Revised one-step method for determination of cardiac output

Cardiac output (Q̇) is a determinant of blood pressure and O ₂ delivery and is critical in the maintenance of homeostasis, particularly during environmental stress and exercise. Cardiac output can be determined invasively in patients; however, indirect methods are required for other situations. Soluble gas techniques are widely used to determine Q̇. Historically, measurements during a breathhold, prolonged expiration and rebreathing to CO₂ equilibrium have been used; however, with limitations, especially during stress. Farhi and co-workers developed a single-step CO₂ rebreathing method, which was subsequently revised by his group, and has been shown to be reliable and compared closely to direct, invasive measures. V̇_CO2, P_ACO2 and Pv _CO2are determined during a 12-25s rebreathing, using the appropriate tidal volume, and Q̇ is calculated. This method can provide accurate data in laboratory and field experiments during exercise, increased or decreased gravity, water immersion, lower body pressure, head-down tilt, altered ambient pressure or changes in inspired gas composition.