At times, patients overdo the well-meaning advice of avoiding chemotherapy toxicity by excessively hydrating. As for hypernatremia, which is much less common at 2.6%, some etiologies include cancer-related hypercalcemia that induces nephrogenic diabetes insipidus (3). Cancer can also interrupt the hypothalamic-pituitary axis and cause central diabetes insipidus. For non-diabetes insipidus reasons, perhaps having to do with diuretics and chemotherapy, the majority of hypernatremia is hospital-acquired (3). Tubulotoxins, such as ifosfamide, may impair the renal concentrating ability and lead to unregulated water loss (6). Neutropenic fever can also contribute to hypernatremia. The basic serum sodium equation is

The Adrogué-Madias (A-M) formula is correct as written, but technically it only works when adding one liter of an intravenous (IV) fluid. For all other volumes, the A-M algorithm gives an approximate answer, one that diverges further from the truth as the IV volume is increased. If one liter of an IV fluid is calculated to change the serum sodium by some amount, then it was long assumed that giving a fraction of the liter would change the serum sodium by a proportional amount. We challenged that assumption and now prove that the A-M change in [sodium] is not scalable in a linear way. Rather, the delta [Na] needs to be scaled in a way that accounts for the actual volume of IV fluid being given. This is accomplished by our improved version of the A-M formula in a mathematically rigorous way. Our equation accepts any IV fluid volume, eliminates the illogical infinities, and, most importantly, incorporates the scaling step so that it cannot be forgotten. However, the non-linear scaling makes it harder to obtain a desired delta [Na]. Therefore, we reversed the equation so that clinicians can enter the desired delta [Na], keeping the rate of sodium correction safe, and then get an answer in terms of the volume of IV fluid to infuse. The improved equation can also unify the A-M formula with the corollary A-M loss equation wherein one liter of urine is lost. The method is to treat loss as a negative volume. Since the new equation is just as straightforward as the original formula, we believe that the improved form of A-M is ready for immediate use, alongside frequent [sodium] monitoring.

No abstract

No abstract

A hyponatremic patient with the syndrome of inappropriate antidiuresis (SIAD) gets normal saline (NS), and the plasma sodium decreases, paradoxically. To explain, desalination is often invoked: If urine is more concentrated than NS, the fluid's salts are excreted while some water is reabsorbed, exacerbating hyponatremia. But comparing concentrations can be deceiving. They should be converted to quantities, as mass balance is key to unlocking the paradox. The [sodium] equation can legitimately be used to track all of the sodium, potassium, and water entering and leaving the body. Each input or output "module" can be counterbalanced by a chosen IV fluid so that the plasma sodium stays stable. This equipoise is expressed in terms of the IV fluid's infusion rate, an easy calculation called the ratio profile. Knowing the infusion rate that maintains steady state, we can prescribe the IV fluid at a faster rate in order to raise the plasma sodium. Rates less than the ratio profile may risk a paradox, which essentially is caused by an IV fluid underdosing. Selecting an IV fluid that is more concentrated than urine is not enough to prevent paradoxes; even 3% saline can be underdosed. Water drinking adds to the ratio profile and is underestimated in its ability to provoke a paradox. In conclusion, the quantitative approach demystifies the paradoxical worsening of hyponatremia in SIAD and offers a prescriptive guide to keep the paradox from happening. The ratio profile method is objective and quickly deployable on rounds, where it may change patient management for the better.