Acid-Base Balance and Fluid Electrolyte Management

Acid-Base Balance

Assess acid-base status via arterial blood gases.

ABG Sample

Freshly oxygenated arterial blood (radial/femoral artery).

ABG Nursing Considerations

Use heparinized syringe; expel air bubbles immediately.

Apply firm pressure ≥ 5 minutes (radial) to prevent hematoma.

Keep sample on ice and transport promptly.

Metabolic Acidosis (simple)

↓ pH, ↓ HCO₃⁻.

Metabolic Alkalosis (simple)

↑ pH, ↑ HCO₃⁻.

Respiratory Acidosis (simple)

↓ pH, ↑ PaCO₂.

Respiratory Alkalosis (simple)

↑ pH, ↓ PaCO₂.

ABG Interpretation Step #1

Determine pH: acidotic (< 7.35) or alkalotic (> 7.45).

ABG Interpretation Step #2

Assess PaCO₂: respiratory component.

ABG Interpretation Step #3

Assess HCO₃⁻: metabolic component.

ABG Interpretation - ROME Mnemonic

Respiratory Opposite - pH ↑ & PaCO₂ ↓ = resp. alkalosis; pH ↓ & PaCO₂ ↑ = resp. acidosis

Metabolic Equal - pH ↑ & HCO₃⁻ ↑ = metabolic alkalosis; pH ↓ & HCO₃⁻ ↓ = metabolic acidosis

Uncompensated ABG

pH abnormal + only one of PaCO₂ or HCO₃⁻ abnormal (not both)

Partially Compensated ABG

pH abnormal + both PaCO₂ & HCO₃⁻ abnormal

Fully Compensated ABG

pH normal + both PaCO₂ & HCO₃⁻ abnormal (use pH 7.40 reference)

Metabolic Acidosis Signs/Symptoms

Kussmaul respirations, ↓ BP, confusion; Causes: DKA, shock, renal failure, diarrhea

Metabolic Alkalosis Signs/Symptoms

Tetany, irritability, shallow respirations; Causes: Vomiting, NG suction, diuretics

Respiratory Acidosis Signs/Symptoms

Dyspnea, warm/flushed skin, headache; Causes: COPD, pneumonia, narcotics

Respiratory Alkalosis Signs/Symptoms

Lightheadedness, tetany, palpitations; Causes: Hyperventilation (anxiety, pain, fever)

Acidosis Monitoring

Monitor K⁺ (risk of hyperkalemia)

Alkalosis Monitoring

Watch Ca²⁺ levels (tetany risk), seizure precautions

Carbonic Acid Production

Cellular metabolism produces carbonic acid (H₂CO₃).

Acidosis Mechanism

If CO₂ ↑ → more H₂CO₃ → lower pH (acidosis).

Alkalosis Mechanism

If CO₂ ↓ → less H₂CO₃ → higher pH (alkalosis).

Nursing Considerations for Respiratory

Monitor ventilator settings, especially tidal volume and rate.

Renal Regulation of pH

The kidneys manage acid-base balance by excreting or retaining hydrogen ions (H⁺) and bicarbonate (HCO₃⁻).

Acidosis Kidney Response

The kidneys will excrete hydrogen ions (H⁺) into the urine and retain or generate bicarbonate (HCO₃⁻) to act as a buffer and raise the pH.

Alkalosis Kidney Response

The kidneys will retain hydrogen ions (H⁺) and excrete bicarbonate (HCO₃⁻) to lower the pH.

Nursing Monitor Labs for Renal

BUN, creatinine, urine pH.

Nursing Process: Acid-Base Balance

Assessment: RR/depth, cognitive status, dizziness, O₂ saturation.

Nursing Interventions Acid-Base Balance

Oxygen therapy, adjust ventilation, IV bicarbonate as ordered; Ensure safety during confusion or dizziness.

Nursing Intervention Evaluation Acid-Base Balance

pH within normal limits, stable RR/depth, improved mental status.

Intracellular Fluid (ICF)

~70% of body fluid; Maintains cell size & metabolism. ~40% of body weight.

Extracellular Fluid (ECF)

~30% of body fluid; Circulates around cells & vascular system.

ECF Subdivisions

Intravascular, Interstitial, and Transcellular.

Intravascular

Plasma in the blood — directly affects heart rate (HR), blood pressure (BP), and perfusion.

Interstitial

Fluid around cells — enables nutrient and waste exchange.

Transcellular

Specialized fluids including cerebrospinal, pleural, peritoneal, synovial, digestive secretions, and sweat.

Osmolarity

Concentration of solutes in a solution, referred to as 'pulling power.'

Osmolality

Measured by weight; often used interchangeably with osmolarity in practice.

Isotonic Solution

Osmolarity equal to plasma; stays in vascular space with no shift.

Hypertonic Solution

Higher osmolarity than plasma; pulls water out of cells causing them to shrink.

Hypotonic Solution

Lower osmolarity than plasma; pushes water into cells causing them to swell.

Common IV Solutions

Includes isotonic (0.9% NaCl, Lactated Ringer's), hypertonic (D5LR), and hypotonic (0.45% NaCl).

Sensible losses

Measurable losses of fluid such as urine, stool, and wound drainage.

Insensible losses

Fluid losses that are hard to measure, such as skin evaporation and respiration.

Kidneys

Filter ~180L plasma/day and excrete ~1.5L urine; adjust Na⁺, K⁺, and water retention or elimination.

Heart & Vascular r/t vascular volume

Stretch receptors detect low volume and stimulate fluid retention.

Lungs r/t vascular volume

Lose ~300mL of water vapor/day;
CO₂ regulation helps maintain pH balance.

Nervous System r/t vascular volume

Osmoreceptors detect osmotic changes and stimulate thirst and ADH release.

GI Tract r/t vascular volume

Absorbs water and nutrients; major site of fluid intake.

Aldosterone

Hormone from adrenal glands that retains sodium and water while excreting potassium.

Cortisol (excess)

Hormone from adrenal cortex that mimics aldosterone's effect: Na⁺/water retention and K⁺ loss.

ADH (Vasopressin)

Hormone from the pituitary that retains water.

Thyroxine

Triggered by: low volume or ↑ osmolarity.

Thyroid r/t vascular volume

Increases renal perfusion & urinary output.

PTH

Raises serum calcium, lowers phosphate.

Clinical Signs of Fluid Volume Deficit

Tachycardia, hypotension, dry mucous membranes, poor skin turgor.

Dehydration

Loss of total body water → ↑ serum Na⁺.

Clinical Signs of Fluid Volume Excess

Bounding pulse, hypertension, JVD, crackles.

Third Spacing

Fluid trapped in transcellular/interstitial spaces.

Normal Findings in Fluid Balance Assessment

Normal BP, HR, O₂ sat.

Fluid Volume Excess Assessment Findings

↑ BP, ↓ O₂, tachycardia, bounding pulses.

Fluid Volume Deficit Assessment Findings

↓ BP, tachycardia, weak/thready pulses.

Electrolytes

Enable nerve conduction, muscle contraction, acid-base balance, fluid balance.

Sodium (Na⁺)

Fluid balance, nerve/muscle signals.

Calcium (Ca⁺)

Muscle contraction, nerve conduction, clotting.

Chloride (Cl⁻)

Acid-base & osmotic balance.

Bicarbonate (HCO₃⁻)

Major buffer in acid-base homeostasis.

Potassium (K⁺)

Cardiac, muscle, neuron function.

Magnesium (Mg⁺)

Enzyme function, muscle & nerve regulation.

Phosphate

Energy (ATP) production, acid-base buffer.

Hyponatremia

Low sodium through GI fluids, water loss via fever, hyperventilation, diarrhea, diuretics, excessive sweating.

Hypernatremia

Excessive sodium intake (diet or IV), water intoxication or overhydration.

Symptoms of Hyponatremia

Restlessness, irritability, confusion, seizures, coma.

Treatment for Hyponatremia

Gradual correction with hypotonic IV fluids, monitor neuro status and serum sodium frequently.

Potassium (K⁺) - Critical Notes

Found mostly inside the cell, essential for cardiac rhythm stability, nerve transmission, muscle contraction.

Fast K+ infusion

fatal arrhythmias.

Calcium (Ca⁺) Where?

99% stored in bones.

Causes of Hypocalcemia

- Hypo + Hyper parathyroidism.

- Vitamin D deficiency.

- Malignancy (bone metastases or paraneoplastic syndrome).

- Renal failure.

- Pancreatitis.

- Prolonged immobility.

- Hypomagnesemia.

Signs of Hypocalcemia

- Neuromuscular excitability: tetany,

- Weakness, fatigue.

- Decreased reflexes.

- Seizures.

- Confusion, altered mental status.

- Positive Chvostek's sign (facial twitching when tapping over facial nerve).

- Bradycardia, heart block.

- Constipation.

- Positive Trousseau's sign (carpopedal spasm when BP cuff inflated).

- Kidney stones.

- Laryngeal spasm or stridor in severe cases.

Treatment for Hypocalcemia

- Oral or IV calcium replacement.

- Increase oral or IV fluids (prefer isotonic fluids) to dilute calcium and promote excretion.

- Treat underlying cause (Vitamin D replacement if deficient).

- Loop diuretics (e.g., furosemide) to promote calcium excretion.

- Cardiac monitoring for severe hypocalcemia..

- Monitor ECG closely for arrhythmias.

Magnesium (Mg²⁺)

Supports enzyme reactions, DNA/RNA synthesis, and nerve/muscle function.

Hypomagnesemia

Low magnesium levels in the blood.

Hypermagnesemia

High magnesium levels in the blood.

Causes of Hypomagnesemia

- GI losses (diarrhea, suctioning).

- Renal failure (most common).

- Chronic alcoholism (poor intake & absorption).

- Excessive antacid or laxative use (especially magnesium-containing).

- Hyperglycemia / DKA.

Signs of Hypomagnesemia

- Increased neuromuscular excitability:

- Muscle weakness, tremors, cramps, seizures.

- Lethargy, drowsiness.

- Positive Chvostek's and Trousseau's signs (can overlap with hypocalcemia).

- Hypotension, bradycardia.

- Respiratory depression (severe cases).

- Cardiac arrhythmias (heart block).

Treatment for Hypomagnesemia

- Oral or IV replacement (must administer slowly to avoid cardiac effects).

- Stop magnesium intake.

- IV calcium gluconate (antagonist) for severe cases.

- Diuretics if renal function intact.

- Monitor reflexes and respiratory status + continuous cardiac and respiratory during IV therapy.

- Dialysis if severe and renal function impaired.

Phosphate (PO₄³⁻)

Involved in energy transfer and bone mineralization.

Hypophosphatemia

Low phosphate levels in the blood.

Hyperphosphatemia

High phosphate levels in the blood.

Causes of Hypophosphatemia

- Renal failure (decreased excretion).

- Malnourishment / malabsorption.

- Alcohol withdrawal.

- Excess phosphate intake (laxatives, enemas).

- Antacid abuse (aluminum, calcium bind phosphate).

- DKA (shifts phosphate into cells)..

- Hyperparathyroidism (↑ PTH lowers phosphate).

Signs of Hypophosphatemia

- Weakness, fatigue.

- Neuromuscular irritability, tetany (often secondary to low calcium).

- Paresthesias.

- Irritability, confusion, seizures.

- Muscle cramps, paresthesias.

- Impaired tissue oxygenation (low ATP).

- Long-term: Muscle weakness, especially diaphragm → risk for respiratory failure.

Treatment for Hypophosphatemia

- Oral or IV replacement depending on severity.

- Administer phosphate binders (calcium-based or sevelamer) with meals.

- Treat / correct underlying cause (renal function support, alcohol abuse, nutrition, PTH control).

- If severe, dialysis may be required.

Fluid and Electrolyte Assessment

Involves history questions, physical exam, and monitoring.

Fluid Management

Encourage or restrict fluids as ordered.

Electrolyte Replacement

Administer carefully — many electrolytes (esp. K⁺, Mg²⁺, Ca²⁺) must be diluted and given slowly (over hours).

Medication Management

Use diuretics to control excess fluid or adjust electrolyte balance.