Module 2 Fluids and Sodium

Intracellular Fluid (ICF)

Fluid located inside cells; accounts for 70% of total body fluid

Extracellular Fluid (ECF)

Fluid located outside cells; accounts for 30% of total body fluid; includes interstitial (22%), intravascular (6%), and transcellular (2%) compartments

Interstitial Fluid

Extracellular fluid located in the spaces between cells; accounts for 22% of total body fluid

Intravascular Fluid

Extracellular fluid located within blood vessels (plasma); accounts for 6% of total body fluid

Transcellular Fluid

Extracellular fluid found in specialized spaces such as cerebrospinal canals, lymphatic tissues, synovial joints, and the eye; accounts for 2% of total body fluid

Isotonic Fluid

A solution with equal concentration on both sides of a semi-permeable membrane; little osmosis occurs; used to increase extracellular fluid volume; examples include 0.9% NS, D5W, and Lactated Ringer's

Lactated Ringer's (LR)

An isotonic IV solution containing NaCl, KCl, and CaCl; used to replace fluids and electrolytes in low blood volume or pressure situations such as surgery, blood loss, and trauma

Hypotonic Fluid

A more dilute IV solution that causes water to move into dehydrated cells; used to treat hypernatremia; examples include 1/2 NS and 1/4 NS

Hypertonic Fluid

A more concentrated IV solution that draws water out of cells into the extracellular space; given very slowly due to risk of pulmonary and cerebral edema; examples include 3% NS, D10W, D5NS, D5LR; 3% NS used for severe hyponatremia

Plasma Expanders (Colloids)

Solutions that pull fluid from the interstitial compartment into the vascular compartment to rapidly increase vascular volume; used in shock, hemorrhage, and severe hypovolemia; examples include albumin, fresh frozen plasma, and PRBCs

Second Spacing (Edema)

Excess accumulation of fluid in the interstitial space; can be localized (trauma, inflammation, burns) or generalized/anasarca (cardiac, renal, liver failure)

Edema - Elevated Hydrostatic Pressure

Increased pressure pushes fluid out of vessels into tissue space, contributing to edema formation

Edema - Decreased Plasma Oncotic Pressure

Reduced plasma proteins result in less force to pull water back into vessels, contributing to edema

Edema - Elevated Interstitial Oncotic Pressure

Inflammation leaks albumin into tissue, drawing water into the interstitial space and worsening edema

Edema - Lymphatic Obstruction

Blocked lymphatic flow decreases removal of interstitial fluid, leading to fluid accumulation

Third Spacing

Accumulation of trapped extracellular fluid in a body cavity that does not normally hold fluid (pericardial, pleural, or peritoneal space); represents a functional volume loss

Isotonic Dehydration (Hypovolemia)

Equal loss of water and electrolytes leading to decreased circulating blood volume and inadequate tissue perfusion

Hypertonic Dehydration

Water loss exceeds electrolyte loss; causes alterations in plasma electrolytes; fluid moves from intracellular space into plasma causing cell shrinkage

Hypotonic Dehydration

Electrolyte loss exceeds water loss; causes fluid shifts between compartments and decreased plasma volume; fluid moves from plasma and interstitial space into cells causing cell swelling

Isotonic Dehydration Causes

Inadequate intake of fluids and solutes, fluid shifts between compartments, excessive loss of isotonic fluids

Hypertonic Dehydration Causes

Conditions that increase fluid loss: excessive perspiration, hyperventilation, ketoacidosis, prolonged fevers, diarrhea, end-stage renal disease, diabetes insipidus

Hypotonic Dehydration Causes

Chronic illness, excessive hypotonic fluid replacement, renal disease, chronic malnutrition

Fluid Volume Deficit - Lab Findings

Increased serum osmolality, increased hematocrit, increased BUN, increased serum sodium, increased urine specific gravity

Fluid Volume Deficit - Assessment Findings

Weak/thready/diminished pulse, decreased BP and orthostatic hypotension, flat neck veins, decreased RR/dyspnea, lethargy to coma, skeletal muscle weakness, fever, decreased urine output, decreased skin turgor, dry mouth, diminished bowel sounds, constipation, thirst

Fluid Volume Deficit Management

Oral rehydration or IV fluids matched to dehydration type (isotonic→isotonic, hypertonic→hypotonic, hypotonic→hypertonic); monitor VS and I/Os; administer antidiarrheal, antibiotics, antiemetics, antipyretics; monitor electrolytes

Isotonic Overload (Hypervolemia)

Excessive fluid in the extracellular space without fluid shifting between spaces; causes circulatory overload and interstitial edema; can lead to heart failure and pulmonary edema

Hypertonic Overload

Rare condition caused by excessive sodium intake; fluid is drawn from intracellular space causing extracellular expansion and intracellular contraction

Hypotonic Overload (Water Intoxication)

Excessive fluid moves into the intracellular space causing all compartments to expand and electrolytes to become diluted

Isotonic Overload Causes

Inadequately controlled IV fluids, kidney disease, long-term corticosteroid therapy

Hypertonic Overload Causes

Excessive sodium ingestion, rapid infusion of hypertonic saline, excessive sodium bicarbonate therapy

Hypotonic Overload Causes

Early kidney disease, heart failure, SIADH, inadequately controlled IV fluids

Fluid Volume Overload - Lab Findings

Decreased serum osmolality, decreased hematocrit, decreased BUN, decreased serum sodium, decreased urine specific gravity

Fluid Volume Overload - Assessment Findings

Bounding/increased pulse, elevated BP, distended neck veins, increased RR/crackles/SOB, altered LOC/HA/visual disturbances/skeletal muscle weakness, increased UO (decreased if kidney damage), pitting edema, pale cool skin, increased GI motility/diarrhea, liver enlargement, ascites

Fluid Volume Overload Management

Restrict fluid and sodium intake; monitor VS, I/Os, and weight; administer diuretics (mild: HCTZ; severe: furosemide; K-sparing: spironolactone); monitor electrolytes; prepare for dialysis if needed

Sodium (Na+)

Most abundant electrolyte in ECF; plays a major role in water distribution control; regulated by ADH, thirst, and the renin-angiotensin-aldosterone system; provides the electrochemical state needed for muscle contraction and nerve impulses

Sodium Regulation

Controlled by ADH, thirst mechanism, and the renin-angiotensin-aldosterone system; salt loss generally leads to water loss, and salt gain leads to water gain

Normal Serum Sodium

135–145 mEq/L

Hyponatremia

Serum sodium below 135 mEq/L; primarily caused by water imbalance rather than sodium imbalance

Hyponatremia - Euvolemic Causes

Normal fluid volume with low sodium; associated with adrenal insufficiency

Hyponatremia - Hypovolemic Causes

Low fluid volume with low sodium; caused by diuretic medications and loss of GI fluids

Hyponatremia - Hypervolemic Causes

High fluid volume with low sodium; caused by renal failure, SIADH, excess water intake (D5W, tube feeds with water), and head trauma

Hyponatremia Clinical Manifestations

Seizures, stupor, lethargy, confusion; abdominal cramping, poor appetite, overactive bowel sounds; muscle spasms, diminished tendon reflexes; decreased urine output; orthostatic hypotension; shallow respirations

Hyponatremia Management

Assess affected systems; restrict water and/or replace sodium as needed; encourage dietary sodium; adjust medications (diuretics, lithium); closely monitor I/Os, daily weights, and labs

Hypernatremia

Serum sodium above 145 mEq/L; caused by excess sodium intake or insufficient water relative to sodium

Hypernatremia Causes

Increased sodium intake, GI tube feeds without supplemental water, hypertonic IV fluids, sodium excretion problems, fluid loss (dehydration, infection, diuresis), impaired thirst or limited fluid access, hyperventilation, hypercortisolism, increased aldosterone production

Hypernatremia Clinical Manifestations

Fatigue; restlessness, muscle twitching, seizures; increased fluid retention, edema, decreased urine output; extreme thirst; dry mouth

Hypernatremia Management

Assess for abnormal water loss and inadequate intake; monitor for CNS changes; gradually lower serum sodium; administer isotonic or hypotonic fluids (0.45% NS) slowly; restrict sodium intake

SIADH (Syndrome of Inappropriate ADH)

Condition of excessive ADH secretion causing the body to retain water, diluting sodium and causing hypervolemic hyponatremia

ADH (Antidiuretic Hormone)

Hormone that promotes water reabsorption in the kidneys; regulates serum osmolality and sodium concentration; excess ADH leads to water retention and hyponatremia

Renin-Angiotensin-Aldosterone System (RAAS)

Hormonal system that regulates sodium and fluid balance; aldosterone causes sodium and water retention in the kidneys

Serum Osmolality

A measure of solute concentration in the blood; elevated in dehydration/fluid deficit; decreased in fluid overload

Urine Specific Gravity

A measure of urine concentration; elevated (concentrated) in fluid deficit; decreased (dilute) in fluid overload

Anasarca

Severe generalized edema affecting the entire body; associated with cardiac, renal, or liver failure

Orthostatic Hypotension

A drop in blood pressure upon standing; a sign of fluid volume deficit and hyponatremia

Pitting Edema

Edema in which finger pressure leaves a temporary indentation; a sign of fluid volume overload