End-tidal carbon dioxide variation after a 100- and a 500-ml fluid challenge to assess fluid responsiveness

EtCO₂ variation has been advocated replacing cardiac output measurements to evaluate fluid responsiveness (FR) during sepsis. The ability of EtCO₂ variation after a fluid challenge to detect FR in the context of general anaesthesia has not been investigated. Forty patients were prospectively studied. They underwent general anaesthesia for major surgeries. CO was measured by transoesophageal Doppler, and EtCO₂ was recorded as well as other haemodynamic parameters [heart rate (HR), mean arterial pressure (MAP), pulse pressure (PP)] at baseline, after 100-ml fluid load over 1 min, and at the end of the 500-ml fluid load. We measured the variation of EtCO₂ at 100 (ΔEtCO₂100) and 500 ml (ΔEtCO₂500), and ROC curves were generated. A threshold for ΔEtCO₂ to predict FR was determined with receiver operating curves (ROC) analysis. The primary end point was the ability of EtCO₂ variation after a 500-ml fluid load to diagnose FR.

Fifteen patients (38 %) were fluid responders. ROC analysis showed that for a threshold of 5.8 % (ΔEtCO₂500), sensitivity was 0.6 IC 95 % [0.33; 0.86] and specificity was 1.0 IC 95 % [1.0; 1.0]. An absolute increase of more than 2 mmHg of EtCO₂ is specific to diagnose fluid responsiveness (spe = 96 [88–100] %, sens = 60 [33–88] %, AUC = 0.80 [0.96–0.65]). HR, MAP, and PP variations and ΔEtCO₂100 did not bring information to predict or diagnose FR. During fluid challenge, the correlation between CI variation and EtCO₂ variation was r = 0.566, p < 0.001.

During surgery, when alveolar ventilation and CO₂ production are constant, ΔEtCO₂500 is fairly reliable to assess FR. When the variation of EtCO₂ is >5.8 %, all patients were responders, but no conclusion could be done when this variation was <5.8 %. ΔEtCO₂100 failed to predict FR.