Clinical approach to disorders of salt and water balance. Emphasis on integrative physiology

Our purpose was to illustrate the utility of an approach that begins with simple principles of physiology to patients who have a disturbance in salt and water balance (Table 1). At times, the physiology is restricted to the kidney and body fluid compartments. In these settings, the goals of therapy are defined by calculating a tonicity balance--electrolyte-free water balances simply do not provide the needed information [3]. At other times, performing balances of other solutes such as urea reveal that another critically important problem is present (tissue catabolism). Thus the physiologic analysis becomes more integrative, extending beyond renal issues. Goals for therapy become clearer once the integrative physiology is known. More modern contributions from molecular studies permit a revised interpretation of the physiology. An example presented was the possible role of gentamicin-like drugs as a cause of high output renal failure that is basically a persistent loop diuretic-like effect. In the patient presenting with hyponatremia, the first step is to determine if the time course is less than 48-hours because emergency therapy is different in this setting. With acute hyponatremia, the objective is to diminish brain cell swelling especially if even mild symptoms are present. In contrast, the objective in the patient with chronic hyponatremia is to prevent ODS. An even slower rate of rise of the PNa is required in patients who are malnourished and/or K+ depleted.