Do fluctuations of PaCO2 impact on the venous-arterial carbon dioxide gradient?

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.

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.

Estimating Arterial Partial Pressure of Carbon Dioxide in Ventilated Patients: How Valid Are Surrogate Measures?

The arterial partial pressure of carbon dioxide (Pa_CO2) is an important parameter in critically ill, mechanically ventilated patients. To limit invasive procedures or for more continuous monitoring of Pa_CO2, clinicians often rely on venous blood gases, capnography, or transcutaneous monitoring. Each of these has advantages and limitations. Central venous Pco₂ allows accurate estimation of Pa_CO2, differing from it by an amount described by the Fick principle. As long as cardiac output is relatively normal, central venous Pco₂ exceeds the arterial value by approximately 4 mm Hg. In contrast, peripheral venous Pco₂ is a poor predictor of Pa_CO2, and we do not recommend using peripheral venous Pco₂ in this manner. Capnography offers measurement of the end-tidal Pco₂ (Pet_CO2), a value that is close to Pa_CO2 when the lung is healthy. It has the advantage of being noninvasive and continuously available. In mechanically ventilated patients with lung disease, however, Pet_CO2 often differs from Pa_CO2, sometimes by a large degree, often seriously underestimating the arterial value. Dependence of Pet_CO2 on alveolar dead space and ventilator expiratory time limits its value to predict Pa_CO2. When lung function or ventilator settings change, Pet_CO2 and Pa_CO2 can vary in different directions, producing further uncertainty. Transcutaneous Pco₂ measurement has become practical and reliable. It is promising for judging steady state values for Pa_CO2 unless there is overt vasoconstriction of the skin. Moreover, it can be useful in conditions where capnography fails (high-frequency ventilation) or where arterial blood gas analysis is burdensome (clinic or home management of mechanical ventilation).

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

Background

To evaluate the effects of acute hyperventilation on the central venous-to-arterial carbon dioxide tension difference (∆PCO₂) in hemodynamically stable septic shock patients.

Methods

Eighteen mechanically ventilated septic shock patients were prospectively included in the study. We measured cardiac index (CI), ∆PCO₂, oxygen consumption (VO₂), central venous oxygen saturation (ScvO₂), and blood gas parameters, before and 30 min after an increase in alveolar ventilation (increased respiratory rate by 10 breaths/min).

Results

Arterial pH increased significantly (from 7.35 ± 0.07 to 7.42 ± 0.09, p < 0.001) and arterial carbon dioxide tension decreased significantly (from 44.5 [41–48] to 34 [30–38] mmHg, p < 0.001) when respiratory rate was increased. A statistically significant increase in VO₂ (from 93 [76–105] to 112 [95–134] mL/min/m², p = 0.002) was observed in parallel with the increase in alveolar ventilation. While CI remained unchanged, acute hyperventilation led to a significant increase in ∆PCO₂ (from 4.7 ± 1.0 to 7.0 ± 2.6 mmHg, p < 0.001) and a significant decrease in ScvO₂ (from 73 ± 6 to 67 ± 8%, p < 0.001). A good correlation was found between changes in arterial pH and changes in VO₂ (r = 0.67, p = 0.002). Interestingly, we found a strong association between the increase in VO₂ and the increase in ∆PCO₂ (r = 0.70, p = 0.001).

Conclusions

Acute hyperventilation provoked a significant increase in ∆PCO₂, which was the result of a significant increase in VO₂ induced by hyperventilation. The clinician should be aware of the effects of acute elevation of alveolar ventilation on ∆PCO₂.

Electronic supplementary material

The online version of this article (doi:10.1186/s13613-017-0258-5) contains supplementary material, which is available to authorized users.