Which of the following criteria is NOT assessed on the Silverman-Anderson Index?

The Silverman-Anderson Index is a clinical scoring system used to assess the degree of respiratory distress in newborns, particularly in preterm infants. It evaluates specific physical signs that indicate the severity of respiratory compromise.

The criteria evaluated in this index include observations of chest movement, such as whether the xiphoid (the lowest part of the sternum) retracts during breathing (indicating work of breathing), whether the nares (nostrils) dilate (showing increased effort in respiration), and the synchronicity of upper chest movement with abdominal movement (to assess overall breathing patterns).

Heart rate variability, on the other hand, is not a component of the Silverman-Anderson Index. While heart rate may provide important clinical information for assessing a newborn's condition, it does not directly relate to the specific criteria outlined in the Silverman-Anderson Index, which focuses solely on respiratory signs. Therefore, this is the correct answer as it identifies a criterion that is indeed excluded from the assessment in this particular index.