Electrolytes & Fluids

TBW=0.60×70 kg=42.0 L |ICF 28.0ISF 10.5plasma 3.5 L

Compartments are drawn to scale for body water: two thirds inside cells, one third outside, and only a quarter of the extracellular third in plasma. Filled dots are infused water; rings are water the fluid pulls from another compartment.

Left in plasma

+0 mL

of 1000 mL infused · plasma volume +0%

Intracellular0 mL

Interstitial0 mL

Plasma0 mL

Classical Starling estimates. The revised Starling model (endothelial glycocalyx) and trials such as SAFE found colloid to crystalloid volume ratios nearer 1 : 1.2–1.5 in practice, so crystalloid stays in plasma longer than this suggests, especially when capillary pressure is low.

Steady state Before the infusion

A 70 kg man holds about 42 L of water: 28 L inside cells, 10.5 L in the interstitium and 3.5 L in plasma. Sodium keeps water outside cells and potassium keeps it inside, so where a fluid ends up depends on what it carries.

Weight70 kg

Sex (TBW fraction)

Composition

What is in the bag

Pick two fluids to set them against the plasma reference range. The differences that matter clinically are chloride, the buffer anion, and how much free water the bag leaves once glucose is metabolised.

Fluid A

Fluid B

ABplasma range

Composition, mmol/L

Fluid	Na⁺	K⁺	Cl⁻	Buffer	Ca²⁺	Mg²⁺	Glucose g/L	mOsm/L

Plasma values are reference ranges (osmolality in mOsm/kg, calcium total). Fluid osmolarity is calculated; measured osmolality is lower, about 286 mOsm/kg for 0.9% NaCl and about 255–265 for Hartmann's. Ringer's lactate is the US equivalent of Hartmann's; check the product sheet for your local brand.

Maintenance calculator, adults (NICE CG174)

Routine maintenance only: no deficit, no ongoing losses. NICE starts at 25–30 mL/kg/day of water and about 1 mmol/kg/day each of Na⁺, K⁺ and Cl⁻, with 50–100 g/day of glucose to limit starvation ketosis.

Use 20–25 mL/kg/day for older or frail adults, renal or cardiac impairment, malnutrition or refeeding risk. With height entered, BMI 30 or above switches to ideal body weight (Devine).

Water1750–2100mL/day

Rate73–88mL/h over 24 h

Na⁺ · K⁺ · Cl⁻≈ 70mmol/day each

Glucose50–100g/day

Cardiac effects

Potassium, calcium and the ECG

Potassium sets the resting membrane potential and repolarisation; calcium sets the length of the plateau. Move the level and watch the monitor rewrite the trace beat by beat.

Serum K⁺4.2 mmol/L

The ECG correlates poorly with the potassium level. A normal ECG does not exclude dangerous hyperkalaemia, and arrhythmia can be the first sign.

Management

Normal

Check local protocol.

Bedside

Electrolyte check

Enter a biochemistry panel in UK units. It adjusts calcium for albumin, grades each abnormality, ranks them by urgency and lists first actions. Leave a field blank to skip it.

Examples are illustrative values, not real patients.

Waiting for values

Enter a panel to start.

Reference

Causes and thresholds

Most potassium disorders come from one of four places: a spurious result, a shift between cells and plasma, the kidney, or the gut. Look for more than one.

Hyperkalaemia

Mechanism	Causes	Clue
Pseudohyperkalaemia	Haemolysis, delayed or chilled sample, thrombocytosis, marked leucocytosis, fist clenching, EDTA contamination (order of draw)	No ECG change; calcium very low with EDTA; repeat with a fresh, careful sample or a blood gas
Reduced renal excretion	AKI, CKD; ACE inhibitors, ARBs, mineralocorticoid receptor antagonists, trimethoprim, NSAIDs, heparin, calcineurin inhibitors, potassium-sparing diuretics	Creatinine up; recent drug start or dose change; dehydration
Shift out of cells	Metabolic acidosis, insulin deficiency (DKA, HHS), β-blockers, digoxin toxicity, rhabdomyolysis, tumour lysis, massive transfusion	Low HCO₃⁻, high glucose, raised CK, urate or phosphate
Mineralocorticoid deficiency	Adrenal insufficiency (Addison's), hyporeninaemic hypoaldosteronism (diabetic nephropathy)	Low Na⁺, hypotension; type 4 RTA pattern

Hypokalaemia

Mechanism	Causes	Clue
Gut loss	Diarrhoea, laxative misuse, high-output stoma or fistula; vomiting and nasogastric drainage lose K⁺ mainly through the kidney	Diarrhoea: low HCO₃⁻; vomiting: high HCO₃⁻, low Cl⁻
Renal loss	Loop and thiazide diuretics, hyperaldosteronism (primary or secondary), Cushing's, Bartter and Gitelman syndromes, renal tubular acidosis, osmotic diuresis	Spot urine K⁺:creatinine above about 1.5 mmol/mmol; check blood pressure, renin and aldosterone
Shift into cells	Insulin, β₂-agonists (salbutamol), alkalosis, refeeding, theophylline, hypokalaemic periodic paralysis	Total body K⁺ may be normal; watch for rebound
Magnesium depletion	Diuretics, alcohol, proton pump inhibitors, diarrhoea	K⁺ fails to rise until Mg²⁺ is replaced

Thresholds used on this page

Analyte	Reference	Mild	Moderate	Severe
K⁺ high	3.5–5.0 (5.5)	5.5–5.9	6.0–6.4	≥ 6.5
K⁺ low	3.5–5.0	3.0–3.4	2.5–2.9	< 2.5
Adjusted Ca²⁺ high	2.2–2.6	2.6–2.9	3.0–3.4	≥ 3.5
Adjusted Ca²⁺ low	2.2–2.6	1.9–2.19	–	< 1.9 or symptoms
Mg²⁺ low	0.7–1.0	0.4–0.69		< 0.4
Phosphate low	0.8–1.5	0.32–0.79		< 0.32
Na⁺ low	135–145	130–134	125–129	< 125

Adjusted Ca²⁺ (mmol/L) = measured + 0.02 × (40 − albumin g/L); in mg/dL, + 0.8 × (4 − albumin g/dL). Many laboratories use their own locally derived equation. Sodium bands follow the 2014 European hyponatraemia guideline. Potassium bands follow the UK Kidney Association.