CLINICAL STATEMENTS AND GUIDELINES mismatch must be distinguished from primary myocardial dysfunction with only moderate AS. Dobutamine stress echocardiography may be useful with measurement of aortic velocity (or mean pressure gradient) and valve area at baseline and at higher flow rates (maximum dose dobutamine 20 mcg/kg per minute) under appropriate clinical and hemodynamic moni- toring. Severe AS is characterized by a fixed valve area, resulting in an increase in trans- aortic velocity to 4 m/s (mean gradient 40 mm Hg) at any flow rate, but with valve area . In contrast, in patients with moderate AS and primary LV dysfunction, there is an increase in valve area as volume flow rate increases, resulting in only a mod- est increase in transaortic velocity or gradient. Some patients fail to show an increase in stroke volume 20% with dobutamine, referred to as lack of contractile reserve or lack of flow 4. The key measurements for clinical decision-mak- ing in patients with AS are the maximum aortic velocity, mean pressure gradient (calculated with the Bernoulli equation), and valve area (calculated with the continuity equation). An additional mea- surement that may be useful when there are dis- crepancies in these measures or in other clinical or imaging data is the ratio of the velocity in the LV outflow tract proximal to the aortic valve and the velocity in the narrowed aortic orifice. The outflow tract to aortic velocity ratio is indepen- dent of body size and eliminates potential errors in calculated valve area related to measurement of LV outflow tract diameter or area. A normal ratio is close to 1.0, whereas a ratio of 0.25 corresponds to a valve area 25% of normal for that patient, which is consistent with severe AS and is a predictor of symptom onset and adverse 5. 