THE IMPACT OF ARTERIAL OXYGEN TENSION ON VENOUS OXYGEN SATURATION IN CIRCULATORY FAILURE

Intraoperative haemodynamic monitoring and management of adults having non-cardiac surgery: Guidelines of the German Society of Anaesthesiology and Intensive Care Medicine in collaboration with the German Association of the Scientific Medical Societies

Haemodynamic monitoring and management are cornerstones of perioperative care. The goal of haemodynamic management is to maintain organ function by ensuring adequate perfusion pressure, blood flow, and oxygen delivery. We here present guidelines on "Intraoperative haemodynamic monitoring and management of adults having non-cardiac surgery" that were prepared by 18 experts on behalf of the German Society of Anaesthesiology and Intensive Care Medicine (Deutsche Gesellschaft für Anästhesiologie und lntensivmedizin; DGAI).

Increased FIO2 influences SvO2 interpretation and accuracy of Fick-based cardiac output assessment in cardiac surgery patients: A prospective randomized study

INTRODUCTION

The study aimed to reveal how the fraction of inspired oxygen (FIO2) affected the value of mixed venous oxygen saturation (SvO2) and the accuracy of Fick-equation-based cardiac output (Fick-CO).

METHODS

Forty two adult patients who underwent elective cardiac surgery were enrolled and randomly divided into 2 groups: FIO2 < 0.7 or >0.85. Under stable general anesthesia, thermodilution-derived cardiac output (TD-CO), SvO2, venous partial pressure of oxygen, hemoglobin, arterial oxygen saturation, arterial partial pressure of oxygen, and blood pH levels were recorded before surgical incision.

RESULTS

Significant differences in FIO2 values were observed between the 2 groups (0.56 ± 0.08 in the <70% group and 0.92 ± 0.03 in the >0.85 group; P < .001). The increasing FIO2 values lead to increases in SvO2, venous partial pressure of oxygen, and arterial partial pressure of oxygen, with little effects on cardiac output and hemoglobin levels. When comparing to TD-CO, the calculated Fick-CO in both groups had moderate Pearson correlations and similar linear regression results. Although the FIO2 <0.7 group presented a less mean bias and a smaller limits of agreement, neither group met the percentage error criteria of <30% in Bland-Altman analysis.

CONCLUSION

Increased FIO2 may influence the interpretation of SvO2 and the exacerbation of Fick-CO estimation, which could affect clinical management.

TRIAL REGISTRATION

ClinicalTrials.gov ID number: NCT04265924, retrospectively registered (Date of registration: February 9, 2020).

Central venous-to-arterial carbon dioxide difference and the effect of venous hyperoxia: A limiting factor, or an additional marker of severity in shock?

Central venous-to-arterial carbon dioxide difference (P_cvaCO₂) has demonstrated its prognostic value in critically ill patients suffering from shock, and current expert recommendations advocate for further resuscitation interventions when P_cvaCO₂ is elevated. P_cvaCO₂ combination with arterial-venous oxygen content difference (P_cvaCO₂/C_avO₂) seems to enhance its performance when assessing anaerobic metabolism. However, the fact that PCO₂ values might be altered by changes in blood O₂ content (the Haldane effect), has been presented as a limitation of PCO₂-derived variables. The present study aimed at exploring the impact of hyperoxia on P_cvaCO₂ and P_cvaCO₂/C_avO₂ during the early phase of shock. Prospective interventional study. Ventilated patients suffering from shock within the first 24 h of ICU admission. Patients requiring FiO₂ ≥ 0.5 were excluded. At inclusion, simultaneous arterial and central venous blood samples were collected. Patients underwent a hyperoxygenation test (5 min of FiO₂ 100%), and arterial and central venous blood samples were repeated. Oxygenation and CO₂ variables were calculated at both time points. Twenty patients were studied. The main cause of shock was septic shock (70%). The hyperoxygenation trial increased oxygenation parameters in arterial and venous blood, whereas PCO₂ only changed at the venous site. Resulting P_cvaCO₂ and P_cvaCO₂/C_avO₂ significantly increased [6.8 (4.9, 8.1) vs. 7.6 (6.7, 8.5) mmHg, p 0.001; and 1.9 (1.4, 2.2) vs. 2.3 (1.8, 3), p < 0.001, respectively]. Baseline P_cvaCO₂, P_cvaCO₂/C_avO₂ and S_cvO₂ correlated with the magnitude of PO₂ augmentation at the venous site within the trial (ρ -0.46, p 0.04; ρ 0.6, p < 0.01; and ρ 0.7, p < 0.001, respectively). Increased P_cvaCO₂/C_avO₂ values were associated with higher mortality in our sample [1.46 (1.21, 1.89) survivors vs. 2.23 (1.86, 2.8) non-survivors, p < 0.01]. P_cvaCO₂ and P_cvaCO₂/C_avO₂ are influenced by oxygenation changes not related to flow. Elevated P_cvaCO₂ and P_cvaCO₂/C_avO₂ values might not only derive from cardiac output inadequacy, but also from venous hyperoxia. Elevated P_cvaCO₂/C_avO₂ values were associated with higher PO₂ transmission to the venous compartment, suggesting higher shunting phenomena.

Early goal-directed therapy in severe sepsis and septic shock: insights and comparisons to ProCESS, ProMISe, and ARISE

Prior to 2001 there was no standard for early management of severe sepsis and septic shock in the emergency department. In the presence of standard or usual care, the prevailing mortality was over 40-50 %. In response, a systems-based approach, similar to that in acute myocardial infarction, stroke and trauma, called early goal-directed therapy was compared to standard care and this clinical trial resulted in a significant mortality reduction. Since the publication of that trial, similar outcome benefits have been reported in over 70 observational and randomized controlled studies comprising over 70,000 patients. As a result, early goal-directed therapy was largely incorporated into the first 6 hours of sepsis management (resuscitation bundle) adopted by the Surviving Sepsis Campaign and disseminated internationally as the standard of care for early sepsis management. Recently a trio of trials (ProCESS, ARISE, and ProMISe), while reporting an all-time low sepsis mortality, question the continued need for all of the elements of early goal-directed therapy or the need for protocolized care for patients with severe and septic shock. A review of the early hemodynamic pathogenesis, historical development, and definition of early goal-directed therapy, comparing trial conduction methodology and the changing landscape of sepsis mortality, are essential for an appropriate interpretation of these trials and their conclusions.

Pitfalls in haemodynamic monitoring in the postoperative and critical care setting

Haemodynamic monitoring is a vital part of daily practice in anaesthesia and intensive care. Although there is evidence to suggest that goal-directed therapy may improve outcomes in the perioperative period, which haemodynamic targets we should aim at to optimise patient outcomes remain elusive and controversial. This review highlights the pitfalls in commonly used haemodynamic targets, including arterial blood pressure, central venous pressure, cardiac output, central venous oxygen saturation and dynamic haemodynamic indices. Evidence suggests that autoregulation in regional organ circulation may change either due to chronic hypertension or different disease processes such as traumatic brain injury, cerebrovascular ischaemia or haemorrhage; this will influence the preferred blood pressure target. Central venous pressure can be influenced by multiple pathophysiological factors and, unless central venous pressure is very low, it is rarely useful as a predictor for fluid responsiveness. Central venous oxygen saturation can be easily increased by a high arterial oxygen tension, making it useless as a surrogate marker of good cardiac output or systemic oxygen delivery in the presence of hyperoxaemia. Many dynamic haemodynamic indices have been reported to predict fluid responsiveness, but they all have their own limitations. There is also insufficient evidence to support that giving fluid until the patient is no longer fluid responsive can improve patient-centred outcomes. With the exception in the context of preventing contrast-induced nephropathy, large randomised controlled studies suggest that excessive fluid treatment may prolong duration of mechanical ventilation without preventing acute kidney injury in the critically ill.

Central Venous-to-Arterial Carbon Dioxide Gradient as a Marker of Occult Tissue Hypoperfusion after Major Surgery

The central venous-arterial carbon dioxide tension gradient (‘CO2 gap’) has been shown to correlate with cardiac output and tissue perfusion in septic shock. Compared to central venous oxygen saturation (SCVO2), the CO2 gap is less susceptible to the effect of hyperoxia and may be particularly useful as an adjunctive haemodynamic target in the perioperative period. This study investigated whether a high CO2 gap was associated with an increased systemic oxygen extraction (O2ER >0.3) or occult tissue hypoperfusion in 201 patients in the immediate postoperative period. The median CO2 gap of all patients was 8 mmHg (IQR 6 to 9), and a large CO2 gap was very common (>6 mmHg in 139 patients [69%], 95% CI 63 to 75; >5 mmHg in 170 patients [85%], 95% CI 79 to 89). A CO2 gap <5 mmHg had a higher sensitivity (93%) and negative predictive value (74%) than a CO2 gap <6 mmHg in excluding occult tissue hypoperfusion. Of the four variables that were predictive of an increased O2ER in the multivariate analysis—CO2 gap, arterial pH, haemoglobin and arterial lactate concentrations—the CO2 gap (odds ratio 4.41 per mmHg increment, 95% CI 1.7 to 11.2, P=0.002) was most important and explained about 34% of the variability in the risk of occult tissue hypoperfusion. In conclusion, a normal CO2 gap (<5 mmHg) had a high sensitivity and negative predictive value in excluding inadequate systemic oxygen delivery and may be useful as an adjunct to other haemodynamic targets in avoiding occult tissue hypoperfusion in the perioperative setting when high inspired oxygen concentrations are used.

Clinical Predictors of a Low Central Venous Oxygen Saturation after Major Surgery: A Prospective Prevalence Study

Optimising perioperative haemodynamic status may reduce postoperative complications. In this prospective prevalence study, we investigated the associations between standard haemodynamic parameters and a low central venous oxygen saturation (ScvO2) in patients after major surgery. A total of 201 patients requiring continuous arterial and central venous pressure monitoring after major surgery were recruited. Simultaneous arterial and central venous blood gases, haemodynamic and biochemical data and perfusion index were obtained from patients at a single time-point within 24 hours of surgery. A low ScvO2 (<70%) was observed in 109 patients (54%). Use of mechanical ventilation, mean arterial pressure, central venous pressure, haemoglobin concentrations, arterial pH and lactate concentrations, arterial oxygen (PaO2) and carbon dioxide tensions (PaCO2) were all associated with a low ScvO2 in the univariate analyses. In the multivariate analysis, only a higher perfusion index (odds ratio [OR] 0.87, 95% confidence interval [CI] 0.78 to 0.98), PaO2 (OR 0.98 per mmHg increment, 95% CI 0.97 to 0.99) and PaCO2 (OR 0.88 per mmHg increment, 95% CI 0.82 to 0.95) and a lower central venous pressure (OR 1.14 per mmHg increment, 95% CI 1.04 to 1.25) were significantly associated with a reduced risk of a low ScvO2, all in a linear fashion. In conclusion, PaO2, PaCO2, perfusion index and central venous pressure were significant predictors of a low ScvO2 in patients after major surgery including cardiac surgery, suggesting that ScvO2 should always be interpreted with the arterial blood gases and that liberal perioperative fluid therapy aiming at a high central venous pressure may be detrimental in optimising ScvO2.

Assessment of global tissue perfusion and oxygenation in neonates and infants after open-heart surgery

OBJECTIVES

Monitoring and preserving adequate perfusion and oxygen balance is a primary objective of critical care. This prospective observational study aimed to assess the relationship between global haemodynamic parameters and variables reflecting tissue oxygenation during the early period following corrective cardiac surgery in neonates and infants. The postoperative time course of oxygen delivery and consumption was evaluated. As surrogate markers of oxygen balance, the central venous oxygen saturation (ScvO2) and venoarterial PCO2 difference (PvaCO2) were thoroughly investigated.

METHODS

Thirteen children <1 year of age who underwent open-heart surgery were prospectively enrolled. In addition to conventional postoperative monitoring, transpulmonary thermodilution (TPTD) was used to monitor cardiac output and calculate oxygen delivery and consumption. In parallel with each TPTD measurement, arterial and central venous blood gas values were recorded. Global haemodynamic parameters and oxygenation measurements were compared with weighted linear regression statistics and Pearson's correlation coefficient.

RESULTS

Data from 145 TPTD measurements and 304 blood gas samples were recorded. The early postoperative period was characterized by a supply-dependent oxygen consumption, as demonstrated by the direct correlation between the change in oxygen delivery and consumption (r = 0.62, P < 0.001). Regarding haemodynamic parameters, none of the heart rate, mean arterial pressure or cardiac index correlated with the measured ScvO2. However, the ScvO2 and PvaCO2 were found to correlate significantly (r = -0.49, P < 0.001), and both strongly related to oxygen extraction.

CONCLUSIONS

Both the ScvO2 and PvaCO2 are reliable and comparable parameters in following tissue oxygen balance during the early postoperative course after open-heart surgery in neonates and infants. As part of multiparameter monitoring, our data highlight the importance of regular ScvO2 measurements and PvaCO2 calculations in paediatric intensive care.

Effects of arterial oxygen tension and cardiac output on venous saturation: a mathematical modeling approach

OBJECTIVES

Hemodynamic support is aimed at providing adequate O2 delivery to the tissues; most interventions target O2 delivery increase. Mixed venous O2 saturation is a frequently used parameter to evaluate the adequacy of O2 delivery.

METHODS

We describe a mathematical model to compare the effects of increasing O2 delivery on venous oxygen saturation through increases in the inspired O2 fraction versus increases in cardiac output. The model was created based on the lungs, which were divided into shunted and non-shunted areas, and on seven peripheral compartments, each with normal values of perfusion, optimal oxygen consumption, and critical O2 extraction rate. O2 delivery was increased by changing the inspired fraction of oxygen from 0.21 to 1.0 in steps of 0.1 under conditions of low (2.0 L.min(-1)) or normal (6.5 L.min(-1)) cardiac output. The same O2 delivery values were also obtained by maintaining a fixed O2 inspired fraction value of 0.21 while changing cardiac output.

RESULTS

Venous oxygen saturation was higher when produced through increases in inspired O2 fraction versus increases in cardiac output, even at the same O2 delivery and consumption values. Specifically, at high inspired O2 fractions, the measured O2 saturation values failed to detect conditions of low oxygen supply.

CONCLUSIONS

The mode of O2 delivery optimization, specifically increases in the fraction of inspired oxygen versus increases in cardiac output, can compromise the capability of the "venous O2 saturation" parameter to measure the adequacy of oxygen supply. Consequently, venous saturation at high inspired O2 fractions should be interpreted with caution.

Optimizing hemodynamic support in septic shock using central and mixed venous oxygen saturation

Global tissue hypoxia is one of the most important factors in the development of multisystem organ dysfunction. In hemodynamically unstable critically ill patients, central venous oxygen saturation (Scvo(2)) and mixed venous oxygen saturation (Svo(2)) monitoring has been shown to be a better indicator of global tissue hypoxia than vital signs and other clinical parameters alone. Svo(2) is probably more representative of global tissue oxygenation, whereas Scvo(2), is less invasive. Svo(2) and Scvo(2) monitoring can have diagnostic and therapeutic uses in understanding the efficacy of interventions in treating critically ill, hemodynamically unstable patients.

Mini ECC vs. conventional ECC: an examination of venous oxygen saturation, haemoglobin, haematocrit, flow, cardiac index and oxygen delivery

In our clinical daily routine, it is noticeable that lower cardiac indices are reached more often without negative consequences when using mini extracorporeal circulation (ECC) systems than when using open ECC systems. We evaluated haemoglobin concentration, haematocrit, blood flow, cardiac index (CI) and mixed-venous oxygen saturation (vSO2) in patients undergoing surgery using mini ECC (n=10 cases) and ECC (n=10 cases). This analysis shows that, with a mini ECC system, the range of oxygen delivery is equal to that achieved when using an open standard ECC system, despite a lower cardiac index. This phenomenon can be explained through a significant lower haemodilution and, therefore, a higher concentration of haemoglobin.

Influence of tissue perfusion on the outcome of surgical patients who need blood transfusion

PURPOSE

The aim this study was to evaluate the clinical outcome of patients needing intra-operative blood transfusion by tissue perfusion markers.

METHODS

A prospective single center cohort study. Adult patients needing blood transfusion during the intra-operative period were recruited.

RESULTS

This study included 61 patients. At the time of blood transfusion the hemoglobin level was 8.4+/-1.8 g/dL. Scv02 has been the best tissue perfusion marker to determine mortality, compared with hematemetric values and other tissue perfusion markers, with a cut-off point at ROC curve equal to 80% (AUC=0.75; sensitivity=80%; specificity=65.2%). Patients who received blood transfusion and had Scv02 <or=80% (N=29), in comparison to those with Scv02>80% (N=32), had lower mortality rates (12.5% vs. 47.1%; p=0.008) and lower incidence of postoperative complications (58.9% vs. 72.9%; p=0.06). Blood transfusion with a Scv02 <or=80% was also associated with reduced use of vasopressors (5.9% vs. 36.8%; p=0.009). Lower incidence of hypoperfusion (17.6% vs. 52.6%; p=0.009), and lower incidence of infection (23.5% vs. 52.6%; p=0.038) in the postoperative period.

CONCLUSIONS

In major surgeries, Scv02 appears to be an important variable to be taken into consideration to decide for or against blood transfusion, since blood transfusion with adequate perfusion, reflected by Scv02>80%, are associated with worse clinical outcomes.

[Arteriovenous difference in O2 content, pulmonary venous O2 saturation, cardiac index: are they equivalent in cardiac surgery?]

OBJECTIVE

To study the concordance of cardiac index (CI), mixed venous oxygen saturation (SvO(2)) and the arterial-mixed venous O(2) content difference, i.e.: C(a-v)O(2), postoperatively to cardiac surgery. We hypothesized that significant discrepancies would be measurable between C(a-v)O(2) and SvO(2), and CI, because the latter two indices encompass less metabolic components than the former.

DESIGN

Analysis of variables collected as part of routine care.

PATIENTS

Eighty anesthetized patients receiving mechanical ventilation after heart surgery.

MEASUREMENTS AND RESULTS

Using linear regression of SvO(2) versus C(a-v)O(2) (Reg 1) and CI versus C(a-v)O(2) (Reg 2), respectively we found that CI=2.2 L min(-1)m(-2) and SvO(2)=70% were equivalent to C(a-v)O(2)=5 ml/100ml. The error reflected by the vertical scatter of points around the regression line, once normalized was 3.24 times greater in Reg 2 than in Reg 1.

CONCLUSIONS

The correspondence of CI, SvO(2) and C(a-v)O(2) values observed in a population of patients studied immediately after scheduled heart surgery match those reported in critically ill patients. SvO(2) and furthermore CI induced a sizeable scatter of points around regression line. Accordingly, they appear as a lesser estimate of the flow/metabolism balance that may at best be inferred from C(a-v)O(2).

A comparison of central and mixed venous oxygen saturation in circulatory failure

OBJECTIVE

The purpose of this study was to evaluate whether central venous oxygen saturation can be used as an alternative to mixed venous oxygen saturation in patients with cardiogenic and septic shock.

DESIGN

Prospective clinical study.

SETTING

A tertiary intensive care unit in a university hospital.

PARTICIPANTS

Twenty patients with cardiogenic or septic shock requiring a pulmonary artery catheter and inotropic support.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The central venous oxygen saturation overestimated the mixed venous oxygen saturation by a mean bias (or an absolute difference) of 6.9%, and the 95% limits of agreement were large (-5.0% to 18.8%). The difference between central and mixed venous oxygen saturation appeared to be more significant when mixed venous oxygen saturation was <70%. The changes in central and mixed venous oxygen saturation did not follow the line of perfect agreement closely in different clinical conditions. The central or mixed venous oxygen saturation had a significant ability to predict the status of cardiac output state, but this ability was reduced when the effect of hyperoxia was not considered.

CONCLUSION

Central and mixed venous oxygen saturation measurements are not interchangeable numerically.