Shock Physiology for Surgeons — Four Categories, Oxygen Delivery, Microcirculation, Lactate Kinetics, Vasoplegia, Compensation, Monitoring

The one-paragraph contract — what this topic owns

The shocked surgical patient rarely offers a single mechanism — so name the four-category classification with its compartment mapping first, then the Sepsis-3 operational definitions, then oxygen delivery and extraction, then microcirculation and mitochondria, then lactate kinetics in context, then vasoplegia and compensation, then the subtype physiologies, then MODS scoring, then responsiveness-based monitoring last; quote every number from the paper named beside it; and fence fluid choice, transfusion ratios, drug doses, antibiotic regimens and full resuscitation ladders to their owners while keeping the physiology that explains them here: because shock is one mismatch with four anatomies, sepsis is a dysregulated host response with a SOFA threshold, delivery failure and cellular failure are different lesions, lactate has two phases, vasoplegia is low resistance with normal-or-high output, compensation masks vitals, and static preload numbers do not predict response.^{[1][2][8][17][20][25][46]}

A 68-year-old man is 6 hours after laparotomy for perforated diverticulitis: MAP 62 mmHg on fluids, lactate 4.2 mmol/L, mottled knees, urine 15 mL/hr, new atrial fibrillation with a soft systolic murmur, distended neck veins absent, calves warm. Which shock category dominates and which coexists, whether his SOFA rise already defines sepsis, whether his lactate means ongoing hypoperfusion or metabolic reprogramming, whether his vessels are vasoplegic or his pump is failing, and which bedside test predicts his next fluid challenge? The examiner will watch you map compartments, apply Sepsis-3 and qSOFA exactly, split flow-phase from metabolic-phase lactate, name vasoplegia criteria, recognise mixed shock, and choose a dynamic responsiveness test over any static pressure. This page teaches each move with every number taken from the paper named beside it.^{[1][2][20][25][46][51]}

Definition — one mismatch, four anatomies

Start from the single sentence that organises everything: a severe mismatch between the supply and demand of oxygen is the common feature of all types of shock.^[1] Then refuse the loose word "shock" and insist on exactly four major categories, each mainly related to one of four organ systems.^[1] Map them aloud: hypovolaemic shock relates to the blood and fluids compartment while distributive shock relates to the vascular system; cardiogenic shock arises from primary cardiac dysfunction; and obstructive shock arises from a blockage of the circulation.^[1] Carry the per-type treatment principle the classification was built to teach: hypovolaemic shock is due to intravascular volume loss and is treated by fluid replacement with balanced crystalloids, while distributive shock is a state of relative hypovolaemia from pathological redistribution of absolute intravascular volume and is treated with a combination of vasoconstrictors and fluid replacement.^[1] State the companion common abnormality in surgical language: all causes possess the common abnormality of oxygen supply not meeting tissue metabolic demands.^[8] Name the life-threatening frame once: shock is a life-threatening circulatory failure that results in inadequate tissue perfusion and oxygenation.^[54] Fence the rest honestly: fluid brand, volume strategy, pressor doses, blood ratios and antibiotic timing live with fluids-electrolytes, transfusion-coagulation, shock-surgical and surgical-infection-antimicrobials — this topic owns the mapping, not the prescription.

Sepsis-3 — dysregulated host response with operational thresholds

Define sepsis exactly: life-threatening organ dysfunction caused by a dysregulated host response to infection.^[2] Operationalise it: an increase in the SOFA score of 2 points or more, which is associated with an in-hospital mortality greater than 10%.^[2] Identify septic shock exactly: a vasopressor requirement to maintain a mean arterial pressure of 65 mm Hg or greater and serum lactate level greater than 2 mmol/L in the absence of hypovolaemia.^[2] Price it: this combination is associated with hospital mortality rates greater than 40%.^[2] Know where the numbers came from: the shock SRMA found a crude mortality of 46.5% with marked heterogeneity, and the derivation cohort requiring pressors plus lactate above 2 mmol/L after fluids died at 42.3% in risk-adjusted comparison with the other five groups.^[4] Run qSOFA at the bedside: respiratory rate of 22/min or greater, altered mentation, or systolic blood pressure of 100 mm Hg or less — one point each.^[2] Know its derivation: among 1.3 million encounters across 12 hospitals, with 1 point each for systolic hypotension, tachypnoea or altered mentation in the qSOFA model.^[3] Tell the host-response story behind the definition: emphasis has moved away from the pathogen toward the abnormal and exaggerated host response, with the pathogen triggering an exaggerated inflammatory-immune response that activates or suppresses endothelial, hormonal, bioenergetic, metabolic and immune pathways into circulatory plus metabolic perturbation and organ dysfunction.^[44] Add the adaptive framing the examiner rewards: these processes, superficially dysfunctional, may be adaptive or protective at first, spilling into maladaptation and harm with the magnitude of the host response.^[44]

Epidemiology — denominators the viva respects

Quote the US clinical-surveillance denominator idle-free: 173,690 sepsis cases among 2,901,019 admissions in 2014, a 6.0% incidence.^[6] Quote the outcome: 15.0% died in hospital and 6.2% went to hospice, with incidence stable by clinical criteria while claims-based incidence rose.^[6] Quote the ICU validation denominator: among 184,875 ICU patients, a total of 34,578 (18.7%) died in hospital, with SOFA rise of 2+ in 90.1%.^[5] Quote the discrimination the examiner will probe: SOFA crude AUROC 0.753 beat SIRS at 0.589 and qSOFA at 0.607 for in-hospital mortality.^[5] Split Sepsis-3 mortality honestly rather than quoting one number: 28-day death was 23.4% for septic shock, 8.8% for vasoplegic shock and 12.2% for cryptic shock, with mortality rising in a lactate gradient inside each group.^[7] Use the split at the bedside: the Sepsis-3 shock definition identifies the highest-risk group needing intensive care, while rising lactate in cryptic shock still warns — some cryptic patients need intensive management too.

Oxygen delivery — supply, demand, extraction, debt

Return to the common abnormality before any device: oxygen supply not meeting tissue metabolic demands.^[8] Teach the cascade in order: when systemic delivery does not meet tissue demands, tissue hypoperfusion begins, and tissue hypoperfusion leads to oxygen debt, cellular injury, organ dysfunction and death.^[9] Read perfusion markers as the interaction, not as isolated numbers: tissue hypoperfusion can be characterised using central venous oxygen saturation and lactate, which reflect the interaction between systemic oxygen delivery and demands.^[9] Carry the phenotype humility from the same review: three recent trials confirmed improving sepsis mortality yet challenged rapid normalisation of perfusion markers as outcome benefit — so define and compare haemodynamic phenotypes with perfusion markers to judge who benefits from early haemodynamic optimisation.^[9] Use the haemorrhage-versus-exercise framework to explain reserve: disturbance of homeostasis occurs when delivery plus energy stores fail to meet cellular energy requirement, and the body's reserve to compensate for inadequate delivery — the VO2max analogy — reframes triage and resuscitation monitoring.^[10] Name the examination consequence: compensatory catecholamines and extracellular fluid shifts initially compensate for shock derangements and may mask severity, and normal vitals after resuscitation can coexist with ongoing tissue metabolic acidosis and lactate.^[8]

Microcirculation — where delivery actually fails

Move the lens from macrocirculation to microvessels: the microvasculature plays the central role in haemorrhagic-shock pathophysiology and in arguably every therapeutic attempt to reverse it.^[13] Describe the septic endothelial shift that explains organ injury despite fluids: during sepsis, endothelial cells shift toward a proapoptotic, proinflammatory, proadhesive and procoagulant phenotype, while glycocalyx damage plus vascular-tone dysfunction impairs microcirculatory flow toward organ injury and potentially life-threatening organ failure.^[12] Enumerate the haemorrhagic microvascular categories the examiner can ask after: oxygen distribution, ischaemia, inflammation, glycocalyx changes, vasomotion, endothelial dysfunction and coagulopathy, studied across skin, muscle, lung, liver, brain, heart, kidney, pancreas, intestines and mesentery.^[13] Define the glycocalyx plainly: a web of membrane bound to the luminal side of the endothelium whose role includes maintenance of the vascular permeability barrier and mediation of shear response.^[14] Explain why fluids are double-edged here: trauma reviews explore the glycocalyx in microcirculatory dysfunction and then investigate fluid administration itself — including its potential to protect the microcirculation — with emphasis on inflammatory modulation and sensible resuscitation.^[14] Price glycocalyx injury with the SRMA: 17 studies with 3,529 patients, syndecan-1 higher in non-survivors, with elevated syndecan-1 carrying death OR 2.32, MODS OR 3.3 and respiratory-failure OR 7.53.^[16] State the bedside moral: microcirculatory impairment should be routinely considered using biomarkers to stratify risk, not inferred from pressure alone.

Cellular energetics — cytopathic hypoxia and mitochondrial humility

State the uncomfortable alternative to every delivery number: cellular energetics are deranged in sepsis, not on inadequate perfusion but on impaired mitochondrial respiration and coupling — organ dysfunction may occur on the basis of cytopathic hypoxia.^[17] Follow the logic to its therapeutic warning: efforts to improve sepsis outcome by monitoring and manipulating cardiac output, systemic DO2 and regional blood flow are doomed to failure if the concept holds — the focus shifts to restoring mitochondrial function and cellular energetics.^[17] Name MODS as the lethal expression: the most common cause of sepsis death is multiple organ dysfunction syndrome, potentially via acquired intrinsic derangements in cellular respiration called cytopathic hypoxia.^[18] Recite the three postulated chemistries: reversible cytochrome-oxidase inhibition by nitric oxide, irreversible respiratory-complex inhibition by peroxynitrite, and activation of poly-(ADP-ribosyl)-polymerase.^[18] Then confess the variability the viva respects: mitochondrial respiration may decrease, increase or stay unchanged by organ and species, with three live explanations — secondary to tissue hypoxia versus toxin- or mediator-impaired oxygen utilisation.^[19] The examination rule follows: quote cytopathic hypoxia as mechanism, refuse to promise that normalising delivery normalises cells, and fence drug-level mitochondrial rescue as experimental.

Lactate — two phases, two shocks, one discipline

Open with the state definition lactate serves: a life-threatening circulatory failure with inadequate tissue oxygen delivery and cellular hypoxia.^[21] Teach biphasic kinetics as the core viva answer: an initial 6-12 h predominantly flow-dependent phase where elevation reflects hypoperfusion and improves with restored delivery via fluids, pressors or hypoxaemia correction — then a later phase beyond 12 h where persistent hyperlactataemia more often reflects metabolic reprogramming, immune activation and mitochondrial dysfunction rather than ongoing hypoperfusion.^[20] State the discipline that follows: persistent hyperlactataemia without hypoperfusion may be misleading and should not drive further resuscitation.^[20] Contextualise the confounders: severe systolic dysfunction may impair clearance despite restored flow, while excessive beta-stimulation particularly with epinephrine can raise lactate without metabolic shift — so persistent lactate with abnormal refill, low ScvO2 or raised venous-arterial CO2 gap carries worse outcomes than isolated persistence.^[20] Explain distributive hyperlactataemia as multifactorial rather than purely anaerobic: increased aerobic glycolysis plus adrenergic stimulation plus mitochondrial dysfunction plus impaired hepatic clearance.^[21] Explain cardiogenic lactate separately: primarily the magnitude and duration of systemic hypoperfusion from reduced output, often compounded by regional especially splanchnic ischaemia — plus adrenergic, post-arrest, reperfusion, inflammatory and mitochondrial contributors — with impaired hepatic and renal clearance sustaining elevation despite restored macrocirculation.^[22] Use Jones narrowly and exactly as physiology illustration: ScvO2 70%+ versus lactate-clearance 10%+ goals after normalising filling pressures and MAP — resuscitation detail and pressor choice stay fenced to shock-surgical.^[24] Close with the synthesis: lactate is both haemodynamic and metabolic biomarker, integrated with perfusion indices to guide escalation, timely de-escalation and adjunct consideration — never an isolated therapeutic target.^[20]

Vasoplegia — low resistance with normal-or-high output

Define vasoplegia in one breath: the syndrome of pathological low systemic vascular resistance whose dominant feature is reduced pressure in the presence of a normal or raised cardiac output.^[25] Place it across surgery: septic shock, post-cardiac-bypass, post-surgical, burn and trauma states — while confessing that uniform clinical definitions are lacking, which challenges translational research.^[25] Name the intrinsic chemistry: nitric oxide, prostanoids, endothelin-1, hydrogen sulphide and reactive oxygen species reviewed as drivers with therapeutic-exploration potential.^[25] Name the extrinsic modulation: glucocorticoid, catecholamine and vasopressin responsiveness of vessels.^[25] Convict the two chief culprits precisely: NO and peroxynitrite are mainly responsible for vasoplegia and vascular hyporeactivity.^[26] Complete the hyporesponsiveness list: COX-2-driven PGI2 rise, KATP and BKCa over-activation in hypotension, plus critical-illness corticosteroid insufficiency, vasopressin depletion, adrenoreceptor dysfunction and desensitisation, and catecholamine inactivation by oxidation.^[26] State the balance humility: the optimum between maintaining SVR and the deleterious effects of catecholamines is as yet unclear, with novel vasoactive agents potentially clarifying pathway roles.^[25] Fence dosing explicitly: receptor physiology and deficiency concepts live here; milligram and unit orders live with shock-surgical and perioperative-pharmacology.

Compensation — excitation, inhibition, pressure control, masked vitals

Run the trauma two-phase story the examiner wants: an initial sympathoexcitatory phase attempting to compensate for acute blood loss — vasoconstriction, tachycardia and preserved MAP — followed by hypotension reflecting a sympathoinhibitory vasodilation phase.^[53] Draw the resuscitation moral carefully: hypotensive-resuscitation advocacy rests on limited data, persistent hypotension and hypoperfusion worsen coagulopathy and organ function, excessive volume harms, and the goals remain restoring intravascular volume with balanced blood products, correcting coagulopathy and maintaining organ perfusion with an appropriate volume-tone balance.^[53] Explain pressure control generally: pressure from blood flow determines perfusion plus nutrition, oxygen and factor transport to every organ, with central pressure near heart and brain distinguished from peripheral pressure — usually correlated, but shocks and cardiovascular disorders disrupt regulation and redistribute vital-versus-accessory flow.^[40] Name the control systems: neural, hormonal, osmotic and cellular regulation of pressure and its haemorrhagic-shock behaviour.^[40] Repeat the masking warning at the bedside: compensatory mechanisms may mask hypoxaemia and hypoperfusion severity, and normal parameters after resuscitation can coexist with ongoing tissue metabolic acidosis and lactate.^[8]

Hypovolaemic and haemorrhagic sequence — volume loss to death

Recite the sequence without inventing classes: rapid significant intravascular-volume loss leading sequentially to haemodynamic instability, decreased oxygen delivery, decreased tissue perfusion, cellular hypoxia, organ damage and death — rapidly fatal without treatment.^[55] State the principle without prescribing ratios: the primary goals are to stop the bleeding and to restore circulating blood volume, with severity-estimated resuscitation using crystalloids, colloids or blood products life-saving in severe shock while the optimal method remains not clearly established.^[55] Note the moderate-hypotension signal with appearance-only humility: delayed massive fluids until definitive care may benefit moderate hypotension from bleeding — an appearance in the review, not a protocol — while severe shock needs volume and blood.^[55] Fence the numbers explicitly: ATLS class percentages, permissive-hypotension targets, TXA timing and transfusion ratios were not verified in this pack and are not examined here — they belong to transfusion-coagulation, damage-control and shock-surgical.

Cardiogenic, septic cardiomyopathy, mixed shock, subphenotypes

Define cardiogenic shock as a spiral: depression of cardiac function that leads to low pressure, coronary ischaemia and further decreased contractility resulting in tissue hypoxaemia.^[31] Price it: high early mortality approaching half, largely driven by aetiology, with rapid coronary-flow restoration substantially reducing infarct-shock death while device selection and timing remain under investigation.^[31] Add the heterogeneity the viva probes: cardiogenic shock can progress rapidly to irreversible multiorgan failure through self-perpetuating processes, with aetiologic, mechanistic and haemodynamic heterogeneity deciding management — early recognition, rapid cause reversal and prompt haemodynamic support.^[32] Handle septic cardiomyopathy honestly: multiple competing definitions hinder understanding; although many septic patients develop cardiac dysfunction, its impact on prognosis and therapy remains to be demonstrated — treat the underlying sepsis with specific supportive care for shock when present.^[33] Explain the mechanics that matter more than ejection fraction: decreased systolic contractility limits ejection and stroke volume, initially compensated by diastolic filling during volume resuscitation plus reduced afterload from vasodilation — while diastolic dysfunction with reduced compliance impedes filling, grows with severity, and tracks mortality more closely than systolic dysfunction when ejection failure meets filling failure.^[34] Classify mixed shock aloud: mixed cardiogenic shock as cardiogenic shock with at least one additional shock state — now the second commonest shock in contemporary coronary intensive care, without standardised frameworks or established haemodynamic definitions, awaiting a classification plus invasive-haemodynamic proposal.^[51] Add the septic subphenotypes: a four-profile model fits best — hyperdynamic with highest output and lowest SVRI, high-preload, hypodynamic with lowest output and highest SVRI, and preserved near-normal — with 48-hour fluid-balance mortality interaction significant only in the preserved group.^[52]

Obstructive mechanics — noncardiac low output that needs release

Define obstructive shock mechanistically: reduction in cardiac output due to noncardiac diseases.^[35] List the recognised causes the examiner expects: pulmonary embolism, tension pneumothorax, pericardial tamponade and aortic dissection.^[35] State the rule that separates it from every other category: obstructive shock typically cannot be stabilised unless the cause is resolved, so diagnosis of the underlying disease is eminent.^[35] Run the structured 3-step diagnosis: clinical examination, then ultrasound using the RUSH protocol, then radiological imaging if needed.^[35] Place the integrative review behind it: distributive — usually septic — shock is by far the commonest, history plus examination plus haemodynamic assessment splits the states, specific management corrects the trigger plus ongoing resuscitation, one state may convert to another with undifferentiated presentation, and continual reassessment is essential.^[36] Add the intensivist's monitoring moral: identifying the cause — hypovolaemic, distributive, cardiogenic or obstructive — leads to entirely different pathways; appropriate fluids save while inadequate or excessive fluids cause organ failure and death through hypovolaemia or overload; and misinterpreted monitoring adds mortality and morbidity.^[37]

Neurogenic and spinal — denervation hypotension with bradycardia

Explain the anatomy first: spinal injury disrupts descendent pathways to sympathetic neurons originating in T1-L2 intermediolateral nuclei.^[38] State the consequence: loss of supraspinal sympathetic control yields reduced sublesional sympathetic activity with unopposed parasympathetic outflow through the intact vagus.^[38] Define the two time frames: spinal shock as transitory suspension of function and reflexes below the injury in the acute phase, with neurogenic shock inside it as severe arterial hypotension plus bradycardia — versus chronic autonomic dysreflexia after resolution as massive imbalanced reflex sympathetic discharge above splanchnic outflow at T5-T6.^[38] Add the T6 rule the viva loves: hypotension both supine and orthostatic, dysreflexia and arrhythmias including persistent bradycardia are attributed to loss of supraspinal sympathetic control commonly with severe lesions at T6 or higher.^[39] Fence the numbers: MAP-85 maintenance, monitoring bundles and heparin choices are guideline recommendations owned by spinal/neurocritical topics — quote the level rule here, not the protocol. Distinguish neurogenic from hypovolaemic early after trauma by the same physiology: warm dry skin with bradycardia at or above T6 versus cold tachycardic shocked trauma — remembering both can coexist.

Anaphylaxis — hypersensitivity with distributive collapse

Define anaphylaxis in one sentence: a severe systemic hypersensitivity reaction rapid in onset, characterised by life-threatening airway, breathing and circulatory problems and usually associated with skin and mucosal changes — triggerable by minute antigen amounts.^[41] Broaden it to multisystem physiology: a severe acute life-threatening multisystem allergic reaction from a plethora of mast-cell mediators culminating in respiratory, cardiovascular and mucocutaneous manifestations that can be fatal — with medications, foods, latex, exercise, hormones, clonal mast-cell disorders, delayed red-meat reactions and hereditary alpha-tryptasaemia described.^[42] Name the manifestations that make it distributive-or-mixed shock: sudden urticaria, pruritus, flushing, erythema, angioedema of lips, tongue, airways and periphery, myocardial dysfunction with hypovolaemia and distributive-or-mixed shock plus arrhythmias, rhinitis, wheeze and stridor — plus vomiting, diarrhoea, uterine cramps, incontinence, dizziness, seizures, confusion and syncope.^[42] Teach the classical pathway stepwise: T cells with Th2 cytokines, B-cell IgE production, crosslinking of high-affinity IgE receptor FcεRI on mast cells and basophils by IgE-antigen complexes, culminating in degranulation.^[42] List the mediators that open vessels: preformed histamine, heparin, tryptase, chymase, carboxypeptidase, cathepsin G and TNF-alpha, plus de-novo leukotrienes, platelet-activating factor, cytokines and VEGF — of which histamine, tryptase, cathepsin G, TNF-alpha, LTC4, PAF and VEGF can increase vascular permeability.^[42] Close the vascular loop: mast-cell histamine and PAF activate endothelial nitric-oxide production, setting a cascade to vasodilation plus barrier dysfunction with adherens-junction opening and capillary leak.^[42] Fence treatment explicitly: adrenaline, positioning, oxygen, fluids and second-line agents with milligram orders belong to anaphylaxis and shock-surgical protocols — mechanism lives here, doses do not.

MODS — host response, endothelial driver, Marshall score

Tell the modern story first: sepsis represents a dysregulated host response to infection leading to organ dysfunction, with the pathogen triggering an exaggerated inflammatory-immune response that activates or suppresses endothelial, hormonal, bioenergetic, metabolic, immune and other pathways into circulatory plus metabolic perturbation.^[44] Name MODS as the terminus: progression to multiple organ dysfunction syndrome represents the most severe sepsis complication and markedly increases clinical mortality.^[45] Name the endothelial driver: endothelial cells regulating microcirculation plus barrier integrity across organs and tissues, with recent work underscoring endothelial function in sepsis-induced MODS development.^[45] Score it with Marshall: 692 patients admitted beyond 24 hours between 1988 and 1990 in a surgical ICU, with 7 systems defining MODS in more than half of 30 reviewed reports.^[43] List what the score kept: five systems with valid descriptors — respiratory PaO2/FiO2, renal creatinine, hepatic bilirubin, haematologic platelets, neurologic Glasgow Coma Scale — plus a new pressure-adjusted heart rate as heart rate times central-venous-to-mean-arterial-pressure ratio for the missing cardiovascular descriptor.^[43] State the examination use: maximal scores summed to 24 correlate in graded fashion with ICU mortality on admission day as prognosis and over the stay as outcome measure.

Monitoring — responsiveness before fluids, pressure components, tolerance

Open with the decision rule: in acute circulatory failure the decision to give fluids or not should not be taken lightly, because overzealous administration harms and volume expansion does not always raise output — so after the very initial phase or without obvious losses, predicting fluid responsiveness should be the first step of fluid strategy.^[46] Abandon static markers explicitly: central venous pressure as well as other static markers of preload has been used for decades, but they are not reliable — robust evidence says this traditional use should be abandoned.^[46] Quote the critical-analysis verdict: dynamic parameters should be used preferentially to static parameters to predict fluid responsiveness in ICU patients.^[47] Explain the dynamic principle: inducing short-term preload changes using heart-lung interactions, passive leg raise or small-volume infusion and observing the resulting cardiac-output effect.^[46] Name the conditions: pulse-pressure and stroke-volume variations came first but are reliable only under strict conditions; caval-diameter variations share many pulse-pressure limitations; passive leg raising is now supported by solid evidence and used more frequently; end-expiratory occlusion suits ventilated patients; and unlike traditional challenge, these tests do not cause overload.^[46] Add the waveform SR: dynamic systolic, pulse-pressure and stroke-volume changes in ventilated patients emerged as useful volume-responsiveness techniques where conventional static variables proved unreliable.^[48] Read the arterial line physiologically: invasive pressure monitoring is a cornerstone of haemodynamic assessment, with systolic, diastolic, mean and pulse-pressure components each carrying therapeutic meaning — pulse pressure as stroke-volume surrogate and responsiveness marker particularly when ventilated, diastolic pressure as vasomotor-tone reflection guiding pressor initiation, with diastolic shock index and the VNERi ratio as emerging tone and responsiveness markers.^[50] Close with tolerance: 60% of ICU patients may not respond after an initial 30 mL/kg, fluids carry complications, volume challenges are not risk-free — so use less-invasive assessment plus dynamic manoeuvres with echocardiography for a more accurate haemodynamic read.^[49]

Pressor physiology — receptors, deficiency, first-agent principle (doses fenced)

State the division of labour once: vasopressors and inotropes are vital in raising systemic vascular resistance and cardiac contractility respectively.^[54] Recite the receptor map the viva wants without prescribing: vasopressors bind adrenergic alpha-1, alpha-2, beta-1 and beta-2; vasopressin AVPR1a, AVPR1B and AVPR2; angiotensin II AG1 and AG2; and dopamine DA1 and DA2 receptors inducing vasoconstriction — with choice and dose varying by patient and practice.^[30] Price the harms physiologically: excessive vasoconstriction, organ ischaemia, hyperglycaemia, hyperlactataemia, tachycardia and tachyarrhythmias.^[30] Explain vasodilatory shock as the final common pathway for all severe shock, with sepsis the commonest primary aetiology and leading critical-illness mortality cause — while confessing its pathophysiology remains incompletely elucidated.^[29] Name vasopressin deficiency as the physiology this topic owns: deficiency of the neuropeptide hormone vasopressin seems to play a significant role in vasodilatory shock.^[29] Quote VASST as deficiency physiology, never as an order: 778 randomised patients infused with study drug — 396 vasopressin, 382 norepinephrine — included in analysis.^[28] Fence explicitly: infusion rates, titration, second-agent timing and steroid adjuncts belong to shock-surgical and perioperative-pharmacology — mechanism and deficiency live here.

Exam synthesis and fence map — what shock-physiology owns

Own the physiology chain end to end: four-category mapping, Sepsis-3 with SOFA and qSOFA, delivery-versus-demand with debt, endothelium and glycocalyx, mitochondria and cytopathic hypoxia, biphasic lactate, vasoplegia chemistry, two-phase compensation, haemorrhagic sequence, pump failure with septic cardiomyopathy and mixed states, obstructive release rule, spinal denervation, mast-cell degranulation, MODS scoring, and dynamic responsiveness with pressure-component reading.^{[1][2][9][12][17][20][25][43][46]} Cross-reference without duplicating: fluids-electrolytes owns brand and strategy numbers; acid-base owns gap, delta and alkali rules; transfusion-coagulation owns ratios, TXA and permissive targets; surgical-infection owns antibiotic choice and redosing; surgical-immunology owns steroid cover and DMARD holds plus anaphylaxis drug orders; perioperative-pharmacology owns holds and rescue doses; shock-surgical owns EGDT, pressor ladders, steroid adjunct verdicts and source-control bundles — quote only their physiology principles here.^[1][24][28] When the viva pushes for a number you were not given, say the gap aloud rather than inventing it: ATLS class percentages, spinal MAP targets as protocol, adrenaline milligram doses and revascularisation trial numbers were not verified in this pack and are not examined from it.

Exam pearls — the one-liners that score

Recite the four-compartment mapping: blood and fluids to hypovolaemic, vascular to distributive, cardiac to cardiogenic, blockage to obstructive.^[1] Recite Sepsis-3 in one breath: dysregulated host response plus SOFA 2+ above 10% mortality; shock as pressor for MAP 65+ plus lactate above 2 above 40% mortality; qSOFA as RR 22+, altered mentation, SBP 100 or less.^[2] Recite the mismatch: supply not meeting demand across every cause, with compensation masking vitals while tissue acidosis persists.^[8] Recite lactate phases: 6-12 h flow-dependent versus later metabolic — persistence without hypoperfusion must not drive resuscitation.^[20] Recite vasoplegia: pathological low resistance with normal-or-raised output, driven by NO plus peroxynitrite.^[25][26] Recite cytopathic humility: deranged energetics from impaired mitochondrial respiration, not perfusion — delivery manipulation alone doomed.^[17] Recite responsiveness: static markers unreliable, dynamic tests first, leg raise supported, overload avoided.^[46] Recite obstruction: noncardiac low output needing release, diagnosed exam to RUSH to imaging.^[35] Recite denervation versus hypersensitivity: sublesional sympathetic loss with vagal excess giving hypotension plus bradycardia — against mast-cell mediator flood giving distributive-or-mixed collapse.^[38][42] Recite MODS: dysregulated host response to MODS via endothelial failure, scored by Marshall across five descriptors plus pressure-adjusted heart rate.^[44][43]

Revision summary

Shock is one supply-demand mismatch in four anatomies with four treatments; sepsis is a dysregulated host response defined by SOFA rise with a pressor-plus-lactate shock subset; delivery failure, microvascular failure and mitochondrial failure are three different lesions; lactate is biphasic and context-dependent; vasoplegia is chemistry plus depleted responsiveness; compensation excites then inhibits and masks numbers; haemorrhage kills sequentially without stop-bleeding plus volume; cardiogenic spirals while septic myocardium stiffens diastolically and mixed states coexist; obstruction needs release; denervation slows and dilates while anaphylaxis leaks and dilates; MODS is endothelial failure scored by Marshall; and fluids follow responsiveness tested dynamically with pressure components read physiologically.^{[1][2][17][20][25][35][43][46]}

Infarct-driven cardiogenic shock kills early at about 50%; revascularisation of the culprit reduces that death.^[43]

References55ShowHide

1. [1]Standl T, et al. The Nomenclature, Definition and Distinction of Types of Shock. Dtsch Arztebl Int, 2018.PMID 30573009
2. [2]Singer M, et al. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA, 2016.PMID 26903338
3. [3]Seymour CW, et al. Assessment of Clinical Criteria for Sepsis: For the Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA, 2016.PMID 26903335
4. [4]Shankar-Hari M, et al. Developing a New Definition and Assessing New Clinical Criteria for Septic Shock: For the Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA, 2016.PMID 26903336
5. [5]Raith EP, et al. Prognostic Accuracy of the SOFA Score, SIRS Criteria, and qSOFA Score for In-Hospital Mortality Among Adults With Suspected Infection Admitted to the Intensive Care Unit. JAMA, 2017.PMID 28114553
6. [6]Rhee C, et al. Incidence and Trends of Sepsis in US Hospitals Using Clinical vs Claims Data, 2009-2014. JAMA, 2017.PMID 28903154
7. [7]Yang WS, et al. A mortality analysis of septic shock, vasoplegic shock and cryptic shock classified by the third international consensus definitions (Sepsis-3). Clin Respir J, 2020.PMID 32438528
8. [8]Cestero RF, et al. Endpoints of resuscitation. Surg Clin North Am, 2015.PMID 25814109
9. [9]Rivers EP, et al. Oxygen extraction and perfusion markers in severe sepsis and septic shock: diagnostic, therapeutic and outcome implications. Curr Opin Crit Care, 2015.PMID 26348417
10. [10]Convertino VA, et al. Physiological comparison of hemorrhagic shock and V˙ O(2)max: A conceptual framework for defining the limitation of oxygen delivery. Exp Biol Med (Maywood), 2019.PMID 31042073
11. [11]Leach RM, et al. The pulmonary physician in critical care * 2: oxygen delivery and consumption in the critically ill. Thorax, 2002.PMID 11828050
12. [12]Joffre J, et al. Endothelial Responses in Sepsis. Am J Respir Crit Care Med, 2020.PMID 32101446
13. [13]Torres Filho I. Hemorrhagic Shock and the Microvasculature. Compr Physiol, 2017.PMID 29357125
14. [14]Tuma M, et al. Trauma and Endothelial Glycocalyx: The Microcirculation Helmet? Shock, 2016.PMID 27082315
15. [15]Fernández-Sarmiento J, et al. The Endothelial Glycocalyx: A Fundamental Determinant of Vascular Permeability in Sepsis. Pediatr Crit Care Med, 2020.PMID 32132499
16. [16]Fernández-Sarmiento J, et al. Biomarkers of Glycocalyx Injury and Endothelial Activation are Associated with Clinical Outcomes in Patients with Sepsis: A Systematic Review and Meta-Analysis. J Intensive Care Med, 2023.PMID 35722738
17. [17]Fink MP. Cytopathic hypoxia. Mitochondrial dysfunction as mechanism contributing to organ dysfunction in sepsis. Crit Care Clin, 2001.PMID 11219231
18. [18]Fink MP. Cytopathic hypoxia. A concept to explain organ dysfunction in sepsis. Minerva Anestesiol, 2000.PMID 10965712
19. [19]Kohoutová M, et al. Variability of mitochondrial respiration in relation to sepsis-induced multiple organ dysfunction. Physiol Res, 2018.PMID 30607965
20. [20]Castro R, et al. What every intensivist should know about the biphasic kinetics of lactate in septic shock. J Crit Care, 2026.PMID 41101140
21. [21]Ahmad R, et al. Shock: Pathophysiology, Classification, and the Role of Lactate in Diagnosis and Prognosis. Cardiol Rev, 2026.PMID 41563115
22. [22]Levy B, et al. Lactate in cardiogenic shock: pathophysiology, prognostic value, and clinical interpretation. Ann Intensive Care, 2026.PMID 42058021
23. [23]Weinberger J, et al. What Is the Utility of Measuring Lactate Levels in Patients with Sepsis and Septic Shock? Semin Respir Crit Care Med, 2021.PMID 34544182
24. [24]Jones AE, et al. Lactate clearance vs central venous oxygen saturation as goals of early sepsis therapy: a randomized clinical trial. JAMA, 2010.PMID 20179283
25. [25]Lambden S, et al. Definitions and pathophysiology of vasoplegic shock. Crit Care, 2018.PMID 29980217
26. [26]Kimmoun A, et al. Mechanisms of vascular hyporesponsiveness in septic shock. Curr Vasc Pharmacol, 2013.PMID 23506493
27. [27]Ratnani I, et al. Vasoplegia: A Review. Methodist Debakey Cardiovasc J, 2023.PMID 37547893
28. [28]Russell JA, et al. Vasopressin versus norepinephrine infusion in patients with septic shock. N Engl J Med, 2008.PMID 18305265
29. [29]Ukor IF, et al. Vasopressin in Vasodilatory Shock. Crit Care Clin, 2019.PMID 30784607
30. [30]Russell JA. Vasopressor therapy in critically ill patients with shock. Intensive Care Med, 2019.PMID 31646370
31. [31]Thiele H, et al. Cardiogenic Shock. N Engl J Med, 2026.PMID 41467651
32. [32]Sarma D, et al. Cardiogenic Shock: Pathogenesis, Classification, and Management. Crit Care Clin, 2024.PMID 37973356
33. [33]Beesley SJ, et al. Septic Cardiomyopathy. Crit Care Med, 2018.PMID 29227368
34. [34]Walley KR. Sepsis-induced myocardial dysfunction. Curr Opin Crit Care, 2018.PMID 29846206
35. [35]Zotzmann V, et al. Obstructive Shock, from Diagnosis to Treatment. Rev Cardiovasc Med, 2022.PMID 39076909
36. [36]Khorsand S, et al. Not all Shock States Are Created Equal: A Review of the Diagnosis and Management of Septic, Hypovolemic, Cardiogenic, Obstructive, and Distributive Shock. Anesthesiol Clin, 2023.PMID 36871993
37. [37]Kashani K, et al. The Intensivist's Perspective of Shock, Volume Management, and Hemodynamic Monitoring. Clin J Am Soc Nephrol, 2022.PMID 35379765
38. [38]Popa C, et al. Vascular dysfunctions following spinal cord injury. J Med Life, 2010.PMID 20945818
39. [39]Furlan JC, et al. Cardiovascular complications after acute spinal cord injury: pathophysiology, diagnosis, and management. Neurosurg Focus, 2008.PMID 18980473
40. [40]Ranjan AK, et al. Controls of Central and Peripheral Blood Pressure and Hemorrhagic/Hypovolemic Shock. J Clin Med, 2023.PMID 36769755
41. [41]Reber LL, et al. The pathophysiology of anaphylaxis. J Allergy Clin Immunol, 2017.PMID 28780941
42. [42]Nguyen SMT, et al. Mechanisms Governing Anaphylaxis: Inflammatory Cells, Mediators, Endothelial Gap Junctions and Beyond. Int J Mol Sci, 2021.PMID 34360549
43. [43]Marshall JC, et al. Multiple organ dysfunction score: a reliable descriptor of a complex clinical outcome. Crit Care Med, 1995.PMID 7587228
44. [44]Arina P, et al. Pathophysiology of sepsis. Curr Opin Anaesthesiol, 2021.PMID 33652454
45. [45]Tang F, et al. Endothelial dysfunction: Pathophysiology and therapeutic targets for sepsis-induced multiple organ dysfunction syndrome. Biomed Pharmacother, 2024.PMID 39068853
46. [46]Monnet X, et al. Prediction of fluid responsiveness: an update. Ann Intensive Care, 2016.PMID 27858374
47. [47]Michard F, et al. Predicting fluid responsiveness in ICU patients: a critical analysis of the evidence. Chest, 2002.PMID 12065368
48. [48]Marik PE, et al. Dynamic changes in arterial waveform derived variables and fluid responsiveness in mechanically ventilated patients: a systematic review of the literature. Crit Care Med, 2009.PMID 19602972
49. [49]Shuman M, et al. The Crossroad: Fluid Responsiveness, Fluid Overload or Fluid Tolerance in the Critically Ill. Crit Care Clin, 2025.PMID 40484613
50. [50]Bertrand M, et al. Applied physiology at the bedside: using invasive blood pressure as a true monitoring tool. Ann Intensive Care, 2025.PMID 41372508
51. [51]van Diepen S, et al. Mixed Cardiogenic Shock: A Proposal for Standardized Classification, a Hemodynamic Definition, and Framework for Management. Circulation, 2024.PMID 39466889
52. [52]Chen Y, et al. Hemodynamic subphenotypes in septic shock and their different responses to fluid balance in relation to ICU mortality. BMC Anesthesiol, 2025.PMID 41444865
53. [53]Richards JE, et al. Vasopressors in Trauma: A Never Event? Anesth Analg, 2021.PMID 33908898
54. [54]Mergoum AM, et al. A Guide to the Use of Vasopressors and Inotropes for Patients in Shock. J Intensive Care Med, 2025.PMID 38613381
55. [55]Gutierrez G, et al. Clinical review: hemorrhagic shock. Crit Care, 2004.PMID 15469601