Oxygen consumption (VO2) is an important part of hemodynamics using the direct Fick principle in children undergoing cardiac catheterization. Accurate measurement of VO2 is vital. Obviously, any error in the measurement of VO2 will translate directly into an equivalent percentage under- or overestimation of blood flows and vascular resistances. It remains common practice to estimate VO2 values from published predictive equations. Among these, the LaFarge equation is the most commonly used equation and gives the closest estimation with the least bias and limits of agreement. However, considerable errors are introduced by the LaFarge equation, particularly in children younger than 3 years of age. Respiratory mass spectrometry remains the “state-of-the-art” method, allowing highly sensitive, rapid and simultaneous measurement of multiple gas fractions. The AMIS 2000 quadrupole respiratory mass spectrometer system has been adapted to measure VO2 in children under mechanical ventilation with pediatric ventilators during cardiac catheterization. The small sampling rate, fast response time and long tubes make the equipment a unique and powerful tool for bedside continuous measurement of VO2 in cardiac catheterization for both clinical and research purposes. © 2012 Wiley Periodicals, Inc.