Catheterization Gradient vs. Severity Index. A. Severity index as determined by cardiac catheterization for patients with aortic sclerosis (maximum gradient <16 mmHg), mild to moderate AS (maximum gradient 16–64 mmHg) and severe AS (maximum gradient >64 mmHg). The severity index increased with increasing severity of aortic valve disease and was significantly different between the three groups (p < 0.0001 by one-way analysis of variance [ANOVA]). B. Correlation of Catheterization Gradient with Severity Index. There was a strong correlation between square root-transformed maximum catheterization gradient and severity index (r = 0.84).