New fetal cardiac z-score equations from William Beaumont Hospital; new online calculator at OBSONO.org
Following up on their abstract from earlier this year, the group at William Beaumont Hospital in Royal Oak, Michigan produced what turns out to be the largest cross-sectional study of normative data for fetal cardiac measurements of all time:
Fetal echocardiography: z-score reference ranges for a large patient population.
Lee W, Riggs T, Amula V, Tsimis M, Cutler N, Bronsteen R, Comstock CH.
Ultrasound Obstet Gynecol. 2010 Jan;35(1):28-34.
Data from over 2700 normal pregnancies was used to construct z-score equations for the following fetal cardiac measures:
- LV minor
- RV minor
- Aortic annulus
- Pulmonary annulus
- Cardiac circumference
By way of comparison, the previous work from Royal Brompton used data from 130 normal pregnancies; the more recent "unpublished" equations from Boston are based on observations made on 232 normal pregnancies. This new data eclipses both of these studies by at least an order of magnitude. For the purpose of generating normative data, "n" is everything, so statistically speaking, this study is HUGE.
The William Beaumont z-score equations deal with the prediction of the standard deviation in a manner similar to that used by the Boston folks, by a separate regression, thus deliberately accounting and controlling for the natural spread of data (heteroscedasticity). And, for what has to be one of the first, if not only, time- the authors also present a beautiful frequency vs. residuals plot and convincingly demonstrate that their data conforms to a normal distribution- a predicate for all z-score comparisons.
To better examine the differences and similarities of the various z-score equations I provide these tools, allowing for side-by-side comparison of the predicted z-scores, mean values, and normal ranges:
Speaking from the perspective of a pediatric cardiac sonographer, I have to recognize and thank the authors of this study for doing the work that we cannot do. Those of us in pediatric cardiology do not see a high volume of unselected, normal pregnancies and thus could never generate this kind of normative data.