Case 1 P atient 1 was a 48-year-old man who had a living relateddonor liver transplant for hepatic cirrhosis. His course was complicated by hospital-acquired pneumonia with initial improvement on IV vancomycin, piperacillin/tazobactam, and tobramycin. However, his condition deteriorated 2 days later, with hypotension and severe leukocytosis. An exploratory laparotomy revealed a biliary leak, and on the same day, blood cultures grew carbapenem-resistant Klebsiella pneumoniae , sensitive to polymyxin B, and with intermediate sensitivity to amikacin. Both IV amikacin and polymyxin B were initiated in consultation with the infectious diseases service. Before infusion of polymyxin B, the patient's serum creatinine level was 0.7 mg/dL and he was resting comfortably on oxygen by nasal cannula with a documented oxygen sat uration (Sa o 2 ) of 100% and a respiratory rate of 18 to 21 breaths/min. Polymyxin 125 mg (1.6 mg/kg) IV every 12 h was started. An hour after initiation of the fi rst polymyxin B infusion, he became apneic and unresponsive to sternal rub, with decreasing Sa o 2 and hypotension. He received emergency airway management including endotracheal intubation, and an infusion of epinephrine. Shortly after resuscitation, his (on an F io 2 of 100%), his sensorium was clear, and his lung mechanics were normal.Case 2 Patient 2 was a 58-year-old man with chronic renal failure who received a cadaveric renal transplant. His postoperative course was complicated by a retroperitoneal hematoma and graft rejection, treated with IV immunoglobulin and steroids, ultimately resulting in acute renal failure that required transient renal replacement therapy. He had a history of multidrug-resistant (MDR) K pneumoniae urinary tract infections sensitive to carbapenems and he received pre-and postoperative meropenem, as well as intraoperative bladder irrigation with polymyxin B. After discharge to the surgical ward, he developed a large perinephric abscess that was drained, and the fl uid culture was positive for MDR K pneumoniae , Enterococcus faecium , and Pseudomonas aeruginosa . He had been treated empirically with meropenem and tigecycline, and IV polymyxin B was added at a loading dose of 200 mg (2.9 mg/kg) IV followed by 80 mg (1.1 mg/kg) IV every 72 h because of his severe renal insuffi ciency. The day of the fourth infusion of polymyxin B, the patient's serum creatinine level was 4.6 mg/dL. Three hours after the start of the infusion the patient had acute respiratory distress. He was speaking with two physicians in his hospital room and his voice suddenly became high-pitched and he then became apneic. He received emergency airway management includ ing endotracheal intubation. An arterial blood gas drawn shortly after intubation revealed hypercapnia (pH 7.14, P co 2 65, and Pa o 2 244 on 100% F io 2 ). He did not require any vasopressor support during the respiratory arrest. The patient later recalled that at the time of the crisis he was fully aware but felt unable to breathe or move his arms. He was transferred to the I...