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GENERAL ARTICLES

The Effect of Duration of Surgery on Fluid Balance During Abdominal Surgery: A Mathematical Model

From the Department of Anesthesiology, Hyogo College of Medicine, Hyogo, Japan.

Address correspondence and reprint requests to Tsuneo Tatara, MD, Department of Anesthesiology, Hyogo College of Medicine, 1-1 Mukogawa-cho, Nishinomiya, Hyogo 663-8501, Japan. Address e-mail to ttatara{at}hyo-med.ac.jp.

BACKGROUND: There is controversy regarding which fluid management regimen provides the best postoperative outcome. Interstitial fluid accumulation may adversely affect postoperative outcome, but the effect of surgical duration on fluid balance is unknown. In this study, we used a mathematical model to describe fluid distribution.

METHODS: Previously published data from bioimpedance analysis in patients undergoing abdominal surgery were used to calculate changes to interstitial volume (V_IT, percent change relative to baseline) in uninjured and injured tissues. Ratios of V_IT in uninjured and injured tissues at the end of surgery to total fluid volume infused during surgery (V_INF, mL/kg) were compared between surgeries of duration <3 h (n = 5) and 3 h (n = 25). Critical values for change in plasma volume (V_PL, percent change relative to baseline) and V_IT, which give rise to adverse outcome, were calculated from previously published data on the physiological effects of IV fluid administration in healthy volunteers. Finally, simulated abdominal surgery in a 70 kg man for 1–8 h was used to determine the effect of crystalloid infusion rate between 2 and 30 mL · kg^–1 · h^–1 on V_PL and V_IT. Fluid infusion rates that maintained V_PL and V_IT in uninjured tissue within critical values were then computationally determined as a function of duration of surgery.

RESULTS: Bioimpedance data showed that the differences in V_IT/V_INF ratios between uninjured and injured tissues were significant only for surgical duration 3 h (0.30 ± 0.17% · kg/mL vs 1.55 ± 0.73% · kg/mL, P < 0.0001). Differences of V_IT/V_INF ratios between surgical durations <3 and 3 h were found only for injured tissue (0.45 ± 0.35% · kg/mL vs 1.55 ± 0.73% · kg/mL, P = 0.003). The range of fluid infusion rates required to maintain V_PL and V_IT within the critical values (>–15% and <20%, respectively) was wide for short-duration surgery (2–18.5 mL · kg^–1 · h^–1 for a 2 h-surgery), whereas it was narrow for long-duration surgery (5–8 mL · kg^–1 · h^–1 for a 6 h-surgery).

CONCLUSIONS: Based on our model, it should be possible to increase the fluid infusion rate without significant interstitial edema for abdominal surgery of <3 h duration. However, our model predicts that restrictive fluid management should be used in abdominal surgery of >6 h duration to avoid excessive interstitial edema.