Acute hyperventilation increases the central venous-to-arterial PCO2 difference in stable septic shock patients

Relationship between the mixed venous-to-arterial carbon dioxide gradient and the cardiac index in acute pulmonary embolism

AIMS

Among patients with acute pulmonary embolism (PE) undergoing mechanical thrombectomy, the cardiac index (CI) is frequently reduced even among those without a clinically apparent shock. The purpose of this study is to describe the mixed venous-to-arterial carbon dioxide gradient (CO2 gap), a surrogate of perfusion adequacy, among patients with acute PE undergoing mechanical thrombectomy.

METHODS AND RESULTS

This was a single-centre retrospective study of consecutive patients with PE undergoing mechanical thrombectomy and simultaneous pulmonary artery catheterization over a 3-year period. Of 107 patients, 97 had simultaneous mixed venous and arterial blood gas measurements available. The CO2 gap was elevated (>6 mmHg) in 51% of the cohort and in 49% of patients with intermediate-risk PE. A reduced CI (≤2.2 L/min/m2) was associated with an increased odds [odds ratio = 7.9; 95% confidence interval (CI) 3.49-18.1, P < 0.001] for an elevated CO2 gap. There was an inverse relationship between the CI and the CO2 gap. For every 1 L/min/m2 decrease in the CI, the CO2 gap increased by 1.3 mmHg (P = 0.001). Among patients with an elevated baseline CO2 gap >6 mmHg, thrombectomy improved the CO2 gap, CI, and mixed venous oxygen saturation. When the CO2 gap was dichotomized above and below 6, there was no difference in the in-hospital mortality rate (9 vs. 0%; P = 0.10; hazard ratio: 1.24; 95% CI 0.97-1.60; P = 0.085).

CONCLUSION

Among patients with acute PE undergoing mechanical thrombectomy, the CO2 gap is abnormal in nearly 50% of patients and inversely related to the CI. Further studies should examine the relationship between markers of perfusion and outcomes in this population to refine risk stratification.

The Evolution of Central Venous-to-arterial Carbon Dioxide Difference (PCO2 Gap) during Resuscitation Affects ICU Outcomes: A Prospective Observational Study

Introduction

The usual methods of perfusion assessment in patients with shock, such as capillary refill time, skin mottling, and serial serum lactate measurements have many limitations. Veno-arterial difference in the partial pressure of carbon dioxide (PCO2 gap) is advocated being more reliable. We evaluated serial change in PCO2 gap during resuscitation in circulatory shock and its effect on ICU outcomes.

Materials and methods

This prospective observational study included 110 adults with circulatory shock. Patients were resuscitated as per current standards of care. We recorded invasive arterial pressure, urine output, cardiac index (CI), PCO2 gap at ICU admission at 6, 12, and 24 hours, and various patient outcomes.

Results

Significant decrease in PCO2 gap was observed at 6 h and was accompanied by improvement in serum lactate, mean arterial pressure, CI and urine output in (n = 61). We compared these patients with those in whom this decrease did not occur (n = 49). Mortality and ICU LOS was significantly lower in patients with low PCO2 gap, while more patients with high PCO2 gap required RRT.

Conclusion

We found that a persistently high PCO2 gap at 6 and 12 h following resuscitation in patients with shock of various etiologies, was associated with increased mortality, need for RRT and increased ICU LOS. High PCO2 gap had a moderate discriminative ability to predict mortality.

How to cite this article

Zirpe KG, Tiwari AM, Kulkarni AP, Vaidya HS, Gurav SK, Deshmukh AM, et al. The Evolution of Central Venous-to-arterial Carbon Dioxide Difference (PCO2 Gap) during Resuscitation Affects ICU Outcomes: A Prospective Observational Study. Indian J Crit Care Med 2024;28(4):349-354.

Unmasking the Impact of Oxygenator Induced Hypocapnia on Blood Lactate in Veno-Arterial Extracorporeal Membrane Oxygenation

Extracorporeal membrane oxygenation (ECMO) is often associated with disturbances in acid/base status that can be triggered by the underlying pathology or the ECMO circuit itself. Extracorporeal membrane oxygenation is known to cause hypocapnia, but the impact of reduced partial pressure of carbon dioxide (pCO2) on biomarkers of tissue perfusion during veno-arterial (VA)-ECMO has not been evaluated. To study the impact of low pCO2 on perfusion indices in VA-ECMO, we placed Sprague-Dawley rats on an established VA-ECMO circuit using either an oxygen/carbon dioxide mixture (O2 95%, CO2 5%) or 100% O2 delivered through the oxygenator (n = 5 per cohort). Animals receiving 100% O2 developed a significant VA CO2 difference (pCO2 gap) and rising blood lactate levels that were inversely proportional to the decrease in pCO2 values. In contrast, pCO2 gap and lactate levels remained similar to pre-ECMO baseline levels in animals receiving the O2/CO2 mixture. More importantly, there was no significant difference in venous oxygen saturation (SvO2) between the two groups, suggesting that elevated blood lactate levels observed in the rats receiving 100% O2 were a response to oxygenator induced hypocapnia and alkaline pH rather than reduced perfusion or underlying tissue hypoxia. These findings have implications in clinical and experimental extracorporeal support contexts.

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.

Endothelial Function and Hypoxic–Hyperoxic Preconditioning in Coronary Surgery with a Cardiopulmonary Bypass: Randomized Clinical Trial

A hypoxic–hyperoxic preconditioning (HHP) may be associated with cardioprotection by reducing endothelial damage and a beneficial effect on postoperative outcome in patients undergoing cardiac surgery with cardiopulmonary bypass (CPB). Patients (n = 120) were randomly assigned to an HHP and a control group. A safe, inhaled oxygen fraction for the hypoxic preconditioning phase (10–14% oxygen for 10 min) was determined by measuring the anaerobic threshold. At the hyperoxic phase, a 75–80% oxygen fraction was used for 30 min. The cumulative frequency of postoperative complications was 14 (23.3%) in the HHP vs. 23 (41.1%), p = 0.041. The nitrate decreased after surgery by up to 20% in the HHP group and up to 38% in the control group. Endothelin-1 and nitric oxide metabolites were stable in HHP but remained low for more than 24 h in the control group. The endothelial damage markers appeared to be predictors of postoperative complications. The HHP with individual parameters based on the anaerobic threshold is a safe procedure, and it can reduce the frequency of postoperative complications. The endothelial damage markers appeared to be predictors of postoperative complications.

ΔPCO2 and ΔPCO2/C(a-cv)O2 Are Not Predictive of Organ Dysfunction After Cardiopulmonary Bypass

Background: Cardiac surgery is associated with a substantial risk of major adverse events. Although carbon dioxide (CO₂)-derived variables such as venous-to-arterial CO₂ difference (ΔPCO₂), and PCO₂ gap to arterial–venous O₂ content difference ratio (ΔPCO₂/C_(a−cv)O₂) have been successfully used to predict the prognosis of non-cardiac surgery, their prognostic value after cardiopulmonary bypass (CPB) remains controversial. This hospital-based study explored the relationship between ΔPCO₂, ΔPCO₂/C_(a−cv)O₂ and organ dysfunction after CPB.

Methods: We prospectively enrolled 114 intensive care unit patients after elective cardiac surgery with CPB. Patients were divided into the organ dysfunction group (OI) and non-organ dysfunction group (n-OI) depending on whether organ dysfunction occurred or not at 48 h after CPB. ΔPCO₂ was defined as the difference between central venous and arterial CO₂ partial pressure.

Results: The OI group has 37 (32.5%) patients, 27 of which (23.7%) had one organ dysfunction and 10 (8.8%) had two or more organ dysfunctions. No statistical significance was found (P = 0.84) for ΔPCO₂ in the n-OI group at intensive care unit (ICU) admission (9.0, 7.0–11.0 mmHg), and at 4 (9.0, 7.0–11.0 mmHg), 8 (9.0, 7.0–11.0 mmHg), and 12 h post admission (9.0, 7.0–11.0 mmHg). In the OI group, ΔPCO₂ also showed the same trend [ICU admission (9.0, 8.0–12.8 mmHg) and 4 (10.0, 7.0–11.0 mmHg), 8 (10.0, 8.5–12.5 mmHg), and 12 h post admission (9.0, 7.3–11.0 mmHg), P = 0.37]. No statistical difference was found for ΔPCO₂/C_(a−cv)O₂ in the n-OI group (P = 0.46) and OI group (P = 0.39). No difference was detected in ΔPCO₂, ΔPCO₂/C_(a−cv)O₂ between groups during the first 12 h after admission (P > 0.05). Subgroup analysis of the patients with two or more failing organs compared to the n-OI group showed that the predictive performance of lactate and Base excess (BE) improved, but not of ΔPCO₂ and ΔPCO₂/C_(a−cv)O₂. Regression analysis showed that the BE at 8 h after admission (odds ratio = 1.37, 95%CI: 1.08–1.74, P = 0.009) was a risk factor for organ dysfunction 48 h after CBP.

Conclusion : ΔPCO₂ and ΔPCO₂/C_(a−cv)O₂ cannot be used as reliable indicators to predict the occurrence of organ dysfunction at 48 h after CBP due to the pathophysiological process that occurs after CBP.

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 .

Venous-to-Arterial Carbon Dioxide Partial Pressure Difference: Predictor of Septic Patient Prognosis Depending on Central Venous Oxygen Saturation

This study aimed to assess the viability of using the venous-to-arterial carbon dioxide partial pressure difference (P(v-a)CO2) to predict clinical worsening of septic shock, depending on central venous oxygen saturation (ScvO2). The prospective, observational, multicentric study conducted in three intensive care units (ICUs) included all patients with a septic shock episode during the first 6 h, with 122 patients assessed. Clinical worsening was defined as an increase of sequential organ failure assessment (SOFA) scores ≥1 (ΔSOFA ≥1) within 2 days. To assess the ability of P(v-a)CO2 to predict clinical worsening, univariate and multivariate analyses were performed according to ΔSOFA. A receiver-operating characteristic (ROC) analysis was used to confirm model predictions. Associations between P(v-a)CO2 and mortality were explored using correlations. Using multivariate analyses, two independent factors associated with ΔSOFA at least 1 were identified: an averaged 6-h value of lactate concentration (Lac [1-6]) (odds ratios [ORs], 2.43 [95% confidence interval, CI, 1.20-4.89]; P = 0.013) and an averaged 6-h value of P(v-a)CO2 (P(v-a)CO2 [1-6]) (OR, 1.49 [95% CI, 1.04-2.15]; P = 0.029). ROC analysis confirmed that Lac [1-6] and P(v-a)CO2 [1-6] were significantly associated with ΔSOFA at least 1, whereas ScvO2 [1-6] was not. Finally, ΔSOFA at least 1 was associated with higher 28-day (76% vs. 10%, P = 0.001) and ICU (83% vs. 12%, P = 0.001) mortality rates, which were higher in patients with P(v-a)CO2 [1-6] more than 5.8 mmHg (57% vs. 33%; P = 0.012). In conclusion, P(v-a)CO2 may help predict outcomes for septic shock patients regardless of ScvO2 values.

Acute hyperventilation increases oxygen consumption and decreases peripheral tissue perfusion in critically ill patients

PURPOSE

This study aimed to evaluate the effects of acute hyperventilation on central venous-to-arterial carbon dioxide tension difference (Pv-aCO₂), central venous oxygen saturation (ScvO₂), central venous-to-arterial CO₂ difference/arterial-central venous O₂ difference ratio (CO₂GAP-Ratio), and peripheral perfusion index (PI) in hemodynamically stable critically ill patients.

METHODS

Fifty-four mechanically ventilated patients were evaluated. The cardiac index, Pv-aCO₂, ScvO₂, CO₂GAP-Ratio, PI, and arterial and venous blood gas parameters were measured in the first set of measurements. Then, alveolar ventilation was increased by raising the respiratory rate (10 breaths/min). After a 30 min hyperventilation period, the second set of measurements was recorded.

RESULTS

Acute hyperventilation induces an increase in Pv-aCO₂ (from 3.87 ± 1.31 to 8.44 ± 1.81 mmHg, P < 0.001) and a decrease in ScvO₂(from 71.78 ± 4.82 to 66.47 ± 5.74%, P < 0.001). The CO₂GAP-Ratio was significantly increased(from 0.97 ± 0.40 to 1.74 ± 0.46, P < 0.001), and the PI showed a remarkable decrease caused by acute hyperventilation(from 1.82 ± 1.14 to 1.40 ± 0.99，P = 0.04). Hyperventilation-induced ∆_Pv-aCO₂ was negatively correlated with ∆PaCO₂(r = -0.572, P<0.001). The change in ∆_PaCO₂ was correlated with ∆_ScvO₂(r = 0.450, P<0.001). However, the left ventricular outflow tract velocity time integral (LVOT-VTI) remained unchanged during hyperventilation.

CONCLUSIONS

Acute hyperventilation induced an increase in oxygen consumption and decreased peripheral tissue perfusion in patients. For critical care patients, it is necessary to pay attention to the influence of hyperventilation on peripheral tissue perfusion indices and oxygen consumption indices.

Relationship between ventilatory pattern and peak VO2 and area M regulates the respiratory system during exercise

BACKGROUND

Exertional dyspnea is a major symptom of heart failure. We investigated the tidal volume (TV)-the respiratory rate (RR) regulation according to the peak O₂ uptake (VO₂) during cardiopulmonary exercise testing (CPET) for clarifying exercise ventilatory pattern.

METHODS

We enrolled 1111 patients (66±13 years old, 68% men) who had undergone CPET at our hospital. We investigated the relationship between TV and RR and drew the TV/height-RR figure according to the %peak VO₂.

RESULTS

During exercise, TV was greater, illustrated as higher %peak VO₂. However, RR was weakly correlated with %peak VO₂. Adjusted with age, height, sex, each point of RR, and %peak VO₂, TV during exercise highly correlated with age, height, each point of RR, and % peak VO₂ (R=0.726 to 0.821, p<0.01). In the figure, regardless of the %peak VO₂, TV/height and RR values were linearly related at rest, as well as at the point of anaerobic threshold, respiratory compensation, and peak exercise point, with each of these lines converging onto a single area (area M). The TV-RR slope values at early phase were also lower at lower %peak VO₂.

CONCLUSIONS

We identified three ventilatory regularities during exercise. First, TV increases as greater %peak VO₂. Second, the line relating TV/height and RR at each reference point during the incremental exercise test converged onto area M. Finally, the TV-RR slope at the early exercise phase was lower in patients with a lower %peak VO₂. These ventilatory regularities may assist in elucidating the excise ventilatory pattern and help the diagnosis of exertional dyspnea.

Effects of respiratory rate on venous-to-arterial CO2 tension difference in septic shock patients undergoing volume mechanical ventilation

Background

To explore the effects of the respiratory rate (RR) on the venous-to-arterial CO₂ tension difference (gapCO₂) in septic shock patients undergoing volume mechanical ventilation.

Methods

Adult patients with septic shock underwent volume mechanical ventilation between October 2015 and October 2016. RR was started at 10 breaths/min, and 2 breaths/min were added every 60 min until 16 breaths/min was reached. At every point, central venous and arterial blood gas measurements were obtained simultaneously.

Results

In this study, gapCO₂ induced by hyperventilation significantly increased, while the central venous carbon dioxide pressure (PvCO₂) and the partial pressure of CO₂ (PaCO₂) in arteries decreased. The decreasing trend of the PaCO₂ was more obvious than that of the PvCO₂. HCO₃⁻ and ctCO₂ were markedly decreased, when the RR was increased (P < 0.05). Central venous oxygen saturation (S_cvO₂) had a decreasing trend between 14 (77.1 ± 8.3%) and 16 (75.2 ± 8.7%) breaths/min; however, the difference was not significant.

Conclusions

In septic patients undergoing ventilation, respiratory alkalosis induced by hyperventilation caused an increase in the gapCO₂. Clinicians should cautiously interpret the gapCO₂ in hemodynamically stable ventilated septic shock patients and its relationship with low cardiac output and inadequate perfusion.

Venous-to-arterial pCO2 difference in high-risk surgical patients

Alteration of tissue perfusion is a main contributor to organ dysfunction in high-risk surgical patients. The difference between venous carbon dioxide and arterial carbon dioxide pressure (pCO₂ gap) has been described as a parameter reflecting tissue hypoperfusion in critically ill patients who are insufficiently resuscitated. The pCO₂ gap/CavO₂ ratio has also been described as an indicator of the respiratory quotient, thus the relationship between DO₂ and VO₂. Most of the knowledge about the pCO₂ gap and the pCO₂ gap/CavO₂ ratio has come from studies in the literature on animal models or intensive care unit (ICU) patients. To date, publications pertaining to the operative setting are sparse. In the present review, we will first discuss the physiological background of the pCO₂ gap and CO₂-O₂ derived parameters used in the operating room. Few studies have focused on the clinical relevance of the pCO₂ gap in high-risk non-cardiac surgical patients. Prospective observational studies with a small sample size and retrospective studies have shown that the pCO₂ gap may be a useful complementary tool to identify patients who remain insufficiently optimized hemodynamically. In a few studies, a high pCO₂ gap was associated with postoperative complications following non-cardiac high-risk surgery. Results of observational studies conducted in patients undergoing cardiac surgery are contradictory. We focused on the divergence between non-cardiac surgery, cardiac surgery, and septic critically ill patients. When analyzing the literature, we can find some explanations for the discrepancies in the published results between cardiac and non-cardiac surgery. Finally, we will discuss the clinical utility of the pCO₂ gap in high-risk surgical patients.

How can CO2-derived indices guide resuscitation in critically ill patients?

Assessing the adequacy of oxygen delivery with oxygen requirements is one of the key-goal of haemodynamic resuscitation. Clinical examination, lactate and central or mixed venous oxygen saturation (S_vO₂ and S_cvO₂, respectively) all have their limitations. Many of them may be overcome by the use of the carbon dioxide (CO₂)-derived variables. The venoarterial difference in CO₂ tension ("ΔPCO₂" or "PCO₂ gap") is not an indicator of anaerobic metabolism since it is influenced by the oxygen consumption. By contrast, it reliably indicates whether blood flow is sufficient to carry CO₂ from the peripheral tissue to the lungs in view of its clearance: it, thus, reflects the adequacy of cardiac output with the metabolic condition. The ratio of the PCO₂ gap with the arteriovenous difference of oxygen content (PCO₂ gap/C_a-vO₂) might be a marker of anaerobiosis. Conversely to S_vO₂ and S_cvO₂, it remains interpretable if the oxygen extraction is impaired as it is in case of sepsis. Compared to lactate, it has the main advantage to change without delay and to provide a real-time monitoring of tissue hypoxia.

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.

Increased admission central venous-arterial CO2 difference predicts ICU-mortality in adult cardiac surgery patients

BACKGROUND

Invasive procedures such as cardiac surgery are associated with perioperative dysfunction of macrocirculation and/or microcirculation and organ failures. Maintenance or resuscitation of an adequate macrocirculation and/or microcirculation is thus crucial in patients after cardiac surgery. We investigated the prognostic power of early central venous-arterial carbon dioxide pressure difference (delta-pCO2) after cardiac surgery.

METHODS

Retrospective analysis of data from 1,019 cardiac surgery patients treated in the ICU of a tertiary medical care academic center. Clinical outcomes and laboratory measures including metabolic indices and calculated delta-pCO₂ were assessed. Receiver operating characteristic (ROC) curves were generated and sensitivity / specificity analysis was performed. Univariate and multivariate regression models were analyzed.

RESULTS

The area under the ROC curve for delta-pCO₂ to predict ICU mortality was 0.72 (sensitivity 65% / specificity 76%) with an optimal delta-pCO₂ cut-off value of 8.6 mmHg. In multivariate regression, delta-pCO₂ was associated with increased ICU mortality (HR 3.72, 95%-CI 1.3-10.66, p = 0.02). After adjustment for typical confounders, delta-pCO₂ remained as independent predictor of ICU mortality after cardiac surgery.

CONCLUSIONS

In a retrospective data analysis in a large sample of adult post cardiac surgery patients treated in the ICU, we observed that admission central venous-arterial delta-pCO₂ independently predicts ICU mortality. Delta-pCO₂ might thus contribute risk stratification in ICU patients after cardiac surgery.