Cardiac output: A view from Buffalo

Cardiac output (Q) is a primary determinant of blood pressure and O₂ delivery and is critical in the maintenance of homeostasis, particularly during environmental stress. Cardiac output can be determined invasively in patients; however, indirect methods are required for other situations. Soluble gas techniques are widely used to determine ̇Q. 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 valid (compared to direct measures) and reliable. (Carbon dioxide output (VCO₂), partial pressure of arterial CO₂ (PaCO₂), and partial pressure of mixed venous CO₂ (Pv_CO2) are determined during 12-25 s of rebreathing, using the appropriate tidal volume, and Q is calculated. This method has the utility to provide accurate data in laboratory and field experiments during exercise, increased and microgravity, water immersion, lower body pressure, head-down tilt, and changes in gas composition and pressure. Utilizing the Buffalo CO₂ rebreathing method it has been shown that the ̇Q can adjust to a wide range of changes in environments maintaining blood pressure and O₂ delivery at rest and during exercise.