Traditional catheterization laboratory assessment of coronary artery disease has consisted of coronary angiography with or without quantitative coronary assessment. Although such information is clinically important, qualitative coronary angiography is limited by inter-observer variability,1 and quantitative assessment may not provide functional information such as coronary flow. Techniques to assess coronary artery flow in the catheterization laboratory have included coronary sinus thermodilution (CST),2,3 digital subtraction angiography4 and xenon washout techniques.5 These techniques have significant drawbacks, including inability to measure beat-to-beat variations.