Fluid & Electrolyte Balance, FVD, CKD, & AKI – Comprehensive Study Notes

Overview of Fluid & Electrolyte (F&E) Balance

Goal: Keep volume/concentration of body fluids & electrolytes within normal physiologic range (homeostasis).
Inter-related with:
- Nutrition → intake of fluids & nutrients.
- Elimination → renal & intestinal output.
- Acid–Base balance → shifts often accompany F&E disorders.

Normal Body-Fluid Compartments

2 major spaces:
- Intracellular Fluid (ICF) – inside cells.
- Extracellular Fluid (ECF) – outside cells; subdivided into:
- Intravascular / Plasma (≈ blood water).
- Interstitial Fluid (IF) – between cells.
- Transcellular (CSF, synovial, pleural, etc.).
Typical distribution in a 70-kg adult male:
- \text{TBW}=42\,L \;(60\%\,\text{body wt})
- ICF =28\,L\;(66\%)
- ECF =14\,L
  - Interstitial =11\,L\;(25\%)
  - Plasma =3\,L\;(8\%)
70-kg female: more adipose → \text{TBW}\approx50\% (≈35 L).
Higher muscle = higher water; fat = lower water.

Sensible vs. Insensible Water Loss

Sensible (measurable): urine, sweat, feces (partially).
Insensible (cannot perceive/measure directly):
- Skin trans-epidermal diffusion.
- Respiratory vapor.
Adult daily average (intake = output ≈ 2.5\,L):
- Intake: fluids 1200\,mL, food 1000\,mL, oxidation 300\,mL.
- Output: urine 1500\,mL, insensible skin/lungs 900\,mL, feces 100\,mL.

Fluid Dynamics Between Compartments

Passive: diffusion, osmosis, filtration (hydrostatic & oncotic pressures).
Active transport: uses ATP to move solutes against gradient (e.g., \text{Na}^+/\text{K}^+ pump).
Key regulatory match-ups (quiz answers):
- Osmosis → free movement of water.
- Hydrostatic pressure → pushes fluid out of capillaries.
- Filtration → fluid moves high→low pressure.
- Active transport → uses energy to reach equilibrium.
- Parathyroid hormone → shifts \text{Ca}^{2+} from bone to ECF & acts on kidney for \text{Ca}^{2+}/\text{PO}_4^{3-} balance.

Principal Electrolytes & Selected Functions

Sodium \text{(Na}^+): chief ECF cation; regulates osmolality, water follows Na; blood pressure.
Chloride \text{(Cl}^-): chief ECF anion.
Potassium \text{(K}^+): chief ICF cation; cardiac rhythm & muscle contraction.
Phosphate \text{(PO}_4^{3-}): chief ICF anion; with Ca in bone/teeth; acid–base buffering; DNA/RNA.
Calcium \text{(Ca}^{2+}): bones/teeth, muscle contraction, coagulation.
Magnesium \text{(Mg}^{2+}): enzyme/immune function, carbohydrate metabolism, neuromuscular stability.

Electrolytes enable:

Water balance, pH regulation, nutrient‐waste transport, neuromuscular & cardiac activity.

Thirst & ADH (Vasopressin) Feedback

Hypothalamic osmoreceptors trigger thirst when osmolality rises.
Posterior pituitary releases ADH → kidneys reabsorb water → ↓ urine output.

Fluid Imbalance Categories

Fluid Volume Deficit (FVD, hypovolemia) – combined salt + water loss > intake.
Dehydration – pure water loss; Na concentration ↑ (hypernatremia); less hemodynamic compromise.
Fluid Volume Excess (hypervolemia) – sodium + water overload.

Risk Factors for Imbalances

Individual:

Intake > output: high salt, excessive IV fluids.
Output > intake: vomiting, diarrhea, hemorrhage, NG suction, wound VAC.
Altered distribution: edema (“third spacing”).
Population:
Infants (↑TBW %, immature kidneys, ↑BMR).
Older adults (↓ thirst & renal function).
Chronic diseases: heart failure, renal disease.

Assessment Framework (Subjective & Objective)

History: intake/output, chronic illnesses, meds (diuretics, supplements), symptom review (GI losses, fatigue, dizziness).
Physical:
- Gas exchange: RR, sounds.
- Perfusion: HR, BP (orthostatics), cap refill.
- Weight: daily, acute changes (1 kg ≈ 1 L water).
- Skin/mucous: turgor, moisture, edema; infants – fontanel, tears; elderly – central sternum.
- Cognitive: LOC, pupils, reflexes, strength.
- Mobility: muscle power.
- Nutrition: detailed intake.
- Elimination: abdominal exam, bowel sounds, stool/urine quantity & color.
Diagnostics:
- Urine: color, specific gravity 1.005\text{–}1.030 (↑ = concentrated).
- Serum: electrolytes, osmolality (expected 275\text{–}295\,mmol/L), creatinine, BUN, CBC (hemoconcentration ↑ Hct in hypovolemia).
- ECG (arrhythmias), imaging (CXR for pulmonary edema; bedside US of IVC collapsibility for volume status).

Urine Output Benchmarks

Adults: < 0.5\,mL/kg/hr or < 30\,mL/hr sustained suggests renal hypoperfusion/AKI.
Infants: diaper weight (1 g = 1 mL).

Fluid Volume Deficit (FVD) – Detailed Exemplar

Major Causes (multisystem):

Poor intake, hemorrhage, GI losses (vomit, diarrhea, NG tube), burns, excessive sweating, fever, mechanical ventilation, renal diuresis, diabetes with glucosuria, third-spacing/burn shifts.
Manifestations:
Dry mucous membranes, poor skin turgor, sunken fontanel, tachycardia, thready pulse, delayed cap refill, orthostatic hypotension, tachypnea, oliguria (< 500\,mL/24 h), weight loss, neuro changes (restlessness → coma), lab signs of electrolyte changes.
Interventions:

Monitoring: VS, weights (“daily wt before 8”), I&O accuracy, neuro checks.
Safety: fall precautions; oral care.
Replacement therapy:
- Oral preferred when possible.
- Isotonic IV (LR, 0.9\% NS) for FVD.
- Avoid hypotonic (e.g., 0.45\% NS) unless indication (DKA); hypertonic (3 % NS) reserved for severe hyponatremia/cerebral edema.
- Additives (KCl) & all IVFs used cautiously in renal impairment.
- Blood products if hemorrhagic.
Education: intake goals, sodium/fluids limits if needed.
Skin care & repositioning.

Key Electrolyte Imbalances

Sodium (135\text{–}145\,mEq/L)

Hypernatremia (> 145): causes – watery diarrhea, renal disease; S&S – thirst, restlessness → seizures (> 160). Treat oral or hypotonic IV water.
Hyponatremia (< 135):
- Hypovolemic: ↓ Na & water (thiazides, GI losses) → isotonic IV.
- Hypervolemic: ↑ Na & water but diluted (HF, cirrhosis, CKD) → fluid restrict, diuretics.
- Symptoms: headache, nausea, gait issues; severe (< 120) → seizures, coma (cerebral edema). Risk ↑ with rapid drop.

Potassium (3.5\text{–}5.0\,mEq/L)

Hypokalemia: diuretics, laxatives, GI loss; weak pulse, constipation, ECG changes; correct Mg first if low.
Hyperkalemia: AKI/CKD, ACEi, NSAIDs; diarrhea, palpitations, ECG changes. Rapid management: insulin + glucose, bicarbonate, calcium gluconate; elimination via diuretics or kayexalate.

Calcium (9.0\text{–}10.5\,mg/dL)

Hypocalcemia: Vit D deficiency, hypoparathyroidism, renal disease; ↑ reflexes, tetany, +Trousseau/Chvostek.
Hypercalcemia: hyperparathyroid, malignancy; ↓ reflexes, fractures.

Magnesium (1.3\text{–}2.1\,mEq/L)

Hypomagnesemia: GI/renal losses, alcoholism; ↑ reflexes, cramps, tachycardia; IV Mg with monitoring.
Hypermagnesemia: kidney failure; ↓ reflexes, bradycardia; treat severe with IV calcium gluconate.

Check-Your-Learning (Sample Answers Incorporated)

FVD risk: vomiting/diarrhea, continuous NG suction, wound VAC → all of the above.
FVD assessment findings: hyperthermia, orthostatic hypotension, ↓ skin turgor (not distended neck veins or bradycardia).
Another name for FVD → Hypovolemia.
Urine SG 1.040 → highly concentrated (possible dehydration).
Half-normal saline = 0.45\% NaCl.
Preferred replacement route = oral.

Chronic Kidney Disease (CKD)

Definition:

Structural/functional kidney abnormalities > 3 months and \text{GFR}<60\,mL/min/1.73 m^2.
Epidemiology:
1 in 3 with diabetes, 1 in 5 with HTN develop CKD.
Causes: diabetes (≈44 %), HTN (29 %), glomerular disease, PKD, others.
Risk Factors: age >60, race (AA, NA, Hispanic), family history, obesity, tobacco, unresolved AKI.
Pediatric: congenital anomalies (CAKUT), genetic syndromes; growth impairment significant.
Staging (GFR):
>90 G1, 60\text{–}89 G2, 45\text{–}59 G3A, 30\text{–}44 G3B, 15\text{–}29 G4,
Waste accumulation ↑ (BUN, creatinine, K).
Dyslipidemia (↑ LDL, ↓ HDL), metabolic acidosis, fluid retention.
Electrolytes: hyperkalemia, hyperphosphatemia → CKD mineral & bone disorder (MBD) with secondary hyperparathyroidism, vascular calcification, fractures.
Hematologic: anemia (↓ EPO), bleeding, infection risk.
Cardiopulmonary: HTN, HF, pericarditis, pulmonary edema.
GI: anorexia, N/V, metallic taste, uremic fetor.
Neuro: fatigue, encephalopathy, peripheral neuropathy, restless legs.
Skin: pruritus, dry/scaly, ecchymosis.
Endocrine/Repro: thyroid issues, infertility, ED.
Interventions:
Control BP (ACEi), manage DM, lipid lowering (statins).
Treat anemia (EPO + iron/folate), hyperkalemia, MBD (phosphate binders, calcitriol, cinacalcet).
Nutrition: protein 0.6–0.8 g/kg (40-54 g for 68 kg), Na ≤ 2 g/day, fluid restriction when on dialysis, K/P restriction.
Renal Replacement: dialysis (hemo/peritoneal) when GFR < 15; transplant (living ≈ 20 y life expectancy).
Fast Facts:
\approx 786 000 ESRD in US; 71 % dialysis, 29 % transplant.
Median adult wait time transplant ≈ 46-59 mo (racial disparities: Black 59.9 mo vs White 41.3 mo).

Acute Kidney Injury (AKI)

Definition: Sudden loss of kidney function (hours–days) with ↑ BUN/creatinine ± oliguria; potentially reversible.
Incidence: 20 % hospitalized; ICU mortality up to 80 % when AKI present.
Risk Factors:

Exposures: shock, sepsis, burns, trauma, cardiac surgery, nephrotoxins.
Susceptibility: volume depletion, age, CKD, DM, cancer, anemia.
Cause Categories:

Prerenal – hypoperfusion (HF, dehydration).
Intrarenal – direct damage (acute tubular necrosis, toxins, drugs, inflammation).
Postrenal – obstruction (stones, tumors, BPH).
Phases after injury:

Oliguric (< 400\,mL/day) → fluid overload, hyperkalemia, metabolic acidosis.
Diuretic (↑ output but unable concentrate) → electrolyte losses.
Recovery (weeks → 12 mo) → GFR improves or progresses to CKD.
Assessment & Labs:
Urine output < 0.5\,mL/kg/hr, SG, casts.
Labs: BUN 6\text{–}20 → ↑, creatinine 0.6\text{–}1.3 → ↑.
Imaging: renal US, CT, biopsy.
Nursing Diagnoses: excess fluid, risk electrolyte imbalance, fatigue, anxiety, nutrition < requirements.
Goals/Interventions:
Treat cause (restore perfusion, remove obstruction).
Manage fluids: restrict to 1\text{–}1.5\,L/day if hypervolemic; loop diuretics.
Correct hyperkalemia (insulin + glucose, bicarb, calcium; kayexalate).
Avoid nephrotoxins (NSAIDs, ACEi, aminoglycosides, contrast).
Nutritional support: controlled Na, K, P; adequate calories & quality protein.
Nursing: VS, weights, I&O, pulmonary care, skin integrity, patient education.

Ethical & Practical Implications

Monitoring disparities in access to transplant & CKD care.
Importance of early recognition (screening high-risk populations) to reduce progression.
Patient safety (fall risk in hyponatremia, infection risk with dialysis access, medication dosing in renal impairment).
End-of-life considerations for ESRD (dialysis withdrawal, transplant candidacy).