Submodule
Fluid Physiology and Assessment
A serum value describes concentration. A bedside assessment explains perfusion, congestion, losses, and the compartment in which water is moving.
- Intracellular and extracellular water
- Osmolality and effective tonicity
- Perfusion versus congestion
- Input, output, weight, and trend
The diagram is proportional in concept, not a patient-specific measurement.
Separate concentration from volume
Sodium is primarily a water-balance concentration, not a direct measurement of total body sodium. A patient may be hyponatremic while intravascularly depleted, clinically euvolemic, or edematous. Volume assessment integrates history, examination, urine output, weight, hemodynamics, kidney function, and the direction of change.
Use osmolality to explain water movement
Estimated serum osmolality can be calculated as two times sodium plus glucose divided by 18 plus blood urea nitrogen divided by 2.8, with values entered in conventional US units. Effective tonicity excludes urea because urea crosses cell membranes relatively freely. Hyperglycemia can therefore pull water from cells and lower measured sodium while increasing tonicity.
Treat the trajectory
A single measurement can be misleading. Acute intake, gastrointestinal losses, diuretics, fever, drains, edema, changes in weight, and urine output create the clinical story. Repeated measurements obtained under comparable conditions are more informative than isolated values.
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Submodule
IV Fluid Selection and the Five Rs
The correct fluid depends on the job. Resuscitation, routine maintenance, replacement, redistribution, and reassessment require different prescriptions.
- Crystalloids and albumin
- Resuscitation versus maintenance
- Ongoing loss replacement
- Balanced solutions and saline
Reassess after every meaningful intervention. The bag is not the treatment endpoint.
Name the indication
Resuscitation treats impaired circulation from hypovolemia. Routine maintenance supplies ordinary water, electrolyte, and limited glucose needs when intake is inadequate. Replacement matches abnormal losses such as gastrointestinal drainage. Redistribution addresses difficult states in which total body water may be high while effective circulating volume is low.
Understand what the bag becomes
Dextrose 5 percent in water is near isotonic in the bag, but after dextrose is metabolized it behaves as electrolyte-free water. It is not a resuscitation fluid. Isotonic crystalloids expand extracellular volume. Hypertonic saline is a monitored therapy for selected sodium emergencies, not a routine volume replacement.
Use evidence without pretending one fluid fits every patient
Balanced crystalloids reduce chloride exposure and are reasonable in many settings. Large pragmatic trials have produced mixed outcome results across populations, so solution choice should reflect the indication, electrolyte and acid-base context, brain injury considerations, compatibility, and local protocol. Hydroxyethyl starch solutions should not be used for routine resuscitation.
Calculate maintenance conservatively
NICE suggests 25 to 30 mL per kilogram per day of water for routine adult maintenance, with lower initial volumes of 20 to 25 mL per kilogram per day for older or frail adults and people with renal impairment, cardiac failure, or refeeding risk. Maintenance is a starting prescription, not a substitute for daily reassessment.
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Submodule
Sodium and Water Disorders
The safest sodium plan begins with tonicity and symptoms, then integrates volume status, cause, chronicity, urine studies, and correction trajectory.
- Hypotonic hyponatremia
- Severe neurologic symptoms
- Hypernatremia and water deficit
- SIADH and diabetes insipidus
Severe neurologic symptoms require urgent monitored care before a complete etiologic workup is finished.
Classify hyponatremia in order
Confirm that hyponatremia is hypotonic. Isotonic results may reflect measurement artifact from marked lipids or proteins, while hypertonic hyponatremia may occur with glucose or another effective osmole. For hypotonic hyponatremia, severe symptoms determine urgency and clinical volume status narrows the cause.
Correct symptoms without creating osmotic injury
European guidance for severe symptomatic hyponatremia targets an initial rise of about 5 mmol/L, then limits the total increase to 10 mmol/L in the first 24 hours and 8 mmol/L in each 24 hours thereafter. People at high risk for osmotic demyelination may require a more conservative ceiling under specialist guidance. The older lecture ceiling above 12 mmol/L per day is not used in this module.
Treat cause and water balance
Hypovolemic hypotonic hyponatremia often responds to isotonic volume restoration, but the resulting suppression of antidiuretic hormone can accelerate correction. SIADH management begins with the cause, medication review, and fluid restriction when appropriate. Tolvaptan has a narrow labeled role, must be initiated or reinitiated in hospital, and is limited by liver injury risk and other contraindications.
Approach hypernatremia as a water problem
Assess duration, symptoms, access to water, renal and extrarenal losses, urine concentration, and volume status. Restore circulation first when shock is present, then replace free water gradually with repeated sodium measurements. Central diabetes insipidus may respond to desmopressin. Nephrogenic disease requires removal of the cause and a tailored strategy.
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Submodule
Potassium Disorders
Potassium emergencies are electrical and physiologic problems. The serum value, ECG, acid-base state, kidney function, medications, and ongoing shifts all matter.
- Hypokalemia and magnesium
- ECG toxicity
- Intracellular redistribution
- Definitive potassium removal
Membrane stabilization, intracellular shifting, and potassium removal are complementary, not interchangeable.
Replace potassium safely
Oral replacement is preferred when the situation is not urgent and the gastrointestinal route is usable. IV potassium is reserved for severe, symptomatic, or otherwise unsuitable cases and must be diluted and delivered by a controlled infusion. Potassium chloride is never administered by IV push. Concentration, access, rate, ECG monitoring, kidney function, and repeat levels follow the product and institutional protocol.
Look for magnesium and ongoing losses
Hypomagnesemia can promote renal potassium wasting and make hypokalemia difficult to correct. Diuretics, gastrointestinal losses, insulin, beta agonists, alkalosis, and poor intake can contribute through different mechanisms. A fixed claim that every 10 mEq changes serum potassium by exactly 0.1 mEq/L is too imprecise for individual dosing.
Sequence acute hyperkalemia treatment
When ECG toxicity is present, IV calcium protects the myocardium but does not lower potassium. Insulin with glucose and an inhaled beta agonist shift potassium into cells. Definitive reduction requires urinary, gastrointestinal, or extracorporeal removal. Dialysis is the most reliable rapid removal strategy when kidney failure or refractory severe hyperkalemia is present.
Know what delayed therapies cannot do
Sodium zirconium cyclosilicate and patiromer can lower potassium over time, but their labels state that they should not be used as emergency monotherapy for life-threatening hyperkalemia because onset is delayed. Sodium bicarbonate is not a universal shifting therapy and is generally considered when clinically important metabolic acidemia is present, with attention to sodium and volume load.
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Submodule
Magnesium Disorders
Magnesium influences neuromuscular function, cardiac repolarization, and potassium handling. Renal function changes both the cause and the safety of treatment.
- Hypomagnesemia
- Refractory hypokalemia
- IV magnesium safety
- Hypermagnesemia
Recognize deficiency and its partners
Diarrhea, malabsorption, alcohol use disorder, proton pump inhibitors, aminoglycosides, cisplatin, and renal wasting can lower magnesium. Deficiency may coexist with hypokalemia and hypocalcemia and can contribute to tremor, weakness, seizures, and ventricular arrhythmia.
Match replacement to risk
Oral magnesium is reasonable for many stable asymptomatic patients but can worsen diarrhea. IV magnesium is used for severe or symptomatic deficiency and selected arrhythmias. Because magnesium is eliminated by the kidneys, impaired renal function increases the risk of accumulation and requires lower exposure, slower administration, and closer monitoring.
Treat excess by severity
Hypermagnesemia can cause diminished reflexes, hypotension, bradycardia, respiratory depression, and conduction abnormalities. Stop magnesium sources, give IV calcium for clinically important toxicity, support elimination when kidney function permits, and use dialysis when severe toxicity cannot be cleared.
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Submodule
Calculations, Administration, and Monitoring
A calculation becomes clinically useful only when units, assumptions, administration constraints, monitoring, and the next decision are explicit.
- Estimated osmolality
- Weight-based maintenance
- Infusion rate and units
- Reassessment and escalation
The monitoring interval must match the acuity, intervention, and risk of overshoot.
Make assumptions visible
Weight-based maintenance is an estimate. Edema, obesity, frailty, kidney failure, cardiac failure, fever, ongoing losses, nutrition support, and oral intake can make a simple result inappropriate. Calculations should support, not replace, a clinical prescription.
Monitor the effect you intended
A fluid plan should define clinical targets such as perfusion, symptoms, urine output, weight, or correction trajectory and safety targets such as respiratory status, edema, glucose, electrolytes, and kidney function. Reassessment may be measured in minutes during resuscitation and hours or days during stable maintenance.
Escalate complexity
Seek experienced help for severe sodium abnormalities, shock, complex redistribution, oliguric kidney failure, significant heart or liver disease, dangerous ECG findings, or correction that is moving faster than intended. A safe clinician recognizes when a formula has reached its limits.
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Module test
Check the connections.
Each attempt draws 10 questions from the complete 100 question bank.
Each attempt draws a fresh set and rearranges the answer choices.
References
Current clinical foundation.
Lecture material was synthesized with the following contemporary guidance. Verify local policy and current guidance before applying clinical information.
- NICE CG174: Intravenous fluid therapy in adults
- European clinical practice guideline on hyponatremia
- Society for Endocrinology emergency guidance for severe symptomatic hyponatremia
- KDIGO conference report on acute hyperkalemia
- FDA label: Sodium zirconium cyclosilicate
- FDA label: Tolvaptan
- FDA label: Potassium chloride injection
- FDA label: Magnesium sulfate injection