End-tidal carbon dioxide is better than arterial pressure for predicting volume responsiveness by the passive leg raising test

Purpose: In stable ventilatory and metabolic conditions, changes in end-tidal carbon dioxide (EtCO2) might reflect changes in cardiac index (CI). We tested whether EtCO2 detects changes in CI induced by volume expansion and whether changes in EtCO2 during passive leg raising (PLR) predict fluid responsiveness. We compared EtCO2 and arterial pulse pressure for this purpose. Methods: We included 65 patients [Simplified Acute Physiology Score (SAPS) II = 57 +/- A 19, 37 males, under mechanical ventilation without spontaneous breathing, 15 % with chronic obstructive pulmonary disease, baseline CI = 2.9 +/- A 1.1 L/min/m(2)] in whom a fluid challenge was decided due to circulatory failure and who were monitored by an expiratory-CO2 sensor and a PiCCO2 device. In all patients, we measured arterial pressure, EtCO2, and CI before and after a fluid challenge. In 40 patients, PLR was performed before fluid administration. The PLR-induced changes in arterial pressure, EtCO2, and CI were recorded. Results: Considering the whole population, the fluid-induced changes in EtCO2 and CI were correlated (r (2) = 0.45, p = 0.0001). Considering the 40 patients in whom PLR was performed, volume expansion increased CI a parts per thousand yen15 % in 21 volume responders. A PLR-induced increase in EtCO2 a parts per thousand yen5 % predicted a fluid-induced increase in CI a parts per thousand yen15 % with sensitivity of 71 % (95 % confidence interval: 48-89 %) and specificity of 100 (82-100) %. The prediction ability of the PLR-induced changes in CI was not different. The area under the receiver-operating characteristic (ROC) curve for the PLR-induced changes in pulse pressure was not significantly different from 0.5. Conclusion: The changes in EtCO2 induced by a PLR test predicted fluid responsiveness with reliability, while the changes in arterial pulse pressure did not.