Effect of increased blood fluidity through hemodilution on general circulation at rest and during exercise in dogs

Recovery of hematocrit and fat deposits varies by cage size in food-restricted captive red crossbills (Loxia curvirostra)

Hematocrit-or the percent volume of red blood cells in whole blood-is thought to fluctuate adaptively in response to changing oxygen demands that occur during different life activities and in different environments. Because red blood cells are made from materials that can be limiting, however, it is thought that hematocrit may also reflect general body condition and access to resources. We tested the effect of hydration state, resource restriction (i.e., time available to forage), and activity (i.e., different cage sizes) on hematocrit in captive red crossbills (Loxia curvirostra). We found no evidence that a mild dehydration protocol impacts hematocrit and only weak support that mild food restriction impacts hematocrit. Food restriction did, however, reduce fat deposits and fat loss was more significant in birds that were also sampled for hematocrit. Furthermore, food-restricted birds housed in flight aviaries recovered hematocrit but not fat stores following repeated blood sampling, whereas birds housed in small cages lost additional hematocrit but mitigated fat loss following successive bleeds. Together these results suggest that different flight demands may determine response to blood loss during food restriction, potentially revealing a trade-off between fat storage and red blood cell development. Our results also demonstrate the need for scientists to carefully record hematocrit data and the time course across which multiple tubes of blood are collected to avoid confounding real patterns with variation generated by sampling protocol.

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

OBJECTIVE

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

DESIGN

Prospective animal study.

SETTING

University research laboratory.

PARTICIPANTS

Nine anesthetized (isoflurane) dogs.

INTERVENTIONS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSIONS

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

Red 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.

Comparison of Hemodynamic Changes After Acute Normovolemic Hemodilution Using Ringer’s Lactate Versus 5% Albumin in Patients on β-Blockers Undergoing Coronary Artery Bypass Surgery

OBJECTIVE

Acute normovolemic hemodilution (ANH) is used cautiously in coronary artery disease (CAD) patients because of concerns of compromised coronary blood flow. This study aimed to compare hemodynamic changes by using either Ringer's lactate or albumin for ANH in CAD patients receiving beta-blockers.

DESIGN

Prospective, randomized study.

SETTING

Postgraduate teaching hospital.

PARTICIPANTS

Thirty patients undergoing coronary artery bypass graft surgery (CABG) (hemoglobin >12 g/dL, on chronic beta-blocker therapy).

INTERVENTIONS

Monitoring, induction, and anesthesia followed a routine protocol for CABG surgery including pulmonary artery catheter placement. Patients were randomly included in group 1 (ANH by Ringer's lactate) or in group 2 (ANH by 5% albumin). A hemodynamic calculation software program was used for parameters recorded before and after ANH.

MEASUREMENTS AND MAIN RESULTS

ANH could not be completed in 5 patients (33%) in group 1 because of a fall in mean arterial pressure (MAP) of more than 25% from baseline. In both groups posthemodilution MAP, heart rate, systemic vascular resistance, and oxygen delivery index decreased, whereas stroke volume index, cardiac index, and tissue oxygen extraction increased significantly as compared to baseline values (p < 0.05). Hemodynamic parameters were better maintained during the study period in group 2 than group 1.

CONCLUSIONS

Hemodynamic stability was better maintained by 5% albumin than Ringer's lactate for ANH in chronic beta-blocked CAD patients. Despite an increase in cardiac index, systemic oxygen delivery was decreased irrespective of the hemodiluting fluid used. ANH to a hemoglobin value of 10 g/dL in chronically beta-blocked CAD patients was well tolerated.

Critical oxygen delivery in conscious septic rats under stagnant or anemic hypoxia

Although evidence shows that critical O2 delivery (QO2crit), the point at which oxygen consumption becomes limited by O2 delivery (QO2), is not affected by the method used to decrease QO2 in healthy subjects, microcirculatory injury caused by sepsis may modify QO2crit in a unique manner. We therefore designed this study to compare QO2crit in anemic and stagnant hypoxia in conscious septic rats. Rats were randomized to control or sepsis induced by cecal ligation and perforation; 24 hours later, oxygen consumption was measured using expired gas analysis, whereas QO2 was calculated from standard formula. Rats were further randomized to anemic hypoxia by isovolemic hemodilution or stagnant hypoxia by stepwise inflation of a balloon-tip catheter in the right atrium. QO2crit and critical hemoglobin concentration were calculated by dual linear regression analysis. We found that (1) QO2crit was not different between anemic and stagnant hypoxia in sepsis and that (2) the critical hemoglobin concentration in anemic hypoxia was similar between sepsis and control, indicating that tolerance to acute anemia is not altered by sepsis. Further studies are needed before the clinical relevance of these conclusions can be fully understood.

Cerebral and peripheral oxygen saturation during red cell transfusion

BACKGROUND

Changes in regional hemoglobin oxygen saturation occur in response to blood transfusion and can be measured by near infrared spectroscopy.

PATIENTS AND METHODS

Cerebral (CsO2) and peripheral (PsO2) oxygen saturation were monitored with an INVOS 4100 near infrared spectroscopy oximeter in 29 patients undergoing 84 intraoperative blood transfusions during aortic or spinal surgery. Hemoglobin concentration was measured before and after transfusion. Mean arterial pressure, end tidal carbon dioxide tension, and arterial oxygen saturation were also monitored.

RESULTS

Mean arterial pressure, arterial oxygen saturation and end tidal carbon dioxide tension remained stable during transfusion, while CsO2 rose by a mean (95% CI) of 4.2 (3.2-5.2%; P = 0.001) and PsO2 rose by a mean (95% CI) of 1.6 (0.3-2.8%; P = 0.016). The rise in CsO2 correlated well with the rise in hemoglobin (r = 0.59, P < 0.001) and with the volume transfused (r = 0.58, P < 0.001). PsO2 correlated with the volume transfused (r = 0.35, P = 0.019) but not with hemoglobin concentration (r = 0.08, P = 0.47).

CONCLUSIONS

Near infrared spectroscopy detected significant rises in tissue oxygenation in response to blood transfusion, particularly in the cerebral cortex. CsO2 may be developed into a blood loss monitor if further research confirms our findings.

Beta-adrenergic stimulation restores oxygen extraction reserve during acute normovolemic hemodilution

Compensatory increases in oxygen extraction (EO(2)) during acute normovolemic hemodilution (ANH) have the effect of decreasing tissue oxygen tension values, thus increasing the threat of tissue hypoxia. We hypothesized that if the beta-adrenergic agonist isoproterenol (ISOP) could augment cardiac output (CO) during ANH, it could reverse the increases in EO(2) and restore the margin of safety for tissue oxygenation. Studies were performed in seven anesthetized (isoflurane) dogs. CO was measured by using thermodilution, and regional blood flow (RBF) was measured by using radioactive microspheres. Systemic oxygen delivery (DO(2)), oxygen consumption (OV0312;O(2)), and EO(2), as well as regional DO(2), were calculated. Measurements were obtained under the following conditions in each dog: 1) baseline-1, 2) ISOP (0.1 micro g. kg(-1). min(-1) IV), 3) baseline-2, 4) ANH, and 5) ISOP during ANH. Hematocrit was 45% +/- 3% under baseline conditions and 18% +/- 3% during ANH. Before ANH, ISOP caused parallel increases in CO and systemic DO(2), which, in the presence of an unchanged OV0312;O(2), reduced EO(2). RBF increased in myocardium and spleen, decreased in pancreas, and did not change in brain, spinal cord, or other tissues. ANH caused increases in CO, which were insufficient to offset the decrease in arterial oxygen content, and thus systemic DO(2) declined; systemic OV0312;O(2) was maintained by an increase in EO(2). ANH-related increases in RBF maintained DO(2) in myocardium, brain, duodenum, and pancreas, whereas DO(2) declined in kidney and spleen. ISOP during ANH increased CO and systemic DO(2), which returned systemic EO(2) to baseline, and it increased RBF in myocardium, kidney, duodenum, and spleen. We conclude that 1) beta-adrenergic stimulation with ISOP restored the systemic EO(2) reserve during ANH, without apparent adverse effects in the individual body tissues, and that 2) the use of inotropic drugs, such as ISOP, may extend the limit to which hematocrit can be reduced safely during ANH.

IMPLICATIONS

By restoring the oxygen extraction reserve, isoproterenol and other inotropic drugs can enhance the margin of safety and extend the limit to which hematocrit can be reduced safely during acute normovolemic hemodilution. The use of this approach will depend on the degree of hemodilution, the extent of mixed venous oxygen desaturation, and whether increases in cardiac output are possible or desirable.

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.

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.

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.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Microcirculation in striated muscle after acute reduction in systemic hematocrit

Capillary blood flow variables were measured in cremaster muscles of anesthetized Golden hamsters after isovolemic hemodilution with donor hamster plasma to mean systemic hematocrits of 49% of normal (Group 1) and 32% of normal (Group 2). Despite their different systemic hematocrits, Groups 1 and 2 showed no significant differences in any measured capillary variable. Capillary cell content decreased significantly after hemodilution from 9.4 +/- 0.8 (SE) in controls to 5.8 +/- 0.7 (Group 1) and 5.0 +/- 0.5 cells/100 microns (Group 2). Hemodilution did not change resting capillary cell velocity or capillary cell flux. Velocities were 115 +/- 14, 186 +/- 25 and 203 +/- 23 microns/sec in controls. Group 1 and Group 2, respectively, while the corresponding mean values for cell flux were 9.8 +/- 2.0, 11.5 +/- 2.1 and 11.9 +/- 2.4 cells/sec. Functional capillary density was unchanged after hemodilution, but tissue cell capacity (estimated from cell content and capillary density) attained only 50% of the expected value, being 26% (Group 1) and 16% (Group 2) of control. Thus, none of the changes in indices of capillary cell flow, capillary cell content or tissue cell capacity were directly proportional to the changes in systemic hematocrit. This suggests that tissue oxygen delivery from capillaries cannot be predicted from measured changes in systemic oxygen transport capacity.

Regional hemodynamics and oxygen supply during isovolemic hemodilution in the absence and presence of high-grade β-adrenergic blockade

Studies were performed in 16 pentobarbital-anesthetized dogs to evaluate regional circulatory effects of isovolemic hemodilution in the absence (group 1) and presence (group 2) of high-grade beta-adrenergic blockade with propranolol. Regional blood flow measured with 15 microm radioactive microspheres was used to calculate regional oxygen supply. In group 1, hemodilution with 5% dextran (40,000 molecular weight) reduced arterial hematocrit and oxygen content by approximately one half and had heterogeneous effects on regional blood flows. Blood flow was unchanged in the renal cortex, liver, and spleen, and it increased in the pancreas, duodenum, brain, and myocardium; however, only in the brain and myocardium were increases in blood flow sufficient to maintain oxygen supply at baseline (pre-hemodilution) levels. In group 2, intravenous administration of propranolol (1 mg/kg) itself decreased blood flow in the spleen and myocardium and had no other regional effects. Hemodilution after propranolol caused regional circulatory changes that were essentially similar to those in the absence of propranolol. It is concluded that (1) during isovolemic hemodilution, oxygen supply to the brain and myocardium is maintained at the expense of oxygen supply to less critical organs, and (2) this pattern of regional circulatory response during hemodilution remains intact in the presence of high-grade beta-adrenergic blockade with propranolol.

A new nomogram facilitating adequate haemodilution

When using deliberate haemodilution to a certain haematocrit value (Hct), the appropriate preoperative blood volume of the patient must be determined and matched with the transfusion volume at a certain blood loss. In order to facilitate such calculations a nomogram was constructed, aiming for a final Hct of 33%. Preoperative Hct, height, weight and sex of the patient are input variables. After drawing three straight lines, the nomogram yields the normal blood volume and the acceptable pre-transfusion blood loss (BL). This nomogram was used during surgery when the preoperative Hct exceeded 35%. Protocols from 100 patients bleeding more than 50% of their BL were studied. Blood loss was 1.1 +/- 0.6 1 (mean +/- s.d.) ranging from 0.4 to 4.0 1. Fifty-one of the patients received blood transfusion. This program resulted in a decrease of Hct (mean +/- s.d.) from 41 +/- 3% preoperatively to 33 +/- 4% during the first 30 min postoperatively. Sixty-three of the patients had a final Hct of 30-35%, 13 had 27-29% and one had 26%. The low values were most likely due to underestimation and consequent unsubstituted blood loss. In summary, the nomogram makes time-consuming mathematical operations unnecessary. It was easy to use and the postoperative Hct was close to that desired in most patients.

Oxygen transport during hemodilution in normoxic and hypoxic dogs treated with verapamil

In order to determine the possible limitations of acute isovolemic hemodilution in patients taking verapamil, the various factors determining oxygen availability were analyzed in an animal experiment. Twenty-four anaesthetized dogs were subjected to a gradual isovolemic hemodilution. Six dogs received a bolus injection of 0.1 mg kg-1 of verapamil followed by 0.01 mg kg-1 min-1. In 12 dogs, mild hypoxic hypoxia was induced by ventilation with 16-17% oxygen; six of them also received verapamil. Six dogs served as a control group. In the latter, due to an increase in heart rate and stroke volume, oxygen availability in the clinically relevant range of hemodilution between HC 25 and 25% was maintained at 81% of its control value. In normoxic animals treated with verapamil, oxygen availability decreased more rapidly and was below the level of the control group once HC reached 25%. Mild hypoxic hypoxia alone did not reduce oxygen availability as much as its combination with verapamil. Even though the verapamil-induced reduction in oxygen availability was similar during normoxia and hypoxia, the sharp rise in serum lactate at HC levels below 35-30% in the hypoxic verapamil group was a sign of impairment of tissue oxygenation. Hematocrit levels below 35-30% and even moderate hypoxemia should be avoided whenever the cardiovascular response to hemodilution is influenced by verapamil.

The effects of hemodilution during pulmonary edema in dogs

Because of their multiple medical problems, patients with the adult respiratory distress syndrome (ARDS) often develop anemia. In order to determine the effects of a low hemoglobin concentration on gas exchange in such patients, the authors studied the effects of isovolemic hemodilution in the dog oleic acid model of ARDS. Twelve splenectomized dogs with oleic acid-induced pulmonary edema and a consequent venous admixture of 31% +/- 5% (mean +/- SEM) (FIO2 = 0.21) underwent two-stage isovolemic hemodilution with Hetastarch followed by retransfusion of the withdrawn red cells. This resulted in hemoglobin levels at each stage of 12.7 +/- 0.7 g/100 ml, 9.1 +/- 0.6 g/100 ml, 6.5 +/- 0.5 g/100 ml, and 10.1 +/- 0.5 g/100 ml (mean +/- SEM). Oxygen transport fell from 363 +/- 25 ml/kg/min to 219 +/- 17 ml/kg/min (p less than 0.001) at maximum hemodilution during air ventilation and from 383 +/- 79 ml/kg/min to 292 +/- 91 ml/kg/min (p less than 0.001) during oxygen ventilation. Since oxygen consumption remained constant throughout the hemoglobin range studied, decreased hemoglobin resulted in declines in P-VO2. Hemodilution with Hetastarch did not affect intrapulmonary shunt or venous admixture despite the significant increase in cardiac output associated with hemodilution.

Hemodilution -- new clothes for an anemic emperor

This review deals with the rationale for the use of hemodilution in patients not subjected to open heart surgery. The claim for an optimum of circulatory oxygen transport at 30% hematocrit has been disproved; hemodilution thus simply means acute normovolemic anemia. Accordingly, it generates a cardiovascular strain and particularly jeopardizes cerebral and myocardial oxygen supply. Potentially serious clinical side effects have been reported. Hemodilution should therefore not be carried beyong the lower normal range for the hemoglobin or hematocrit level, i.e. 12--12.5 g% or 35--36%.

Optimal hematocrit for canine skeletal muscle during rhythmic isotonic exercise

Contractile power, blood flow, O2-uptake, and O2-extraction during isotonic, rhythmic exercise were determined in the isolated canine gastrocnemius muscle during perfusion with blood with hematocrits between 0.21 and 0.81. The results obtained in 36 measurements on nine muscles showed that maximal O2-delivery to the muscle if found at hematocrits between 0.5 and 0.6. Both in the range of hemodilution, and in the range of extreme hemoconcentration, O2-delivery decreases significantly. O2-consumption and contractile power of the muscles are almost unaffected in the hematocrit range between 0.4 and 0.7; beyond and below this hematocrit range both parameters decrease. O2-extraction is virtually constant in the hematocrit range between 0.3 and 0.6, but increase both below and above these hematocrit levels. It is concluded that due to reduced vasodilatory reserve in working skeletal muscle compared to resting muscle the optimal hematocrit is shifted to higher values.

Exercise induced augmentation of myocardial oxygen extraction in spite of normal coronary dilatory capacity in dogs

Myocardial O2-extraction rate was studied during exercise induced augmentation of cardiac work in dogs. The O2-extraction rate at rest was 75% of arterial content. Progressive levels of exercise increased the animals' O2-consumption from 7 ml/min-kg up to 91 ml/min-kg. Cardiac output rose from 108 ml/min-kg at rest to 484 ml/min-kg at the highest exercise level. The increase in myocardial O2-consumption from 9 ml/min-100 g at rest up to 57 ml/min-100g at the highest exercise level was met by an increase in coronary flow from 59 to 256 ml/min-100 g and a rise of myocardial AVDO2 from 15 to 22 Vol%. Thus the latter contributed 40% to the augmented myocardial O2-requirements. Coronary venous O2-saturation decreased to 9% saturation during highest levels of exercise. This low value was not the result of a limited coronary dilatory capacity, of inadequate state of exercise training, or of a relative underperfusion of the inner layers of the left ventricle. Thus, augmentation of myocardial O2-extraction rate seems to be a mechanism of physiological relevance during exercise induced elevation of myocardial O2-requirements in dogs and may be explained by capillary recruitment in the myocardium.

Transmural differences in myocardial blood flow and in coronary dilatory capacity in hemodiluted conscious dogs

In 16 conscious resting dogs regional myocardial blood flow and the local coronary dilatory capacity were studied with the particle distribution technique during isovolemic hemodilution (hct = 13%). Postischemic peak coronary hyperemia following release of temporary circumflex coronary artery occlusion was used for quantification of regional coronary dilatory capacity. In hemodilution (arterial blood oxygen content less than one third of normal) left ventricular blood flow (LVBF) was 460 +/- 36 ml/100 g - min, subendocardial/subepicardial flow amounted to 1.3 +/- 0.1. During postischemic peak hyperemia LVBF increased by 33% up to 606 +/- 63 ml/100 g - min. This 33% increase in LVBF was distributed mainly to the subepicardial layer, while in the subendocardial layer there was no significant flow increase. It is concluded that the increase in heart rate and systolic coronary vascular compression in addition to the lowered arterial oxygen content lead to exhaustion of the dilatory reserve in the subendocardium during hemodilution. Therefore the remaining overall dilatory capacity is without functional significance.

Effect of increased blood fluidity through hemodilution on coronary circulation at rest and during exercise in dogs

Coronary flow and myocardial oxygen consumption were measured in conscious dogs at rest and during two levels of submaximal treadmill exercise (3 and 7 km/h at 15% grade, respectively) during adaptation to progressive hemodilution with dextran 60. At rest coronary flow increased to more than seven-fold with diminishing hematocrit to 12.5% in order to cover myocardial oxygen consumption which increased from 6.5 +/- 0.3 ml/min with 100 g at hematocrit 47.5% to 13.5 +/- 0.8 ml/min with 100 g at hematocrit 12.5%. The dilatory capacity of the coronary vessels, estimated from the reactive hyperemia after a 12 sec occlusion of the left circumflex coronary artery, dropped from 602% at control to 45% at lowest hematocrit levels. During the superimposed stress of exercise coronary flow and myocardial oxygen consumption increased further, so that the dilatory capacity of the coronaries was exhausted at hematocrit levels between 16 and 22%. Myocardial oxygen consumption per unit of oxygen delivered to peripheral tissues increased substantially with progressive hemodilution. In the presence of the reduced arterial oxygen content the augmented myocardial oxygen demand limits the overall adaptability to hemodilution by an exhaustion of the coronary dilatory capacity.