We appreciate the comments of Cantinotti et al . Since the introduction of Z scores in cardiology, limitations in the use of current pediatric nomograms for the mitral valve and aortic valve annulus, and all other cardiac structures, have been discussed in the past by several authors. This is successfully summarized in the report by den Dekker et al . mentioned by Cantinotti et al .

When using Z scores in pediatric cardiology, it is crucial to distinguish between “fixed” and “dynamic” structures of the heart, especially when an underlying congenital heart disease is present.

The only structures that are “fixed” are the valve rings, because they consist of fibrous tissue. The sizes of all other structures, such as the ventricles, or an atrial septal defect or a patent foramen ovale, vary with loading conditions, pressure, compliance, and heart rate. Cantinotti et al . should therefore separately discuss “fixed” and “dynamic” structures in their comment on our article. We agree concerning the limitations of Z scores of dynamic structures, as these must be judged and used in the light of physiology. Furthermore, recommendations for defining and classifying the severity of valvular heart disease, grading the severity of defects characterized by left-to-right shunting, and the general lack of classifying the severity of congenital heart disease were not the intention of our report.

Our report dealt with a well-defined group of patients with hypoplastic left heart complex and focuses on the fixed-sized mitral valve and aortic valve annulus. Z scores in this category are extremely useful to describe the severity of hypoplastic left heart complex, to compare data of others, and to complete follow-up in patients with hypoplastic left heart complex after surgical intervention.

After critical analysis, the Z -score method of Pettersen et al . seems the most appropriate, as these authors used the most advanced calculation model and had the largest number of normal infants in their cohort. However there is no golden standard.