Assessment of Cardiac Output With End-tidal Carbon Monoxide

Hemodynamic monitoring, especially cardiac output assessment, is a key feature for the management of critically ill patients. Although the use of invasive methods, such as thermodilution with a pulmonary artery catheter, remains the GOLD standard for the evaluation of the cardiac output, several non-invasive techniques are currently used in practice. An acceptable estimation of the cardiac output can be made by standard transthoracic echocardiography. Cardiac output can be calculated from subaortic velocity time integral (VTI). However, this technique requires a trained operator and depends on the echogenicity of the patient. The best method for assessing cardiac output depends on the patient's needs, the clinical scenario and the physician's experience with the monitoring device itself. No simple and rapid tool currently exist for assessing cardiac output in critically ill patients.

The measurement of end-tidal carbon dioxide (EtCO2) used in routine in critically ill patients requiring mechanical ventilation could be an interesting alternative. Indeed, the amount of carbon dioxide (CO2) exhaled depends on the production of CO2 by the body, the pulmonary blood flow (corresponding to cardiac output) and its elimination by alveolar ventilation. In controlled ventilation, ie for constant alveolar ventilation, EtCO2 should therefore depend only on cardiac output. It has been shown in a porcine model that EtCO2 and cardiac output are strongly related under stable respiratory and metabolic conditions. In humans, only the variation of EtCO2 after volume expansion has been studied and EtCO2 seems to reflect changes in cardiac output. Nevertheless, the usefulness of EtCO2 in assessing cardiac output has never been evaluated.

The objective of this study is therefore to determine the relationship between EtCO2 and cardiac output evaluated by the measurement of subaortic VTI in critically ill patients.