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

Perioperative blood conservation strategies for major spine surgery

BACKGROUND

Orthopedic surgery, especially spine and spinal deformity surgery, may be associated with high perioperative blood loss. In order to reduce the risk of excessive blood loss and unnecessary blood transfusions, strategies such as Patient Blood Management including goal-directed coagulation management have been developed.

RECENT FINDINGS

Adverse effects of allogeneic blood transfusions have been shown for most surgical fields including orthopedic surgery. Several efforts have been made to increase the preoperative red blood cell (RBC) mass, to reduce the intraoperative blood loss, and to use restrictive transfusion triggers in order to minimize or avoid RBC transfusions. Measures to reduce intraoperative blood loss include new surgical techniques, use of cell salvage where possible, bedside coagulation management with point-of-care devices, substitution of coagulation factors, antifibrinolytic agents, and desmopressin, induced hypotension, and avoidance of hypothermia.

SUMMARY

Blood conservation in spinal surgery is a multidisciplinary approach and the efficacy of most single measures has been shown. Cost-effectiveness and the benefits of long-term patient outcomes are the subjects of current and future research.

Evidence base for restrictive transfusion triggers in high-risk patients

Liberal versus restrictive red blood cell (RBC) transfusion triggers have been debated for years. This review illustrates the human body's physiologic response to acute anemia and summarizes the evidence from prospective randomized trials (RCTs) for restrictive use of RBC transfusions in high-risk patients. During progressive anemia, the human body maintains the oxygen delivery to the tissues by an increase in cardiac output and peripheral oxygen extraction. Seven RCTs with a total of 5,566 high-risk patients compared a restrictive hemoglobin (Hb) transfusion trigger (Hb < 70 or < 80 g/l) with a liberal Hb transfusion trigger (Hb < 90 or < 100 g/l). Unanimously these studies show non-inferiority, safety, and a significant reduction in RBC transfusions in the restrictive groups. In one RCT mortality was higher in the liberal Hb transfusion group, and in two additional RCTs mortality of subgroups or after risk adjustment was significantly higher in the liberal Hb transfusion trigger groups.

CONCLUSION

Strong RCT evidence suggests the safety of restrictive transfusion triggers. As a consequence, an Hb transfusion trigger of <70 g/l is recommended for high risk patients.

Thoracic epidural anesthesia with ropivacaine does not compromise the tolerance of acute normovolemic anemia in pigs

BACKGROUND

The initial treatment of an acute blood loss with acellular fluids leads to the dilution of the red cell mass remaining in the vasculature, that is, to acute normovolemic anemia. Whether the compensation and, thus, the tolerance of acute anemia, are affected by sympathetic block induced by thoracic epidural anesthesia has not yet been investigated.

METHODS

Eighteen anesthetized and mechanically ventilated pigs were instrumented with thoracic epidural catheters and randomly assigned to receive an epidural injection of either 5-ml ropivacaine 0.2% (n = 9) aiming for a Th5-Th10 block or saline (n = 9) followed by continuous epidural infusion of 5 ml/h of either fluid. Subsequently, acute normovolemic anemia was induced by replacement of whole blood with 6% hydroxyethyl starch solution until a "critical" limitation of oxygen transport capacity was reached as indicated by a sudden decrease in oxygen consumption. The critical hemoglobin concentration quantified at this time point was the primary endpoint; secondary endpoints were hemodynamic and oxygen transport parameters.

RESULTS

Thoracic epidural anesthesia elicited only a moderate decrease in mean arterial pressure and cardiac index and a transient decrease in oxygen extraction ratio. During progressive anemia, the compensatory increases in cardiac index and oxygen extraction ratio were not compromised by thoracic epidural anesthesia. Critical hemoglobin concentration was reached at identical levels in both groups (ropivacaine group: 2.5 ± 0.6 g/dl, saline group: 2.5 ± 0.6 g/dl).

CONCLUSION

Thoracic epidural anesthesia with ropivacaine 0.2% does not decrease the tolerance to acute normovolemic anemia in healthy pigs. The hemodynamic compensation of acute anemia is fully preserved despite sympathetic block, and the critical hemoglobin concentration remains unaffected.

The limit of anemia tolerance during hyperoxic ventilation with pure oxygen in anesthetized domestic pigs

BACKGROUND

During acellular replacement of an acute blood loss, hyperoxic ventilation (HV) increases the amount of O2 physically dissolved in the plasma and thereby improves O2 supply to the tissues. While this effect could be demonstrated for HV with inspiratory O2 fraction (FiO2) 0.6, it was unclear whether HV with pure oxygen (FiO2 1.0) would have an additional effect on the physiological limit of acute normovolemic anemia.

METHODS

Seven anesthetized domestic pigs were ventilated with FiO2 1.0 and subjected to an isovolemic hemodilution protocol. Blood was drawn and replaced by a 6% hydroxyethyl starch (HES) solution (130/0.4) until a sudden decrease of total body O2 consumption (VO2) indicated the onset of O2 supply dependency (primary endpoint). The corresponding hemoglobin (Hb) concentration was defined as 'critical Hb' (Hbcrit). Secondary endpoints were parameters of myocardial function, central hemodynamics, O2 transport and tissue oxygenation.

RESULTS

HV with FiO2 1.0 enabled a large blood-for-HES exchange (156 ± 28% of the circulating blood volume) until Hbcrit was met at 1.3 ± 0.3 g/dl. After termination of the hemodilution protocol, the contribution of O2 physically dissolved in the plasma to O2 delivery and VO2 had significantly increased from 11.7 ± 2 to 44.2 ± 9.7% and from 29.1 ± 4.2 to 66.2 ± 11.7%, respectively. However, at Hbcrit, cardiovascular performance was found to have severely deteriorated.

CONCLUSION

HV with FiO2 1.0 maintains O2 supply to tissues during extensive blood-for-HES exchange. In acute situations, where profound anemia must be tolerated (e.g. bridging an acute blood loss until red blood cells become available for transfusion), O2 physically dissolved in the plasma becomes an essential source of oxygen. However, compromised cardiovascular performance might require additional treatment.

Monitoring of intense neuromuscular blockade in a pig model

Intense neuromuscular blockade (NMB) measured by post tetanic count (PTC) was monitored, reversed and verified in this pig model. In a cross-over assessor blinded design six pigs were randomized to either no NMB followed by intense NMB, or intense NMB followed by no NMB. Neuromuscular measurements were performed with acceleromyography [train-of-four (TOF) Watch SX]. In all pigs, the response to TOF nerve stimulation was stable and intense NMB (PTC 0-1) was established with rocuronium 3 mg/kg. For reversal, the pigs received sugammadex 20-35 mg/kg and returned to TOF-ratio above 0.90 within 2.15 min after injection. We established a pig model for monitoring intense NMB with surface stimulation electrodes and acceleromyography. We verified total relaxation of the diaphragm and the abdominal muscles at the PTC 0-1 by suction test and with surface electromyography. This pig model is suitable for studies with experimental abdominal surgery with monitoring of intense NMB, and where relaxation of the diaphragm and the abdominal muscles are required.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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