The main intracellular cation is Potassium, which plays a crucial role in maintaining the cell's resting membrane potential and proper cellular function. Its concentration is significantly higher inside cells compared to outside, essential for nerve impulse transmission, muscle contraction, and heart rhythm regulation. In surgical and critical care settings, monitoring potassium levels is vital since imbalances can lead to life-threatening arrhythmias or muscle weakness. Sodium, by contrast, predominates extracellularly. Maintaining proper intracellular potassium levels is crucial for cellular homeostasis and patient stability during perioperative management.

Reference: Guyton and Hall Textbook of Medical Physiology, Guyton AC, Hall JE, 14th Edition.