BACKGROUND: We investigated whether cardiac output measured with pulse wave transit time (esCCO, Nihon Kohden, Tokyo, Japan) is able to track changes in cardiac output induced by an increase in preload (volume expansion/passive leg-raising) or by changes in vasomotor tone (variation in norepinephrine dosage) in critically ill patients. METHODS: Eighty patients for whom the decision to give fluid (500 mL of saline over 15 min) (n=20), to perform passive leg-raising (n=20), and to increase (n=20) or to decrease (n=20) norepinephrine were included by the physician. Cardiac output was measured with pulse wave transit time (CO-esCCO) and transthoracic echocardiography (CO-TTE) before and after therapeutic intervention. RESULTS: Comparison between CO-TTE and CO-esCCO showed a bias of -0.7 l min(-1) and limits of agreement of -4.4 to 2.9 l min(-1), before therapeutic intervention and a bias of -0.5 l min(-1) and limits of agreement of -4.2 to 3.2 l min(-1) after therapeutic intervention. Bias was correlated with systemic vascular resistance (r(2)=0.60, P<0.0001). Percentage error was 61% before and 59% after therapeutic intervention. Considering the overall data (n=80), the concordance rate was 84%, polar plot analysis revealed an angular bias (sd) of -11°(35°) and radial limits of agreement of (sd 50°). With regard to passive leg-raising and volume expansion groups (n=40), the concordance rate was 83%, the angular bias (sd) was -20°(36°) and radial limits of agreement ( 50°). Considering variations in norepinephrine dosage groups (n=40), the concordance rate was 86%, the angular bias (sd) was -1.8°(33°) and radial limits of agreement (40°). CONCLUSIONS: esCCO was not able to track changes in cardiac output, induced by an increase in preload or by variations in vasomotor tone. Therefore, esCCO cannot guide haemodynamic interventions in critically ill patients.