Saturday, April 18, 2009

Critical Aortic Stenosis: Echo Calculations

Calculators for Critical Aortic Stenosis: Rhodes Score, Discriminant Score, and CHSS Survival Benefit

In a previous post I presented my first version of the Discriminant Score calculator. Since then, we (sonographers) still get asked to calculate a Rhodes Score (this score has achieved virtual Brand Name recognition at this point) for patients with what appear to be borderline anatomy- even though the Discriminant Score now updates and improves upon the older score. In the process of developing the calculator for the Rhodes Score I was also clued-in to the Congenital Heart Surgeons Society (CHSS) Survival Benefit score. So, I thought I could present calculators based on each of these manuscripts (references included):

A few procedural notes related to the actual calculations are probably worth mentioning:

  • Rhodes Score
    • an erratum was published in 1995 (the original article was published in 1991). This is not to be missed, as is corrects the misprinted formula for the area of an ellipse used to calculate the MV (annulus) area, thus the indexed MV area, and thus the overall score
    • I omit the calculation of LV mass as the authors note the technical difficulty of the measurement (particularly, I might add, in patients where the LV is misshaped)
  • CHSS Survival Benefit Score
    • this is not the CHSS's current survival benefit calculator (I still can't figure that one out); they prefer you not play "what if..." with theirs  :)
    • necessary calculations of the z-scores use the only published data available at the the time: the Wessex z-score data (discussed previously here)
    • swapping the aortic root z-score equations for the competition (i.e., the Boston data) can have a pronounced effect (try it yourself)

It is this last point that I find both fascinating and more than a little disturbing: the CHSS survival benefit score, the way it is published- referring to the Wessex z-score data- appears to have a built in bias against biventricular repair. That is to say, in my experience (see for yourself) the Wessex data has a small standard deviation, and thus, less tolerance for deviations from the mean, and calls "abnormal" too soon. Way, way too soon. So, if the choice to go down the single ventricle pathway is (somewhat) dependent upon the relative size of the measured structures, and the relative size is gauged by the z-score, and the z-scores are biased...

If the choice of z-score equations perches neonates on the balance of biventricular vs. univentricular repair, we should probably be thinking pretty hard about how and where we want to derive our reference values.

A consensus *cough* Z-Score Writing Project *cough* can't come soon enough.