Aortic Valve Z-Score Smackdown

After overcoming my perplexity about the distribution of left atrial scores, I thought it might be interesting to look a little closer at how other normative data is predicted.

For the sake of simplicity and to illustrate the point, the estimation of BSA from height and weight has been omitted and the Cincinnati z-score calculation is limited to "boys".

Aortic Valve Z-Score Comparison

What's The Difference?

In part: Skewness.

That is to say, the Cincinnati, Michigan, and Wessex data are all modeled as having positive skew, whereas the Boston data demonstrates a normal distribution about the mean:

Their difference is perhaps most apparent as an overlay (_B=Boston):

Fortunately, most of the predictions perform well when evaluating for hypoplasia. The "big" question is how do you want to deal with dilation? Compared to the Boston predictions, the Cincinnati predictions will call an abnormal, dilated aortic valve "normal" (a false negative). Conversely, The Boston data will call an abnormal on what would otherwise be considered normal in Cincinnati (a false positive).

Which type of error are you willing- or unwilling- to make?

LV Mass Z-Scores

"...these could easily be included in echocardiography software, which would allow automated generation of an LV mass-for-height z score and percentile for each child undergoing echocardiography."

A Novel Method of Expressing Left Ventricular Mass Relative to Body Size in Children [link]

Bethany J. Foster, MD, MSCE; Andrew S. Mackie, MD, SM; Mark Mitsnefes, MD; Huma Ali;
Silvia Mamber, MD; Steven D. Colan, MD

Circulation. 2008;117:2769-2775 Published online before print May 19, 2008

Apart from debunking the practice of simply indexing LV mass by dividing mass by height, the "novel method" is the LMS (lambda, mu, sigma) method of analysis. While I couldn't paint my way out of a Box-Cox transformation, I get the idea: the lambda (power transformation to deal with skew), mu (mean), and sigma (coefficient of variation) are determined for each of many groups, elegantly- and deliberately- addressing the matters of skewness and heteroscedasticity.

Lots to read up on with this technique:

• The LMS method for constructing normalized growth standards [link]
• Smoothing reference centile curves: the LMS method and penalized likelihood [link]
• download LMS chart-making software from Tim Cole's website

The authors acknowledge that their data should not be taken to represent the definitive model of LV mass reference values, only that they are proposing the LMS technique as an alternate, superior, method.

Note: LV Mass was estimated from m-mode using the Devereux equation.

LV Mass Z-Score Calculator

Echo Z-Score Suggested Reading

 

Lots of people have asked me something like:

We are finding discrepancies in Z scores calculated by your method and the Z scores calculated by software provided by Boston...
WHY ?

I usually feel obligated to first set the record straight: these are not my z-score calculations. They are calculators based upon published literature, and I cite the source literature on the same page as the calculator... In some cases it is clear that this misconception is a language barrier issue, and I must apologize for that. English is the only language I know (well, apart from some Spanish, but most of that I can't repeat in polite company).

To really answer the question I have to admit: I don't really have all the words, or even some of the right words, and I am basically incapable of organizing them in a meaningful order.

Fortunately, the lucid discussion about reference values for pediatric echo, the matters of predicting echo normal values, and the general application of z-scores towards pediatric cardiology has already been done– and by people far brighter and more eloquent than me. I have read and (mostly) understand, and I therefore highly recommend, the following:

• the invited editorial Establishing norms for echocardiographic measurements of cardiovascular structures and function in children
• the original article Theoretical and empirical derivation of cardiovascular allometric relationships in children
• the excellent chapter in Nadas' Pediatric Cardiology: Methodological Issues in Clinical Research
• the recent work, A Novel Method of Expressing Left Ventricular Mass Relative to Body Size in Children

Why are your z-scores different? I don't know for sure, but I can take some guesses. Certainly, I recognize that there are differences.

The inevitable, and, possibly, better question is "Which one is most correct ?"

Universal Z-Score Calculations

According to the Spring issue of the ASE's Pediatric and Congenital Heart Disease Council News, interesting things are afoot:

The council has developed a Normative Database working group... to develop consensus methods for standardization of measures acquired during the pediatric echocardiogram with a long-term goal of creating a normative database of universally available standardized z-score calculations for the pediatric and young adult population.

I have toyed around a bit with making a few z-score calculators for pediatric echo universally available, and in the process one thing became clear: we are a long way from a consensus on z-scores. And, while enormously useful, the z-score calculators by themselves seem to only scratch the surface of what is now possible.

What is truly fascinating to me is the idea of a universal normative database. While I have yet to implement such a design through ParameterZ.com, what I have learned is: technology is not a barrier to creating an online pediatric echo reference values database.

What would be really cool is to marry the reference values with something that would...

... allow participants to create and generate web-based, secure echo reports that are standardized and complete.

(That is the ASE's promised echo toolbox reporting tool).

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I have been contemplating the concept of web based pediatric echo z-scores and standardized echo reports for some time.

I have to say that I am excited and eager to see what more and greater minds come up with.

Fetal Ventricular Wall Dimensions

Reference values for fetal ventricular wall thickness are not easy to come by. This gadget is based on one of few available references.

Evaluation of Fetal Heart Dimensions from 12 Weeks to Term
Cora Firpo, MD, Julien I.E. Hoffman, MD, and Norman H. Silverman, MD
Am J Cardiol 2001 [link]

Measurements of the ventricular walls and septum were made from the "4-chamber" views, below the coapted AV valve leaflets, in diastole.