Catecholamines and regional hemodynamics during isovolemic hemodilution in anesthetized pigs

Hypervolemia suppresses dilutional anaemic injury in a rat model of haemodilution

Background and Objectives

Haemodilution leads to complications in clinical practice. It is exactly unknown whether this damage is caused by the fluid or by the stretching of the vascular bed. We aimed to compare two different haemodilution techniques at the same anaemic level.

Methods

Normovolemic or hypervolemic haemodilution was performed on twelve adult male Wistar rats. In the normovolemic procedure, blood was withdrawn and instantaneously administered with similar amounts of 6% hydroxyethyl starch (HES 130/0.4). Fluid was administered without withdrawing blood in the hypervolemic procedure. In both models, a 25% haematocrit level was targeted and kept at this level for 90 min to deepen the anaemia effect. Besides haemodynamics measurement, renal function (creatinine, blood urea nitrogen) and injury (tissue norepinephrine, malondialdehyde) were evaluated. Also, systemic hypoxia (lactate), oxidative stress (malondialdehyde, ischaemia-modified albumin), inflammation (tumour necrosis factor-alpha [TNF-α]), osmotic stress, adrenal stress (norepinephrine, epinephrine), and vascular stretching (atrial natriuretic peptide [ANP]) were assessed.

Results

Arterial pressure in the normovolemic group was lower than in the hypervolemic group. Serum creatinine, blood urea nitrogen, and lactate levels were higher in the normovolemic group. Tissue norepinephrine and malondialdehyde levels were higher in the normovolemic group. Serum ANP, malondialdehyde, ischaemia-modified albumin, free haemoglobin, syndecan-1, and TNF-α were higher in both groups compared to respective baseline.

Conclusions

Normovolemic haemodilution may lead to hypoxic kidney injury. The hypervolemic state may be advantageous if fluid is to be administered. Thus, the effect of the fluid itself can be relatively masked.

Hemodilution on microvascular oxygen delivery potential of the blood during coronary bypass surgery

PURPOSE

The hematocrit-to-whole blood viscosity ratio (Hct/WBV) reflects the blood O₂ delivery potential (O₂-DP). WBV is variable to the dynamic vascular shear rate (SR), 1-5/s at microcirculation and 300/s at larger vessels. To estimate the impact of hemodilution on the blood O₂-DP to the myocardium, we analyzed the hemodilution-induced change of Hct/WBV at SR 5/s (Hct/WBV₅) during off-pump coronary bypass (OPCAB) surgery.

METHODS

During OPCAB surgery (n = 21), 10% acute normovolemic hemodilution (HD 10%) was applied. Arterial blood samples were taken: one before and two after HD 10%. One of which after HD 10% underwent an additional 33% in vitro hemodilution (reaching 40% hemodilution in total, HD 40%). WBV of all blood samples was determined using a scan-capillary tube viscometer (Hemovister^™). The changes of Hct/WBV₅ were analyzed as a primary measure of the study and compared with those of Hct/WBV at SR 300/s (Hct/WBV₃₀₀).

RESULTS

Median[IQR] of Hct/WBV₅ [3.5 (2.8-4.2)%/cPoise] was significantly increased by HD 10 and HD 40% [3.6 (3.2-4.6)%/cPoise and 4.2 (3.3-5.2)%/cPoise, respectively, all P < 0.001], but the degrees of changes after HD 10 and HD 40% were not different. Median[IQR] of Hct/WBV₃₀₀ [10.3(8.6‒10.8)%/cPoise] was not changed by HD 10% [10.3(9.1-11.1)%/cPoise], but it was significantly decreased by HD 40% [8.4(7.4‒9.2)%/cPoise, P < 0.001].

CONCLUSION

The increased Hct/WBV₅ suggests that 10-40% hemodilution improves the blood O₂-DP to the myocardium during OPCAB surgery. The SR-specific discrepancy in Hct/WBV changes advocates using microvascular WBV and Hct/WBV to evaluate the blood O₂-DP changes to the myocardium. Further study is warranted to assess the actual changes in myocardial O₂ delivery.

Disentangling the Gordian knot of local metabolic control of coronary blood flow

Recognition that coronary blood flow is tightly coupled with myocardial metabolism has been appreciated for well over half a century. However, exactly how coronary microvascular resistance is tightly coupled with myocardial oxygen consumption (MV̇o₂) remains one of the most highly contested mysteries of the coronary circulation to this day. Understanding the mechanisms responsible for local metabolic control of coronary blood flow has been confounded by continued debate regarding both anticipated experimental outcomes and data interpretation. For a number of years, coronary venous Po₂ has been generally accepted as a measure of myocardial tissue oxygenation and thus the classically proposed error signal for the generation of vasodilator metabolites in the heart. However, interpretation of changes in coronary venous Po₂ relative to MV̇o₂ are quite nuanced, inherently circular in nature, and subject to confounding influences that remain largely unaccounted for. The purpose of this review is to highlight difficulties in interpreting the complex interrelationship between key coronary outcome variables and the arguments that emerge from prior studies performed during exercise, hemodilution, hypoxemia, and alterations in perfusion pressure. Furthermore, potential paths forward are proposed to help to facilitate further dialogue and study to ultimately unravel what has become the Gordian knot of the coronary circulation.

Regulation of myocardial oxygen delivery in response to graded reductions in hematocrit: role of K+ channels

This study was designed to identify mechanisms responsible for coronary vasodilation in response to progressive decreases in hematocrit. Isovolemic hemodilution was produced in open-chest, anesthetized swine via concurrent removal of 500 ml of arterial blood and the addition of 500 ml of 37 °C saline or synthetic plasma expander (Hespan, 6% hetastarch in 0.9% sodium chloride). Progressive hemodilution with Hespan resulted in an increase in coronary flow from 0.39 ± 0.05 to 1.63 ± 0.16 ml/min/g (P < 0.001) as hematocrit was reduced from 32 ± 1 to 10 ± 1% (P < 0.001). Overall, coronary flow corresponded with the level of myocardial oxygen consumption, was dependent on arterial pressures ≥ ~ 60 mmHg, and occurred with little/no change in coronary venous PO₂. Anemic coronary vasodilation was unaffected by the inhibition of nitric oxide synthase (L-NAME: 25 mg/kg iv; P = 0.92) or voltage-dependent K⁺ (K _V) channels (4-aminopyridine: 0.3 mg/kg iv; P = 0.52). However, administration of the K _ATP channel antagonist (glibenclamide: 3.6 mg/kg iv) resulted in an ~ 40% decrease in coronary blood flow (P < 0.001) as hematocrit was reduced to ~ 10%. These reductions in coronary blood flow corresponded with significant reductions in myocardial oxygen delivery at baseline and throughout isovolemic anemia (P < 0.001). These data indicate that vasodilator factors produced in response to isovolemic hemodilution converge on vascular smooth muscle glibenclamide-sensitive (K _ATP) channels to maintain myocardial oxygen delivery and that this response is not dependent on endothelial-derived nitric oxide production or pathways that mediate dilation via K _V channels.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Intestinal and sublingual microcirculation are more severely compromised in hemodilution than in hemorrhage

The alterations in O2 extraction in hemodilution have been linked to fast red blood cell (RBC) velocity, which might affect the complete release of O2 from Hb. Fast RBC velocity might also explain the normal mucosal-arterial Pco2 (ΔPco2). Yet sublingual and intestinal microcirculation have not been completely characterized in extreme hemodilution. Our hypothesis was that the unchanged ΔPco2 in hemodilution depends on the preservation of villi microcirculation. For this purpose, pentobarbital-anesthetized and mechanically ventilated sheep were submitted to stepwise hemodilution ( n = 8), hemorrhage ( n = 8), or no intervention (sham, n = 8). In both hypoxic groups, equivalent reductions in O2 consumption (V̇o2) were targeted. Microcirculation was assessed by videomicroscopy, intestinal ΔPco2 by air tonometry, and V̇o2 by expired gases analysis. Although cardiac output and superior mesenteric flow increased in hemodilution, from the very first step (Hb = 5.0 g/dl), villi functional vascular density and RBC velocity decreased (21.7 ± 0.9 vs. 15.9 ± 1.0 mm/mm2 and 1,033 ± 75 vs. 850 ± 79 μm/s, P < 0.01). In the last stage (Hb = 1.2 g/dl), these variables were lower in hemodiution than in hemorrhage (11.1 ± 0.5 vs. 15.4 ± 0.9 mm/mm2 and 544 ± 26 vs. 686 ± 70 μm/s, P < 0.01), and were associated with lower intestinal fractional O2 extraction (0.61 ± 0.04 vs. 0.79 ± 0.02, P < 0.01) but preserved ΔPco2 (5 ± 2 vs. 25 ± 4 mmHg, P < 0.01). Therefore, alterations in O2 extraction in hemodilution seemed related to microvascular shunting, not to fast RBC velocity. The severe microvascular abnormalities suggest that normal ΔPco2 was not dependent on CO2 washout by the villi microcirculation. Increased perfusion in deeper intestinal layers might be an alternative explanation.

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.

Heart failure with preserved ejection fraction: chronic low-intensity interval exercise training preserves myocardial O2 balance and diastolic function

We have previously reported chronic low-intensity interval exercise training attenuates fibrosis, impaired cardiac mitochondrial function, and coronary vascular dysfunction in miniature swine with left ventricular (LV) hypertrophy (Emter CA, Baines CP. Am J Physiol Heart Circ Physiol 299: H1348-H1356, 2010; Emter CA, et al. Am J Physiol Heart Circ Physiol 301: H1687-H1694, 2011). The purpose of this study was to test two hypotheses: 1) chronic low-intensity interval training preserves normal myocardial oxygen supply/demand balance; and 2) training-dependent attenuation of LV fibrotic remodeling improves diastolic function in aortic-banded sedentary, exercise-trained (HF-TR), and control sedentary male Yucatan miniature swine displaying symptoms of heart failure with preserved ejection fraction. Pressure-volume loops, coronary blood flow, and two-dimensional speckle tracking ultrasound were utilized in vivo under conditions of increasing peripheral mean arterial pressure and β-adrenergic stimulation 6 mo postsurgery to evaluate cardiac function. Normal diastolic function in HF-TR animals was characterized by prevention of increased time constant of isovolumic relaxation, normal LV untwisting rate, and enhanced apical circumferential and radial strain rate. Reduced fibrosis, normal matrix metalloproteinase-2 and tissue inhibitors of metalloproteinase-4 mRNA expression, and increased collagen III isoform mRNA levels (P < 0.05) accompanied improved diastolic function following chronic training. Exercise-dependent improvements in coronary blood flow for a given myocardial oxygen consumption (P < 0.05) and cardiac efficiency (stroke work to myocardial oxygen consumption, P < 0.05) were associated with preserved contractile reserve. LV hypertrophy in HF-TR animals was associated with increased activation of Akt and preservation of activated JNK/SAPK. In conclusion, chronic low-intensity interval exercise training attenuates diastolic impairment by promoting compliant extracellular matrix fibrotic components and preserving extracellular matrix regulatory mechanisms, preserves myocardial oxygen balance, and promotes a physiological molecular hypertrophic signaling phenotype in a large animal model resembling heart failure with preserved ejection fraction.

Red blood cell transfusion in critically ill children: a narrative review

OBJECTIVE

To review the pathophysiology of anemia, as well as transfusion-related complications and indications for red blood cell (RBC) transfusion, in critically ill children. Although allogeneic blood has become increasingly safer from infectious agents, mounting evidence indicates that RBC transfusions are associated with complications and unfavorable outcomes. As a result, there has been growing interest and efforts to limit RBC transfusion, and indications are being revisited and revamped. Although a so-called restrictive RBC transfusion strategy has been shown to improve morbidity and mortality in critically ill adults, there have been relatively few studies on RBC transfusion performed in critically ill children.

DATA SOURCES

Published literature on transfusion medicine and outcomes of RBC transfusion. STUDY SELECTION, DATA EXTRACTION, AND SYNTHESIS: After a brief overview of physiology of oxygen transportation, anemia compensation, and current transfusion guidelines based on available literature, risks and outcomes of transfusion in general and in critically ill children are summarized in conjunction with studies investigating the safety of restrictive transfusion strategies in this patient population.

CONCLUSIONS

The available evidence does not support the extensive use of RBC transfusions in general or critically ill patients. Transfusions are still associated with risks, and although their benefits are established in limited situations, the associated negative outcomes in many more patients must be closely addressed. Given the frequency of anemia and its proven negative outcomes, transfusion decisions in the critically ill children should be based on individual patient's characteristics rather than generalized triggers, with consideration of potential risks and benefits, and available blood conservation strategies that can reduce transfusion needs.

Cardiovascular performance of adult breeding sows fails to obey allometric scaling laws

In view of the remarkable decrease of the relative heart weight (HW) and the relative blood volume in growing pigs, we investigated whether HW, cardiac output (CO), and stroke volume (SV) of modern growing pigs are proportional to BW, as predicted by allometric scaling laws: HW (or CO or SV) = a·BW(b), in which a and b are constants, and constant b is a multiple of 0.25 (quarter-power scaling law). Specifically, we tested the hypothesis that both HW and CO scale with BW to the power of 0.75 (HW or CO = a·BW(0.75)) and SV scales with BW to the power of 1.00 (SV = a·BW(1.0)). For this purpose, 2 groups of pigs (group 1, consisting of 157 pigs of 50 ± 1 kg; group 2, consisting of 45 pigs of 268 ± 18 kg) were surgically instrumented with a flow probe or a thermodilution dilution catheter, under open-chest anesthetized conditions to measure CO and SV, after which HW was determined. The 95% confidence intervals of power-coefficient b for HW were 0.74 to 0.80, encompassing the predicted value of 0.75, suggesting that HW increased proportionally with BW, as predicted by the allometric scaling laws. In contrast, the 95% confidence intervals of power-coefficient b for CO and SV as measured with flow probes were 0.40 to 0.56 and 0.39 to 0.61, respectively, and values obtained with the thermodilution technique were 0.34 to 0.53 and 0.40 to 0.62, respectively. Thus, the 95% confidence limits failed to encompass the predicted values of b for CO and SV of 0.75 and 1.0, respectively. In conclusion, although adult breeding sows display normal heart growth, cardiac performance appears to be disproportionately low for BW. This raises concern regarding the health status of adult breeding sows.

Red blood cell transfusion in the treatment and management of anaemia: the search for the elusive transfusion trigger

Therapeutic red blood cell (RBC) transfusion is widely utilized in the management of anaemia. Critically ill intensive care unit (ICU) patients in particular, as well as medical and haematology-oncology patients, are among the largest groups of users of RBC products. While anaemia is common in these patients, its treatment and management, including appropriate thresholds for RBC transfusion, remain controversial. We review here the function of RBCs in oxygen transport and physiology, with a view to their role in supporting and maintaining systemic tissue oxygenation. Adaptive and physiological compensatory mechanisms in the setting of anaemia are discussed, along with the limits of compensation. Finally, data from clinical studies will be examined in search of evidence for, or against, a clinically relevant transfusion trigger.

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.

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.

Acute decrease in renal microvascular PO2 during acute normovolemic hemodilution

Large differences in the tolerance of organ systems to conditions of decreased O(2) delivery such as hemodilution exist. The kidney receives approximately 25% of the cardiac output and O(2) delivery is in excess of the oxygen demand under normal circumstances. In a rat model of acute normovolemic hemodilution (ANH), we studied the effect of reduced hematocrit on renal regional and microvascular oxygenation. Experiments were performed in 12 anesthetized male Wistar rats. Six animals underwent four steps of ANH (hematocrit 25, 15, 10, and <10%). Six animals served as time-matched controls. Systemic and renal hemodynamic and oxygenation parameters were monitored. Renal cortical (c) and outer medullary (m) microvascular PO(2) (microPO(2)) and the renal venous PO(2) (P(rv)O(2)) were continuously measured by oxygen-dependent quenching of phosphorescence. Despite a significant increase in renal blood flow in the first two steps of ANH, cmicroPO(2) and mmicroPO(2) dropped immediately. From the first step onward oxygen consumption (VO(2(ren))) became dependent on oxygen delivery (DO(2(ren))). With a progressive decrease in hematocrit, a significant correlation between microPO(2) and VO(2(ren)) could be observed, as well as a PO(2) gap between microPO(2) and P(rv)O(2). Furthermore, there was a high correlation between VO(2(ren)) and RBF over a wide range of flows. In conclusion, the oxygen supply to the renal tissue is becoming critical already in an early stage of ANH due to the combination of increased VO(2(ren)), decreased DO(2(ren)), and intrarenal O(2) shunt. This has clinical relevance as recent publications reporting that hemodilution during surgery forms a risk factor for postoperative renal dysfunction.

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 Po2 but elevated microcirculatory hemoglobin oxygen (HbO2) 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 HbO2 saturation (μHbO2)] and palladium-porphyrin phosphorescence quenching [microcirculatory oxygen pressure in plasma/tissue (μPo2)]. Microcirculatory oxygen shunting was assessed as the disparity between mucosal and mesenteric venous HbO2 saturation (HbO2-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 μHbO2 (68 ± 2 to 41 ± 3%) and μPo2 (28 ± 1 to 17 ± 1 Torr). Thus the proposed constellation indicative for oxygen off-load deficits (sustained μHbO2 at decreased μPo2) did not develop. A twofold increase in the HbO2-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.

Cardiovascular response to acute normovolemic hemodilution in patients with coronary artery diseases: Assessment with transesophageal echocardiography

OBJECTIVE

Preoperative acute normovolemic hemodilution induces an increase in circulatory output that is thought to be limited in patients with cardiac diseases. Using multiple-plane transesophageal echocardiography, we investigated the mechanisms of cardiovascular adaptation during acute normovolemic hemodilution in patients with severe coronary artery disease.

DESIGN

Prospective case-control study.

SETTING

Operating theater in a university hospital.

PATIENTS

Consecutive patients treated with beta-blockers, scheduled to undergo coronary artery bypass (n = 50).

INTERVENTIONS

After anesthesia induction, blood withdrawal and isovolemic exchange with iso-oncotic starch (1:1.15 ratio) to achieve a hematocrit value of 28%.

MEASUREMENTS AND MAIN RESULTS

In addition to heart rate and intravascular pressures, echocardiographic recordings were obtained before and after acute normovolemic hemodilution to assess cardiac preload, afterload, and contractility. In a control group, not subjected to acute normovolemic hemodilution, hemodynamic variables remained stable during a 20-min anesthesia period. Following acute normovolemic hemodilution, increases in cardiac stroke volume (+28 +/- 4%; mean +/- sd) were correlated with increases in central venous pressure (+2.0 +/- 1.3 mm Hg; R = .56) and in left ventricular end-diastolic area (+18 +/- 5%, R = .39). The unchanged left ventricular end-systolic wall stress and preload-adjusted maximal power indicated that neither left ventricular afterload nor contractility was affected by acute normovolemic hemodilution. Diastolic left ventricular filling abnormalities (15 of 22 cases) improved in 11 patients and were stable in the remaining four patients. Despite reduction in systemic oxygen delivery (-20.5 +/- 7%, p < .05), there was no evidence for myocardial ischemia (electrocardiogram, left ventricular wall motion abnormalities).

CONCLUSIONS

In anesthetized patients with coronary artery disease, moderate acute normovolemic hemodilution did not compromise left ventricular systolic and diastolic function. Lowering blood viscosity resulted in increased stroke volume that was mainly related to increased venous return and higher cardiac preload.

Myocardial O2 consumption in porcine left ventricle is heterogeneously distributed in parallel to heterogeneous O2 delivery

Myocardial blood flow is unevenly distributed, but the cause of this heterogeneity is unknown. Heterogeneous blood flow may reflect heterogeneity of oxygen demand. The aim of the present study was to assess the relation between oxygen consumption and blood flow in small tissue regions in porcine left ventricle. In seven male, anesthetized, open-chest pigs, local oxygen consumption was quantitated by computational model analysis of the incorporation of 13C in glutamate via the tricarboxylic acid cycle during timed infusion of [13C]acetate into the left anterior descending coronary artery. Blood flow was measured with radioactive microspheres before and during acetate infusion. High-resolution nuclear magnetic resonance 13C spectra were obtained from extracts of tissue samples (159 mg mean dry wt) taken at the end of the acetate infusion. Mean regional myocardial blood flow was stable [5.0 ± 1.6 (SD) and 5.0 ± 1.4 ml·min−1·g dry wt−1 before and after 30 min of acetate infusion, respectively]. Mean left ventricular oxygen consumption measured with the NMR method was 18.6 ± 7.7 μmol·min−1·g dry wt−1 and correlated well ( r = 0.85, P = 0.02, n = 7) with oxygen consumption calculated from blood flow, hemoglobin, and blood gas measurements (mean 22.8 ± 4.7 μmol·min−1·g dry wt−1). Local blood flow and oxygen consumption were significantly correlated ( r = 0.63 for pooled normalized data, P < 0.0001, n = 60). We calculate that, in the heart at normal workload, the variance of left ventricular oxygen delivery at submilliliter resolution is explained for 43% by heterogeneity in oxygen demand.

Strategies for minimizing blood loss in orthopedic surgery

Several major orthopedic surgical procedures including hip arthroplasty, femoral osteotomy, and spinal fusion may result in significant blood loss and the need for allogeneic blood transfusions. Due to the heightened awareness of the potential deleterious effects of allogeneic blood product administration, several techniques have been evaluated to determine their efficacy in limiting perioperative blood loss. The following article will discuss the options to limit the need for allogeneic blood product administration during orthopedic surgical procedures. These techniques include: general considerations, autologous transfusion therapy, intraoperative and postoperative blood salvage, pharmacologic manipulation of the coagulation cascade, and controlled hypotension. Undoubtedly, many of these techniques are effective alone; however, the goal of performing major orthopedic surgical procedures without the use of allogeneic blood products can only be accomplished by combining several of these techniques.

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.

Hyperoxic ventilation at the critical haematocrit

OBJECTIVE

During normovolaemic haemodilution arterial O(2)-content decreases exponentially. Nevertheless, tissue oxygenation is first maintained initially by increased organ perfusion and O(2)-extraction. As soon as these compensatory mechanisms are exhausted, myocardial ischaemia and tissue hypoxia occur at an individual 'critical' haematocrit (Hct) value. This study was conducted in order to assess whether tissue hypoxia at the critical Hct is reversed by hyperoxic ventilation with 100% O(2).

METHOD

Eighteen anaesthetized pigs were ventilated with room air and were hemodiluted by 1:1 exchange of blood with 6% pentastarch to their individual critical Hct (onset of myocardial ischaemia; significant ECG changes). At the critical Hct, hyperoxic ventilation was initiated. In nine complete datasets, global O(2) delivery and consumption, local tissue O(2) partial pressure (tpO(2)) (MDO-Electrode, Eschweiler, Kiel, Germany) and organ blood flow (microsphere method) in skeletal muscle were analyzed at baseline, after haemodilution to the critical Hct and after 15 min of hyperoxic ventilation.

RESULTS

At the critical Hct (7.2+/-1.2%), tpO(2) was reduced from 23+/-3 to 10+/-2 Torr with 50% of all values in the hypoxic range (<10 Torr, all P<0.05). During hyperoxic ventilation, contribution of physically dissolved O(2) to the O(2) delivery and O(2) consumption increased by 400 and 563% (P<0.05) and instantly restored tpO(2) to 18+/-2 Torr, (hypoxic values 25%, P<0.05).

CONCLUSION

Hyperoxic ventilation reversed tissue hypoxia at the critical Hct due to preferential utilization of plasma O(2) and allowed temporary preservation of tissue oxygenation. During haemodilution, hyperoxic ventilation might offer an effective bridge until red cells are ready for transfusion.

The effects of acute normovolemic hemodilution on left ventricular systolic and diastolic function in the absence or presence of beta-adrenergic blockade in dogs

Acute normovolemic hemodilution (ANH) increases cardiac output because of a reduction in blood viscosity and enhancement of left ventricular (LV) contractility. The status of LV function, especially LV diastolic function during ANH, remains controversial. We therefore examined LV systolic and diastolic function during ANH. Sixteen dogs were anesthetized with isoflurane in the absence (Group 1) and presence (Group 2) of beta-adrenergic blockade (propranolol 1 mg/kg). LV contractility was quantified by the slope (M(w)) of the stroke work and end-diastolic volume relation. Diastolic function was evaluated with the time constant of LV relaxation (T), chamber stiffness constant (K(c)), peak LV diastolic filling rate during early filling (peak E) and atrial contraction (peak A), and ratio of peak E to peak A (E/A). Normovolemic exchange of blood (50 mL/kg) for 6% hydroxyethyl starch (ANH50) significantly increased M(w) in Group 1 but not in Group 2. In both groups, ANH50 significantly decreased T. ANH50 significantly increased peak E in both groups and peak A in Group 1, and it did not change the E/A ratio or K(c) in either group. ANH causes positive inotropic effects and enhances diastolic function without beta-blockade. Even after beta-adrenergic blockade, ANH improves diastolic function through the reduction of LV ejection impedance.

IMPLICATIONS

Acute normovolemic hemodilution enhances LV (left ventricular) diastolic function by alterations in the LV loading condition produced by hemodilution, which mainly contributes to a compensatory increase in cardiac output.

Effects of profound hemodilution on small-intestinal wound healing in rabbits

BACKGROUND

Wound healing is influenced by tissue oxygen tension and blood perfusion, but not by moderate anemia or hemodilution. The effect of perioperative profound hemodilution on small-intestinal wound healing remains unclear.

METHODS

We performed jejunectomy followed by end-to-end anastomosis in rabbits subjected to a variety of perioperative hemodilutions: HD((HES)), hemodiluted with hydroxyethylstarch; HD((P+HES)), hemodiluted with autologous plasma and hydroxyethylstarch; HD((HES))/R, hemodiluted with hydroxyethylstarch and retransfused afterward. Intraoperative hemoglobin levels were 5 g 100 ml(-1). On Postoperative Day 5, the tensile strength (TS) of the anastomosis was measured and histological specimen was obtained. The time courses of hemoglobin, serum albumin (Alb), plasma fibrinogen (Fbg), and plasma activity of factor XIII (F XIII) were measured.

RESULTS

TS in HD((HES))/R (236.0 +/- 52.2 gf) was similar to that in control (266.5 +/- 41.6 gf); however, TS in HD((HES)) (179.8 +/- 17.9 gf) and HD((P+HES)) (165.5 +/- 14.7 gf) decreased significantly. The histological findings in HD((HES))/R were similar to those of control, whereas they demonstrated a delayed healing process in HD((HES)) and HD((P+HES)). Hemoglobin levels were still lower on Postoperative Day 5 in HD((HES)) and HD((P+HES)), but increased to 10.0 g 100 ml(-1) after retransfusion in HD((HES))/R. Hemodilution caused significant decreases in Alb, Fbg, and F XIII, but the values after retransfusion in HD((HES))/R were similar to postoperative values in HD((P+HES)).

CONCLUSION

Intraoperative profound hemodilution does not interfere with small-intestinal wound healing as long as postoperative hemoglobin levels were maintained above 10 g 100 ml(-1). Postoperative levels of other plasma constituents may not influence wound healing.

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.

Nitric oxide does not play a major role in the regulation of systemic hemodynamic responses to acute normovolemic hemodilution

BACKGROUND

The mechanisms of cardiovascular changes following acute normovolemic hemodilution (ANH) have not been fully elucidated. We tested the hypothesis that inhibition of nitric oxide synthesis attenuates ANH-induced cardiovascular responses.

METHODS

We observed the effects of N(omega)-nitro-L-arginine methyl ester (L-NAME) pretreatment on ANH-induced cardiovascular responses and compared these effects with those elicited by phenylephrine (PHE). Twenty dogs anesthetized with isoflurane were divided into two groups: one group was pretreated with L-NAME and the other with PHE. Both groups were normovolemically hemodiluted using 6% hydroxyethyl starch to reduce the hemoglobin concentration to approximately 50% of the pretreatment value.

RESULTS

Pretreatment with either L-NAME or PHE caused a significant increase in mean aortic blood pressure (MAP) and systemic vascular resistance (SVR) with a significant decrease in cardiac output (CO) and stroke volume (SV). However, no remarkable differences in these variables were seen between groups. In both groups ANH produced increases in heart rate, CO, SV, and maximal left ventricular dP/dt with a significant decrease in SVR. No significant differences in these variables were apparent after ANH except that MAP was decreased in the PHE group but not in the L-NAME group.

CONCLUSION

Our results suggest that nitric oxide does not play a major role in mediation or modulation of the systemic vascular responses to ANH.

Effects of modest anemia on systemic and coronary circulation of septic sheep

Although a lower transfusion trigger is generally recommended, little evidence is available about the physiological mechanisms of mild anemia in diseases with an imbalance between O2 supply and O2 demand such as sepsis. This study was undertaken to describe the systemic and coronary metabolic O2 reserve in an awake sheep model of hyperdynamic sepsis comparing two different hemoglobin levels. Twenty-four hours after sheep were rendered septic by cecal ligation and perforation (CLP), blood transfusion (n = 7, hemoglobin = 120 g/l) and isovolemic hemodilution (n = 8, hemoglobin = 70 g/l), respectively, were performed. Another 24 h later, we measured hemodynamics, organ blood flows, and systemic and myocardial O2 metabolism variables at baseline and through four stages of progressive hypoxia. Maximum coronary blood flow was 766.3 +/- 87.4 ml. min(-1). 100 g(-1) in hemodiluted sheep group versus 422.7 +/- 53.7 ml. min(-1). 100 g(-1) in the transfused sheep (P < 0.01). Myocardial O2 extraction was higher in the transfusion group (P = 0.03) throughout the whole hypoxia trial. In the hemodilution group, coronary blood flow increased more per increase in myocardial O(2) uptake than in transfused sheep (P < 0.01). This was accompanied by a lower left ventricular epicardial-to-endocardial flow ratio in hemodiluted sheep (1.13 +/- 0.07) than in transfused sheep (1.34 +/- 0.02, P < 0.05). We conclude that the lower coronary blood flow and greater myocardial O2 extraction in transfused septic sheep preserves transmyocardial O2 metabolism better in comparison to hemodiluted sheep.

Uncompensated blood loss is not tolerated during acute normovolemic hemodilution in anesthetized pigs

Clinically, hemodilution to a hematocrit of 9% has been studied, but the effects of hypovolemia during this degree of hemodilution have not been elucidated. We studied the response to blood loss during extreme hemodilution and evaluated indicators of hypovolemia. Systemic and myocardial hemodynamics, oxygen transport, and blood lactate concentrations were measured in 12 anesthetized pigs exposed to a graded blood loss of 10, 20, 30, and 40 mL/kg. Six animals were hemodiluted (hematocrit 10.8% +/- 1.4%, mean +/- SD), and six animals served as controls (hematocrit 34.6% +/- 1.5%). Hemodilution decreased systemic oxygen delivery to 9.5 +/- 0.6 mL x kg(-1) x min(-1) (controls 21.7 +/- 3.9 mL x kg(-1) x min(-1)) (P < 0.01) despite a 31% increase in cardiac output. Systemic oxygen uptake was unchanged. Arterial lactate increased to 3.3 +/- 1.1 mM/L (controls 1.6 +/- 0.6 mM/L) (P < 0.05), and mixed venous oxygen saturation (SvO2) decreased to 38.2% + 4.8% (controls 68.6% +/- 2.9%) (P < 0.01). At a blood loss of 10 mL/kg, cardiac output continued to be greater in the hemodiluted animals (P < 0.01). Arterial blood pressure decreased to 61 +/- 8 mmHg (controls 84 +/- 18 mm Hg) (P < 0.05), whereas heart rate was unchanged. Systemic oxygen delivery decreased to 8.8 +/- 1.2 mL x kg(-1) x min(-1) (controls 14.1 +/- 2.5 mL x kg(-1) x min(-1)) (P < 0.01). Systemic oxygen uptake was maintained by a further increase in oxygen extraction, and SvO2 decreased to 29.7% +/- 7.3%, compared with 55.3% +/- 9.0% in controls (P < 0.01). Arterial lactate increased to 4.9 +/- 1.4 mM/L (controls 1.8 +/- 0.8 mM/L) (P < 0.01). Myocardial oxygen delivery and lactate uptake were unchanged. When the blood loss equaled 30 mL/kg, myocardial lactate production occurred, and two hemodiluted animals died of circulatory failure. Central venous and capillary wedge pressures changed minimally during the blood loss and did not differ between groups. We conclude that a decrease in arterial blood pressure and SvO2 were early signs of hypovolemia during hemodilution, whereas central venous pressure and pulmonary capillary wedge pressure were insensitive indicators.

IMPLICATIONS

Anesthetized pigs with extremely low hemoglobin levels (one third of normal) showed poor tolerance to blood loss >10 mL/kg. A decreasing arterial blood pressure, a decreasing oxygen saturation in the venous blood, and an increase in arterial blood lactate concentration were useful indicators of blood loss.

Haemodynamic and metabolic changes induced by repeated episodes of hypoxia in pigs

BACKGROUND

Repeated hypoxia and surgical trauma trigger a potent neuroendocrine response and their association is thought to play a pivotal role in the pathogenesis of multi-organ dysfunction. We investigated the cardiovascular and metabolic responses to repeated acute hypoxia in anaesthetised and surgically instrumented pigs.

METHODS

Under ketamine-midazolam anaesthesia, 15 pigs were surgically instrumented for measurements of cardiac output, vascular pressures and organ blood flows. Lactate production and O2 uptake were determined in the brain, liver, kidney and intestine. Ten animals were subjected to two 12-min periods of ventilatory hypoxia (FIO2 = 7%) followed by re-oxygenation and 5 animals underwent 120-min normoxic ventilation (Control group).

RESULTS

Both hypoxic challenges produced a comparable release of catecholamines that was associated with increased cardiac output and redistribution of blood flow away from the intestinal and renal areas towards the brain and the liver; O2 uptake was markedly reduced in the intestine (-56 +/- 10%, P < 0.05) and least affected in the brain and the kidney (-19 +/- 12% and -23 +/- 21%, respectively). During the second hypoxic test, lethal cardiovascular depression occurred in 5 animals; these non-survivors demonstrated impaired hyperdynamic response and incomplete recovery of intestinal O2 uptake during the first hypoxia/reoxygenation test. In the Control group, normoxic ventilation was not associated with significant haemodynamic and metabolic changes.

CONCLUSION

Intraoperative hypoxia causes marked heterogeneity in organ blood flow and metabolism. The inability to develop a hyperdynamic cardiovascular response during a first hypoxic event, as well as a persistent intestinal O2 debt following re-oxygenation, predict the occurrence of death during the second hypoxic insult.

No release of cardiac troponin I during major orthopedic surgery after acute normovolemic hemodilution

BACKGROUND

Normovolemic hemodilution is a well-accepted method for intraoperative blood salvage. However, some controversy exists concerning the possible risk of myocardial fiber injury as consequence of the reduced oxygen content. Laboratory diagnosis of perioperative myocardial fiber injury is difficult, since biochemical markers are elevated postoperatively due to the surgical trauma. Cardiac troponin I (cTnI) is a new, highly sensitive and specific marker for the detection of myocardial injury. The aim of our study was to investigate whether normovolemic hemodilution in patients with major orthopedic surgery (13 hemodiluted patients, 15 control) induces a release of cTnI.

METHODS

cTnI as a highly specific and sensitive cardiac parameter, as well as total creatine kinase (CK), creatine kinase isoenzyme MB mass (CKMB mass) and myoglobin were measured after induction of anesthesia, after normovolemic hemodilution, prior to retransfusion of blood components, 3 h after surgery, and on the first and third postoperative days.

RESULTS

Prior to retransfusion of blood components the hematocrit was decreased to 25.4 +/- 1.2% (mean +/- SEM; range: 18%-34%) in the control group and to 20.2 +/- 0.8% (mean +/- SEM; range: 17%-24%) in the hemodilution group. Total CK, CKMB mass as well as myoglobin concentration increased significantly in both groups, reaching their maxima within the first day of surgery. In contrast, cTnI was below the detection limit of assay (< 0.5 micrograms/L) at any time.

CONCLUSIONS

We suggest that pre- and intraoperative hemodilution to a hematocrit of approximately 20% by maintaining normovolemia does not induce myocardial fiber injury in patients without preexisting cardiac diseases.

Central and mixed venous blood oxygen correlate well during acute normovolemic hemodilution in anesthetized pigs

BACKGROUND

Central venous oxygen saturation (ScvO2) and oxygen tension (PcvO2), obtained from the superior vena cava, correlate well with mixed venous (pulmonary arterial) oxygen saturation (SvO2) and tension (pvO2) when the hematocrit is normal. The present study was undertaken to assess whether extreme hemodilution affects this relation.

METHODS

We compared mixed and central venous blood during graded arterial desaturation (inspired fraction of oxygen (FIO2) between 1.0 and 0.10) in 10 hemodiluted pigs, and in 10 pigs with normal hematocrit (control), during fentanyl-ketamine-pancuronium anesthesia and mechanical ventilation.

RESULTS

Arterial oxygen saturation decreased from 100% at FIO2 = 1.0 to 44 +/- 12% at FIO2 = 0.10 (mean +/- SD). Venous oxygen saturation ranged from 3.5% to 97.3%. The regression coefficient between SvO2 and ScvO2 was 0.97 (R2 = 0.93, bias -2.4 +/- 5.8%) in the hemodiluted and 0.99 (R2 = 0.97, bias -3.0 +/- 5.0%) in the control group. Venous oxygen tension values ranged from 0.5 kPa to 9.5 kPa, and the regression coefficient for oxygen tension was 0.94 (R2 = 0.89, bias -0.20 +/- 0.47 kPa) in the hemodiluted and 0.99 (R2 = 0.97, bias -0.43 +/- 0.48 kPa) in the control group. The regression coefficient for pH was 0.95 in the hemodiluted and 0.98 in the control animals.

CONCLUSION

The findings indicate that also during hemodilution monitoring of central venous blood oxygen may be as useful as monitoring of mixed venous blood oxygen.

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.

Nitrous oxide reduces inspired oxygen fraction but does not compromise circulation and oxygenation during hemodilution in pigs

BACKGROUND

The use of nitrous oxide (N2O) during hemodilution has been questioned. Nitrous oxide reduces the inspired oxygen fraction (F1O2), depresses myocardial function and may reduce cardiac output (CO) and systemic oxygen delivery (DO2SY). The aim of this study was to evaluate the importance of the effects of nitrous oxide on systemic and myocardial circulation and oxygenation during extreme, acute, normovolemic hemodilution.

METHODS

Ten midazolam-fentanyl-pancuronium anesthetized pigs were exposed to 65% N2O before and after extreme isovolemic hemodilution (hematocrit 33 +/- 1% and 10 +/- 1%, respectively). Systemic and myocardial hemodynamics, oxygen delivery and consumption and blood lactate were measured before (at F1O2 1.0 and 0.35) and during N2O exposure.

RESULTS

Hemodilution caused an increase in CO from 137 +/- 43 to 229 +/- 32 ml.kg-1.min-1 (P < 0.01), a decrease in systemic vascular resistance (from 42 +/- 14 to 20 +/- 4 mmHg.L-1.min-1, P < 0.05), a decrease in mean arterial blood pressure (from 119 +/- 19 to 100 +/- 26 mmHg, P < 0.05) and a decrease in DO2SY from 21.1 +/- 6.9 to 13.7 +/- 2.1 ml.kg-1.min-1 (P < 0.01). Cardiac venous blood flow increased by 135% (P < 0.01) and cardiac venous saturation from 25 +/- 6 to 41 +/- 5% (P < 0.05). After hemodilution, changing F1O2 from 1.0 to 0.35 reduced arterial blood oxygen content from 59.4 +/- 3.7 to 52.3 +/- 5.1 ml.L-1 (P < 0.01), mixed venous saturation (SvO2) from 75 +/- 9 to 47 +/- 7% (P < 0.05) and DO2SY from 13.7 +/- 2.1 to 11.9 +/- 2.3 ml.kg-1.min-1 (P < 0.05). Dissolved oxygen at F1O2 = 1.0 and F1O2 = 0.35 constituted 25.4 +/- 3.1% and 10.1 +/- 1.5%, respectively, of systemic oxygen delivery after hemodilution, compared with 10.7 +/- 1.2% and 3.9 +/- 0.5% before hemodilution (P < 0.01). Left ventricular oxygen delivery and consumption were unchanged. Exposure to N2O did not affect mean arterial blood pressure or systemic vascular resistance before or after hemodilution. After hemodilution during N2O-exposure, CO and DO2SY decreased by 9% (P < 0.01 and P < 0.05, respectively), but no changes in SvO2, systemic oxygen uptake or arterial lactate were observed. The effect of N2O on myocardial oxygenation was similar before and after hemodilution; cardiac venous blood flow, left ventricular oxygen delivery and uptake decreased, but no animals showed left ventricular lactate production.

CONCLUSION

Nitrous oxide did not compromise systemic and myocardial circulation and oxygenation during acute normovolemic hemodilution in pigs. Possible adverse effects from the use of nitrous oxide during hemodilution seem to be related to a reduced F1O2, reducing the safety margin for systemic oxygen delivery.

The effects of progressive anemia on jejunal mucosal and serosal tissue oxygenation in pigs

Anemia may promote intestinal hypoxia. We studied the effects of progressive isovolemic hemodilution on jejunal mucosal (Po2muc), and serosal tissue oxygen tension (Po2ser, Clark-type surface electrodes), mucosal microvascular hemoglobin oxygen saturation (Hbo2muc), and hematocrit (Hctmuc; tissue reflectance spectophotometry) in a jejunal segment. Twelve domestic pigs were anesthetized, paralyzed, and mechanically ventilated. Laparatomy was performed, arterial supply of a jejunal segment isolated, and constant pressure pump perfused. Seven animals were progressively hemodiluted to systemic hematocrits (Hctsys) of 20%, 15%, 10%, and 6%. Baseline for Po2muc, Po2ser and Hbo2muc was 23.5 +/- 2.1 mm Hg, 57.5 +/- 4 mm Hg, and 47.0% +/- 6.4% which were not different from the five controls. Despite a significant increase in jejunal blood flow, jejunal oxygen delivery decreased and oxygen extraction ratio increased significantly at Hctsys 10% and 6%. Po2ser decreased significantly below or at Hctsys of 15%, whereas Po2muc and Hbo2muc were maintained to Hctsys of 10%, but less than 10% Hbo2muc and mesenteric venous pH decreased significantly, implying that physiological limits of jejunal microvascular adaptation to severe anemia were reached. Decrease of Hctmuc was less pronounced than Hctsys. In conclusion, redistribution of jejunal blood flow and an increase in the ratio of mucosal to systemic hematocrit are the main mechanisms maintaining mucosal oxygen supply during progressive anemia.

Hemodilution significantly decreases tolerance to isoflurane-induced cardiovascular depression

BACKGROUND

Hemodilution is used to reduce the need for allogenic blood transfusion. The aim of this study was to evaluate to what extent acute extreme normovolemic hemodilution affects the circulatory response to isoflurane.

METHODS

Ten midazolam-fentanyl-pancuronium anesthetized pigs were exposed to isoflurane at end-tidal concentrations of 0, 0.5, 1.0, 1.5 and 2%, before and after extreme normovolemic hemodilution (hematocrit 33 +/- 3% and 11 +/- 1%, respectively). Systemic and myocardial hemodynamics and oxygen delivery and consumption were measured.

RESULTS

At zero end-tidal isoflurane concentration, hemodilution caused an increase in cardiac output (from 157 +/- 12 to 227 +/- 39 ml kg min-1, P < 0.01) a decrease in systemic vascular resistance (from 39 +/- 7 to 18 +/- 5 mmHg.L-1.min-1, P < 0.01) a decrease in mean arterial blood pressure (MAP) (from 130 +/- 13 to 91 +/- 13 mmHg, P < 0.01) and a decrease in systemic oxygen delivery (from 23.1 +/- 2.7 to 11.8 +/- 1.7 ml.kg-1.min-1, P < 0.01). When the end-tidal isoflurane concentration was increased from 0 to 2% after hemodilution, cardiac output decreased by 86 +/- 37 ml.kg-1.min-1, as compared with 36 +/- 20 ml.kg-1.min-1 (P < 0.01) before hemodilution. Likewise, systemic vascular resistance decreased with increasing isoflurane concentrations; at 2%, the decrease was 7 +/- 4 mmHg.L-1.min-1 after hemodilution and 18 +/- 5 mmHg.L-1.min-1 before hemodilution (P < 0.01). At an end-tidal isoflurane concentration of 2%, MAP had decreased to 43 +/- 6 mmHg after hemodilution, and to 61 +/- 15 mmHg before hemodilution (P < 0.01). After hemodilution, isoflurane concentrations above 1% decreased systemic oxygen delivery enough to cause delivery-dependent oxygen consumption and hyperlactemia; and at 2% isoflurane, myocardial blood flow became insufficient, as indicated by myocardial lactate production.

CONCLUSIONS

isoflurane-induced cardiovascular depression had adverse effects on cardiac output and oxygen delivery during extreme hemodilution because: 1) The vasodilatory effect of isoflurane was insufficient to compensate for the myocardial depression, and also contributed to a critically low arterial blood pressure; 2) A decrease in cardiac output produced delivery-dependent oxygen consumption and hyperlactemia; and 3) A decrease in myocardial blood flow caused myocardial ischemia which may have exacerbated the myocardial depression.

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.

Does increasing oxygen delivery improve outcome? Yes

Increasing DO2 to supranormal levels, spontaneously or therapeutically, correlates with better survival in the critically ill patient, but not all patients who attain a DO2I greater than 600 mL/min/m2 survive. Conversely, there is often a 50% or greater survival rate in patients who do not reach normal DO2I values. No investigator has been able to show an incremental increase in survival with increasing DO2I; but studies have shown improved survival rates with increasing SVO2. The observations support the idea that absolute values for DO2I are not as important as the ability to normalize SVO2 when SVO2 is low. Therapeutic interventions may be most effective in those patients demonstrating increased peripheral oxygen extraction (SVO2 = 40% to 60%). These "type A" patients are mounting an appropriate response to increased needs. Several authors have noted increased mortality rates for patients unable to increase a low VO2 despite increased DO2. This is McClave's "type B" physiologic response. Flow dependency is not correlated with mortality. In fact, it is the patient who can raise VO2 when DO2 is increased who tends to survive. Dantzker, Giunta, and Hotchkiss propose that the flow dependency of VO2 may be a normal physiologic response. Clinical outcomes continue to support the necessity of maintaining an optimal DO2 in critically ill patients. The question of what is optimal DO2 has yet to be answered. Vincent nicely summaries the present "state of the art" in treating the critically ill: "Rather than aim at achieving arbitrary target values in all patients, we believe that this process should be based on a careful clinical evaluation of the individual patient, complemented by measurements of cardiac output, determinations of mixed venous oxygen saturation (or the oxygen-extraction ratio), and other measurements of tissue perfusion, such as the base deficit, blood lactate level, or gastric intramucosal pH." In addition, the type or stage of physiologic response should be identified. Independent markers of tissue ischemia should be sought and abnormalities corrected by increasing DO2. SVO2 should be normalized when low, again by increasing DO2. Data continue to support clinical interventions aimed at optimizing DO2. Does increasing DO2 increase the survival rates of critically ill patients? Sometimes.

Mechanism of systemic vasodilation during normovolemic hemodilution

In the nonfailing heart, normovolemic hemodilution increases cardiac output and decreases total peripheral resistance (TPR). Putative mechanisms mediating the decrease in TPR include reflex vasodilation and changes in the local regulation of blood flow. Our objectives were to determine whether ablation of reflex neural mechanisms or the inhibition of nitric oxide (NO) synthase, the enzyme responsible for the synthesis of the endothelium-derived relaxing factor (EDRF-NO), modulates the systemic vasodilator response to normovolemic hemodilution. Three groups of male Sprague-Dawley rats were subjected to acute normovolemic hemodilution, which was achieved by exchanging a volume of blood equivalent to 3.8% of body weight with hydroxyethyl starch. Hemodilution increased cardiac output and decreased TPR. Subsequent administration of the NO synthase inhibitor, L-nitroarginine (LNA), returned both cardiac output and TPR to control values. Pretreatment with LNA prior to hemodilution increased TPR, an effect that was partially reversed by the NO donor, sodium nitroprusside. In this setting, hemodilution failed to decrease TPR. After spinal cord destruction by "pithing," hemodilution decreased TPR to the same extent as that observed in intact rats. This hemodilution-induced decrease in TPR was abolished by the subsequent administration of LNA. These results indicate that neural reflexes do not modulate the systemic vascular response to hemodilution. Moreover, the systemic vasodilator response to hemodilution is abolished after inhibition of endogenous NO synthesis.