In this prospective study, cardiac output was measured in 38 intensive care unit patients before and after a fluid challenge, using both pulse contour analysis (Nexfin®; BMEYE, Amsterdam, the Netherlands) and transthoracic echocardiography. The ability of the Nexfin device to detect significant changes in the velocity–time integral was evaluated. The pulse wave could not be detected by the Nexfin device in five patients (13%), leaving 33 patients for analysis. The Nexfin device adequately tracked changes in the velocity–time integral in 20 (61%) patients. Using a cut-off of a 10% increase in cardiac output estimated by the Nexfin or by echocardiography, the sensitivity of the Nexfin device to detect a response to fluid challenge was 47%, with specificity 81% and accuracy 64%. The percentage error between the Nexfin and echocardiography was 448%; lower limit of agreement −48% (95% CI −62 to −36%) and upper limit of agreement, 32% (95% CI 20–45%). We conclude that the Nexfin device does not adequately track changes in cardiac output in critically ill patients.