['To test whether cardiac function index could actually behave as an indicator of left ventricular systolic function by testing if 1) it increased with inotropic stimulation; 2) it was not altered by fluid loading; 3) it correlated with the echographic left ventricular ejection fraction, considered as the clinical gold standard for measuring left ventricular systolic function, and it reliably tracked the changes in left ventricular ejection fraction during therapeutic intervention. The transpulmonary thermodilution calculates the cardiac function index, which is the ratio of cardiac output over global end-diastolic volume.', 'Prospective study.', 'Medical intensive care unit of a university hospital.', 'Thirty-nine patients (n = 48 cases) with acute circulatory failure.', 'A 500-mL saline administration (n = 24 cases) and a dobutamine infusion (n = 24 cases).', 'We simultaneously measured left ventricular ejection fraction (monoplane or biplane Simpson method) and cardiac function index at baseline and after saline and dobutamine administration. As volume expansion altered neither left ventricular ejection fraction (47 +/- 11% to 47 +/- 11%) nor cardiac function index (4.5 +/- 2.2 to 4.5 +/- 2.1 min(-1)), dobutamine infusion significantly increased left ventricular ejection fraction by 32 +/- 28% and cardiac function index by 29 +/- 22%. Considering the 96 cardiac function index:left ventricular ejection fraction pairs of measurements, cardiac function index and left ventricular ejection fraction were correlated significantly (r = .67, p < .05). A cardiac function index <3.2 min(-1) predicted a left ventricular ejection fraction of <35% with a sensitivity of 81% and a specificity of 88%. Importantly, the changes in left ventricular ejection fraction and in cardiac function index during fluid and dobutamine administration were correlated significantly (r = .79, p < .05).'] Does cardiac function index provided by transpulmonary thermodilution behave as an indicator of left ventricular systolic function?