Coupling of cardiac index and global end-diastolic volume index: is it mathematical or something else?

Introduction

Our goal was to analyse the relationship between the cardiac index (CI) and the global end-diastolic volume index (GEDVI), a volumetric preload parameter measured by the single transpulmonary indicator dilution technique, in a mixed population of intensive care patients. We hypothesized a close mathematical connection underlying observed clinical changes.

Methods

An observational study (OTKA T 046538) in the medical–surgical ICU of a teaching hospital. Hemodynamic data from 32 patients (altogether 122 datasets) were included in the analysis using the PiCCO system (Pulsion, Germany). The CI–GEDVI relationship was investigated using a regression analysis between the main components of the equation: GEDVI = CI × (MTt - DSt) (MTt = mean transit time, DSt = downslope time). To illustrate purely mathematical relationships, the theoretical correlation lines were calculated and compared with the measured data. To demonstrate the complex relationship among all three parameters a three-dimensional (3D) presentation was applied.

Results

The 3D presentation resulted in a good fit of the measured values onto the theoretical surface (r = 0.98). In GEDVI–CI and GEDVI–(MTt–DSt) there were aspects of the 3D surface where the correlation was weak (r = 0.35 and 0.34). However, classifying the data according to the ranges of the third parameter, a positive linear regression was observable in each range with high correlation coefficients. In the CI–(MTt–DSt) aspect of the 3D surface the regression was better (r = 0.76), supporting the role of hydrodynamic rules, and the correlation curve crossed the theoretical iso-GEDVI lines, suggesting the effect of the Frank–Starling mechanism.

Conclusion

Our study has demonstrated that there are three mechanisms working at the same time in the relationship between GEDVI and CI. The basic mathematical coupling is modified by hydrodynamic rules and the final relationship is adjusted by physiological factors. The complex analysis revealed that CI and GEDVI are related parameters even in clinical situations. The physiological influences can modify this relationship significantly, however, especially in low ranges of CI. Evaluating the GEDVI we have to consider the CI values because the intravascular volume status depends on the relationship of these two parameters (high GEDVI with low CI – volume overload, the same GEDVI with high CI – normovolaemia).