Mathematical coupling of data between global-end diastolic volume index and cardiac index calculated by the PiCCO device: myth or reality?

Background: Aim of the study was to investigate whether cardiac index (CI) and global end diastolic volume index (GEDVi) determined from the same thermodilution curve are mathematically coupled during the infusion of an inotropic agent in critically ill patients.

Methods: Seventeen patients were prospectively studied. CI and GEDVi were evaluated in triplicate by the transpulmonary thermodilution technique with the PiCCO system before and 20 to 30 minutes after increases in dobutamine infusion rate. Mixed linear model was used to determine the within-subject correlation coefficient between changes in CI and GEDVi induced by changes in dobutamine infusion rate.

Results: Dobutamine administration significantly increased CI by 48±35%, whereas the average increase in GEDVi was only 8.2±12.3% but statistically significant (P<0.0001). The increase of GEDVi in response to dobutamine infusion was unexpected given that dobutamine has no recognized effect on right and left ventricular dimensions. Intriguingly, we observed a significant correlation coefficient, in individual patients, between changes in CI and GEDVi (r=0.58, P=0.002).

Conclusion: Our study provides evidence that changes in GEDVi are mathematically coupled to changes in CI during dobutamine infusion. Therefore, clinicians using PiCCO device to evaluate GEDVi must be aware of the underlying formula to avoid placing undue reliance on artifactual correlations due to mathematical coupling.