Sodium balance, not fluid balance, is associated with respiratory dysfunction in mechanically ventilated patients: a prospective, multicentre study

Abstract

BACKGROUND: Large positive sodium balances, independent of fluid balance, may lead to expanded extracellular fluid volumes and adverse clinical outcomes in the critically ill, including impaired oxygenation. OBJECTIVES: To estimate sodium and fluid balances in critically ill patients needing invasive mechanical ventilation (MV) for more than 48 hours and to evaluate the relationship between fluid balance, sodium balance and respiratory function (PaO2/FiO2 ratio and length of MV). DESIGN AND SETTING: A prospective, observational study of 50 patients on MV in four tertiary intensive care units. MAIN OUTCOME MEASURES: Daily sodium and fluid input and output, biochemistry, haemodynamic variables, oxygenation (PaO2/FiO2) and steroid and vasopressor administration were recorded for 3 days after study enrolment. Outcome data included the duration of invasive MV, ICU and hospital mortality and ICU and hospital lengths of stay. RESULTS: Fifty patients (33 men [66%]) with a mean age of 62.8 years (standard deviation, 14.6 years) and a median admission Acute Physiology and Chronic Health Evaluation III score of 82 (interquartile range [IQR], 61-99) were studied. By Day 3 after enrolment, the median cumulative fluid balance was 2668 mL (IQR, 875-3507 mL) and the cumulative sodium balance was +717 mmol (IQR, +422 to +958 mmol). Intravenous steroids and the presence of shock led to a lower daily sodium excretion (P=0.004 and P=0.01, respectively). A positive sodium balance was associated with a reduction in the next day's PaO2/FiO2 ratio (ρ=-0.36, P=0.001) and an increased length of MV (linear regression analysis, P<0.01). The cumulative fluid balance was not associated with either parameter. CONCLUSIONS: The cumulative positive sodium balance, not the cumulative positive fluid balance, is associated with respiratory dysfunction and an increased length of MV.