Role of cardiopulmonary bypass (CPB) in the intraoperative decline in vitamin D levels. A, Compares 25 hydroxyvitamin D (25OHD) levels in study participants who did (open squares) and did not require CPB (black circles). Graph shows the group mean percent of the preoperative value at each perioperative time. Time points: t0, preoperative; t1, pediatric intensive care unit (PICU) admission; t2, 4–8 h after PICU admission; t3, morning postoperative day 1; t4, morning postoperative day 2. B, Graph shows group mean 25OHD levels with SDs (error bars) in preoperative and intraoperative samples for 12 study participants. Time points: t0, preoperative; ta, after CPB initiation; tb, on CPB before modified ultrafiltration (MUF); tc, following MUF; UF, final ultrafiltrate sample.