CHAPTER 21. Fluid, Electrolyte, and Acid-Base Balance
Kim A. Noble
OBJECTIVES
At the conclusion of this chapter, the learner will be able to:
1. Identify the three primary fluid compartments of the body and the volume and distribution of fluid in each.
2. Differentiate between the individual forms of crystalloid and colloid solutions and their indications for use.
3. Identify fluid and electrolyte imbalances and the nursing assessment and management for each.
4. Describe the primary mechanisms responsible for the regulation of fluid and electrolyte balance.
5. Identify the physiological origination of acid-base balance in the body and the potential implications of abnormalities in the perianesthetic patient.
6. Describe the components of arterial blood gas (ABG) results specific to acid-base interpretation and their physiological rationale for analysis.
7. Identify common abnormalities in acid-base balance and their application to the perianesthetic population.
I. FLUID AND ELECTROLYTE BALANCE OVERVIEW
A. Body cells function in a tightly regulated fluid- and electrolyte-filled environment.
1. Fluid and electrolyte homeostasis
a. Maintained via hormonal mechanisms
b. Maintained via neural mechanisms
2. Alterations in fluid and electrolyte homeostasis impact cellular function.
a. Change the electrical potential of excitable cells
b. Lead to intracompartmental fluid shifts
c. Directly impact organ system function
3. There is a constant flux or movement of water and solutes between the three primary body fluid compartments.
a. Extracellular fluid compartment
(1) Intravascular fluid
(2) Interstitial fluid
b. Intracellular compartment
c. Transcellular compartment
4. Fluid flux is constrained by:
a. Compartmental membranes
b. Solute and plasma protein concentrations
5. Sodium/potassium (Na +/K +) adenosine triphosphatase (ATPase) pump
a. Active (energy dependent) pump on cell membranes
b. Functions in the maintenance of solute concentration gradients across the cell membranes
B. Fluid balance requires both:
1. Normal volume of water
2. Normal concentrations of particles in solution (Table 21-1)
| ADH, Antidiuretic hormone; ATPase, adenosine triphosphatase; BP, blood pressure; CNS, central nervous system; DI, diabetes insipidus; DKA, diabetic ketoacidosis; ECF, extracellular fluid; ECG, electrocardiogram; GI, gastrointestinal; ICF, intracellular fluid; NSS, normal saline solution; N/V, nausea/vomiting; PTH, parathyroid hormone; PVCs, premature ventricular contractions; RR, respiratory rate; SIADH, syndrome of inappropriate secretion of ADH. | |||
| *Electrolytes found in high concentration in ECF are in low concentration in ICF; similarly, the primary electrolytes of the ICF are present, but in low concentrations, in the ECF. | |||
| ECF Ion | Normal Serum Value | Indicators | |
|---|---|---|---|
| Deficit (Hypo-) | Excess (Hyper-) | ||
| Sodium (Na +) Regulates ECF osmolality and vascular fluid volume ↓ICF content; ↑ECF content maintained by Na +/K + ATPase pump | 135-145 mEq/L | Hyponatremia <130 mEq/L, ↓serum osmolality Salt diluted by excess retained water Bladder irrigations, electrolyte-free IV infusions, ADH oversecretion (SIADH) Weak muscles Confusion Nausea/vomiting Hypotension Seizure Coma if <115 mEq/L | Hypernatremia >145 mEq/L, ↑serum osmolality Excess salt from water losses Inadequate osmotic diuresis; poor fluid intake; lack of ADH (DI) Thirst Flushed skin Hypotension Oliguria Seizures, coma if extreme |
| Chloride (Cl −) Preserve acid-base balance Reciprocal: if Cl − depleted,  risesCombines with Na + to maintain osmolality | 96-106 mEq/L | Hypochloremia ~ <98 mEq/L Prolonged Cl − loss: gastric suction, diuresis Patient hypoventilates | Hyperchloremia ~ >108 mEq/L Cl − gain; NSS resuscitation Patient hyperventilates |
| Bicarbonate (HCO 3−) | 22-28 mEq/L | Metabolic Alkalosis pH >7.45 Acid loss/HCO 3− gain: N/V; ↑GI suction Patient hypoventilates; compensatory ↓K + | Metabolic Acidosis pH <7.35 Acid gain/HCO 3− loss: renal failure; DKA Patient hyperventilates; compensatory ↑K + |
| Osmolality (mOsm) | 280-300 Osm/kg | Dehydration ECF concentrated (DI) ↑ Risk of thrombosis | Overhydration ECF dilute (SIADH) |
| Potassium (K +) ↓ECF content; ↑ICF content maintained by Na +/K + ATPase pump Potent effect on cell and neuromuscular irritability Acidosis, catabolism: move K + to serum Insulin, glucose shift K + back to cell ↑ Concentration in ECF maintained by Na +/K + ATPase pump | 3.5-5.0 mEq/L | Hypokalemia ~ <3.5 mEq/L Reflects ECF loss: diuretics, diarrhea, N/V, digitalis, bowel preps ↓ECF K + → ↓ICF K + Muscle weakness Hypoventilation Flaccid paralysis Cardiac arrhythmias: more PVCs, U wave classic, conduction blocks Slow KCI doses: 10 mEq/h peripheral; 20 mEq/h central line | Hyperkalemia ~ >5.0 mEq/L ↑ Serum K +: tissue lysis, acidosis (renal or DKA) Malignant hyperthermia: LETHAL Muscle weakness Hypoventilation Paralysis Cardiac arrhythmias: peaked T waves; wide QRS; asystole Stat insulin (glucose), bicarbonate and Ca + drives K + back into ICF Dialyze renal patients Stop any K + intake |
| Magnesium (Mg +) Promotes acetylcholine release at neuromuscular junction Regulates K + Opposes Ca ++ | 1.5-2.5 mEq/L | Hypomagnesemia <1.5 mEq/L Diarrhea Mal-absorption Long-term N/V ↑ Aldosterone Neuromuscular irritability, seizures Cardiac: long PR, wide QRS, flat T; torsades risk Affects serum K +, Ca ++, and  | Hypermagnesemia >2.5 mEq/L MgSO 4 infusion (eclampsia) Ketoacidosis Chronic renal failure CNS depression, sedation, muscle weakness, ↓ reflexes ↓BP; ↓heart rate If Mg + >12 → ↓RR |
| Phosphate (PO 4−) Most stored in bone Essential for energy and acid-base balance Inverse relationship with calcium: if PO 4−↑, Ca ++↓ Need parathyroid hormone (PTH) to excrete | 1-2 mEq/L (3-4.5 mg/dL) | Hypophosphatemia <1.5 mg/dL Aspirin overdose Ketoacidosis Steroids Malabsorption ↑Ca ++ Energy depletion: weak muscle, seizures, cardiorespiratory failure | Hyperphosphatemia >4.5 mg/dL Laxative excess Supplement in diet Trauma Cell death, renal failure, PTH decreases |
| Calcium (Ca ++) Critical for impulse conduction, contraction, and coagulation Is stored in bone Present in blood (ECF): ionized (50%), protein bound Inverse relationship with  : when Ca ++↑,  | 4.5-5.3 mEq/L (8.5-10.5 mg/dL) | Hypocalcemia <4.5–5.3 mEq/L Low albumin Renal failure (chronic) Hypoparathyroidism Tingling/weakness Twitching/tetany Low BP ECG change Postoperative laryngospasm | Hypercalcemia >4.5 mEq/L Immobility Malignancy Low  Hyperparathyroidism Lethargy Short QT |
II. BODY FLUID DISTRIBUTION
A. Body water accounts for:
1. Approximately 60% of adult total body weight
2. As much as 75% to 77% of infant total body weight
3. Average male (154 lb or 70 kg) has 42 L of total body water.
a. Extracellular fluid (ECF) accounts for approximately 14 L of fluid.
(1) Intravascular fluid: accounts for approximately one third of total ECF
(2) Interstitial fluid: accounts for approximately two thirds of total ECF
b. Intracellular fluid (ICF) accounts for approximately 28 L of fluid.
4. Percentage of water varies with percentage of body fat.
a. Muscle: high water content
b. Fat: low water content
c. Female body contains a higher proportion of fat than male.
B. Body fluid compartments
1. ECF compartment: accounts for one third of total body water.
a. Fluid circulating outside of cells
b. Volume: 33% to 40% of adult’s total body weight, nearly 75% of a young child’s body weight
c. Three subcomponents of ECF
(1) Intravascular fluid: fluid within the vascular system
(a) Crucial for cardiovascular function
(b) Accounts for one third of ECF volume or 8% of total body water
(2) Interstitial fluid: fluid between the cells
(a) Returns to circulation via lymphatics
(b) Controlled by capillary cell wall integrity, oncotic and hydrostatic pressures
(c) About two thirds of ECF volume (20% of adult total body water
(3) Transcellular fluid
(a) Includes:
(i) Synovial
(ii) Cerebrospinal
(iii) Intestinal, hepatic
(iv) Biliary
(v) Pancreatic
(vi) Sweat
(vii) Pleural
(viii) Pericardial
(ix) Peritoneal
(x) Intraocular fluids
(b) Accounts for about 1% of adult total body weight
d. Anesthetic medications dilate vasculature and expand ECF capacity.
(1) Ease fluid overload and improve diastolic filling in the heart.
(2) If ECF volume insufficient, significant hypotension results.
2. ICF compartment: accounts for two thirds of total body water
a. Volume accounts for 66% to 75% of total body water.
b. Fluid found within cells
C. Three processes that govern water movement
1. Osmosis
a. The movement of water from a dilute space with few particles across a semipermeable membrane to a more densely concentrated space; “salt sucks”
b. Osmosis seeks to establish equilibrium between ECF and ICF.
c. The unequal numbers and size of particles controls fluid movement between ECF and ICF.
(1) Glucose, urea, and protein are large molecules that normally cannot pass from blood (ECF) through selectively permeable cell walls.
(2) Because of large particles in the blood, ECF contains more particles, and is therefore more concentrated, than in cells (ICF).
(3) Water shift is constant.
(a) Net movement of water is towards the ECF.
(b) Prevents cells from becoming:
(i) Waterlogged
(ii) Edematous
(iii) Bursting
d. Factors influencing osmosis or the movement of water
(1) Cell wall permeability (integrity)
(2) Serum sodium levels
(3) Na +/K + ATPase pump
(a) Active pump found on cell membranes
(b) Functions in the maintenance of intracellular to extracellular ion concentration gradients
2. Oncotic pressure
a. Also called colloid osmotic pressure
b. Colloids are large particles, such as protein, that normally cannot cross cell membrane.
c. Plasma colloid osmotic pressure: primarily contained in serum and pulls fluid from interstitial space into capillaries across a pressure gradient
3. Hydrostatic pressure
a. Pump pressure exerted by blood against blood vessel (capillary) walls
(1) Elevated capillary hydrostatic pressure with rise in arterial pressure or vessel resistance
(2) Low capillary resistance or low arterial pressure reduces capillary hydrostatic pressure.
b. Principal force causing capillary filtration, or the movement of fluid out of the capillary into the interstitial space
c. Greater at arterial end of the capillary (32 mm Hg) than venous (15 mm Hg)
d. Opposes oncotic or osmotic pressure
III. PHYSIOLOGICAL PARTICLE (SOLUTE) DISTRIBUTION
A. Components (solute) distributed within body water
1. Electrolytes: electrically active ions with either a positive or negative charge when dissolved in solution (Box 21-1). NOTE: A measure of the serum (ECF) concentration of an electrolyte does not necessarily reflect the electrolyte content of intracellular electrolytes (ICF).
a. Primary extracellular (ECF) electrolytes
(1) Cation: positively charged ion
(a) Sodium (Na +)
(i) Reflects serum osmolality
(ii) Regulates fluid balance
(iii) The cation in highest concentration in the ECF
(b) Expect fluid imbalance if serum sodium increased or decreased.
(c) Inverse relationship with serum potassium
(i) If Na + rises, expect low K +.
(d) Na + concentration gradient (ICF: ECF) maintained by the activity of the Na +/K + ATPase pump
(2) Anion: negatively charged ion
(a) Chloride (Cl −) competes with bicarbonate ( 
) to combine with sodium.
) to combine with sodium.(b) Bicarbonate: immediately available acid-base buffer
b. Primary ICF electrolytes: cannot directly measure; reflected by ECF values. NOTE: Status of ICF electrolytes is not necessarily reflected by a laboratory measure of an electrolyte in the serum (ECF).
(1) Cations: positively charged ions, critical for cardiac function
(a) Potassium (K +): poorly stored, deficits occur quickly with loss or reduced intake; cation in highest concentration in ICF
(b) Magnesium (Mg +)
(c) Calcium (Ca ++); stored in ICF; released for cellular activity
(d) Replace all cations slowly.
(i) Always in diluted solution
(ii) Never intravenous (IV) push
(2) Anions: negatively charged ions
(a) Phosphorus (P), present in body fluid as phosphate (PO 4)
BOX 21-1
TONICITY OF REPLACEMENT IV SOLUTIONS
Normal Serum Osmolarity: 290 mOsm/L
Tonicity
Hypotonic: Osmolality <240 mOsm/L
ECF concentration < ICF
Causes water to move from serum into cells
Isotonic: Osmolality 240-340 mOsm/L
Concentration of dissolved particles in ECF = ICF
Hypertonic: Osmolality >340 mOsm/L
ECF concentration > ICF
Causes water to move from cell to serum
IV Solution Tonicity
Half normal saline (0.45 NS): 154 mOsm/L
5% dextrose in water (D 5W): 252 mOsm/L
2.5% dextrose in one-half NS (D 2.5 0.45NS): 265 mOsm/L
Lactated Ringer’s (LR): 5% dextrose in one-half normal saline: 310 mOsm/L
0.9 normal saline (NS): 308 mOsm/L
5% dextrose in one-fourth NS (D 5 0.225 NS): 326 mOsm/L
5% dextrose in one-half NS (D 5 0.45 NS): 406 mOsm/L
10% dextrose in water (D 10W): 505 mOsm
5% dextrose in LR (D 5LR): 524 mOsm/L
5% dextrose in NS (D 5 0.9 NS): 560 mOsm/L
ECF, Extracellular fluid; ICF, intracellular fluid.
2. Nonelectrolyte particles, undissolved
a. Large, osmotically active molecules
b. Influence movement of water across permeable cell membranes
c. Examples
(1) Sugar
(2) Urea
(3) Protein
3. Buffers: physiological controls to regulate acids and bases
a. Bicarbonate: immediate chemical buffer
(1) Present in ECF
(2) Regulate (buffer) pH by accepting or releasing acidic hydrogen ions (H +).
(3) Maintain serum’s chemical neutrality, specifically pH – 7.4: a mathematic representation of hydrogen ion in ECF.
(4) Maintain bicarbonate-to–carbonic acid ratio of 20:1.
b. Phosphate, hemoglobin, and protein: chemical buffers
(1) Present in all body fluids to help maintain acid-base balance and coagulation
(2) Proteins create colloid osmotic pressure to regulate fluid distribution.
(a) Low-protein conditions include:
(i) Hemorrhage (red blood cell loss)
(ii) Malnutrition
(iii) Severe infections
(iv) Fistulas
(v) Fluid imbalances
(b) Low-protein conditions allow fluid to leak from vascular space (ECF) to ICF because of loss of oncotic pressure.
(c) Need serum albumin level greater than 4 g/dL for adequate protein level
4. Salts: potassium chloride (KCl) is one example.
B. Osmolality is a measure of the amount of solute per volume of solution.
1. An index of the body’s hydration status
2. Normal value: 280 to 294 milliosmoles (mOsm)/kg
3. Total number of “osmotically active” particles in solution
a. Determined by total of electrolyte and nonelectrolyte particles
b. Creates osmotic pressure per liter of solution to maintain water in appropriate compartment
c. Serum sodium is the most important determinant.
(1) Water follows sodium to equalize concentration and establish equilibrium; “salt sucks.”
(2) When serum sodium elevated, water shifts into serum (ECF) by osmosis, diluting sodium and normalizing osmolality.
(3) When serum sodium low, water shifts from serum by osmosis to concentrate sodium and normalize osmolality.
4. Serum osmolarity monitored by the hypothalamus
a. Increased osmolarity (increased Na +; decreased water) causes thirst and the negative feedback release of antidiuretic hormone (ADH).
(1) ADH causes the kidney to:
(a) Reabsorb water from the distal tubule
(b) Expand the water in the ECF
(c) Normalize the osmolarity
(2) The normal osmolarity leads to the negative feedback (decreased) release of ADH.
b. Decreased osmolarity (decreased Na +; increased water) decreases the release of ADH.
(1) As ADH secretion decreases:
(a) The water reabsorbed from the distal tubule decreases, causing an increased urinary output.
(b) This leads to decreased water in the ECF and normalizes the osmolarity.
5. Osmolality, in the form of volume or pressure, is also sensed by baroreceptors in the right atrium, leading to the release of atrial natriuretic peptide (ANP).
a. Osmolality high
(1) Low volume and pressure
(2) ECF-concentrated (hypertonic) patient is dehydrated.
b. Osmolality low
(1) High volume and pressure
(2) ECF-dilute (hypotonic) patient is overhydrated.
c. Primarily adjusted by titrating the release of ADH
C. Mechanisms of solute transport
1. Passive or non–energy-expending transport
a. Diffusion: results in the movement of particles in solution across a selectively permeable cell membrane “down” the concentration gradient, or from an area of high solute concentration to an area of lower solute concentration
(1) Purpose: try to equalize concentration of particles between compartments
(2) Electrolytes are small; pass easily across cell walls
(3) Larger particles inhibited from crossing selectively permeable membrane
(4) Although individual ions move constantly, passively, and randomly between ECF and ICF mostly towards the dilute solution
(5) Particle concentration dissolved in ECF or ICF determines water movement (osmosis) and fluid balance.
(6) Solute concentration difference between areas is a concentration gradient.
b. Facilitated diffusion: a substance (for example, insulin) facilitates the diffusion of particles (e.g., glucose) across the semipermeable membrane.
c. Filtration: transfer of water and dissolved substances through the semipermeable gradient via a pressure gradient from higher to lower pressure (hydrostatic pressure).
(1) Pressure created by the weight of the solute-laden solution
(2) Glomerular filtration in kidney’s nephron is an example
(a) Arterial blood pressure is greater than intrarenal pressure.
(b) This pressure gradient forces blood into the glomerulus for filtration.
(3) A force opposing oncotic pressure
d. Osmotic pressure: pressure exerted within a compartment by osmotically active particles in solution
(1) Differences in particle concentration between two compartments create a concentration gradient.
(2) Pressure across this gradient moves (redirects) water across the gradient to equalize water between cells or fluid compartments.
(3) After water equilibrates:
(a) Concentrations of particles in solution equalize.
(b) Volume of water in the compartments may not be equal.
(4) Opposes interstitial fluid pressure
2. Active or energy-dependent solute transportation; primarily through the action of the Na +/K + ATPase pump
a. Metabolic energy in the form of adenosine triphosphate (ATP) is consumed to move substances against their concentration gradient(s) through semipermeable cell membranes.
b. Oxygen also required
(1) During cellular processes, Na + diffuses down concentration gradient, through cell wall and into ICF; K + moves passively in opposite fashion out of the cell.
(2) Na +/K + ATPase (active, energy-dependent pump) returns Na + (against concentration gradient; uphill) to ECF and K + (uphill) to ICF.
IV. HORMONAL REGULATORS OF BLOOD VOLUME
A. ADH: adjusts serum osmolality, concentrates electrolytes
1. Regulates reabsorption or elimination of water, but not Na +, in the distal renal tubules, thereby concentrating or diluting Na +
2. Released by the pituitary’s posterior (hypophysis) in response to a 1% to 2% increase or decrease in serum osmolality, as sensed by osmoreceptors in hypothalamus
3. Increased ADH secretion: response to increased serum osmolality
a. Prompts water reabsorption at kidney’s collecting ducts: urine concentrates and output decreases.
(1) Normal urine specific gravity: 1.010-1.025
(2) Specific gravity increases: more concentrated with dissolved solutes
b. Secretion stimulated by stress such as:
(1) Pain
(2) Trauma
(3) Surgery
(4) Hypovolemia
(5) Opioids
(6) Hypoxia
(7) Hypercapnia
4. Decreased ADH secretion: response to decreased serum osmolality
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a. Promotes water elimination through collecting ducts
