Cardiac output (CO) measurement is an essential part of haemodynamic management in critically ill patients, especially in the intensive care unit (ICU). Since 1970 the ‘clinical reference standard’ for CO monitoring is the use of a pulmonary artery catheter (PAC) for thermodilution (TDPAC). Because of concerns about the safety and overall benefit of the PAC, less invasive and non-invasive techniques have emerged to measure CO in the ICU and these techniques are developing quickly. The aim of this review is to give an overview of the currently available non-invasive techniques for continuous CO monitoring such as oesophageal Doppler, partial carbon dioxide rebreathing, bioimpedance, bioreactance and volume clamping. Furthermore, we evaluate their accuracy in the setting of the ICU by comparing them with the ‘gold standard’ TDPAC. Although the non-invasive techniques show reasonably good results in elective postoperative ICU patients, the non- invasive techniques are not able to replace TDPAC for accurate CO measurements in non-elective ICU patients. Overall the non-invasive techniques seem to perform better as a trend- monitoring device as opposed to measuring absolute CO.
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