Urinary alkalinization is beneficial in the treatment of certain ingestions. The urinary pH is manipulated to exploit physiologic properties of specific toxins. Only nonionized substances are free to move passively across membranes. Ionized particles must remain in the fluid-filled compartments in which they were formed. Weak acids are mostly uncharged at physiologic pH, but become charged in alkaline environments. By significantly raising the urinary pH with intravenous sodium bicarbonate, toxins that are weak acids are converted from their nonionized form to their ionized form and are therefore held within the urinary collection system. This "ion trapping" keeps the toxin within the renal tubules, thereby enhancing its excretion. This creates an imbalance in the concentration gradient of the toxin between the blood and the urinary collection system, drawing more toxin into the renal lumen. The ideal toxin that can be eliminated by this technique is one with substantial urinary excretion and with a pKa less than the serum pH (Table 151-4).
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