Little is known regarding the incidence and clinical impact of AI in neonates during the acute postoperative period following separation from CPB. In a randomized control pilot study performed by the UAB CVICU research team, prophylactic post-CPB hydrocortisone infusions improved some postoperative outcomes, especially in those that acquired AI. In an attempt to further explore post-CPB AI, a retrospective analysis of data from this study was performed. Of the 40 neonates included in the study, one-third (32.5%) developed AI following CPB (as determined by low-dose, 1 µg, cosyntropin stimulation test). Almost all of these subjects had normal response to cosyntropin stimulation pre-CPB. Subjects that developed AI demonstrated more hemodynamic instability, increased serum lactate and required more colloid resuscitation in the immediate post-CPB period in the operating room.