Potassium Disorders I: Hypokalemia

Mechanism

Mechanisms of Development of Hypokalemia

Hypokalemia is defined as plasma potassium less than 3.5 mEq/L. It is produced through three pathophysiological mechanisms:

1. Intracellular Redistribution (Cell Shift): Extracellular potassium is quickly introduced into the cell without any real losses from the organism. It occurs in the presence of metabolic alkalosis (due to H⁺/K⁺ exchange), administration of beta-2 adrenergic agonists (such as salbutamol, which directly stimulates the mitochondrial Na⁺/K⁺-ATPase pump), insulin infusion or due to episodes of severe hypothermia.
2. Extrarenal Losses (Urine K⁺ < 20 mEq/g creatinine): Severe gastrointestinal losses due to profuse diarrhea or chronic laxative use. Note: Vomiting induces hypokalemia not due to direct loss of potassium in the gastric juice (which contains only 10 mEq/L of K⁺), but due to metabolic alkalosis and secondary hypovolemia that netly stimulate renal aldosterone, forcing the excretion of potassium through urine.
3. Renal Losses (Urine K⁺ > 20 mEq/g creatinine): Caused by the active use of loop diuretics (furosemide) or thiazides, states of primary or secondary hyperaldosteronism, renal tubular acidosis (type 1 and 2) or genetic transporter syndromes (Bartter and Gitelman syndromes).

Progressive Electrocardiographic Manifestations

Hypokalemia hyperpolarizes the myocardial cell membrane, delaying the active cellular repolarization phase. The signs on the electrocardiogram (ECG) appear correlatively with the decrease in plasma potassium:

• Progressive decrease or flattening of the T wave.
• Marked depression of the ST segment (mimicing ischemic events).
• Appearance of a prominent U wave (preferentially visible in precordial leads V2 to V4).
• Progression to pathological prolongation of the QT/QU interval, predisposing to the development of lethal ventricular arrhythmias such as torsades de pointes or ventricular fibrillation.

The Compulsory Physiological Relationship between Potassium and Magnesium

It is a law of clinical practice that severe hypokalemia is refractory to exogenous potassium replacement if magnesium deficiency is not corrected concomitantly and as a priority. This phenomenon is mediated by the behavior of the renal secretory potassium channel ROMK (Renal Outer Medullary Potassium channel), located in the distal convoluted and collecting tubule.

Under conditions of physiological homeostasis, intracellular magnesium acts as a "physical plug" or blocking gate that is located in the pore of the ROMK channel, preventing the uncontrolled exit of potassium ions from the cellular cytosol into the urine. If there is cellular magnesium depletion, the ROMK channel remains open permanently and without restriction, allowing massive and irreversible excretion of potassium through the urine, regardless of the amount of exogenous potassium that is administered intravenously.

Therefore, in the event of any hypokalemia with serum magnesium less than 1.8 mg/dL, 2 to 4 grams of Magnesium Sulfate should be co-administered intravenously.