--- module: 018-02 language: en chapter: 18 title: "Sodium, Water, Potassium, and Volume Regulation" module_title: "Effective osmoles, renal responses, and safe electrolyte correction" source_sha256: 3f832435e57a1666e269e112028456cd398a0cf8581b3efe0ca6f0d2d2f72b9c --- # Effective osmoles, renal responses, and safe correction ## Concentration versus content ### Plasma sodium is a ratio, not a store ### Extracellular volume follows total sodium content ### Saline treats hypovolaemia, not every hyponatraemia ### Oedema with low sodium despite sodium excess ### Isotonic sodium expands extracellular volume ### Free water spreads across both compartments ### Hypotonic loss cuts volume and raises sodium ### Translocational: glucose draws water from cells ### Pseudohyponatraemia is a measurement artefact ## Reading the urine ### Osmolality shows whether hormone is suppressed ### Maximally dilute urine: excess intake or low solute ### Concentrated urine means water is retained #### Volume defence, nausea, pain, cortisol, drugs ### Urine sodium infers renal sodium avidity #### Low value: underfilling, heart failure, cirrhosis #### Higher value: salt loss, diuretics, adrenal deficiency ### A late sample describes the treatment response ### Falling osmolality after volume restoration ## Hyponatraemia and brain adaptation ### Acute hypotonicity drives water into brain cells ### Cells lose electrolytes, then organic osmolytes ### Adapted brain is vulnerable to rapid rises ### Oligodendrocytes and myelin can be injured ### Risk: low sodium, malnutrition, alcohol, liver ### Severe symptoms: controlled modest prompt rise ### Sodium accelerates when hormone switches off ### Potassium replacement also raises sodium ### Follow the trajectory, not the formula ## Hypernatraemia and water loss ### Inadequate water relative to solute ### Severe cases signal dependence or impaired thirst ### The deficit estimate excludes ongoing loss ### Very concentrated urine: extrarenal loss or sodium gain ### Dilute urine: hormone deficiency or renal resistance ### Central lacks production, nephrogenic lacks response ### Causes: neurosurgery, lithium, calcium, potassium, tubules ### Chronic hypertonicity accumulates brain osmolytes ### Isotonic resuscitation first, then free water ## Potassium distribution and excretion ### Small extracellular fraction shifts concentration fast ### Insulin and beta two stimulation drive uptake ### Acidosis depends on anion and transport response ### Principal cells perform the secretion #### Needs potassium, aldosterone, delivery, flow #### Low effective volume limits distal delivery ### Acid-base with blood pressure organises causes #### Alkalosis with hypertension: mineralocorticoid effect #### Alkalosis without hypertension: vomiting, diuretics, salt wasting #### Hyperkalaemic normal-gap acidosis: low aldosterone effect ### Urine potassium read with volume and intake ## Emergency potassium treatment ### Hyperkalaemia kills through conduction failure ### Calcium stabilises but does not lower potassium ### Insulin with glucose shifts potassium into cells ### Bicarbonate suits significant metabolic acidosis ### Removal: urine, gastrointestinal binding, dialysis ### Monitor glucose for delayed hypoglycaemia ### Potassium rebounds if nothing was removed ### Oral replacement is safer where feasible ### Correct magnesium to hold the replacement ## Calcium, magnesium, and refeeding ### Hydrogen and calcium compete for albumin binding ### Acute alkalosis lowers the ionised concentration ### Direct measurement in critical illness and transfusion ### Magnesium governs channels, parathyroid, repolarisation ### Deficiency makes potassium and calcium refractory ### Kidney failure: accumulation suppresses reflexes ### Refeeding drives phosphate, potassium, magnesium inward ### Thiamine demand and sodium-water retention rise ### Prevention: cautious energy, thiamine, monitoring ## Prescribing and verifying ### Resuscitation, maintenance and replacement differ ### One bag cannot serve all three purposes ### Losses differ in water and electrolyte proportions ### Daily weight beats an incomplete balance chart ### Look for underfilling and venous congestion ### Cumulative sodium exposure from fluids and medicines ### The plan states mechanism, pace and monitoring ### Unexpected value: check sampling site and handling #### Line contamination, haemolysis, counts, assay method #### Danger with electrocardiogram or symptoms: treat now #### Trajectory from repeats, tracing, urine, what was given