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J Thorac Cardiovasc Surg 2007;133:235-240
© 2007 The American Association for Thoracic Surgery

Surgery for Congenital Heart Disease

Unmeasured anions in children after cardiac surgery

Paediatric Intensive Care Unit, Royal Children’s Hospital, Melbourne, Australia.

Received for publication April 14, 2006; revisions received August 2, 2006; accepted for publication September 6, 2006.

^_* Address for reprints: Deirdre Murray, MD, Department of Paediatrics, Clinical Investigations Unit, Cork University Hospital, Cork, Ireland. (Email: d.murray{at}ucc.ie).

OBJECTIVES: Acidosis caused by increased unmeasured anion levels occurs frequently after cardiac surgery, with uncertain significance. We examined the ability of unmeasured anions and lactate to predict major events after cardiac surgery, in addition to lactate/increased unmeasured anion levels during low cardiac output states.

METHODS: In the initial 6 months, all patients admitted after cardiac surgery were enrolled. Arterial samples were taken at 0, 4, 8, 12, 24, and 36 hours postoperatively. The Stewart method was used to calculate excess acid and unmeasured anion levels. Major adverse events were defined as low cardiac output states requiring cardiac massage or mechanical support. In the second 6-month period, data were collected from a further 8 infants during cardiac arrest/extracorporeal membrane oxygenation cannulation.

RESULTS: One hundred thirteen patients were analyzed. Major adverse events occurred in 8 (7.1%) of 113 patients. On admission, metabolic acidosis occurred in 94 of 113 samples: lactate alone (n = 25); mixed lactate and unmeasured anions (n = 44); and unmeasured anions alone (n = 25). All of the patients who experienced major adverse events had unmeasured anion levels of greater than 3 mEq/L on admission. Initial unmeasured anion levels were significantly higher in those infants with major adverse events (10.6 mEq/L [standard deviation, 8.2 mEq/L] vs 4.8 mEq/L [standard deviation, 6.6 mEq/L], P = .024). Lactate levels did not differ between the 2 groups. In the 16 patients sampled during major adverse events, metabolic acidosis occurred in 15 of 16, with a mean excess acid level of 14.9 mEq/L (standard deviation, 8.3 mEq/L). Although unmeasured anions made a significant contribution, lactate was the predominant acid.

CONCLUSIONS: After cardiac surgery, unmeasured anion levels were significantly higher in those children with major adverse events. The greatest risk of major adverse events was found in children with both increased lactate levels and increased unmeasured anion levels on admission.