CHAPTER 4 Fluids (AS9.3)

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).

Box 4.1 Distribution of total body water between fluid compartments

Intracellular (55%).
Extracellular (45%).
- Functional (27.5%).
  - Interstitial (20%).
  - Intravascular (7.5%).
- Plasma (5.5%).
- Subglycocalyceal layer (SGL) (2%).
- Sequestered^a (17.5%).
  - Bone and connective tissue (15%).
  - Transcellular (2.5%).

Table 4.1 Age-related variation in total body water and extracellular fluid as percentage of body weight

**Table 4.1** Age-related variation in total body water and extracellular fluid as percentage of body weight
Age	TBW (%)	ECF (%)	Blood volume (%)
Neonate	80	45	9
6 months	70	35
1 year	60	28
5 year	65	25	8
Young adult (male)	60	22	7
Young adult (female)	50	20	7
Elderly	50	20

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).

Table 4.2 Differentiating features between osmolality, tonicity, and osmolarity

**Table 4.2** Differentiating features between osmolality, tonicity, and osmolarity
Osmolality	Tonicity	Osmolarity
It is defined as the number of moles per kg of solvent It is not affected by temperature Plasma osmolality is calculated as (moles/kg) = 2 × Na + (glucose/18) + (urea/2.8) Na is in mmol/L, glucose and urea in mg/dL	It is the number of effective osmoles The solute particles which can move freely across a membrane do not contribute to osmolality (e.g., urea and glucose) It can be calculated by subtracting glucose and urea concentration from calculated osmolality	It is defined as the number of moles per liter of solution It is affected by temperature It is calculated as: osmolarity in (mmoles/L) = (concentration in mg/dL × 10)/ molecular weight

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.

Table 4.3 Comparison of crystalloids and colloids

**Table 4.3** Comparison of crystalloids and colloids
Crystalloids	Colloids
Crystalloid is a solution of electrolytes or smaller particles	Colloid consists of larger size particles in a solution of electrolytes
They expand intravascular volume for lesser duration because of short intravascular half-life^a	They expand intravascular compartment for longer duration
Cheap	Expensive
Move easily between intravascular and interstitial compartment, therefore can precipitate edema	Stay mainly in intravascular compartment and decreases cerebral and pulmonary edema
Do not interfere with clotting, blood grouping	Can interfere with clotting Dextrans can interfere with blood grouping by causing rouleaux formation
Usually do not cause renal dysfunction, but infusion of large doses of 0.9% NaCl can cause renal dysfunction (due to hyperchloremic metabolic acidosis)	Colloid can cause renal dysfunction
No allergy potential (unless contaminated)	Allergic reactions are common
Examples: 0.9% NaCl, ringer lactate, and plasmalyte	Examples: Hydroxyethyl starch, albumin, gelatins, and dextrans

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.

Table 4.4 Comparative composition of various crystalloids

**Table 4.4** Comparative composition of various crystalloids
Fluid	Na	K	Cl	Ca	Mg	HCO₃	Lactate	Acetate	Gluconate	Glucose (g/L)	Osmolarity (mosm/L)
Plasma	140	5	100	4.4	2	24	1	–	–	1	285
0.9% NaCl	154	–	154	–	–	–	–	–	–	–	308
5% Dextrose	–	–	–	–	–	–	–	–	–	50	252
25% Dextrose	–	–	–	–	–	–	–	–	–	250	1,260
50% Dextrose	–	–	–	–	–	–	–	–	–	500	2,520
0.45% NaCl	77	–	77	–	–	–	–	–	–	–	154
5% Dextrose/0.45% NaCl	77	–	77	–	–	–	–	–	–	–	406
Ringer lactate	130	4	109	3	–	–	28	–	–	–	273
Plasmalyte A	140	5	98	–	3	–	–	27	23	–	293