Body compartment volumes and composition after giving a vasopressin antagonist: changes are revealed by a tonicity balance

Hyponatraemia, the most common electrolyte abnormality in hospitalized patients, is associated with an increased mortality rate w1x. This unfavourable outcome probably reflects the severity of the underlying condition unless hyponatraemia was acute and caused brain cell swelling or there was osmotic demyelination induced by overly rapid correction of hyponatraemia w2x. In acute care settings, nephrologists are often asked to explain why there was a sudden rise or fall in the plasma sodium (Na) concentration (PNa), what this change in PNa implies for the patient, and how this electrolyte disorder should be managed. The traditional approach views the problem in terms of changes in the excretion of electrolyte-free water w3x. We shall illustrate in this teaching exercise that this is not adequate for clinical decision-making. Accordingly, our purpose is to provide a simple and reliable alternative to define the basis for a change in PNa and to deduce its impact on body fluid compartment volumes and composition because this information is essential to design proper therapy. For the PNa to change, the content of Na q anduor water in the extracellular fluid (ECF) compartment must be altered. Two other factors must be considered when assessing the basis for a change in PNa. First, balance for Na plus potassium (K) rather than just Na must be calculated w4x. Second, one must be certain that there was not a shift of water across cell membranes due to a gain of particles restricted to the ECF compartment (e.g. glucose w5x) or in the intracellular fluid (ICF) compartment (e.g. during a seizure w6x). Changes in balance of water and NaqK should be related to total body water to determine their quantitative impact on the PNa w7x. Thus, for every mmol of NaqK positive or negative balance per litre of total body water, the change in PNa should be 1 mmolul. When there is a positive or negative balance of 1 litre of water, the PNa should change by the formula, PNa3 (1 litreutotal body water). The initial body weight and an estimate of body composition are used to deduce total body water in all calculations w8x.