Physiology of the Developing Kidney: Fluid and Electrolyte Homeostasis and Therapy of Basic Disorders (Na/H 2 O/K/Acid Base)

The spectrum of clinical conditions requiring the prescription of fluid and electrolyte therapy by pediatric clinicians is vast, ranging from rehydrating otherwise healthy children with acute gastrointestinal illness to correcting life-threatening abnormalities in children with complex chronic disease. Recognition of each child’s individual clinical situation, notably any changes in normal physiology and homeostatic mechanisms that accompany the acute or chronic illness, and each situation’s ultimate goal in respect to volume resuscitation or electrolyte correction is crucial for the provision of the correct combination of fluid and electrolytes in the proper amount of time. Historically, an understanding of the morbidity and mortality that accompanies significant perturbations in fluid and electrolyte balance dates to clinical observations made in epidemics of cholera and other diarrheal illness in the eighteenth century [1]. The recognition that moribund patients could be saved with provision of salt solutions spurred interest in defining the fluid and electrolyte needs in healthy individuals and in developing clinical parameters for fluid and electrolyte therapy. Over time, this work helped to define the threshold for the minimum daily provision of fluid and electrolytes – so-called maintenance requirements – as well as a threshold of maximal tolerance. In the early twentieth century, clinicians began to try to restore circulation in children with volume compromise by intraperitoneal injection of saline or intravenous infusion of isotonic solutions [2, 3]. Fluid spaces were defined in terms of intracellular and extracellular compartments, and the kidney’s role in the regulation of overall body volume and specific solute gradients between these fluid spaces became better appreciated [4, 5]. By the middle of the twentieth century, simple equations to link average daily metabolic rate to daily fluid requirements were devised, and the practice of calculating daily fluid and electrolyte needs for an ill child based on consideration of both “maintenance” needs and past and current losses or “deficits” was taught as the best approach to minimize complications and improve outcomes [6–8]. Despite caveats by the founders of this approach to individualize therapy and be cognizant of clinical situations where they may not pertain, emphasis on these empiric equations led to formulaic hydration protocols that became problematic in the setting of reduced urine output or non-osmotically stimulated ADH release. Reassessment of this approach in the late twentieth and early twenty-first centuries has lead to more widespread understanding that such elaborate maneuvers are often unnecessary and that there needs to be ongoing focus on the child’s response to initial fluid and electrolyte therapy with consideration for adjustments to optimize outcome [9]. More widespread recognition that oral rehydration solutions are simple, safe, and efficacious alternatives for most children has also impacted this tradition grounded on arbitrarily calculated intravenous fluid volumes [10]. In more complex disorders of fluid and electrolyte pathophysiology, such as seen in critically ill children with sepsis, burns, trauma, or postoperatively or in children and adolescents with conditions