Fluids & Electrolytes in Surgical Patients — Compartments, Crystalloids, Strategy, Sodium, Potassium, Acid-Base, Calcium

The surgical patient who survives the operation can still be injured by the drip — so choose the crystalloid by the trial ladder with its brain-injury exception, dose volume by RELIEF rather than dogma, direct fluids to stroke volume in elective abdominal cases while refusing the same protocol in emergencies, expect only a fifth of each litre to stay intravascular at half an hour, name overload at 10% weight gain and de-escalate it, correct sodium at a rate matched to chronicity, replete magnesium with potassium, sequence calcium-insulin-dialysis for hyperkalaemia and hydration-bisphosphonate for hypercalcaemia: because balanced solutions probably trim mortality a little except in head injury, restrictive regimens injure kidneys without buying survival, goal-directed therapy prevents infections and leaks in elective but not emergency surgery, and every number below comes from the paper named beside it.^{[1][3][10][15][23][32][52][67]}

A 68-year-old woman is day 1 after emergency laparotomy for perforated diverticulitis with faecal peritonitis: 6 litres positive, sodium 150, potassium 2.8, magnesium low, chloride high with a normal-gap acidosis on top of her resolving lactate, urine output falling. Which fluid, how much more, how fast to move the sodium, what to do about the potassium that will not rise, and what her kidneys will do if you keep pouring? The examiner will watch you default to balanced crystalloids while naming the TBI exception you checked, refuse both dogmatic restriction and blind liberality with RELIEF numbers, read the kinetics behind her oedema, pair magnesium with potassium, and start de-escalation before the 10% line. This page teaches each move with every number taken from the paper named beside it.^{[5][10][23][29][39][50]}

Crystalloid Choice — the balanced-vs-saline trial ladder

Resuscitate the general surgical patient with balanced crystalloids first: the BEST-Living individual-patient meta-analysis of 34,685 ICU patients found high posterior probability (0.895) that balanced solutions reduce in-hospital mortality, though certainty is moderate and the absolute reduction small at 0.4 percentage points.^[1] The NEJM Evidence synthesis of low-bias trials agrees — 90-day mortality risk ratio 0.96 with 89.5% posterior probability of benefit — framing the effect as a 9% relative reduction to a 1% relative increase in death.^[2] The sepsis subgroup strengthens the default where it matters most to surgeons managing peritonitis: composite mortality risk ratio 0.91 with balanced solutions.^[4] The two positive single-centre trials anchor the ladder: SMART randomised 15,802 critically ill adults to saline or balanced crystalloids (lactated Ringer's or Plasma-Lyte A) and cut the MAKE30 composite of death, new renal replacement or persistent renal dysfunction from 15.4% to 14.3% (odds ratio 0.91, p=0.04), with 30-day in-hospital mortality 10.3% versus 11.1%.^[5] SALT-ED randomised 13,347 non-critically-ill ED patients with identical fluids: hospital-free days identical at a median of 25 each, but MAKE30 lower with balanced solutions at 4.7% versus 5.6% (adjusted odds ratio 0.82, p=0.01).^[6] Set against them the three nulls honestly: SPLIT (2,278 ICU patients, AKI 9.6% versus 9.2%) found no renal difference with buffered crystalloid;^[7] PLUS (5,037 patients, 90-day death 21.8% versus 22.0%) found no evidence that Plasma-Lyte 148 beats saline on death or kidney injury;^[8] and BaSICS (11,052 patients, nearly half planned surgical admissions, 90-day death 26.4% versus 27.2%) likewise does not support routine balanced solution use — noting both arms received only a median 1.5 L on day one, diluting any fluid-type signal.^[9] Teach the ladder, not the mandate: probable small benefit, moderate certainty, and an overview of 14 reviews warning that most carry critical AMSTAR-2 weaknesses — the point estimate is real but small.

TBI Exception — saline protects the injured brain

Reverse the default the moment the head is injured: in BEST-Living, traumatic-brain-injury patients died more often with balanced solutions (19.1% versus 14.7%, odds ratio 1.424) with 0.975 probability of harm.^[1] The dedicated TBI meta-analysis splits the effect cleanly — non-TBI mortality odds ratio 0.93 favouring balanced, TBI odds ratio 1.31 favouring saline — concluding the fluid effect depends on the illness, particularly TBI.^[3] The French physiology review states the bedside rule directly: prefer lactated Ringer's over saline to avoid hyperchloraemic acidosis except with the specific contraindication of intracranial hypertension.^[63] For the polytrauma laparotomy with severe head injury, resuscitate with saline and defend it with the TBI subgroup numbers; for the same laparotomy without brain injury, resuscitate balanced and defend that with the non-TBI numbers.^[3][63]

Sepsis Volume Strategy — phases, CLOVERS, CLASSIC, ESICM

Think in the four overlapping JAMA phases — resuscitation to restore perfusion, optimization weighing each further bolus, stabilization giving fluid only with responsiveness signals, evacuation eliminating accumulated excess — because each trial answers a different phase.^[35] The old EGDT recipe (CVP 8-12, MAP 65-90, ScvO2 above 70%) did not beat unstructured care across three trials (24.9% versus 25.4% mortality), so pressor and transfusion detail belongs to the shock topic, not here.^[35] For early hypotension after 1-3 L, CLOVERS separated the arms by 2,134 mL over 24 hours — restrictive-first with earlier, longer vasopressors versus liberal fluids — and found 90-day pre-discharge death 14.0% versus 14.9% (p=0.61): neither strategy wins, and early vasopressors are safe.^[33] CLASSIC corroborates from the ICU phase (42.3% versus 42.1% mortality restricting without severe hypoperfusion).^[35] Apply the ESICM 2025 volume rules as written: up to 30 mL/kg crystalloids initially in sepsis/septic shock with frequent reassessment (very low certainty); restrictive strategy after blunt trauma (moderate certainty) and penetrating trauma (low certainty); no primary fluid resuscitation for left-sided cardiogenic shock; and explicitly no recommendation for or against restrictive-versus-liberal in the optimization phase (moderate certainty of no effect).^[34] In evacuation, the FACTT result is the prize: conservative fluids plus diuretics after ARDS stabilization added 2.5 ventilator-free days (14.6 versus 12.1), while hydroxyethyl starch raised kidney replacement (7.0% versus 5.8%) — starch stays banned.^[35] Keep the precision future in view without practising it yet: CLOVERS subphenotype SP2 (endothelial injury/inflammation) died 41% with liberal versus 27% with restrictive fluids while SP1 sat at 9% versus 9% (interaction p=0.02).^[38]

Perioperative Restrictive vs Liberal — RELIEF ends the dogma

RELIEF randomised 3,000 high-risk major-abdominal-surgery patients to restrictive (median 3.7 L to 24 hours) versus liberal (6.1 L) regimens: disability-free survival at one year was identical (81.9% versus 82.3%, hazard ratio 1.05), while acute kidney injury rose with restriction (8.6% versus 5.0%, p less than 0.001) alongside more surgical-site infection (16.5% versus 13.6%) and more renal replacement (0.9% versus 0.3%) before multiplicity adjustment.^[10] Messina's 18-trial, 5,567-patient synthesis agrees — severe complications identical (risk difference 0.009) but major renal events fewer with liberal fluids (risk difference 0.06, p=0.001).^[20] The nuance that saves restriction from the bin is contrast: Shen showed restrictive regimens cut complications only when the trial arms truly separated — weight-gain difference of 2 kg or more (risk ratio 0.67) or intake ratio of 1.8 or more (risk ratio 0.72) — so small-contrast trials are uninformative, not negative.^[19] The colorectal ERAS review aligns practice with this: perfusion rate per kg per hour predicts less than total-volume and weight-gain thresholds, standardized restrictive protocols suit selected ERAS patients, and invasive goal-directed therapy is reserved for polymorbid and frail presentations.^[71] Zero-balance is a starting aim, not a religion — the kidney pays for dogma.

Goal-Directed Therapy — elective yes, emergency no

For elective major abdominal surgery, direct fluids to flow: Sun's 45-trial synthesis found perioperative goal-directed haemodynamic therapy cut short-term mortality (risk ratio 0.75), long-term mortality (0.80) and overall complications (0.76), while bringing flatus 0.4 days and diet 0.74 days earlier.^[13] Yuan's abdominal-surgery synthesis adds the wound argument — goal-directed fluid therapy cut surgical-site infection (risk ratio 0.74) and stay (1.16 days).^[14] Jessen's 76-trial synthesis tempers the enthusiasm honestly: mortality odds ratio 0.84 and shorter stay at low certainty, with only infectious outcomes and anastomotic leakage reaching moderate certainty.^[15] FEDORA gives the examined protocol: oesophageal-Doppler guidance to maximal stroke volume with mean pressure above 70 and cardiac index at or above 2.5 halved moderate-or-severe complications (8.6% versus 16.6%) and shortened stay in low-to-moderate-risk elective patients, without moving 180-day mortality.^[11] Laparoscopic HPB surgery corroborates at smaller scale: stroke-volume/cardiac-index guidance used less crystalloid (5.1 versus 6.3 mL/kg/h) with fewer patients harmed (57.8% versus 70.1%), driven by fewer pleural effusions (9.5% versus 19.7%) and fewer ICU admissions.^[22] Now the brakes, each examined. Emergency laparotomy for obstruction or perforation: flow-guided to near-maximal stroke volume changed nothing (30% versus 25% complications, odds ratio 1.24) and lengthened stay (7 versus 6 days).^[17] High-risk elective abdominal surgery with cardiac-index/stroke-volume-variation algorithms: 0.79 versus 0.69 major complications per patient with more fluids and more dobutamine — no benefit.^[18] And the 2026 BMJ synthesis of 255 abdominal-surgery trials (55,260 patients) finds goal-directed haemodynamic therapy, targeted pressure management and restrictive fluids show no pulmonary-complication benefit at moderate certainty — only low FiO2 (high certainty), lung-protective ventilation, physiotherapy, analgesia and nutrition clear the bar.^[16] State the Cochrane honesty clause alongside any GDT claim: restrictive-versus-goal-directed evidence rests on 6 trials and 562 mostly ASA I-II elective patients.^[12] Where invasive monitoring is unavailable, pleth variability index guidance individualises volume non-invasively — cutting about 761 mL total fluid and 655 mL crystalloid with no stay or metabolic difference — a volume-sparing tool, not an outcome therapy.^[21]

Monitoring & Responsiveness — dynamic beats static, MAP governs kinetics

Monitor shock by the ESICM 2025 hierarchy: skin perfusion (capillary refill with temperature and mottling), serial central-venous saturation and veno-arterial CO2 gap where a central line exists, dynamic over static preload indices wherever applicable, cardiac output or stroke volume when initial therapy fails, an arterial line for refractory or pressor-dependent shock, and echocardiography first-line to type the shock.^[36] The single most examined rule: in persistent shock after initial resuscitation, assess fluid responsiveness before giving more fluid.^[36] Hypotension itself retains fluid — Hahn's kinetic analyses show elimination falling in proportion to mean pressure (over tenfold less excretion at MAP 50 than 100), and anaesthesia induction dropping pressure from 110 to 60 cuts distribution by 75% and elimination by 90%, leaving infused fluid intravascular.^[26][24] So a hypotensive postoperative patient is automatically fluid-retaining: fix pressure, expect retention, and re-measure responsiveness rather than pouring. Bioimpedance remains research, not prescription: in HPB perioperative patients only the fluid-imbalanced subset showed postoperative rises in extracellular water and ECW/TBW ratio, suggesting — not proving — a mechanism for ascites and collections.^[28]

Compartments & Kinetics — where each litre goes

Teach Hahn's numbers because the viva rewards them: crystalloid expands plasma by 50-60% of the infused volume while the infusion runs, but only 15-20% remains at 30 minutes; saline's half-life is twice Ringer's; small volumes barely reach the interstitium while rapid infusions guarantee oedema.^[23] Past a threshold the interstitium overflows into a slow-exchange reservoir: once fast-exchange volume gains 600-800 mL — roughly 1.3-1.5 L of crystalloid over 30 minutes — the remote space fills, and the known sinks are skin, bowel wall and lung, with animal microscopy adding cardiac interstitial swelling and hypoxia beyond 100 mL/kg.^[25] Hyper-oncotic albumin 20% may recruit that lymphatic fluid and drive diuresis, but evidence is explicitly limited.^[25] Use the Bhave vocabulary precisely because examiners test it: dehydration means total-body-water loss producing hypertonicity (intracellular contraction), while volume depletion means extracellular-fluid deficit (blood-volume contraction) — different words, different fluids.^[27] In inflammation a second mechanism stacks on overhydration: cytokine and vasoactive suction withdraws fast-exchange fluid, slows lymph flow, and produces hypovolaemia with hypoalbuminaemia and oedema together — the sepsis/preeclampsia pattern in which pressors barely shift distribution.^[25]

Overload, Creep & De-resuscitation — the evacuation phase at the bedside

Define overload at 10% cumulative weight gain and quote its price without flinching: it independently predicts morbidity and mortality,^[29] each extra positive litre multiplies mortality risk by 1.19, and the surgery subgroup carries adjusted risk 6.17 — with sepsis at 1.66, kidney injury at 2.63 and respiratory failure at 1.19 for cumulative balance.^[32] Non-survivors ran 4.4 L more positive at one week, and restrictive management in that systematic review halved the odds of death (24.7% versus 33.2%, odds ratio 0.42).^[31] Hunt fluid creep first: maintenance fluids in patients already drinking, drug carriers and flushes accumulate silently — fluids are drugs, never routine maintenance without indication.^[29] Then de-escalate in order: limit intake to physiological needs, then actively remove with diuretics or renal replacement; removing 4.9 L dropped intra-abdominal pressure from 19.3 to 11.5 mmHg in the de-resuscitation series.^[30][31] Name the syndrome when present — fluid accumulation syndrome is any weight-percentage gain with new organ failure — and run its stepwise programme: minimise intake and sodium/chloride load, maximise output.^[37] Put overload on the round as a working diagnosis like difficult ventilator weaning, because unrecognised accumulation blocks recovery the same way; whether permissive haemodynamic instability is acceptable to achieve it remains explicitly unanswered.^[30]

Hypernatraemia & Diabetes Insipidus — water lost, brain at risk

Sort every hypernatraemia first into sodium gain versus free-water loss using bedside assessment plus urine electrolytes — the test that discriminates the mechanism.^[40] Treat by restoring water: free water with or without diuretics that promote renal sodium excretion — and remember the critically ill cannot drink to thirst, so the physician owns the balance.^[40] The rate is the safety variable: match correction speed to onset rapidity, because both directions injure.^[40] Split by the 48-hour line: acute hypernatraemia (under 48 hours) needs rapid but controlled correction to avert vascular rupture and cerebral bleeding, while chronic hypernatraemia (48 hours or more) needs gradual correction to avert cerebral oedema — and the critically ill carry excess mortality from correction errors both ways.^[39] Always work up the drivers behind the number, especially diabetes insipidus and volume depletion.^[39] Central DI is AVP deficiency that abolishes concentrating ability and produces hypotonic polyuria with compensatory thirst; the copeptin assay has modernised its diagnosis, and management centres on fluid-intake control with pharmacological AVP replacement — including the adipsic, pregnancy and perioperative variants.^[41] The examined perioperative protocol (paediatric DI, neurosurgical risk) titrated aqueous vasopressin to antidiuresis while restricting intravenous fluids to normal saline at two-thirds maintenance plus loss replacement: sodium held between 130 and 150, the 24-hour swing nearly halved against historical controls (8.36 versus 17.6 mEq/L), and hyponatraemia fell.^[42]

Hyponatraemia — hypotonic fluids harm children; adults need formula discipline

The paediatric harm is quantified and examined: on postoperative hypotonic-fluid protocols, 48.6% of children were hyponatraemic within 12-24 hours — subclinical but significant, worse in hot months — grounds to abandon hypotonic maintenance.^[43] The mechanism is ADH: postoperative non-osmotic antidiuretic-hormone release leaves surgical patients unable to excrete free water, and hypotonic maintenance as currently given raises hyponatraemia rates while isotonic fluid reduces the risk on the best available data.^[44] Prescribe the paediatric surgical rule as examined: isotonic balanced crystalloids for maintenance and replacement because they lower hyponatraemia and metabolic-acidosis risk; add 1-2.5% glucose to maintenance only to prevent hypoglycaemia, lipid mobilisation, ketosis and hyperglycaemia; cut clear-fluid fasting to about 1 hour; and reduce postop isotonic infusion rates to avoid dilutional hyponatraemia.^[45] Know the history that caused the harm: Holliday and Segar's 1957 weight-based hypotonic maintenance fitted their original population, not the postoperative child — tradition, not physiology, produced hospital-acquired hyponatraemia with its morbidity.^[46] Under ADH release with low urine output, halve the calculated hourly maintenance rate — volume restraint matters as much as tonicity.^[47] For adults, attribute the prediction formula honestly: Adrogue-Madias (NEJM 2000) for expected sodium change per litre of infusate — an existence-only landmark here, cited for attribution while correction-rate discipline (calculate, recheck, respect chronicity) carries the bedside weight.^[48][39]

Hypokalaemia & Magnesium — the refractory pair

Define hypokalaemia below 3.5 mmol/L and teach the distribution trap first: a mere 1% transcellular shift moves plasma potassium by 50%, so a low number may mean shift, loss, or both.^[49] Balance is external (about 100 mmol daily intake, 95% renal and 5% colonic excretion) and internal (intra-versus-extracellular distribution under insulin, beta-agonist and aldosterone control); once extrarenal causes are excluded, interrogate distal sodium delivery, mineralocorticoid status, and distal-nephron defects that govern secretion.^[49] The refractory mechanism is magnesium: intracellular depletion releases magnesium-mediated ROMK inhibition and wastes potassium — concomitant deficiency aggravates hypokalaemia and makes it potassium-resistant — yet deficiency alone need not cause it without distal sodium delivery or aldosterone drive.^[50] So the rule is absolute: check magnesium in every refractory hypokalaemia and replete together. The surgical drivers to list are vomiting and nasogastric losses, diuretics, secondary aldosteronism, starvation and poor intake, unrepleted parenteral nutrition, and DKA therapy shifts. Repletion itself stays empirical-with-feedback: published ICU dosing equations exist but rest on thin randomised evidence, so treat the cause, dose, recheck, and adjust to the patient's response.^[59]

Hyperkalaemia Emergencies — membrane, shift, remove (+BRASH)

Run the emergency in membrane-shift-remove order: stabilise membrane with calcium — 10 mL of 10% calcium gluconate intravenously, chloride in arrest — indicated with ECG changes or potassium at or above 6.5 mmol/L, then shift potassium intracellularly.^[51][52] Shifting rests on insulin-glucose as the cornerstone with its considerable hypoglycaemia price — worst in non-diabetics with low baseline glucose, demanding serial glucose checks — synergised by inhaled salbutamol combined with insulin.^[52] Bicarbonate's acute role is equivocal and limited to severe acidotic comorbidity; diuretics support elimination only with volume overload and no prospective ED evidence; binders (patiromer, zirconium cyclosilicate) hold chronic promise but need acute-care validation; dialysis remains definitive in refractory or end-stage renal cases.^[52] The Cochrane randomised base agrees: salbutamol and insulin-glucose each work, the combination beats either, bicarbonate is equivocal, resin fails by four hours, and dialysis works.^[53] Retire sodium polystyrene sulfonate — inefficacious acutely with gastrointestinal harm, no longer recommended.^[51][52] Dose honestly per Sterns: standard insulin regimens under-deliver both insulin (below maximal kalemic effect) and glucose (below hypoglycaemia protection), with short-acting insulins theoretically better in severe kidney disease and ZS-9 showing acute promise that might defer emergency dialysis.^[54] Finally, recognise BRASH when bradycardia, renal failure, AV blockade, shock and hyperkalaemia spiral together — hyperkalaemia plus AV-nodal blockers initiating synergistic bradycardia — and treat the cycle (fluids, full hyperkalaemia therapy, epinephrine for bradycardia, advanced rescue as needed), not the potassium alone.^[55]

Refeeding & Nutrition Electrolytes — phosphate crashes first

Recognise the surgical starvation face: the NEJM parenteral-nutrition vignette describes mesenteric-ischaemia resection with jejunostomy, 15% six-month weight loss with poor intake, and hypomagnesaemia plus hypophosphataemia on labs — the gut-failure postoperative patient is the refeeding candidate, with feeding-regimen detail fenced to icu-nutrition-surgical.^[58] Apply the Boot selection rule: a phosphate fall exceeding 0.16 mmol/L from normal within 72 hours of starting feeding identifies patients whose long-term mortality falls with hypocaloric or restricted intake for at least 48 hours — with thiamine and electrolyte supplementation as standard in all cases.^[56] Expect the crash early on parenteral nutrition: moderate-severe hypophosphataemia runs significantly commoner in ICU than non-ICU recipients, worst over days 1-4, demanding early monitoring and consideration of hypercatabolic-tailored regimens.^[57] Repletion discipline mirrors potassium: correction is empirical and individualised to response, not protocol-fixed.^[59]

Acid-Base — saline acidosis vs chloride-depletion alkalosis

Name the saline acidosis correctly: large-volume normal saline (and saline-solvent colloids) cause hyperchloraemic, normal-gap acidosis by chloride loading and strong-ion-difference fall — call it hyperchloraemic acidosis, never dilutional acidosis — and separate the effects of the acidosis from the effects of whatever caused it.^[60][63] Prove balanced fluids where it shows fastest, DKA resuscitation: RINSE-DKA found lactated Ringer's resolved high-gap acidosis faster than saline (adjusted hazard ratio 1.325), and the blinded Mahler trial showed balanced solution leaving chloride lower (105 versus 111) and bicarbonate higher (20 versus 17) — balanced crystalloids prevent the superimposed normal-gap acidosis that prolongs insulin infusions.^[61][62] Then teach alkalosis as generation plus maintenance: vomiting, mineralocorticoid excess, liquorice, chloruretic diuretics, calcium-alkali excess and Bartter/Gitelman generate it, while volume contraction, low filtration, potassium deficiency, hypochloraemia, aldosterone excess and high CO2 maintain it — so assess effective arterial volume and urine electrolytes first, reverse the contributors, and reserve acetazolamide, acid infusion or low-bicarbonate dialysis for severe cases.^[64][66] Severe alkalosis (pH 7.55 or above) carries significantly increased mortality in the critically ill, so aggressive management has its place.^[64] Replace the outdated phrase with Luke's verdict: chloride repletion corrects alkalosis renally despite persisting volume contraction, potassium and sodium depletion — it is chloride-depletion alkalosis, not contraction alkalosis, orchestrated at the collecting duct's pendrin exchanger.^[65] The surgical face is vomit-loss alkalosis — SMA-obstruction vomiting producing metabolic alkalosis with aspiration risk is the examined example — and the tubular face is Bartter/Gitelman salt-wasting with the classic hypokalaemic, hypochloraemic metabolic-alkalosis triad.^[69][70]

Hypercalcaemia Emergencies — hydrate, bisphosphonate, then cause

Stratify by Walker's JAMA numbers: hypercalcaemia affects about 1% of people; mild disease (total calcium below 12 mg/dL) is usually asymptomatic with constitutional symptoms in a fifth; severe disease (14 mg/dL or above, or rapid onset over days to weeks) brings nausea, vomiting, dehydration, confusion, somnolence and coma.^[67] Nine in ten cases are primary hyperparathyroidism or malignancy: elevated or normal PTH means PHPT, suppressed PTH below 20 pg/mL means another cause — order intact PTH first.^[67] Sequence emergency therapy: aggressive IV hydration with normal saline, then intravenous bisphosphonate (zoledronic acid or pamidronate) when rehydration alone is inadequate; renal failure redirects to denosumab with or without dialysis; vitamin-D intoxication, granulomatous disease and some lymphomas answer to glucocorticoids as primary therapy.^[67][68] Then treat the cause — parathyroidectomy for surgical PHPT disease, palliation where malignancy drives it — because treatment lowers calcium transiently while prognosis follows aetiology.^[67]

Exam Synthesis & Fence Map — what fluids-electrolytes owns

This topic owns six things and fences everything else: compartment physiology and volume kinetics; crystalloid choice with the TBI exception; perioperative volume strategy (restrictive-versus-liberal, goal-directed therapy, monitoring); sodium, potassium, magnesium and phosphate management; acid-base interpretation and correction; and overload with de-resuscitation.^{[23][3][10][48][65][30]} Pressor choice, MAP targets and lactate resuscitation belong to shock-surgical; bundles and source control to postoperative-sepsis; ventilation to ards-surgical; KDIGO and RRT timing to acute-kidney-injury-surgical; ISTH/JAAM detail to disseminated-intravascular-coagulation; IAH grades and decompression to abdominal-compartment-syndrome; feeding regimens to icu-nutrition-surgical; products and ratios to massive-transfusion and damage-control-resuscitation; SOFA trajectories and support-drug verdicts to multiorgan-dysfunction; PPC mechanics to postoperative-respiratory-failure.^{[33][35][31][11]} The one-paragraph fluids story for the viva: balanced crystalloids probably save a few per hundred except in head injury where they kill; RELIEF forbids dogmatic restriction because the kidney pays first; goal-directed therapy prevents wound infection and leak in elective abdominal surgery but fails emergencies and high-risk protocols; kinetics leaves a fifth of crystalloid intravascular at 30 minutes and parks the rest in skin, bowel, lung and heart; 10% weight gain predicts death and demands named de-escalation; sodium moves at chronicity speed; potassium never rises without magnesium; hyperkalaemia dies without calcium-insulin-dialysis sequencing and hypercalcaemia without hydration-bisphosphonate sequencing.^{[1][3][10][15][23][32][39][50][52][67]}

Exam Pearls — the one-liners that score

• Balanced probably beats saline a little (posterior probability ~0.9, absolute difference 0.4 points) — except TBI, where balanced kills (odds ratio 1.42, harm probability 0.975).^[1]
• SMART cut MAKE30 (14.3 versus 15.4%, odds ratio 0.91); SALT-ED matched hospital-free days but cut MAKE30 (4.7 versus 5.6%); SPLIT, PLUS and BaSICS were null — teach the ladder whole.^{[5][6][7][8][9]}
• RELIEF: restrictive 3.7 versus liberal 6.1 L, survival identical, kidney injury 8.6 versus 5.0% — restriction needs a true contrast (2 kg or 1.8x) to help.^[10][19]
• Goal-directed therapy halves elective complications (FEDORA 8.6 versus 16.6%) and wound infection (risk ratio 0.74) but fails emergencies, high-risk protocols and 255-trial PPC synthesis.^{[11][14][17][16]}
• Only 15-20% of crystalloid stays intravascular at 30 minutes; saline lasts twice Ringer's; 1.3-1.5 L in 30 minutes opens the third-space overflow.^[23][25]
• Dehydration is hypertonicity; volume depletion is ECF loss — different words, different fluids.^[27]
• Overload is 10% weight gain; each extra litre multiplies death risk 1.19; surgery subgroup risk 6.17 — name it and de-escalate.^[29][32]
• CLOVERS: early vasopressors with less fluid neither help nor harm (14.0 versus 14.9%) — ESICM caps initial sepsis fluids at 30 mL/kg with reassessment.^[33][34]
• Acute sodium (under 48 h) moves fast but controlled; chronic (over 48 h) moves slowly — both errors kill the critically ill.^[39]
• Half of children on hypotonic postop fluids turn hyponatraemic — isotonic balanced plus halved rates under ADH.^[43][44][47]
• A 1% potassium shift moves plasma levels 50%; magnesium depletion unlocks ROMK wasting — replete together or fail.^[49][50]
• Hyperkalaemia: calcium at ECG changes or 6.5+, insulin plus salbutamol synergy, bicarbonate equivocal, resin out, dialysis definitive — BRASH spirals need epinephrine plus the cycle broken.^[52][53][55]
• Phosphate fall over 0.16 within 72 hours of feeding means restrict calories 48 hours plus thiamine — the refeeding rule.^[56]
• Saline acidosis is chloride-load, self-correcting; vomit alkalosis is chloride-depletion, saline-responsive — pendrin, not contraction.^[63][65]
• Hypercalcaemia: 90% PHPT or malignancy; PTH below 20 means look elsewhere; saline then bisphosphonate, denosumab/dialysis in renal failure.^[67][68]

Revision summary

Fluids and electrolytes in surgical patients reduce to six examined moves: resuscitate balanced except in head injury where saline saves; dose perioperative volume by RELIEF (liberal kidneys beat restricted dogma) and direct elective-abdominal fluids to stroke volume while refusing emergency protocols; expect kinetics to park four-fifths of crystalloid outside vessels within half an hour and de-escalate overload at the 10% line; move sodium at chronicity speed with isotonic discipline in children; pair magnesium with potassium and sequence calcium-insulin-dialysis against hyperkalaemia; and read acidosis as chloride load versus chloride loss while hydrating hypercalcaemia into bisphosphonates — every number from the live PubMed abstract beside it.^{[3][10][11][23][32][39][50][52][65][67]}

SMART MAKE30 14.3 vs 15.4% (OR 0.91, p=0.04) and SALT-ED MAKE30 4.7 vs 5.6% (OR 0.82, hospital-free days null) as the positive single-centre pair against the null multicentre pair PLUS (90-day death 21.8 vs 22.0%) and BaSICS (26.4 vs 27.2%, 48.4% planned surgical) with SPLIT (AKI 9.6 vs 9.2%) as the early null — and state the pooled synthesis (BEST-Living OR 0.962, posterior probability 0.895; NEJM Evidence RR 0.96, 89.5%).^[9]

-2134 mL separation, 90-day pre-discharge death 14.0 vs 14.9% (p=0.61) — restrictive-first with early vasopressors neither helps nor harms — and CLASSIC (42.3 vs 42.1%) as the ICU-phase corroboration.^[33]

References71ShowHide

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