Understanding elevated Pv-aCO2 gap and Pv-aCO2/Ca-vO2 ratio in venous hyperoxia condition

The ∆Pv-aCO2/∆Ca-vO2 ratio as a predictor of mortality in patients with severe acute respiratory distress syndrome related to COVID-19

OBJECTIVE

To evaluate the central venous-to-arterial carbon dioxide difference combined with arterial-to-venous oxygen content difference (∆Pv-aCO2/∆Ca-vO2 ratio) as a predictor of mortality in patients with COVID-19-related severe acute respiratory distress syndrome (ARDS).

METHODS

Patients admitted to the intensive care unit with severe ARDS secondary to SARS-CoV-2, and invasive mechanical ventilation were included in this single-center and retrospective cohort study performed between April 18, 2020, and January 18, 2022. The tissue perfusion indexes (lactate, central venous oxygen saturation [ScvO2], and venous-to-arterial carbon dioxide pressure difference [∆Pv-aCO2]), anaerobic metabolism index (∆Pv-aCO2/∆Ca-vO2 ratio), and severity index (Simplified Acute Physiology Score II [SAPSII]) were evaluated to determine its association with the mortality through Cox regression analysis, Kaplan-Meier curve and receiver operating characteristic (ROC) curve.

RESULTS

One hundred fifteen patients were included in the study and classified into two groups, the survivor group (n = 54) and the non-survivor group (n = 61). The lactate, ScvO2, ∆Pv-aCO2, and ∆Pv-aCO2/∆Ca-vO2 ratio medians were 1.6 mEq/L, 75%, 5 mmHg, and 1.56 mmHg/mL, respectively. The ∆Pv-aCO2/∆Ca-vO2 ratio (Hazard Ratio (HR) = 1.17, 95% confidence interval (CI) = 1.06-1.29, p = 0.001) was identified as a mortality biomarker for patients with COVID-19-related severe ARDS. The area under the curve for ∆Pv-aCO2/∆Ca-vO2 ratio was 0.691 (95% CI 0.598-0.774, p = 0.0001). The best cut-off point for ∆Pv-aCO2/∆Ca-vO2 ratio was >2.14 mmHg/mL, with a sensitivity of 49.18%, specificity of 85.19%, a positive likelihood of 3.32, and a negative likelihood of 0.6. The Kaplan-Meier curve showed that survival rates were significantly worse in patients with values greater than this cut-off point.

CONCLUSIONS

The ∆Pv-aCO2/∆Ca-vO2 ratio could be used as a predictor of mortality in patients with severe ARDS secondary to SARS-CoV-2.

Hemodynamic monitoring with two blood gases: “a tool that does not go out of style”

Introduction. Hemodynamic monitoring of a critically ill patient is an indispensable tool both inside and outside intensive care; we currently have invasive, minimally invasive and non-invasive devices; however, no device has been shown to have a positive impact on the patient's evolution; arterial and venous blood gases provide information on the patient's actual microcirculatory and metabolic status and may be a hemodynamic monitoring tool. Objective. To carry out a non-systematic review of the literature of hemodynamic monitoring carried out through the variables obtained in arterial and venous blood gases. Material and methods. A non-systematic review of the literature was performed in the PubMed, OvidSP and ScienceDirect databases with selection of articles from 2000 to 2019. Results. It was found that there are variables obtained in arterial and venous blood gases such as central venous oxygen saturation (SvcO2), venous-to-arterial carbon dioxide pressure (∆pv-aCO2), venous-to-arterial carbon dioxide pressure/arteriovenous oxygen content difference (∆pv-aCO2/∆Ca-vO2) that are related to cellular oxygenation, cardiac output (CO), microcirculatory veno-arterial flow and anaerobic metabolism and allow to assess tissue perfusion status. Conclusion. The variables obtained by arterial and venous blood gases allow for non-invasive, accessible and affordable hemodynamic monitoring that can guide medical decision-making in critically ill patients.

Interpretation of venous-to-arterial carbon dioxide difference in the resuscitation of septic shock patients

The venous-to-arterial carbon dioxide difference [P(v-a)CO2] was calculated from the difference of venous CO2 and arterial CO2, which has been used to reflect the global flow in the circulatory shock. Moreover, recent clinical studies found the P(v-a)CO2 was related to the sublingual microcirculation perfusion in the sepsis. However, it is still controversial that whether P(v-a)CO2 could be used to assess the microcirculatory flow in septic patients. Moreover, the related influent factors should be taken into account when interpreting P(v-a)CO2 in clinical practice. This paper reviews the relevant experimental and clinical scenarios of P(v-a)CO2 with the aim to help intensivists to use this parameter in the resuscitation of septic shock patients. Furthermore, we propose a conceptual framework to manage a high P(v-a)CO2 value in the resuscitation of septic shock. The triggers of correcting an elevated P(v-a)CO2 should take into consideration the other tissue perfusion parameters. Additionally, more evidence is required to validate that a decreasing in P(v-a)CO2 by increasing cardiac output would result in improvement of microcirculation. Further investigations are necessary to clarify the relationship between P(v-a)CO2 and microcirculation.

Resuscitation incoherence and dynamic circulation-perfusion coupling in circulatory shock

OBJECTIVE

Poor tissue perfusion/cellular hypoxia may persist despite restoration of the macrocirculation (Macro). This article reviewed the literatures of coherence between hemodynamics and tissue perfusion in circulatory shock.

DATA SOURCES

We retrieved information from the PubMed database up to January 2018 using various search terms or/and their combinations, including resuscitation, circulatory shock, septic shock, tissue perfusion, hemodynamic coherence, and microcirculation (Micro).

STUDY SELECTION

The data from peer-reviewed journals printed in English on the relationships of tissue perfusion, shock, and resuscitation were included.

RESULTS

A binary (coherence/incoherence, coupled/uncoupled, or associated/disassociated) mode is used to describe resuscitation coherence. The phenomenon of resuscitation incoherence (RI) has gained great attention. However, the RI concept requires a more practical, systematic, and comprehensive framework for use in clinical practice. Moreover, we introduce a conceptual framework of RI to evaluate the interrelationship of the Macro, Micro, and cell. The RI is divided into four types (Type 1: Macro-Micro incoherence + impaired cell; Type 2: Macro-Micro incoherence + normal cell; Type 3: Micro-Cell incoherence + normal Micro; and Type 4: both Macro-Micro and Micro-cell incoherence). Furthermore, we propose the concept of dynamic circulation-perfusion coupling to evaluate the relationship of circulation and tissue perfusion during circulatory shock.

CONCLUSIONS

The concept of RI and dynamic circulation-perfusion coupling should be considered in the management of circulatory shock. Moreover, these concepts require further studies in clinical practice.

Oxygen-Flow-Pressure Targets for Resuscitation in Critical Hemodynamic Therapy

Far from traditional "vital signs," the field of hemodynamic monitoring (HM) is rapidly developing. However, it is also easy to misunderstand hemodynamic therapy as merely HM and some concrete bundles or guidelines for circulation support. Here, we describe the concept of "critical hemodynamic therapy" and clarify the concepts of the "therapeutic target" and "therapeutic endpoint" in clinical practice. Three main targets (oxygen delivery, blood flow, perfusion pressure) for resuscitation are reviewed in critically ill patients according to the sepsis guidelines and hemodynamic consensus. ScvO2 at least 70% has not been recommended as a directed target for initial resuscitation, and the directed target of mean arterial pressure (MAP) still is 65 mmHg. Moreover, the individual MAP target is underlined, and using flow-dependent monitoring to guide fluid infusion is recommended. The flow-directed target for fluid infusion might be a priority, but it remains controversial in resuscitation. The interpretation of these targets is necessary for adequate resuscitation and the correction of tissue hypoxia. The incoherence phenomenon of resuscitation (macrocirculation and microcirculation, tissue perfusion, and cellular oxygen utilization) is gaining increased attention, and early identification of these incoherences might be helpful to reduce the risk of over-resuscitation.

The Prognostic Value of Central Venous-to-Arterial CO2 Difference/Arterial-Central Venous O2 Difference Ratio in Septic Shock Patients with Central Venous O2 Saturation ≥80%

Background:

It is a great challenge for physician to assess the relationship between O2 delivery and O2 consumption in septic shock patients with high ScvO2. Recently, the venous-to-arterial CO2 difference/arterial-central venous O2 difference ratio (P(v-a)CO2/C(a-v)O2) has shown potential for reflecting anaerobic metabolism. Therefore, we evaluated the value of using the P(v-a)CO2/C(a-v)O2 ratio to predict mortality and assess anaerobic metabolism in septic shock patients with high ScvO2 (≥ 80%).

Methods:

This was a clinical investigation of septic shock patients on the P(v-a)CO2/C(a-v)O2 ratio in the intensive care unit (ICU) department. The patients’ arterial and central venous blood gas levels were measured simultaneously at enrollment (T0) and 24 h (T24) after resuscitation.

Results:

A total of 61 patients with high ScvO2 at T24 after resuscitation were selected for analysis. The ICU mortality rate in the septic shock patients was 20% (12/61). The nonsurvivors had a significantly higher P(v-a)CO2, P(v-a)CO2/C(a-v) O2 ratio, arterial lactate level and lower lactate clearance at T24 after resuscitation. The P(v-a)CO2/C(a-v)O2 ratio had the biggest the areas under the receiver operating characteristic (AUC) for predicting ICU mortality. For predicting ICU mortality, a threshold of P(v-a)CO2/C(a-v)O2 ratio ≥1.6 was associated with a sensitivity of 83% and a specificity of 63%. Multivariate analysis showed P(v-a)CO2/C(a-v)O2 ratio at both T0 (RR 5.597, P = 0.024) and T24 (RR 5.812, P = 0.031) was an independent predictor of ICU mortality. Including the ratio into the regression model showed a bigger AUC than without the ratio (0.886 vs. 0.833).

Conclusions:

The P(v-a)CO2/C(a-v)O2 ratio is an independent predictor of ICU mortality in septic shock patients with high ScvO2 after resuscitation. It is worthy of consideration to recruit microcirculation to correct the high ratio in high ScvO2 case.