• Shailesh Bihari, Claire E Baldwin, and Andrew D Bersten.
• Flinders Medical Centre, Adelaide, SA, Australia. biharishailesh@gmail.com
• Crit Care Resusc. 2013 Jun 1;15(2):89-96.

BackgroundDistribution of total body water (TBW) depends on local and systemic factors including osmolality, relative sodium content and permeability. Although positive fluid balance has been associated with increased morbidity and mortality in critically ill patients, the mechanisms and relative roles of sodium balance and water distribution are uncertain.ObjectiveTo track changes in sodium and fluid balance, respiratory function and body composition in patients who required mechanical ventilation for ≥48 hours.Design, Setting And ParticipantsProspective observational study, set in a tertiary intensive care unit, of 10 patients (seven men) with a mean age of 60 years (standard deviation [SD],12 years) and mean admission Acute Physiology and Chronic Health Evaluation (APACHE) III score of 71 (SD, 26).MethodsSodium and fluid balances were estimated daily for up to 5 days, following institution of mechanical ventilation on Day 0. Serum sodium level, oxygenation (PaO(2)/FIO(2)), body weight, intracellular and extracellular fluid (ECF) distribution (bioelectrical impedance spectroscopy), and blinded chest x-ray oedema scores were performed daily.ResultsAfter 5 days of mechanical ventilation, the cumulative fluid balance was - 954 mL (SD, 3181 mL) and estimated cumulative sodium balance was 253 mmol (SD, 346 mmol). Serum sodium had increased from 140 mmol/L (SD, 4 mmol/L) to 147 mmol/L (SD, 5 mmol/L). Cumulative sodium balance was weakly correlated with worsening chest x-ray score (r = 0.35, P = 0.004), a reduction in PaO(2)/ FIO(2) ratio (r = - 0.52, P = 0.001) and 24-hour urinary sodium (r = - 0.24, P = 0.02). Between Days 1 and 5, body weight decreased (- 2.7 kg; SD, 1.4 kg) and TBW decreased (- 3.4 L; SD, 1.3 L), despite a rise in ECF distribution (1.4% of TBW; SD, 1.9% of TBW).ConclusionsFluid balance may not reflect sodium balance in critically ill patients. As sodium balance correlates with respiratory dysfunction and increased extracellular volume, further studies examining sodium balance and morbidity seem warranted.

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