Postoperative fluid retention after heart surgery is accompanied by a strongly positive sodium balance and a negative potassium balance

The conventional model on the distribution of electrolyte infusions states that water will distribute proportionally over both the intracellular ( ICV ) and extracellular ( ECV ) volumes, while potassium homes to the ICV and sodium to the ECV . Therefore, total body potassium is the most accurate measure of ICV and thus potassium balances can be used to quantify changes in ICV . In cardiothoracic patients admitted to the ICU we performed complementary balance studies to measure changes in ICV and ECV . In 39 patients, fluid, sodium, potassium, and electrolyte‐free water ( EFW ) balances were determined to detect changes in ICV and ECV . Cumulatively over 4 days, these patients received a mean ±  SE infusion of 14.0 ± 0.6 L containing 1465 ± 79 mmol sodium, 196 ± 11 mmol potassium and 2.1 ± 0.1 L EFW . This resulted in strongly positive fluid (4.0 ± 0.6 L) and sodium (814 ± 75 mmol) balances but in negative potassium (−101 ± 14 mmol) and EFW (−1.1 ± 0.2 L) balances. We subsequently compared potassium balances (528 patients) and fluid balances (117 patients) between patients who were assigned to either a 4.0 or 4.5 mmol/L blood potassium target. Although fluid balances were similar in both groups, the additionally administered potassium (76 ± 23 mmol) in the higher target group was fully excreted by the kidneys (70 ± 23 mmol). These findings indicate that even in the context of rapid and profound volume expansion neither water nor potassium moves into the ICV .