The Use of Central Venous to Arterial Carbon Dioxide Tension Gap for Outcome Prediction in Critically Ill Patients: A Systematic Review and Meta-Analysis

Monitoring CO2 kinetics as a marker of cardiopulmonary efficiency

PURPOSE OF REVIEW

To describe current and near future developments and applications of CO2 kinetics in clinical respiratory and cardiovascular monitoring.

RECENT FINDINGS

In the last years, we have witnessed a renewed interest in CO2 kinetics in relation with a better understanding of volumetric capnography and its derived parameters. This together with technological advances and improved measurement systems have expanded the monitoring potential of CO2 kinetics including breath by breath continuous end-expiratory lung volume and continuous noninvasive cardiac output. Dead space has slowly been gaining relevance in clinical monitoring and prognostic evaluation. Easy to measure dead space surrogates such as the ventilatory ratio have demonstrated a strong prognostic value in patients with acute respiratory failure.

SUMMARY

The kinetics of carbon dioxide describe many relevant physiological processes. The clinical introduction of new ways of assessing respiratory and circulatory efficiency based on advanced analysis of CO2 kinetics are paving the road to a long-desired goal in clinical monitoring of critically ill patients: the integration of respiratory and circulatory monitoring during mechanical ventilation.

Correlation and Agreement Between the CO2 Gap Obtained From Peripheral Venous Blood and From Mixed Venous Blood in Mechanically Ventilated Septic Patients

BACKGROUND

Venous-arterial CO2 difference (Pv-aCO2) is a valuable marker that can identify a subset of patients in shock with inadequate cardiac output to meet tissue metabolic requirements. Some authors have found that Pv-aCO2 levels calculated from mixed vs central venous blood demonstrate a linear relationship. The purpose of this study is to determine whether there is a linear relationship between Pv-aCO2 obtained with peripheral venous blood (Pv-aCO2p) and with mixed venous blood, and the agreement between the 2 measures.

METHODS

This was a prospective, single-center, observational clinical study enrolling mechanically ventilated patients in septic shock during the first 24 hours following admission to the intensive care unit.

RESULTS

The Bravais-Pearson r-coefficient between Pv-aCO2 and Pv-aCO2p was .70 in 38 determinations (95%CI .48-.83; P-value = 1.25 x 10^-6). The Bland-Altman bias was 4.11 mmHg (95%CI 2.82-5.39), and the repeatability coefficient was 11.05. Using the Taffe approach, the differential and proportional biases were 2.81 (95%CI .52-5.11) and 1.29 (95%CI .86-1.72), respectively.

CONCLUSION

There was linear correlation between Pv-aCO2p and Pv-aCO2 in mechanically ventilated patients with septic shock. The bias showed a gradual increase in high Pv-aCO2 values in an upward trend.

Venous Minus Arterial Carbon Dioxide Gradients in the Monitoring of Tissue Perfusion and Oxygenation: A Narrative Review

According to Fick's principle, the total uptake of (or release of) a substance by tissues is the product of blood flow and the difference between the arterial and the venous concentration of the substance. Therefore, the mixed or central venous minus arterial CO₂ content difference depends on cardiac output (CO). Assuming a linear relationship between CO₂ content and partial pressure, central or mixed venous minus arterial PCO₂ differences (P_cv-aCO₂ and P_mv-aCO₂) are directly related to CO. Nevertheless, this relationship is affected by alterations in the CO₂Hb dissociation curve induced by metabolic acidosis, hemodilution, the Haldane effect, and changes in CO₂ production (VCO₂). In addition, P_cv-aCO₂ and P_mv-aCO₂ are not interchangeable. Despite these confounders, CO is a main determinant of P_cv-aCO₂. Since in a study performed in septic shock patients, P_mv-aCO₂ was correlated with changes in sublingual microcirculation but not with those in CO, it has been proposed as a monitor for microcirculation. The respiratory quotient (RQ)-RQ = VCO₂/O₂ consumption-sharply increases in anaerobic situations induced by exercise or critical reductions in O₂ transport. This results from anaerobic VCO₂ secondary to bicarbonate buffering of anaerobically generated protons. The measurement of RQ requires expired gas analysis by a metabolic cart, which is not usually available. Thus, some studies have suggested that the ratio of P_cv-aCO₂ to arterial minus central venous O₂ content (P_cv-aCO₂/C_a-cvO₂) might be a surrogate for RQ and tissue oxygenation. In this review, we analyze the physiologic determinants of P_cv-aCO₂ and P_cv-aCO₂/C_a-cvO₂ and their potential usefulness and limitations for the monitoring of critically ill patients. We discuss compelling evidence showing that they are misleading surrogates for tissue perfusion and oxygenation, mainly because they are systemic variables that fail to track regional changes. In addition, they are strongly dependent on changes in the CO₂Hb dissociation curve, regardless of changes in systemic and microvascular perfusion and oxygenation.

Use of CO2-Derived Variables in Cardiac Intensive Care Unit: Pathophysiology and Clinical Implications

Shock is a life-threatening condition, and its timely recognition is essential for adequate management. Pediatric patients with congenital heart disease admitted to a cardiac intensive care unit (CICU) after surgical corrections are particularly at risk of low cardiac output syndrome (LCOS) and shock. Blood lactate levels and venous oxygen saturation (ScVO₂) are usually used as shock biomarkers to monitor the efficacy of resuscitation efforts, but they are plagued by some limitations. Carbon dioxide (CO₂)-derived parameters, namely veno-arterial CO₂ difference (ΔCCO₂) and the VCO₂/VO₂ ratio, may represent a potentially valuable addition as sensitive biomarkers to assess tissue perfusion and cellular oxygenation and may represent a valuable addition in shock monitoring. These variables have been mostly studied in the adult population, with a strong association between ΔCCO₂ or VCO₂/VO₂ ratio and mortality. In children, particularly in CICU, few studies looked at these parameters, while they reported promising results on the use of CO₂-derived indices for patients' management after cardiac surgeries. This review focuses on the physiological and pathophysiological determinants of ΔCCO₂ and VCO₂/VO₂ ratio while summarizing the actual state of knowledge on the use of CO₂-derived indices as hemodynamical markers in CICU.

Sepsis and the microcirculation: the impact on outcomes

PURPOSE OF REVIEW

Advances in the treatment of septic shock have historically focused on resuscitation endpoints, mainly mean arterial pressure and cardiac output. As the definitions of sepsis and septic shock have shifted to focus on the diversity of causes of dysregulated host-response we have seen an emerging phenotype where tissue hypoxia persists despite adequate macrocirculatory parameters. Interest in the topic of microcirculation is re-emerging as validated bedside techniques for hemodynamic monitoring, such as video microscopes, are becoming available. We review the current understanding of how sepsis induced hypoperfusion with a focus on recent advances in monitoring the microcirculation, and how a proliferation of biomarkers and emerging therapeutic targets may impact future research.

RECENT FINDINGS

Conventional hemodynamic monitoring systems fail to assess the microcirculation, and it's response to treatment. Lactate and venous oxygen saturations often drive biomarker-guided sepsis management. Visual assessments such as mottling and capillary refill time are often associated with predicting outcomes, but sometimes can have issues with inter-provider reliability. Microcirculatory damage can be observed sublingually and appears to have prognostic value.

SUMMARY

Sepsis is associated with changes in the microcirculation that can lead to tissue hypoxia and organ dysfunction. Further studies are needed to validate the usefulness of microcirculatory bedside tools in guiding resuscitative efforts.

Veno-Arterial Partial Pressure of Carbon Dioxide Difference as a Metric of Systemic Oxygen Delivery: Insights from a Correlative Meta-Analysis

The assessment of cardiac output and adequacy of systemic oxygen delivery in children after cardiac surgery require the use of an aggregate of hemodynamic monitors and blood tests. There are previously published data regarding the utility of the veno-arterial partial pressure of carbon dioxide difference (AVDco2) to help with this. This study pooled data on the correlation of AVDco2 with other metrics of cardiac output and systemic oxygen delivery such as arteriovenous oxygen saturation difference, venous saturation, and serum lactate. A systematic review of the literature was done to identify studies analyzing the correlation of AVDco2 with other hemodynamic and laboratory values. Data were extracted, and correlation coefficients were pooled for each specific comparison to create a point estimate for the overall correlation. A total of four studies with 350 patients and 809 paired blood gases were pooled. Adequate data were available to assess the correlation of AVDco2 with arteriovenous oxygen saturation difference, venous saturation, and serum lactate. There was a significant, moderate correlation with arteriovenous oxygen saturation difference and venous saturation. A significant, weak correlation with serum lactate was found. The AVDco2 has significant, moderate correlations with other metrics of the adequacy of systemic oxygen delivery such as arteriovenous oxygen saturation difference and venous saturation. There was a significant but only weak correlation with serum lactate. AVDco2 may be complementary to assess the adequacy of cardiac output and systemic oxygen delivery.

PCO2 gap, its ratio to arteriovenous oxygen content, ScvO2 and lactate in high-risk abdominal surgery patients: An observational study

BACKGROUND

The difference between arterial and central venous carbon dioxide partial pressure (PCO₂ gap), a marker of oxygen delivery (DO₂) and oxygen consumption (VO₂) adequacy, has been evaluated as a promising prognostic tool in intensive care unit (ICU) patients. We therefore sought to study the association between intraoperative PCO₂ gap and postoperative complications (POC) in the perioperative setting of elective major abdominal surgery.

METHODS

We conducted a single-centre prospective observational study. All adult patients who underwent major planned abdominal surgery were eligible. PCO₂ gap was measured every 2 hours during surgery, at ICU admission and repeated 12 hours and 24 hours later. Severe POC within 28 days after surgery were defined as complications graded 3 or more according to Clavien-Dindo classification. Following a univariate analysis, a multivariable analysis using a logistic regression model was performed.

RESULTS

Ninety patients were included and divided into two groups according to the occurrence of POC. No significant difference was found between groups regarding baseline characteristics at inclusion. Thirty-nine (43%) patients developed postoperative complications. The median [IQR] intraoperative PCO₂ gap was significantly higher in patients who had complications (6.5 [5.5-7.3] mmHg) compared to those who did not (5.0 [3.9-5.8] mmHg; p < 0.001). The area under the receiver operating characteristic curve for occurrence of POC was 0.78 for the PCO₂ gap. After multivariable analysis, PCO₂ gap was found independently associated with POC (OR: 14.9, 95% CI [4.68-60.1], p < 0.001) with a threshold value of 6.2 mmHg. The duration of surgery (OR: 1.01, 95% CI [1.00; 1.01], p = 0.04) and the need for vasoactive support during surgery (OR: 5.76, 95% CI [1.72; 24.1], p = 0.006) were also independently associated with POC.

CONCLUSION

Intraoperative PCO₂ gap is a relevant predictive factor of severe postoperative complications in high-risk elective surgery patients.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT03914976.

Central Venous-to-Arterial CO2 Difference-Assisted Goal-Directed Hemodynamic Management During Major Surgery-A Randomized Controlled Trial

BACKGROUND

Different goals have guided goal-directed therapy (GDT). Protocols aiming for central venous-to-arterial carbon dioxide gap (DCO2) <6 mm Hg have improved organ function in septic shock. Evidence for use of DCO2 in the perioperative period is scarce. We aimed to determine if a GDT protocol using central venous saturation of oxygen (SCvo2) and DCO2 reduced organ dysfunction and intensive care unit (ICU) stay in American Society of Anesthesiologist (ASA) I and II patients undergoing major surgeries compared to pragmatic goal-directed care.

METHODS

One hundred patients were randomized. Arterial and venous blood-gas values were recorded every 2 hours perioperatively for all patients. Intervention group (GrI) with access to both values was managed per protocol based on DCO2 and SCvo2. Dobutamine infusion 3 to 5 µg/kg/min started if DCO2 >6 mm Hg after correcting all macrocirculatory end points. Control group (GrC) had access only to arterial-gas values and managed per "conventional" goals without DCO2 or SCvo2. Patients were followed for 48 hours after surgery. Organ dysfunction, sequential organ failure assessment (SOFA) scores-primary outcome, length of stay in ICU, and duration of postoperative mechanical ventilation and hospital stay were recorded. The patient, surgeons, ICU team, and analyzer were blinded to group allocation.

RESULTS

The groups (44 each) did not significantly differ with respect to baseline characteristics. Perioperative fluids, blood products, and vasopressors used did not significantly differ. The GrI had less organ dysfunction although not significant (79% vs 66%; P = .2). Length of ICU stay in the GrI was significantly less (1.52; standard deviation [SD], 0.82 vs 2.18; SD, 1.08 days; P = .002). Mechanical ventilation duration (0.9 days in intervention versus 0.6 days in control; P = .06) and length of hospital stay did not significantly differ between the groups. Perioperative DCO2 (5.8 vs 8.4 mm Hg; P < .001) and SCvo2 (73.5 vs 68.4 mm Hg; P < .001) were significantly better in the GrI.

CONCLUSIONS

GDT guided by DCO2 did not improve organ function in our cohort. It resulted in greater use of dobutamine, improved tissue oxygen parameters, and decreased length of ICU stay. More evidence is needed for the routine use of DCO2 in sicker patients. In the absence of cardiac output monitors, it may be a readily available, less-expensive, and underutilized parameter for major surgical procedures.

Pathophysiology and clinical implications of the veno-arterial PCO2 gap

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency Medicine 2021. Other selected articles can be found online at https://www.biomedcentral.com/collections/annualupdate2021 . Further information about the Annual Update in Intensive Care and Emergency Medicine is available from https://link.springer.com/bookseries/8901 .

Individualized Hemodynamic Management in Sepsis

Hemodynamic optimization remains the cornerstone of resuscitation in the treatment of sepsis and septic shock. Delay or inadequate management will inevitably lead to hypoperfusion, tissue hypoxia or edema, and fluid overload, leading eventually to multiple organ failure, seriously affecting outcomes. According to a large international survey (FENICE study), physicians frequently use inadequate indices to guide fluid management in intensive care units. Goal-directed and "restrictive" infusion strategies have been recommended by guidelines over "liberal" approaches for several years. Unfortunately, these "fixed regimen" treatment protocols neglect the patient's individual needs, and what is shown to be beneficial for a given population may not be so for the individual patient. However, applying multimodal, contextualized, and personalized management could potentially overcome this problem. The aim of this review was to give an insight into the pathophysiological rationale and clinical application of this relatively new approach in the hemodynamic management of septic patients.