Section 8.3: Fluids, Electrolytes, and Acid-Base Balance

Fluid Volume Imbalances

Maintaining fluid balance is critical for cellular homeostasis. The body regulates fluids through osmoreceptors, antidiuretic hormone (ADH), the renin-angiotensin-aldosterone system (RAAS), and atrial natriuretic peptide (ANP).

Fluid Volume Deficit (Hypovolemia / Dehydration)

• Causes: Gastrointestinal losses (vomiting, diarrhea), excessive sweating, hemorrhage, burns, diuretic therapy, or decreased oral intake.
• Manifestations: Tachycardia, weak and thready pulse, hypotension (especially orthostatic), tachypnea, dry mucous membranes, poor skin turgor (tenting), oliguria, and significant weight loss. Laboratory findings typically show hemoconcentration (elevated hematocrit, BUN, and urine specific gravity).
• Nursing Interventions: Administer oral and intravenous fluids (typically isotonic solutions like 0.9% Normal Saline or Lactated Ringer's) as ordered, monitor daily weights closely, and accurately track strict intake and output.

Fluid Volume Excess (Hypervolemia)

• Causes: Heart failure, renal failure, liver cirrhosis, excessive sodium intake, or overzealous administration of intravenous fluids.
• Manifestations: Bounding pulse, hypertension, jugular venous distention (JVD), peripheral pitting edema, rapid weight gain, dyspnea, and crackles in the lungs. Lab findings often reflect hemodilution (decreased hematocrit and BUN).
• Nursing Interventions: Implement fluid and sodium restrictions, administer prescribed diuretics, elevate the head of the bed to ease breathing, and monitor daily weights as the most accurate indicator of fluid status.

Electrolyte Imbalances

Electrolytes govern neuromuscular and cardiac function. Understanding their normal ranges and clinical presentations of imbalance is essential for the PNLE.

Sodium (Na) - Normal: 135-145 mEq/L Sodium is the primary extracellular cation, crucial for water balance and nerve impulse transmission.

• Hyponatremia (<135): Caused by water intoxication or sodium loss (diuretics, vomiting). Symptoms are primarily neurological due to cerebral edema: confusion, lethargy, muscle cramps, seizures. Treat with water restriction or hypertonic saline (3% NaCl) in severe cases, administered slowly.
• Hypernatremia (>145): Caused by water loss (insensible losses, diabetes insipidus) or excessive sodium intake. Manifests as extreme thirst, dry and flushed skin, agitation, and twitching. Treat with hypotonic IV fluids.

Potassium (K) - Normal: 3.5-5.0 mEq/L Potassium is the primary intracellular cation, vital for cardiac and skeletal muscle contraction.

• Hypokalemia (<3.5): Often caused by potassium-wasting diuretics (e.g., Furosemide), diarrhea, or vomiting. Symptoms include muscle weakness, leg cramps, hypoactive bowel sounds, and ECG changes (prominent U wave, flattened T wave, ST depression). Treatment involves potassium replacement. Crucial Rule: Never administer IV potassium via IV push or bolus; it must be diluted and infused slowly via a pump to prevent cardiac arrest.
• Hyperkalemia (>5.0): Caused by renal failure, potassium-sparing diuretics, or massive cellular damage (burns, trauma). Symptoms include muscle twitching progressing to weakness, flaccid paralysis, and life-threatening ECG changes (tall peaked T waves, widened QRS). Treatment includes administering Calcium Gluconate (protects the heart), Insulin with Dextrose (shifts K into cells), Sodium Polystyrene Sulfonate (Kayexalate to excrete K via feces), or dialysis.

Calcium (Ca) - Normal: 8.5-10.5 mg/dL Calcium is necessary for bone structure, muscle contraction, and blood clotting. It has an inverse relationship with phosphorus.

• Hypocalcemia (<8.5): Caused by hypoparathyroidism, renal failure, or vitamin D deficiency. Manifests as neuromuscular irritability: numbness/tingling, hyperactive reflexes, muscle spasms (tetany), a positive Trousseau's sign (carpal spasm with BP cuff inflation), and a positive Chvostek's sign (facial twitching when the facial nerve is tapped).
• Hypercalcemia (>10.5): Caused by hyperparathyroidism, prolonged immobilization, or bone malignancies. Manifests as depressed neuromuscular activity: muscle weakness, lethargy, constipation, kidney stones, and bone pain.

Magnesium (Mg) - Normal: 1.5-2.5 mEq/L Magnesium acts similarly to calcium in neuromuscular function.

• Hypomagnesemia (<1.5): Commonly seen in chronic alcoholism and malnutrition. Symptoms mirror hypocalcemia (hyperactive deep tendon reflexes, tremors, positive Trousseau/Chvostek).
• Hypermagnesemia (>2.5): Rare, usually due to renal failure or excessive antacid use. Manifests as loss of deep tendon reflexes, respiratory depression, and bradycardia. Antidote is Calcium Gluconate.

Acid-Base Balance and ABG Interpretation

The body maintains a narrow pH range (7.35 - 7.45) using chemical buffers, respiratory regulation (lungs managing CO2), and renal regulation (kidneys managing HCO3).

Normal ABG Values:

• pH: 7.35 - 7.45 (Acidic < 7.35 / Alkalotic > 7.45)
• PaCO2: 35 - 45 mmHg (Respiratory parameter. Alkalotic < 35 / Acidic > 45)
• HCO3 (Bicarbonate): 22 - 26 mEq/L (Metabolic parameter. Acidic < 22 / Alkalotic > 26)

The ROME Mnemonic:

• Respiratory Opposite: pH and PaCO2 move in opposite directions.
  • Respiratory Acidosis: pH ↓, PaCO2 ↑ (Causes: Hypoventilation, COPD, narcotics)
  • Respiratory Alkalosis: pH ↑, PaCO2 ↓ (Causes: Hyperventilation, anxiety, pain)
• Metabolic Equal: pH and HCO3 move in the same direction.
  • Metabolic Acidosis: pH ↓, HCO3 ↓ (Causes: Diabetic ketoacidosis, renal failure, severe diarrhea)
  • Metabolic Alkalosis: pH ↑, HCO3 ↑ (Causes: Severe vomiting, continuous NG suction, excessive antacids)

Compensation: The body attempts to correct the pH. If the primary issue is respiratory, the kidneys compensate by altering HCO3. If metabolic, the lungs compensate by altering ventilation (PaCO2).

• Uncompensated: pH is abnormal; one parameter is abnormal, the other is normal.
• Partially Compensated: pH is abnormal; both PaCO2 and HCO3 are abnormal (moving in the same direction to correct pH).
• Fully Compensated: pH is normal; both PaCO2 and HCO3 are abnormal, but the pH has returned to the normal range.