Hemodilution significantly decreases tolerance to isoflurane-induced cardiovascular depression

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.

Maximal hysteresis: a new method to set positive end-expiratory pressure in acute lung injury?

BACKGROUND

No methods are superior when setting positive end-expiratory pressure (PEEP) in acute lung injury (ALI). In ALI, the vertical distance (hysteresis) between the inspiratory and expiratory limbs of a static pressure-volume (PV) loop mainly indicates lung recruitment. We hypothesized that PEEP set at the pressure where hysteresis is 90% of its maximum (90%MH) would give similar oxygenation, but less cardiovascular depression than PEEP set at the pressure at lower inflection point (LIP) on the inspiratory limb or at the point of maximal curvature (PMC) on the expiratory limb in ALI.

METHODS

In 12 mechanically ventilated pigs, ALI was induced in a randomized fashion by lung lavage, lung lavage plus injurious ventilation, or by oleic acid. From a static PV loop obtained by an interrupted low-flow method, the pressures at LIP [25 (25, 25) cmH(2)O, mean and 25, 75 percentiles], at PMC [24 (20, 24) cmH(2)O], and at 90% MH [19 (18, 19) cmH(2)O] were determined and used for the PEEP-settings. We measured lung inflation (by computed tomography), end-expiratory lung volume (EELV), airway pressures, compliance of the respiratory system (Crs), blood gases, cardiac output and arterial blood pressure.

RESULTS

There were no differences between the PEEP settings in EELV or oxygenation, but the 90%MH setting gave lower end-inspiratory pause pressure (P<0.025), higher Crs (P<0.025), less hyper-aeration (P<0.025) and better maintained hemodynamics.

CONCLUSION

In this porcine lung injury model, PEEP set at 90% MH gave better lung mechanics and hemodynamics, than PEEP set at PMC or LIP.

Subcutaneous Po2 as an index of the physiological limits for hemodilution in the rat

This study was designed to test the hypothesis that changes in subcutaneous Po2 (PscO2) during progressive hemodilution will reliably predict a “critical point” at which tissue O2 consumption (V̇o2) becomes dependent on O2 delivery (Q̇o2). Twelve pentobarbital-anesthetized male Sprague-Dawley rats (315–375 g) underwent stepwise exchange of plasma for blood (1.5 ml of plasma for each 1 ml of blood lost). The initial exchange was equal to 25% of the estimated circulatory blood volume, and each subsequent exchange was equal to 10% of the estimated circulatory blood volume. After nine exchanges, the hematocrit (Hct) fell from 42 ± 1 to 6 ± 1%. Cardiac output and O2 extraction rose significantly. PscO2 became significantly reduced ( P < 0.05) after exchange of 45% of the blood volume (Hct = 16 ± 1%). V̇o2 became delivery dependent when Q̇o2 fell below 21 ml·min−1·kg body wt−1 (mean Hct = 13 ± 1%). Eight control rats undergoing 1:1 blood-blood exchange showed no change in PscO2, pH, HCO3−, or hemodynamics. Measurement of PscO2 may be a useful guide to monitor the adequacy of Q̇o2 during hemodilution.

Anesthetic management of a patient with β-thalassemia intermedia undergoing splenectomy: a case report

A 37-year-old man with beta-thalassemia intermedia (betaTI), a rare disease caused by partial or complete deficiency of beta-globin chain synthesis, fell into a hemolytic crisis. Severe anemia persisted despite frequent transfusions. Therefore, he was scheduled for splenectomy to alleviate the anemia. The preoperative laboratory data showed marked anemia and liver dysfunction. Echocardiography revealed hyperkinetic left ventricular motion and increased cardiac index (CI), indicating a compensatory hyperdynamic circulation induced by persistent, severe anemia. Our strategy during general anesthesia was to keep the hyperkinetic cardiovascular system steady. Hence, the hemodynamic parameters including the CI were measured using a Swan-Ganz catheter, and other physiological parameters were monitored perioperatively. Anesthesia was maintained with balanced anesthesia: isoflurane at low concentrations and fentanyl to avoid cardiovascular depression. Throughout the operation, vital signs were kept stable and the lactate/pyruvate ratio was unchanged, indicating that anaerobic metabolism did not increase. We report successful anesthetic management with attention to hemodynamic changes in a patient with betaTI.

Influence of crystalloid and colloid replacement solutions on hemodynamic variables during acute normovolemic hemodilution

STUDY OBJECTIVE

To determine whether, in maintaining normovolemia during acute normovolemic hemodilution, replacement fluid choice influences intraoperative hemodynamic variables.

DESIGN

Prospective, randomized, single-blinded study.

SETTING

Operating room of a tertiary-care university hospital.

PATIENTS

40 adult, ASA physical status I, II, and III patients scheduled for acute normovolemic hemodilution during radical prostatectomy.

INTERVENTIONS

Patients were randomly assigned to four replacement fluid groups to receive 1) Ringer's lactate, 2) 5% albumin, 3) 6% dextran 70, or 4) 6% hetastarch. A standardized general anesthetic was used, and patients underwent moderate hemodilution to a target hemoglobin of 9 gm/dL.

MEASUREMENTS

Hemodynamic variables were recorded using standard monitors, 5-lead electrocardiography, radial arterial catheter, and pulmonary artery catheter. Red blood cell loss for the entire hospitalization was calculated.

MAIN RESULTS

Demographic and clinical outcome data were similar among the groups. During acute normovolemic hemodilution, heart rate and pulmonary capillary wedge pressure were unchanged from baseline in all groups, but patients receiving Ringer's lactate or albumin had greater declines in mean arterial pressure at the end of acute normovolemic hemodilution. Cardiac and oxygen consumption indexes were stable during acute normovolemic hemodilution, but oxygen extraction increased.

CONCLUSIONS

During hemodilution, anesthetized patients maintain whole body oxygenation by increasing oxygen extraction. The administration of hetastarch or dextran as the replacement fluid during acute normovolemic hemodilution is associated with a more stable mean arterial pressure, but overall acute normovolemic hemodilution is well tolerated irrespective of the replacement fluid used.

Comparison of transfusion with DCLHb or pRBCs for treatment of intraoperative anemia in sheep

Isoflurane-anesthetized sheep were transfused with packed red blood cells (pRBCs) or diaspirin cross-linked hemoglobin (DCLHb) for treatment of intraoperative hemorrhage. A rapid 15-min hemorrhage with lactated Ringer (LR) infusion maintained filling pressure at baseline and reduced blood hemoglobin (Hb) to ~5 g/dl. Sheep received 2 g/kg Hb, DCLHb (n = 6), or pRBCs (n = 7); control group received LR alone (n = 6). After 2 h, anesthesia was discontinued; sheep were monitored in the animal intensive care unit for 48 h. DCLHb expanded blood volume more, but increased total blood Hb less, than pRBCs. Lower Hb and increased methemoglobin resulted in lower arterial oxygen content compared with the pRBCs. DCLHb caused pulmonary hypertension (from 13 to 30 mmHg) and elevated filling pressure (from 6 to 15 mmHg). Cardiac outputs (CO) were similar for all groups during anesthesia; however, during recovery CO increased only in the LR and packed pRBCs groups. DCLHb may limit the reflex ability to increase CO after volume expansion. Hemodynamic effects of DCLHb may be exaggerated when infused after large-volume LR.

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.