+/-2.7). The levels of ALDO in the hypertensive subjects compared to the normotensive subjects were similar in the JV (119+/-58 vs. 139+/-66 pg/ml) but were higher in Irbid City (199+/-112 vs. 146+/-84). The levels of ANH in the hypertensive subjects compared to the normotensive subjects were lower in the JV (13.9+/-9.3 vs. 28.0+/-12.7 ng/ml) and were also lower in Irbid City (21.0+/-12.2 vs. 26.7+/-11.6).     

Consequences of fetomaternal haemorrhage after intrauterine transfusion.

Fetomaternal haemorrhage was studied after 68 consecutive fetal intravascular transfusions performed in 20 patients with Rh isoimmunisation. alpha Fetoprotein concentration was assayed in maternal blood taken before, and immediately after each transfusion and three and 24 hours later. An increase of 50% or more in the concentration in any of the samples after transfusion was considered to indicate fetomaternal haemorrhage. Fetal alpha fetoprotein concentration in blood sampled before transfusion was also assayed and the amount of fetomaternal haemorrhage calculated. Fetomaternal haemorrhage occurred in 21 of 32 patients with an anterior placenta and in six of 36 with a posterior or fundal placenta. The mean estimated volume of haemorrhage was 2.4 ml, which was on average equal to 3.1% of the total fetoplacental blood volume. When the volume of fetomaternal haemorrhage at the first transfusion was greater than 1 ml there was a greater increase in maternal Rh (D) antibody titres and a greater fall in fetal packed cell volume. Sampling of fetal blood should not be routinely done early in patients with Rh isoimmunisation, and intrauterine transfusion should be delayed as long as possible. Sampling sites other than the placental cord insertion reduces the risk of fetomaternal haemorrhage.     

Effects of volume and rate of blood loss on indicators of auto-hemodilution

The results of synchronous monitoring of blood impedance versus body impedance in dog experiments showed that the speed of physiological hemodilution related to blood loss up to 30 ml/kg is poorly dependent on the rate of blood loss (0.4-1.0 ml/kg/min) and averages 0.186 +/- 0.02 ml/min/kg. Termination of blood loss is followed by marked reduction in the rate of auto-hemodilution. In bloodletting the studies revealed a linear dependence between the volume of blood loss up to 30 ml/kg and tissue fluid volume entering the blood stream. In blood loss intestinal absorption of fluid does not block the fluid introduction from the interstice. When associated, both these flows increase volume of fluid filling vascular bed and promote stable recovery of the baseline circulating blood volume.     

Oxygen release from arterioles with normal flow and no-flow conditions.

The rate of oxygen release from arterioles ( approximately 55 microm diameter) was measured in the hamster window chamber model during flow and no-flow conditions. Flow was stopped by microvascular transcutaneous occlusion using a glass pipette held by a manipulator. The reduction of the intra-arteriolar oxygen tension (Po2) was measured by the phosphorescence quenching of preinfused Pd-porphyrin, 100 microm downstream from the occlusion. Oxygen release from arterioles was found to be 53% greater during flow than no-flow conditions (2.6 vs. 1.7 x 10(-5) ml O2.cm(-2).s(-1), P < 0.05). Acute hemodilution with dextran 70 was used to reduce vessel oxygen content, significantly increase wall shear stress (14%, P < 0.05), reduce Hct to 28.4% (SD 1.0) [vs. 48.8% (SD 1.8) at baseline], lower oxygen supply by the arterioles (10%, P < 0.05), and increase oxygen release from the arterioles (39%, P < 0.05). Hemodilution also increased microcirculation oxygen extraction (33% greater than nonhemodilution, P < 0.05) and oxygen consumption by the vessel wall, as shown by an increase in vessel wall oxygen gradient [difference in Po2 between the blood and the tissue side of the arteriolar wall, nonhemodiluted 16.2 Torr (SD 1.0) vs. hemodiluted 18.3 Torr (SD 1.4), P < 0.05]. Oxygen released by the arterioles during flow vs. nonflow was increased significantly after hemodilution (3.6 vs. 1.8 x 10(-5) ml O2.cm(-2).s(-1), P < 0.05). The oxygen cost induced by wall shear stress, suggested by our findings, may be >15% of the total oxygen delivery to tissues by arterioles during flow in this preparation.     

Effect of plasma expander viscosity on the cell free layer

The effect of low and high viscosity hemodilution with plasma expanders on the extent of the cell free layer (CFL) width was analyzed in the microcirculation of the exteriorized cremaster muscle preparation of Sprague-Dawley male rats. Anesthetized animals were subjected to 40% hemodilution by blood volume, using 5% human serum albumin (HSA) or 6% Hetastarch (hydroxyethyl starch 670 kDa). Arterioles (n=5 for each treatment) were investigated. Mean arterial pressure, heart rate, vessel flow velocity and CFL width were measured at baseline and 5, 20 and 40 min post-exchange transfusion. Blood and plasma viscosity was determined from terminal blood collections. CFL width and pseudoshear rate, diameter and flow, normalized to baseline, were significantly elevated at all post-exchange assessments. Peripheral vascular resistance decreased. The increase of the CFL width was greater with HSA by comparison with Hetastarch hemodilution (p<0.05). Hetastarch blood and plasma viscosities increased significantly compared to those of HSA (p<0.05). This study shows that CFL widths are influenced by plasma expander viscosity, a phenomenon proportional to the increase in molecular weight of the colloids in solution.     

Comparison of PEG-modified albumin and hemoglobin in extreme hemodilution in the rat.

We have reported a new polyethylene glycol (PEG)-modified, hemoglobin-based O2 carrier (MP4) with novel properties, including a large molecular excluded volume and low PO2 necessary to obtain 50% O2 (approximately 6 Torr). To evaluate the ability of MP4 to transport O2, we compared it with PEG-modified albumin (MPA) using the identical chemistry of attachment of PEG chains. The resulting solutions were well matched with respect to all physical properties except that MP4 is an O2 carrier, whereas MPA is not. An additional solution, 10% pentastarch, was matched with the PEG-modified proteins with regard to oncotic activity and viscosity but does not contain PEG. The model used to evaluate O2 transport was continuous exchange transfusion in the rat until the hematocrit was virtually unmeasurable. Objective end points included survival and the onset of anaerobic metabolism, signaled by acid-base derangement and accumulation of lactic acid. Continuous exchange transfusion of 2.5 blood volumes in rats (n=5 in each treatment group) was carried out over 60 min, such that the final hematocrit was between 0 and 5% in all animals. Animals were observed for an additional 70 min, when survivors were killed. Overall survival for the MP4 animals was 100%; no animal that received either pentastarch or MPA survived. The hematocrit at which lactic acid began to rise was approximately 14.8% in both pentastarch and MPA animals and 7.4% in the animals that received MP4. In all groups, the total hemoglobin was approximately 5 g/dl at this point. We conclude that, despite its low PO2 necessary to obtain 50% O2, MP4 effectively substitutes for red blood cell hemoglobin in its ability to oxygenate tissues in extreme hemodilution.     

Redistribution of intestinal microcirculatory oxygenation during acute hemodilution in pigs.

Acute normovolemic hemodilution (ANH) compromizes intestinal microcirculatory oxygenation; however, the underlying mechanisms are incompletely understood. We hypothesized that contributors herein include redistribution of oxygen away from the intestines and shunting of oxygen within the intestines. The latter may be due to the impaired ability of erythrocytes to off-load oxygen within the microcirculation, thus yielding low tissue/plasma Po(2) but elevated microcirculatory hemoglobin oxygen (HbO(2)) saturations. Alternatively, oxygen shunting may also be due to reduced erythrocyte deformability, hindering the ability of erythrocytes to enter capillaries. Anesthetized pigs underwent ANH (20, 40, 60, and 90 ml/kg hydroxyethyl starch; ANH group: n = 10; controls: n = 5). We measured systemic and mesenteric perfusion. Microvascular intestinal oxygenation was measured independently by remission spectrophotometry [microcirculatory HbO(2) saturation (muHbO(2))] and palladium-porphyrin phosphorescence quenching [microcirculatory oxygen pressure in plasma/tissue (muPo(2))]. Microcirculatory oxygen shunting was assessed as the disparity between mucosal and mesenteric venous HbO(2) saturation (HbO(2)-gap). Erythrocyte deformability was measured as shear stress-induced cell elongation (LORCA difractometer). ANH reduced hemoglobin concentration from 8.1 to 2.2 g/dl. Relative mesenteric perfusion decreased (decreased mesenteric/systemic perfusion fraction). A paralleled reduction occurred in mucosal muHbO(2) (68 +/- 2 to 41 +/- 3%) and muPo(2) (28 +/- 1 to 17 +/- 1 Torr). Thus the proposed constellation indicative for oxygen off-load deficits (sustained muHbO(2) at decreased muPo(2)) did not develop. A twofold increase in the HbO(2)-gap indicated increasing intestinal microcirculatory oxygen shunting. Significant impairment in erythrocyte deformability developed during ANH. We conclude that reduced intestinal oxygenation during ANH is, in addition to redistribution of oxygen delivery away from the intestines, associated with oxygen shunting within the intestines. This shunting appears to be not primarily caused by oxygen off-load deficit but rather by oxygen/erythrocytes bypassing capillaries, wherein a potential contributor is impaired erythrocyte deformability.