11–17% of the exam, and the category candidates find most intimidating — unnecessarily. Fluid and electrolyte imbalances, ABG interpretation, and emergency responses all follow patterns. Learn the patterns and the memorization shrinks dramatically.
Fluid balance basics
Think in two directions — too much volume or too little — and the signs mirror each other.
- Fluid volume deficit — tachycardia, hypotension, poor skin turgor, dry mucous membranes, dark concentrated urine, rising BUN. Treat the cause and replace fluids.
- Fluid volume excess — bounding pulse, hypertension, edema, crackles, weight gain, distended neck veins. Think heart failure, renal failure, over-infusion.
The best fluid-status monitor is daily weight. A 1 kg change roughly equals 1 L of fluid — weight change beats intake/output for tracking trends.
The electrolytes that matter
For each, ask: is it high or low, and what does the heart or the neuromuscular system do about it?
| Electrolyte | Low | High |
|---|---|---|
| Potassium (3.5–5.0) | Flat/inverted T, U waves, weakness, ileus | Peaked T waves, wide QRS, cardiac arrest risk |
| Sodium (135–145) | Confusion, seizures (correct slowly) | Thirst, dry, restless, edema if fluid-related |
| Calcium (9.0–10.5) | Tetany, Chvostek/Trousseau, +reflexes | Weakness, stones, decreased reflexes |
| Magnesium (1.3–2.1) | +reflexes, tremor, torsades | –reflexes, respiratory depression |
Notice the pattern: calcium and magnesium generally move reflexes in the same direction as their own level (low → hyperexcitable), while potassium hits the heart hardest.
Reading an ABG
Normals: pH 7.35–7.45, PaCO₂ 35–45, HCO₃ 22–26. Use ROME — Respiratory Opposite, Metabolic Equal:
- Respiratory: pH and CO₂ move in opposite directions (low pH + high CO₂ = respiratory acidosis).
- Metabolic: pH and HCO₃ move in the same direction (low pH + low HCO₃ = metabolic acidosis).
- Then check compensation: if the “other” value has shifted to pull pH toward normal, it’s compensating.
Recognizing emergencies
- Shock — falling BP, rising HR, cool clammy skin, decreasing urine output. Position, oxygen, fluids, find the cause.
- Increased intracranial pressure — rising BP with widening pulse pressure, bradycardia, irregular respirations (Cushing’s triad), decreasing consciousness. Elevate the head, avoid anything that raises pressure.
- Anaphylaxis — airway swelling, wheeze, hypotension. Epinephrine first, then support airway.
Practice it: reinforce this category with explained questions in the question bank, and see how much of your exam it fills on the test plan page.
