Abstract Optimization of fluid status, perfusion pressure and cardiac output in intraoperative and critical care settings improves clinical outcomes. Cardiac output is a measure of the blood pumped around the circulation. Modelling of the circulation facilitates estimation of cardiac output from readily measured variables. Methods include thermodilution, analysis of arterial pressure waveforms, Doppler measurements of blood flow velocity, electrical bioimpedance and regional oxygenation. Physical limitations of each measurement technique produce predictable as well as unquantifiable errors, leading to over- or underestimates of cardiac output.