The Limit of Anemia Tolerance during Hyperoxic Ventilation with Pure Oxygen in Anesthetized Domestic Pigs

Abstract

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' (Hb_crit). 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 Hb_crit 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 Hb_crit, 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.

The effects of hyperoxaemia on tissue oxygenation in patients with a nadir haematocrit lower than 20% during cardiopulmonary bypass

“…28 They found that HV with FiO 2 100% only provided a marginal increase in anaemia tolerance compared with animals ventilated with FiO 2 60% in their previous study. 27,28…”

“…27 Pape et al also examined whether HV with FiO 2 100% could increase anaemia tolerance and allow for a more extensive haemodilution compared to HV with FiO 2 60%. 28 They found that HV with FiO 2 100% only provided a marginal increase in anaemia tolerance compared with animals ventilated with FiO 2 60% in their previous study. 27,28 Much less information is available about the relationship between hyperoxaemia and tissue oxygenation during CPB.…”

“…28 They found that HV with FiO 2 100% only provided a marginal increase in anaemia tolerance compared with animals ventilated with FiO 2 60% in their previous study. 27,28 Much less information is available about the relationship between hyperoxaemia and tissue oxygenation during CPB. Parolari et al examined the relationship between VO 2 , DO 2 and haemodynamics in a clinical perfusion setting during a moderately hypothermic CPB.…”

Evidence Base for Restrictive Transfusion Triggers in High-Risk Patients

“…As a consequence and without further treatment, animals die within 3 h after reaching the Hb(crit) [15]. By increasing the inspired O 2 concentration, an exaggerated level of hemodilution with a lower Hb(crit) can be reached [16]. …”

Thoracic Epidural Anesthesia with Ropivacaine Does Not Compromise the Tolerance of Acute Normovolemic Anemia in Pigs

“…[13][14][15] Hb crit has been chosen as the primary endpoint in a variety of studies investigating the impact of interventions on the limit of tolerance to acute anemia. As reflected by reduced values of Hb crit (listed in table 2), tolerance to anemia was found to be increased by (1) hypothermia (moderate reduction of body core temperature reduces total body oxygen demand), 16 (2) hyperoxic ventilation (excellent bioavailability of oxygen physically dissolved in the plasma), 13,[17][18][19] (3) infusion of norepinephrine (stabilization of coronary perfusion pressure during hemodilution) 20 or infusion of artificial oxygen carriers (maintenance of arterial oxygen content despite reduced hematocrit), 15,21,22 (4) deep neuromuscular block (lowering skeletal muscular oxygen demand), 23 and (5) use of colloidal rather than crystalloid fluids for volume replacement (maintenance of microvascular perfusion due to reduced extravasation rate). 24 In contrast, tolerance to anemia was reduced by (1) hypovolemia (reduction of microcirculatory oxygen supply), (2) profound anesthesia (pharmacologic reduction of cardiac output), 25 and (3) coronary artery disease (reduced coronary flow reserve).…”