Given the task definition and input, reply with output. In this task, you are given a passage which has a question and the context. You have to generate an answer to the question based on the information present in the context.

Context: The prognosis of patients with severe aortic stenosis, low aortic gradient and preserved ejection fraction is controversial. Our study analyzed the prognosis of these patients and its relation to pressure gradient and aortic valve flow.', 'We performed a retrospective cohort study of 363 consecutive patients with severe aortic stenosis and preserved ejection fraction, divided into 4 groups, based on the presence of a systolic volume index greater or lower than 35 mL/m(2) and the presence of a mean aortic gradient greater or lower than 40 mmHg. Group I: normal flow, high gradient (n=169, 47%); group II: normal flow, low gradient (n=98, 27%); group III: low flow, high gradient (n=54, 15%), and group IV: low flow, low gradient (n=42, 12%). The primary endpoint was overall mortality.', 'Independent risk factors for mortality were age (hazard ratio=1.04; 95% confidence interval, 1.01-1.08) and atrial fibrillation (hazard ratio=2.21; 95% confidence interval, 1.24-3.94). Surgical treatment was associated with longer survival in all groups (hazard ratio=0.25; 95% confidence interval: 0.13-0.49). Mortality was higher in patients with low flow than in those with with normal flow (26.6% vs 13.6%; P=.004). The most favorable mean prognosis was found in group II (hazard ratio=0.4; 95% confidence interval, 0.2-0.9).\Question: Is systolic volume index by Doppler echocardiography an useful marker for stratification and prognostic evaluation in patients with severe aortic stenosis and preserved ejection fraction?
Patients with severe aortic stenosis, normal ejection fraction and low aortic flow have a worse prognosis. Analysis of aortic flow by Doppler echocardiography is useful in risk stratification and therapeutic decision-making in patients with aortic stenosis.