Nitrogen balance, protein loss, and the open abdomen

Objective: Goal-directed nutritional support is essential to improving morbidity and mortality. Open abdominal decompression is similarly crucial to the successful treatment of intra-abdominal hypertension and abdominal compartment syndrome. The open abdomen, however, places the patient at risk for potentially significant fluid, electrolyte, and presumably protein losses from the exposed viscera. Although nutritional protein assessments are frequently utilized to measure urinary nitrogen, these calculations do not consider the loss of protein from the open abdomen. We hypothesize that accurate assessment of nitrogen balance in the patient requiring an open abdomen must include either a measurement or estimation of abdominal fluid nitrogen loss. Design., Prospective, observational cohort study. Setting., Adult surgical/trauma intensive care unit of a level I trauma center. Patients. Surgical/trauma patients requiring laparotomy. Interventions. Serial 24-hr collections of urine and abdominal fluid protein were performed to characterize abdominal fluid protein loss and evaluate the clinical effect of accounting for abdominal fluid nitrogen as part of nitrogen balance calculations. Measurements and Main Results: Nitrogen intake correlates with urinary nitrogen loss but not with abdominal fluid nitrogen loss. Abdominal fluid nitrogen loss is significant and remains relatively stable in the early postoperative period. Nutritional calculations that fail to account for abdominal fluid nitrogen loss significantly overestimate actual nitrogen balance by an average of 3.5 g/24 hrs. Conclusions. The open abdomen represents a significant source of protein/nitrogen loss in the critically ill. Failure to account for this loss in nutritional calculations may lead to underfeeding and inadequate nutritional support with a direct effect on patient outcome. Although direct measurement of abdominal fluid protein loss may be optimal, an estimate of 2 g of nitrogen per liter of abdominal fluid output should be included in the nitrogen balance calculations of any patient with an open abdomen.