We read with interest the recent article by Lytrivi et al. and would like to thank the investigators for this important reference material. For clarification, we hope that they will respond to these questions:

• 1.
  

  The units of measure used for the relationship volume/(body surface area) ^1.38 are expressed as mL/m ^2.6 . Using the rule of exponents, the exponent 2 raised to the power 1.38 should result in 2 × 1.38 = 2.76. Could the authors explain the derivation of the exponent used in this model?

  
  
• 2.
  

  Inspection of the model (Figure 2) at the extreme small end (<0.2 m ^2.6 ) appears to show an inadequate fit, suggesting that some bias may exist in the model. Had the authors in this case considered a groupwise analysis of body surface area versus volume?

In light of the recent critical appraisal of coronary artery Z score equations by Dallaire and Dahdah, it seems appropriate to reiterate their imperative: “When Z scores are computed, it is critical that their distribution be normal and that these proportions be respected across the entire population.”