Clinical Use of Venoarterial PCO2 Difference in Septic Shock

Assessing Acid–Base Status in Circulatory Failure: Relationship Between Arterial and Peripheral Venous Blood Gas Measurements in Hypovolemic Shock

Background and Objectives:

In severe circulatory failure agreement between arterial and mixed venous or central venous values is poor; venous values are more reflective of tissue acid–base imbalance. No prior study has examined the relationship between peripheral venous blood gas (VBG) values and arterial blood gas (ABG) values in hemodynamic compromise. The objective of this study was to examine the correlation between hemodynamic parameters, specifically systolic blood pressure (SBP) and the arterial–peripheral venous (A-PV) difference for all commonly used acid–base parameters (pH, Pco 2, and bicarbonate).

Design, Setting, Participants, and Measurements:

Data were obtained prospectively from adult patients with trauma. When an ABG was obtained for clinical purposes, a VBG was drawn as soon as possible. Patients were excluded if the ABG and VBG were drawn >10 minutes apart.

Results:

The correlations between A-PV pH, A-PV Pco 2, and A-PV bicarbonate and SBP were not statistically significant ( P = .55, .17, and .09, respectively). Although patients with hypotension had a lower mean arterial and peripheral venous pH and bicarbonate compared to hemodynamically stable patients, mean A-PV differences for pH and Pco 2 were not statistically different ( P = .24 and .16, respectively) between hypotensive and normotensive groups.

Conclusions:

In hypovolemic shock, the peripheral VBG does not demonstrate a higher CO2 concentration and lower pH compared to arterial blood. Therefore, the peripheral VBG is not a surrogate for the tissue acid–base status in hypovolemic shock, likely due to peripheral vasoconstriction and central shunting of blood to essential organs. This contrasts with the selective venous respiratory acidosis previously demonstrated in central venous and mixed venous measurements in circulatory failure, which is more reflective of acid–base imbalance at the tissue level than arterial blood. Further work needs to be done to better define the relationship between ABG and both central and peripheral VBG values in various types of shock.

Central venous-arterial carbon dioxide difference as an indicator of cardiac index

OBJECTIVE

The mixed venous-arterial (v-a) pCO(2) difference has been shown to be inversely correlated with the cardiac index (CI). A central venous pCO(2), which is easier to obtain, may provide similar information. The purpose of this study was to examine the correlation between the central venous-arterial pCO(2) difference and CI.

DESIGN

Prospective, cohort study.

SETTING

Intensive care unit of an urban tertiary care hospital.

PATIENTS AND PARTICIPANTS

Eighty-three consecutive intensive care unit patients.

MEASUREMENTS

Simultaneous blood gases from the arterial, pulmonary artery (PA), and central venous (CV) catheters were obtained. At the same time point, cardiac indices were measured by the thermodilution technique (an average of three measurements). The cardiac indices obtained by the venous-arterial differences were compared with those determined by thermodilution.

RESULTS

The correlation (R(2)) between the mixed venous-arterial pCO(2) difference and cardiac index was 0.903 (p <0.0001), and the correlation between the central venous-arterial pCO(2) difference and cardiac index was 0.892 (p <0.0001). The regression equations for these relationships were natural log (CI)=1.837-0.159 (v-a) CO(2) for the PA and natural log (CI)=1.787-0.151 (v-a) CO(2) for the CV (p <0.0001 for both). The root-mean-squared error for the PA and CV regression equations were 0.095 and 0.101, respectively.

CONCLUSION

Venous-arterial pCO(2) differences obtained from both the PA and CV circulations inversely correlate with the cardiac index. Substitution of a central for a mixed venous-arterial pCO(2) difference provides an accurate alternative method for calculation of cardiac output.