| Abstract: Introduction: Nitrogen (N2) balance calculations commonly account for urinary, cutaneous, fecal, and respiratory N2 loss, but not abdominal fluid protein (AFP) loss. We hypothesize that accurate assessment of N2 balance must consider AFP loss from the open abdomen.
Methods: Surgical / trauma patients requiring open abdominal decompression (OAD) and temporary abdominal closure (TAC) for ≥ 5 days were studied. Prospective 24-hour collections of urine and abdominal fluid on days 1, 3, and 5 following abdominal decompression were performed for measurement of urine urea nitrogen (UUN) and AFP. Indirect calorimetry was utilized to determine caloric requirements. N2 balance was calculated both with and without the AFP loss component. Data were analyzed using Kruskal-Wallis analysis of variance with Wilcoxon rank-sum for within-group comparisons.
Results: Data from 20 consecutive patients were analyzed. Mean age (± standard deviation) was 43 ± 17 years. Mean APACHE-2 was 19 ± 8, SAPS-2 39 ± 12, and ISS 28 ± 14. Overall survival was 70%. Patients underwent TAC following either emergent laparotomy (80%) or development of secondary abdominal compartment syndrome (20%). Patients were receiving 42 ± 25% of their measured caloric requirement and 45 ± 34% of their estimated protein requirement by Day 5. Mean AFP loss was 1.9 ± 1.3 gm N2/L. Traditional N2 balance calculations significantly underestimated actual N2 balance by 3.5 ± 1.7 gm (p=0.007).
Conclusions: The open abdomen represents a significant source of protein/N2 loss. Failure to account for this loss in nutritional calculations may lead to underfeeding and inadequate nutritional support. While direct measurement of AFP loss is optimal, an estimate of 1.9 gm N2/L of abdominal fluid output appears reasonable.
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