Isotonic Crystalloid Solutions

Mechanism

Physiopathology of Physiological Saline Serum (0.9%)

The term "physiological" is a historical anachronism. Massive infusion of 0.9% saline exposes the kidney to a supraphysiological chloride concentration (154 mEq/L compared to 100-106 mEq/L in plasma). Excess chlorine infused systemically alters the state of metabolic balance and induces the appearance of Hyperchloremic Metabolic Acidosis.

Pharmacokinetics

Pharmacological Pearl: The Metabolism of Lactate and Calcium in the Lactated Ringer

The lactate present in Lactated Ringer's does not cause clinical lactic acidosis. Under aerobic conditions and with preserved hepatic metabolic function, each lactate molecule infused is taken up by the hepatocyte and metabolized mitochondrially, consuming a hydrogen ion to produce net bicarbonate:

L-Lactate^- + H^{+ + 3 O₂ 3 CO₂ + 3 H₂O Generation of Sodium Bicarbonate}

However, two strict pharmacological limitations must be considered:

• Incompatibility of Routes with Blood Derivatives: Lactated Ringer's contains Ca²⁺ in its formulation (3 mEq/L). If coinfused by the same route with citrate-preserved red blood cell concentrates, exogenous calcium binds to citrate, exceeding the precipitation threshold of the coagulation cascade, inducing the formation of obstructive microclots. Their joint infusion in the same venous line is formally contraindicated.
• Risk in Intracranial Hypertension: Its actual measured osmolarity is 273 mOsm/L (hypotonic with respect to the plasma 285-295 mOsm/L). Its massive infusion decreases plasma osmotic pressure, favoring the movement of water to the interstitial space of the brain parenchyma, worsening cerebral edema in patients with traumatic brain injury (TBI).

Indicators and dose

Mechanism of Hyperchloremic Acidosis and Renal Vasoconstriction

1. Reduction of Strong Ion Difference (SID): By infusing a solution with a Na⁺:Cl^- ratio of 1:1, the plasma strong ion difference decreases from its normal value (≈ 40-42 mEq/L). According to quantitative physicochemistry, to maintain electroneutrality in a system with decreased SID, plasma water dissociates, releasing hydrogen ions (H⁺), which causes an immediate decrease in systemic pH.

2. Active Tubuloglomerular Feedback: Excess filtered chlorine is actively sensed by the macula dense cells in the thick portion of the ascending loop of Henle. This increases chloride entry through the Na⁺-K⁺-2Cl^- cotransporter (NKCC2), inducing the local release of adenosine. Adenosine binds to A1 receptors located in the afferent arteriole, inducing deep renal vasoconstriction, a 20% decrease in renal blood flow, a decrease in the glomerular filtration rate (GFR) and a propensity for the development of acute kidney injury (AKI).

Balanced Solutions

Balanced solutions (such as Lactated Ringer's, Plasmalyte or Isofundin) replace part of the chloride ion with metabolizable organic buffer anions. By presenting a Na⁺:Cl^- ratio similar to the plasma one, they do not alter the systemic strong ion difference and minimize the occurrence of hyperchloremic acidosis and associated kidney injury.