Venous oxygen saturation as a physiologic transfusion trigger

Abstract

This article is one of ten reviews selected from the Yearbook of Intensive Care and Emergency Medicine 2010 (Springer Verlag) and co-published as a series in Critical Care. Other articles in the series can be found online at http://ccforum.com/series/yearbook. Further information about the Yearbook of Intensive Care and Emergency Medicine is available from http://www.springer.com/series/2855.

Introduction

Venous oxygen saturation is a clinical tool which integrates the whole body oxygen uptake-to-delivery (VO₂-DO₂) relationship. In the clinical setting, in the absence of pulmonary artery catheter (PAC)-derived mixed venous oxygen saturation (SvO₂), the central venous oxygen saturation (ScvO₂) is increasingly being used as a reasonably accurate surrogate [1]. Central venous catheters (CVCs) are simpler to insert, and generally safer and cheaper than PACs. The CVC allows sampling of blood for measurement of ScvO₂ or even continuous monitoring if an oximetry catheter is being used. The normal range for SvO₂ is 68 to 77% and ScvO₂ is considered to be 5% above these values [2].

A decrease in hemoglobin (Hb, g/dl) is likely to be associated with a decrease in DO₂ when cardiac output (CO) remains unchanged, since DO₂ = CO × CaO₂, where CaO₂ is arterial oxygen content and is ≈ Hb × SaO₂ × 1.34 (where SaO₂ is the arterial oxygen saturation in%; and 1.34 is the oxygen-carrying capacity of Hb in mlO₂/g Hb), when one ignores the negligible oxygen not bound to Hb [1]. A decrease in Hb is one of the four determinants responsible for a decrease in SvO₂ (or ScvO₂), alone or in combination with hypoxemia (decrease in SaO₂), an increase in VO₂ without a concomitant increase in DO₂, or a fall in cardiac output.

When DO₂ decreases, VO₂ is maintained (at least initially) by an increase in oxygen extraction (O₂ER) since O₂ER = VO₂/DO₂. As VO₂ ≈ (SaO₂ - SvO₂) × (Hb × 1.34 × CO) and DO₂ ≈ SaO₂ × Hb × 1.34 × CO, O₂ER and SvO₂ are thus linked by a simple equation: O₂ER ≈ (SaO₂ - SvO₂)/SaO₂ or even simpler: O₂ER ≈ 1 - SvO₂. Assuming SaO₂ = 1 [3], if SvO₂ is 40%, then O₂ER is 60%.

Because it integrates Hb, cardiac output, VO₂ and SaO₂, the venous oxygen saturation therefore helps to assess the VO₂-DO₂ relationship and tolerance to anemia during blood loss.

Venous oxygen saturation as a physiologic transfusion trigger

When DO₂ decreases beyond a certain threshold, it induces a decrease in VO₂. This point is known as the critical DO₂ (DO₂crit), below which there is a state of VO₂-DO₂ dependency also called tissue dysoxia. In humans, dysoxia is usually present when SvO₂ falls below a critical 40-50% (SvO₂crit); this may, however, also occur at higher levels of SvO₂ when O₂ER is impaired. Usually efforts in correcting cardiac output (by fluids or inotropes), and/or Hb and/or SaO₂ and/or VO₂ must target a return of SvO₂ (ScvO₂) from 50 to 65-70% [4]. In sedated critically ill patients in whom life support was discontinued, the DO₂crit was found to be approximately 3.8 to 4.5 mlO₂/kg/min for a VO₂ of about 2.4 mlO₂/g/min; O₂ER reached an O₂ERcrit of 60% [5] with SvO₂crit being ≈ 40%.

In a landmark study by Rivers et al. [6], patients admitted to an emergency department with severe sepsis and septic shock were randomized to standard therapy (aiming for a central venous pressure [CVP] of 8-12 mmHg, mean arterial pressure (MAP) ≥ 65 mmHg, and urine output ≥ 0.5 ml/kg/h) or to early goal-directed therapy where, in addition to the previous parameters, an ScvO₂ of at least 70% was targeted by optimizing fluid administration, keeping hematocrit ≥ 30%, and/or giving dobutamine to a maximum of 20 μg/kg/min. The initial ScvO₂ in both groups was low (49 ± 12%), suggesting a hypodynamic condition before resuscitation was started. From the 1^st to the 7^th hour, the amount of fluid received was significantly larger in the early goal-directed therapy patients (≈ 5,000 ml vs 3,500 ml, p < 0.001), fewer patients in the early goal-directed therapy group received vasopressors (27.4 vs 30.3%, p = NS), and significantly more patients were treated with dobutamine (13.7 vs 0.8%, p < 0.001). It is noticeable that the number of patients receiving red blood cells (RBCs) was significantly larger in the early goal-directed therapy group than in the control group (64.1 vs 18.5%) suggesting that the strategy of targeting a ScvO₂ of at least 70% was associated with more decisions to transfuse once fluid, vasopressors, and dobutamine had been titrated to improve tissue oxygenation. In the follow-up period between the 7^th and the 72^nd hour, mean ScvO₂ was higher, mean arterial pH was higher, and plasma lactate levels and base excess were lower in patients who received early goal-directed therapy. Organ failure score and mortality were significantly different in patients receiving standard therapy compared to early goal-directed therapy patients. This was the first study to demonstrate that initiation of early goal-directed therapy to achieve an adequate level of tissue oxygenation by DO₂ (as judged by ScvO₂ monitoring) could significantly reduce mortality.

In a prospective observational study [7], we tested how well the ScvO₂ corresponded to the French recommendations for blood transfusion and to the anesthesiologist's decision to transfuse. The French recommendations for blood transfusion were presented during a consensus conference organized in 2003 by the French Society of Intensive Care Medicine (Société de Réanimation de Langue Française, SRLF) [8]. These recommendations are based on plasma Hb concentration and associated clinical state (Table 1), and apart from in cardiac and septic patients, the threshold Hb value for blood transfusion is 7 g/dl. Sixty high risk surgical patients in whom the need for a blood transfusion was discussed postoperatively were included in the study [7]. They were eligible for study inclusion if they were hemodynamically stable and equipped with a CVC. The decision to transfuse was taken by the anesthesiologist in charge of the patient. The anesthesiologist was aware of the SRLF recommendations; if requested, he/she was provided with the ScvO₂ value that was obtained at the same time as the blood was sampled for the Hb concentration. The following parameters were registered: Age, a history of cardiovascular disease, presence of sepsis, number of blood units transfused, agreement with the SRLF recommendations. A decision to transfuse was made in 53 of the 60 general and urologic surgical patients. ScvO₂ and Hb were measured before and after blood transfusion, together with hemodynamic parameters (heart rate, systolic arterial pressure). Patients were retrospectively divided into two groups according to the ScvO₂ before blood transfusion (< or = 70%); each of these groups was further divided into two groups according to agreement or not with the SRLF recommendations for blood transfusion. The ScvO₂ threshold value of 69.5% (sensitivity 82%; specificity 76%) was validated with a receiver operator characteristic (ROC) curve analysis (Figure 1).

Table 1 The French recommendations for blood transfusion in critically ill patients are based on a recent consensus by the French Society of Intensive Care Medicine (Société de Réanimation de Langue Française; SRLF) using threshold values for hemoglobin (Hb) together with the clinical context to indicate blood transfusion [8].

Figure 1

ROC curve analysis illustrating the usefulness of ScvO ₂ measurement before blood transfusion in order to predict a minimal 5% increase in ScvO ₂ after BT. The threshold value for ScvO₂ with the best sensitivity and best specificity was 69.5% (*sensitivity: 82%, specificity: 76%; area under the curve: 0.831 ± 0.059). Adapted from [7] with permission.

Overall, demographic characteristics were similar (age, weight, number of blood units transfused) among the groups. Blood transfusion provided a significant and approximately similar increase in hemoglobin concentration for all patients in the four groups but the ScvO₂ value increased significantly only in patients with ScvO₂ < 70% before blood transfusion (Figure 2 and Table 2). The heart rate and systolic arterial pressure did not help in the decision to transfuse.

Table 2 Central venous O₂ saturation (ScvO₂), hemoglobin (Hb), heart rate (HR) and systolic arterial pressure (SAP) values (median [CI 95%]) in 53 hemodynamically stable postoperative patients who received blood transfusion (BT), divided into two groups according to their ScvO₂ before blood transfusion (< or ≥ 70%); and then into four groups according to agreement or not with the SRLF recommendations for transfusion.

Figure 2

Individual evolutions in ScvO ₂ before and after blood transfusion (BT) according to agreement (Reco+) or not (Reco-) with the SRLF recommendations for transfusion and according to the ScvO ₂ before transfusion (< or ≥ 70%). Adapted from [7] with permission.

The conclusions of this observational study were as follows: 1) Twenty of the 53 patients (37.7%) received a blood transfusion against SRLF recommendations; 2) thirteen of these 20 patients (65%) had an ScvO₂ < 70% and nevertheless seemed to benefit from the blood transfusion (according to the VO₂/DO₂ relationship), and one may speculate that the fact that they did not comply with the SRLF recommendations for blood transfusion could have contributed to a "lack of blood transfusion" in these patients; indeed, according to the ScvO₂ (which remained largely below 70%) blood transfusion may even have been insufficient (n = 2 blood units) in this subgroup; 4) 54.5% of the patients (18/33) met the SRLF recommendation had an ScvO₂ = 70% and received a blood transfusion although VO₂/DO₂ may have been adequate; one may speculate that transfusion in these patients could have contributed to an "excess of blood transfusion".

Following the study by Rivers et al. [6] and our own observations [7] we can conclude that ScvO₂ appears to be an interesting parameter to help with transfusion decisions in hemodynamically unstable patients with severe sepsis or in stable high-risk surgical patients equipped with a CVC. ScvO₂ can be proposed as a simple and universal physiologic transfusion trigger. This suggestion merits a controlled randomized study in which patients would be separated into two treatment groups: 1) A control group in which the decision to transfuse would be made according to Hb threshold values (similar to those presented by the SRLF); 2) an ScvO₂ goal-directed group in which the decision to transfuse would be made according to an ScvO₂ value < 70% as soon as the Hb value was less than 10 g/dl (hematocrit < 30%) providing that the CVP was 8 to 12 mmHg.

The concept of physiologic transfusion trigger

In an 84-year-old male Jehovah's Witness undergoing profound hemodilution, the DO₂crit was 4.9 mlO₂/kg/min for a VO₂ of about 2.4 mlO₂/kg/min; the Hb value at the DO₂crit was 3.9 g/dl [9]. This Hb value can be defined as the critical Hb value. Consistent with these results, in young, healthy, and conscious (which means higher VO₂) volunteers undergoing acute hemodilution with 5% albumin and autologous plasma, DO₂crit was found to be less than 7.3 mlO₂/kg/min for a VO₂ of 3.4 mlO₂/kg/min [10] and an Hb value of 4.8 g/dl. The same investigators studied healthy resting humans to test whether acute isovolemic reduction of blood hemoglobin concentration to 5 g/dl would produce an imbalance in myocardial oxygen supply and demand, resulting in myocardial ischemia [11]. Heart rate increased from 63 ± 11 (baseline measured before hemodilution began) to 94 ± 14 beats/min (a mean increase of 51 ± 27%; p < 0.0001), whereas MAP decreased from 87 ± 10 to 76 ± 11 mmHg (a mean decrease of 12 ± 13%; p < 0.0001), mean diastolic blood pressure decreased from 67 ± 10 to 56 ± 10 mmHg (a mean decrease of 15 ± 16%; p < 0.0001), and mean systolic blood pressure decreased from 131 ± 15 to 121 ± 16 mmHg (a mean decrease of 7 ± 11%; p = 0.0001). Electrocardiographic (EKG) changes were monitored continuously using a Holter EKG recorder for detection of myocardial ischemia. During hemodilution, transient, reversible ST-segment depression developed in three asymptomatic subjects at hemoglobin concentrations of 5 g/dl. The subjects who had EKG ST-segment changes had significantly higher maximum heart rates (110 to 140 beats/min) than those without EKG changes, despite having similar baseline values. The higher heart rates that developed during hemodilution may have contributed to the development of an imbalance between myocardial oxygen supply and demand resulting in EKG evidence of myocardial ischemia. An approach to the myocardial oxygen balance is offered by the product systolic arterial pressure × heart rate which should remain below 12,000. For heart rate = 110 beats/min, if systolic arterial pressure is 120 mmHg, systolic arterial pressure × heart rate = 13,200 and may be considered too high for the myocardial VO₂.

In 20 patients older than 65 years and free from known cardiovascular disease, Hb was decreased from 11.6 ± 0.4 to 8.8 ± 0.3 g/dl [12]. With stable filling pressures, cardiac output increased from 2.02 ± 0.11 to 2.19 ± 0.10 l/min/m² (p < 0.05) while systemic vascular resistance (SVR) decreased from 1796 ± 136 to 1568 ± 126 dynes/s/cm5 (p < 0.05) and O₂ER increased from 28.0 ± 0.9 to 33.0 ± 0.8% (p < 0.05) resulting in stable VO₂ during hemodilution. While no alterations in ST segments were observed in lead II, ST segment deviation became slightly less negative in lead V₅ during hemodilution, from -0.03 ± 0.01 to -0.02 ± 0.01 mV (p < 0.05). The authors concluded that isovolemic hemodilution to a hemoglobin value of about 8.8 g/dl was the limit that could be tolerated in these patients [12].

In 60 patients with coronary artery disease receiving chronic beta-adrenergic blocker treatment and scheduled for coronary artery bypass graft (CABG) surgery, Hb was decreased from 12.6 ± 0.2 to 9.9 ± 0.2 g/dl (p < 0.05) [13]. With stable filling pressures, cardiac output increased from 2.05 ± 0.05 to 2.27 ± 0.05 l/min/m² (p < 0.05) and O₂ER from 27.4 ± 0.6 to 31.2 ± 0.7% (p < 0.05), resulting in stable VO₂. No alterations in ST segments were observed in leads II and V₅ during hemodilution. Individual increases in cardiac index and O₂ER were not linearly related to age or left ventricular ejection fraction [13].

Healthy young volunteers were also tested with verbal memory and standard computerized neuropsychologic tests before and twice after acute isovolemic reduction of their Hb concentration to 5.7 ± 0.3 g/dl [14]. Heart rate, MAP, and self-assessed sense of energy were recorded at the time of each test. Reaction time for Digit-Symbol Substitution Test (DSST) increased, delayed memory was degraded, MAP and energy level decreased, and heart rate increased (all p < 0.05). Increasing PaO₂ to 406 ± 47 mmHg reversed the DSST result and the delayed memory changes to values not different from those at the baseline Hb concentration of 12.7 ± 1.0 g/dl, and decreased heart rate (p < 0.05) although MAP and energy level changes were not altered with increased PaO₂ during acute anemia. In that study, the authors confirmed that acute isovolemic anemia subtly slows human reaction time, degrades memory, increases heart rate, and decreases energy levels [14].

Subsequent studies identified the cause of the observed cognitive function deficits in impaired central processing as quantified by measurement of the P300 latency. The P300 response was significantly prolonged when unmedicated healthy volunteers were hemodiluted from hemoglobin concentrations of 12.4 ± 1.3 to 5.1 ± 0.2 g/dl [15]. The increased P300 latencies could be reversed to values not significantly different from baseline when inspired oxygen concentration was increased from 21 (room air) to 100%. These results suggest that P300 latency is a variable that is sensitive enough to predict subtle changes in cognitive function. Accordingly, increase in the P300 latency above a certain threshold may serve as a monitor of inadequate cerebral oxygenation and as an organ-specific transfusion trigger in the future. Spahn and Madjdpour recently commented [16] that Weiskopf et al. [15, 17] have opened a "window to the brain" with respect to monitoring the adequacy of cerebral oxygenation during acute anemia.

These observations and results clearly indicate that there is no 'universal' Hb threshold that could serve as a reliable transfusion trigger and that transfusion guide lines should take into account the patient's individual ability to tolerate and to compensate for the acute decrease in Hb concentration. Useful transfusion triggers should rather consider signs of inadequate tissue oxygenation that may occur at various hemoglobin concentrations depending on the patient's underlying disease(s) [18].

Conclusion

Physiologic transfusion triggers should progressively replace arbitrary Hb-based transfusion triggers [19]. The same conclusions were drawn by Orlov et al. in a recent trial using a global oxygenation parameter for guiding RBC transfusion in cardiac surgery [20]. The use of goal-directed erythrocyte transfusions should render the management of allogeneic red cell use more efficient and should help: 1) in saving blood and avoiding unwanted adverse effects; and 2) in promoting and optimizing the adequacy of this life-saving treatment [16]. These 'physiologic' transfusion triggers can be based on signs and symptoms of impaired global (lactate, SvO₂ or ScvO₂) or, even better, regional tissue (EKG ST-segment, DSST or P300 latency) oxygenation; they do, however, have to include two important simple hemodynamic targets: heart rate and MAP or systolic arterial pressure.