What question did this study set out to answer?

Investigate the ability of CO2-derived variables from passive leg raising to detect preload responsiveness in mechanically ventilated patients.

February 19, 2026Open Access

Changes in CO2-Derived Variables, Induced by Passive Leg Raising Test, Detect Preload Responsiveness in Mechanically Ventilated Patients: A Pilot Study

Key Points

Investigate the ability of CO2-derived variables from passive leg raising to detect preload responsiveness in mechanically ventilated patients.
Studied 30 mechanically ventilated patients with acute circulatory failure
Performed passive leg raising tests and measured hemodynamic variables and CO2-derived variables
Calculated differences in P(cv-a)CO2 and P(cv-a)CO2/C(a-cv)O2 between PLR and pre-PLR
Utilized ROC-AUC for predictive value assessment of changes in CO2-derived variables
50% of patients classified as preload responsive
Significant correlation between ΔP(cv-a)CO2 and ΔP(cv-a)CO2/C(a-cv)O2 with VTI changes
PLR-induced change in P(cv-a)CO2 associated with preload responsiveness (OR 0.48)
AUCs for predicting preload responsiveness: 0.89 for ΔP(cv-a)CO2 and 0.85 for ΔP(cv-a)CO2/C(a-cv)O2

Abstract

Background/Objectives. Changes in CO2-derived variables during a fluid challenge have been proposed as markers of fluid responsiveness. We investigated whether, instead of fluid administration, passive leg raising (PLR)-induced changes in the CO2-derived variables, namely central venous-arterial carbon dioxide partial pressure (P(cv-a)CO2) and the ratio between P(cv-a)CO2 and the arterial-central venous oxygen content (P(cv-a)CO2/C(a-cv)O2), could detect preload responsiveness in critically ill patients. Methods. We studied 30 mechanically ventilated patients in whom a PLR test was performed due to acute circulatory failure. Routine hemodynamic variables, velocity-time integral (VTI), in the left ventricular outflow tract, and CO2-derived variables, were measured before, during, and after a PLR test. A PLR-induced increase in VTI of ≥10% defined preload responsiveness. The differences (Δ) of P(cv-a)CO2 and P(cv-a)CO2/C(a-cv)O2 between PLR and pre-PLR were calculated. The predictive values of PLR-induced changes in the CO2-derived variables was determined by receiver operating characteristic area under curves (ROC-AUCs). Results. Fifteen patients (50%) were classified as preload responsive. ΔP(cv-a)CO2 and ΔP(cv-a)CO2/C(a-cv)O2 were correlated with VTI changes and differed significantly between responders and non-responders −1.3 (−2–−0.6) vs. 0.6 (−0.1–1.1) mmHg, p < 0.001, and −0.38 (−0.97–−0.34) vs. 0.1 (−0.15–0.57) mmHg/mL O2, p < 0.001, respectively. The PLR-induced decrease in P(cv-a)CO2 was significantly associated with preload responsiveness (OR 0.48, CI 0.20–0.89, p = 0.016, bootstrap CI 0–0.85). The AUC curves for both ΔP(cv-a)CO2 and ΔP(cv-a)CO2/C(a-cv)O2 ratio to predict preload responsiveness were 0.89 (CI 0.74–1), p < 0.001, and 0.85 (CI 0.70–1), p < 0.001, respectively. Conclusions. In mechanically ventilated ICU patients with circulatory shock, PLR-induced changes in P(cv-a)CO2 and P(cv-a)CO2/C(a-cv)O2 ratio were correlated with VTI changes. The change in P(cv-a)CO2 was the only variable detecting preload responsiveness assessed by PLR; therefore, it could serve as an indirect marker, useful to guide fluid resuscitation when cardiac output measurement is not feasible.

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