Chloride: The Forgotten Ion

Chloride (Cl-) is the most common anion in intracellular and extracellular fluid. It maintains acid-base balance, plasma oncotic pressure, osmolarity, the resting action potential, and carbon dioxide transport in erythrocytes. It keeps serum concentrations within a very narrow range through hormonal control by the renin-angiotensin-aldosterone system and cortisol. In practice, we have rarely asked ourselves how to calculate the Cl- when we prescribing intravenous solutions to a patient, but we must remember that even a minor change in the Cl- concentration is enough to produce not only a profound alteration of the acid-base balance and thus adverse effects in practically any organ system, as explained by the Stewart model through the strong ion difference (SID), where the smaller the difference between cations and anions [(Na+, K+, Ca+ and Mg+) - (Cl- and lactate)], the higher the concentration of hydrogen ions [H+] and therefore hyperchloremic metabolic acidosis. The measured use of chloride solutions should guide medical conduct at the time of their administration.