Venoarterial PCO₂ gradient does not correlate with cardiac output, base excess and arterial lactate concentration in septic shock patients

Introduction

According to modified Fick equation, the venoarterial PCO₂ gradient (ΔPCO₂) is inversely correlated to cardiac output so that an increase in the ΔPCO₂ value can alert to the presence of low-flow states or at least to a mismatch between tissue blood flow and metabolism. In septic shock patients, however, the relation between cardiacoutput and ΔPCO₂ is quite complex due to possible distributive abnormalities of macrocirculatory and microcirculatory blood flow. The objectives of this study was to determine whether there is a good correlation between cardiac output and ΔPCO₂ in a group of septic shock patients and between ΔPCO₂ base excess (BE) and arterial lactate concentration (LAC), these two latter already demonstrated as good predictors of mortality in critically ill patients.

Methods

We retrieved 21 patients from our prospective collected data base from January to December 2000. APACHE II scores were calculated at admission and ΔPCO₂, cardiac output, BE and LAC were recorded at the moment of pulmonary artery catheter insertion and 24 hours after. All four variables had a normal distribution in our sample. Correlation between ΔPCO₂ and the other three variables was performed, using the Pearson coefficient.

Results

The patients' age was 53.1 ± 13.9 years and the APACHE II score was 26.0 ± 7.4. ΔPCO₂ was not correlated with any of the other three variables. The Pearson correlation coefficient was -0.173 (P = 0.29), 0.291 (P = 0.07) and 0.08 (P = 0.66) for cardiac output, BE and LAC, respectively.

Conclusions

ΔPCO₂ is not correlated with cardiac output, BE and LAC in septic shock patients.