Fluid, Electrolyte, and Acid-Base Balance
Fluid, electrolyte, and acid-base balance is a fundamental topic in clinical nursing, particularly relevant in the UAE where diarrhoeal diseases, heat-related illness, and the high prevalence of chronic diseases (diabetes, kidney disease, heart failure) mean that nurses frequently care for patients with disturbances in these balances. The DOH (UAE) examination tests your ability to assess, identify, and respond to common imbalances.
🟢 Lite — Quick Review (1h–1d)
Rapid summary for last-minute revision before your exam.
Daily Water Balance:
- Intake: ~2,500 mL (drinks 1,500 + food 700 + metabolism 300)
- Output: Urine 1,500 + faeces 200 + insensible losses (skin/lungs) 800
Key Electrolyte Normal Ranges:
| Electrolyte | Normal Range |
|---|---|
| Sodium (Na⁺) | 135–145 mEq/L |
| Potassium (K⁺) | 3.5–5.0 mEq/L |
| Calcium (Ca²⁺) | 2.1–2.6 mmol/L (total); 1.1–1.3 mmol/L (ionised) |
| Magnesium (Mg²⁺) | 0.7–1.1 mmol/L |
| Phosphate (PO₄³⁻) | 0.8–1.5 mmol/L |
Acid-Base Quick Reference:
- pH < 7.35 = Acidosis; pH > 7.45 = Alkalosis
- For any ABG, check: Is pH normal? → Is PaCO₂ or HCO₃⁻ abnormal? → What is compensating?
⚡ Exam Tip: K⁺ levels are critical before administering digoxin. If K⁺ is low, digoxin toxicity risk increases significantly even at normal digoxin levels. Always check electrolytes when administering cardiac glycosides.
🟡 Standard — Regular Study (2d–2mo)
Standard content for students with a few days to months.
1. Sodium Imbalances
Hyponatraemia (Na⁺ < 135 mEq/L):
- Hypovolaemic (volume depleted): Vomiting, diarrhoea, diuretics, burns
- Euvolaemic: SIADH (inappropriate ADH secretion — lung cancer, CNS disorders, medications), adrenal insufficiency
- Hypervolaemic (volume overloaded): CHF, cirrhosis, nephrotic syndrome
Hypernatraemia (Na⁺ > 145 mEq/L):
- Usually indicates water deficit or sodium gain
- Causes: Dehydration, diabetes insipidus, hypertonic IV fluids, hyperaldosteronism
- Risk: Brain cell shrinkage → seizures, coma, subdural haemorrhage
- Management: Correct slowly (risk of cerebral oedema if corrected too fast)
2. Potassium Imbalances
Hypokalaemia (K⁺ < 3.5 mEq/L):
- Causes: Diuretics (especially thiazides), vomiting, diarrhoea, alkalosis, insulin, steroids
- Symptoms: Muscle weakness, cramps, constipation (can progress to ileus), cardiac arrhythmias
- ECG changes: Flattened T waves, prominent U waves, ST depression
- Dangerous: Can precipitate fatal arrhythmias (especially with digoxin)
Hyperkalaemia (K⁺ > 5.0 mEq/L):
- Causes: Renal failure, potassium-sparing diuretics, ACEi/ARBs, tissue breakdown (rhabdomyolysis, tumour lysis), metabolic acidosis
- Symptoms: Muscle weakness, paraesthesia, arrhythmias
- ECG changes (progressive): Tall peaked T waves → prolonged PR interval → widened QRS → sine wave → VF/asystole
- Treatment: Calcium gluconate (cardioprotection) → Insulin + dextrose (shifts K⁺ into cells) → Salbutamol nebuliser (shifts K⁺ into cells) → Sodium bicarbonate (if acidosis) → Loop diuretics → Dialysis (definitive in severe cases)
3. Calcium Imbalances
Hypocalcaemia:
- Causes: Vitamin D deficiency (very common in veiled women in UAE), hypoparathyroidism, chronic kidney disease, pancreatitis, massive transfusion
- Symptoms: Tetany (muscle cramps, carpopedal spasm), perioral numbness, Chvostek’s sign (facial twitching when tapping facial nerve), Trousseau’s sign (carpopedal spasm with BP cuff inflation), seizures, arrhythmias
- Treatment: IV calcium gluconate (preferred over calcium chloride — less caustic to veins)
Hypercalcaemia:
- Causes: Hyperparathyroidism, malignancy (bone metastases), granulomatous diseases, thiazide diuretics
- Symptoms: “Stones, bones, groans, and psychiatric overtones” — kidney stones, bone pain, abdominal pain/constipation, depression/confusion
- Treatment: Aggressive IV fluids (saline); bisphosphonates (pamidronate, zoledronic acid); treat underlying cause
🔴 Extended — Deep Study (3m+)
Comprehensive coverage for students on a longer study timeline.
4. Acid-Base Disorders — Full Interpretation
Compensation Rules:
| Primary Disorder | Expected Compensation |
|---|---|
| Metabolic acidosis | PaCO₂ falls by 1.2 mmHg for each 1 mEq/L ↓ in HCO₃⁻ |
| Metabolic alkalosis | PaCO₂ rises by 0.7 mmHg for each 1 mEq/L ↑ in HCO₃⁻ |
| Respiratory acidosis | HCO₃⁻ rises by 1 mEq/L for each 10 mmHg ↑ in PaCO₂ (acute) / 3.5 mEq/L (chronic) |
| Respiratory alkalosis | HCO₃⁻ falls by 2 mEq/L for each 10 mmHg ↓ in PaCO₂ (acute) / 4.5 mEq/L (chronic) |
Example: pH 7.18, PaCO₂ 50, HCO₃⁻ 18:
- pH is acidotic → primary acidosis
- HCO₃⁻ is low → metabolic acidosis
- Expected PaCO₂ for metabolic acidosis: 1.2 × (24−18) = 7.2 mmHg fall; Expected PaCO₂ = 40−7.2 = ~33 mmHg
- Actual PaCO₂ is 50 (higher than expected) → partially compensated metabolic acidosis
Anion Gap (AG):
- AG = Na⁺ − (Cl⁻ + HCO₃⁻); Normal = 8–12 mEq/L
- Elevated anion gap metabolic acidosis (MUDPILES):
- Methanol
- Uraemia (renal failure)
- Diabetic/propylene glycol (lactic acidosis)
- Paracetamol/poisoning
- Iron, isoniazid
- Lactic acidosis
- Ethylene glycol
- Salicylates
5. IV Fluids — Crystalloids vs Colloids
| Fluid | Na⁺ | Use |
|---|---|---|
| Normal Saline 0.9% (NS) | 154 | Fluid resuscitation, compatible with blood, resuscitation |
| Ringer’s Lactate (Hartmann’s) | 130 | Preferred for trauma, burns, surgery; has K⁺ (avoid in renal failure) |
| Half NS 0.45% | 77 | Maintenance; diabetic patients |
| D5W | 0 | Free water; hypoglycaemia |
| D5 0.9% NS | 154 | Surgical maintenance |
Exam Watch: For fluid resuscitation in a shocked patient, crystalloids are first-line (3–4 mL per mL of estimated blood loss). For every litre of crystalloid given, only ~250 mL remains intravascular (the rest distributes to the interstitial space). Blood products should be given early in massive haemorrhage (1:1:1 ratio of PRBCs:FFP:platelets is associated with improved survival in trauma).
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