- Perform study
- Obtain standardized measurements
- Compare measurements to established criteria
The problem of relating normal values over a wide variety of body sizes is at least as old as pediatric cardiology itself. Knowing if a newborn's tricuspid valve is too small or a two year-old's coronary artery is too big, is essential to the modern practice of pediatric echo.Enter the nomogram: Found in the backs of textbooks and throughout the literature, nomograms soon became the staple that replaced my flashcards. Now, all I needed was a copy machine and notebook. Nomograms aren't perfect, however. If the nomogram is printed too small (as I have deviously done with the above sample) it is quite difficult to resolve small differences between our measurement and the printed reference- and almost all of them require some interpolation on our part. Still, for the most part, the power and the glory had returned. Soon however, the absurdity of sitting in front of a computer/echo reading station, while interpolating hash-marks in a notebook caught up with me. This is the Age of Information? Besides- what if an unscrupulous cardiology fellow absconded with our Precious Notebook of Nomograms? What if?
Underlying each nomogram is the theory that a predictable relationship exists between the independent measure (age, weight, BSA, etc.) and a dependent variable (coronary artery diameter, annulus dimension, etc.). Further underlying each of these relationships, is the assumption that these variables have a normal distribution in our population.When we perform an echocardiogram and measure, for instance, the left coronary artery, and then ask "is it normal?"-- what we are really asking is: "how does our measurement compare to the mean of the population of other (normal) humans of this size?" The answer is best given with one number: the z-score. The z-score tells us in one simple, elegant number how our measure relates to the population.
- Exactly normal: z = 0
- pretty much normal: ± 1
- too small: -3
- gigantic: +7