End-tidal CO2 in the diagnosis of fluid responsiveness - a systematic review

An indication of the adequacy of the intra-vascular volume is of importance in critically ill patients. The status of the intravascular volume can be determined from a fluid challenge test. Most tests involve invasive monitoring. An exception is the capnographic measurement of changes in end-tidal (ET) CO2 after a fluid challenge. The method is appealing as it rests on solid physiological ground - the Fick principle and the Frank-Starling mechanism. Furthermore, it is non-invasive and convenient. We report the results of a systematic review of the merits of this method.

After a registration with PROSPERO, we searched MEDLINE, EMBASE, the Cochrane Library database and trial registers for studies on the diagnostic accuracy of changes in ET-CO2 in fluid responsiveness testing. Test sensitivity, specificity and area under the receiver operating charac-teristics curve (AUROC) were the primary outcome meas-ures.

Seven papers met the inclusion criteria. The test was found to have a median sensitivity of 0.75 (range: 0.60-0.91) and a median specificity of 0.94 (range: 0.70-1.00). The median AUROC was 0.82 (range: 0.67-0.94); the diagnostic threshold was an increase in ET-CO2 of 2 mmHg/5%.

Monitoring of ET-CO2 during fluid responsiveness testing provides good diagnostic value with few false negative tests and fewer false positive tests. The included studies have important methodological flaws. It must therefore be acknowledged that the diagnostic value of ET-CO2 monitoring found in the review is overrated and overrated to an unknown degree. Therefore, at the present state of affairs, implementation of the test cannot be considered evidence-based.