Introduction
Fluid therapy is an integral component of perioperative care by anesthesia providers. Administration of fluid helps to prevent dehydration, maintains effective circulatory volume, and ensures adequate tissue perfusion during the period when the patient is unable to achieve these goals through normal oral intake.
Therefore, a basic knowledge of different types of fluids, their physicochemical properties, and their pathophysiological effects is necessary to achieve optimal patient care.
Water constitutes 60% of total body weight in the average adult, varying with age, gender and body composition (Box 4.1 and Table 4.1).
Abbreviations: ECF, extracellular fluid; TBW, total body water.
The human plasma maintains its osmolarity in a very narrow range (280–290 mosm/L). The administration of fluids with different osmolarity can alter the plasma osmolarity, leading to water shift from or into cells. Change in plasma osmolarity can also stimulate or suppress the hypothalamic osmoreceptors, therefore altering the thirst response.
Note: a “sequestered” extracellular compartment refers to water present in bone and dense connective tissue or within transcellular compartment and therefore not readily available for equilibration with other fluid compartments.
So, it would be noteworthy to overview the concept of osmolality, osmolarity, and tonicity (Table 4.2).
A given fluid can be isotonic, hypotonic, or hypertonic with respect to a reference solution.
Isotonic solution has identical osmolality as the reference solution. No movement of water across the semipermeable membrane occurs.
Hypotonic solution has lesser osmolality than that of the reference solution. Water moves into cells when kept inside hypotonic solution and results in cellular edema.
Hypertonic solution has higher osmolality than that of the reference solution. Water moves out of cells when kept inside hypertonic solution and leads to cellular dehydration and shrinkage.
The fluids commonly used during perioperative period belong either to crystalloid or the colloid category. Table 4.3 illustrates crystalloids and colloids.
Note: a The studies on effects of fluids on red blood cells (RBC) dilution and change in hematocrit suggests that the crystalloids do not leave the intravascular compartment immediately but move in to subglycocalyceal layer (SGL), which is a subcompartment of intravascular chamber (volume 700–1,000 mL), while the colloids remain in the intravascular chamber and therefore cause more dilution of hematocrit than crystalloids.
Crystalloids
Crystalloids are solutions of electrolytes or small particles and are the most commonly used fluids in the perioperative period during resuscitation and intensive care. The comparative composition of various crystalloids is depicted in Table 4.4. Table 4.5 lists the properties, indications, and side effects of crystalloids.
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