22 SHOCK, VASOPRESSORS & FLUIDS
# 23 SHOCK, VASOPRESSORS & FLUIDS
What is shock?
Shock is circulatory failure causing inadequate tissue oxygen delivery. It is not defined by blood pressure — a normotensive patient can be in shock and a hypotensive one can be well perfused. The defining problem is cellular: O₂ supply fails to meet demand, mitochondria switch to anaerobic metabolism, lactate rises, and organs fail.
➡ DO₂ = CO × (1.34 × Hb × SaO₂)
➡ CO = HR × SV; SV depends on preload, afterload, contractility
➡ MAP = CO × SVR — the equation behind every vasopressor decision
Classification — the four buckets
| Type | Mechanism | HR | CVP | SVR | CO | Classic example |
|---|---|---|---|---|---|---|
| Hypovolaemic | Loss of volume → ↓ preload | ↑ | ↓ | ↑ | ↓ | Haemorrhage, burns, GI losses |
| Cardiogenic | Pump failure | ↑ | ↑ | ↑ | ↓ | MI, arrhythmia, valve rupture |
| Obstructive | Mechanical block to filling/output | ↑ | ↑ | ↑ | ↓ | Tamponade, tension PTX, massive PE |
| Septic (early — "warm") | Vasodilation + capillary leak | ↑ | ↓ | ↓ | ↑ | Sepsis |
| Septic (late — "cold") | + myocardial depression | ↑ | variable | ↓ | ↓ | Late sepsis |
| Anaphylactic | IgE → histamine → vasodilation | ↑ | ↓ | ↓ | ↓ | Bee sting, penicillin |
| Neurogenic | Loss of sympathetic outflow | ↓ | ↓ | ↓ | ↓ | Cord injury above T6 |
👩⚕️ Two facts that unlock the topic: distributive shock is the only category with low SVR, and neurogenic is the only shock with bradycardia because the sympathetic chain itself is offline.
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Hypovolaemic shock — ATLS haemorrhage classes
Total blood volume ≈ 70 ml/kg (≈ 5 L in a 70 kg adult).
| Class | Loss | Volume | HR | BP | Pulse pressure | RR | Urine | Mental state |
|---|---|---|---|---|---|---|---|---|
| I | <15% | <750 ml | <100 | Normal | Normal | 14–20 | >30 ml/hr | Slightly anxious |
| II | 15–30% | 750–1500 | 100–120 | Normal | Narrowed | 20–30 | 20–30 | Mildly anxious |
| III | 30–40% | 1500–2000 | 120–140 | ↓ | Narrowed | 30–40 | 5–15 | Confused |
| IV | >40% | >2000 | >140 | ↓↓ | Narrowed | >35 | Negligible | Lethargic |
👩⚕️ Classic exam trap: systolic BP only falls in Class III. A young trauma patient with normal BP, tachycardia and a narrow pulse pressure has already lost up to 1.5 L. Pulse pressure narrows early because diastolic rises (↑ SVR) before systolic falls. Beta-blockers blunt the tachycardia — an elderly patient on bisoprolol can be Class III without a fast HR.
Cardiogenic shock
Pump failure: CO collapses despite adequate filling, so CVP rises. Sympathetic compensation drives SVR up — the patient is cold, clammy, oliguric, with crackles and a raised JVP. Causes: acute MI (commonest), arrhythmia, acute valve failure, myocarditis.
The therapeutic dilemma is raising CO without flogging a failing heart. Dobutamine (β1 inotrope) is first-line. Pure vasoconstrictors raise afterload and worsen pump failure. Refractory cases need IABP or ECMO.
Obstructive shock
A mechanical obstruction prevents filling or output. All three produce ↑CVP, ↑SVR, ↓CO — distinguish them by context:
➡ Cardiac tamponade — Beck's triad: raised JVP, muffled heart sounds, hypotension. Pulsus paradoxus (>10 mmHg drop in systolic on inspiration).
➡ Tension pneumothorax — tracheal deviation, absent breath sounds, hyperresonance. Treat before imaging: needle decompression in the 4th/5th ICS mid-axillary line (recently revised from 2nd ICS mid-clavicular).
➡ Massive PE — acute right heart strain, raised JVP, hypoxia, often a normal CXR.
Distributive shock
Leaky vessels, relaxed smooth muscle, plummeting SVR. MAP = CO × SVR, so even a hyperdynamic heart cannot defend pressure.
Septic shock
Sepsis 3: life-threatening organ dysfunction from a dysregulated host response to infection. Septic shock = vasopressors needed to keep MAP ≥ 65 and lactate > 2 despite fluid resuscitation.
Screening tools:
- qSOFA (bedside): RR ≥ 22, altered mentation, SBP ≤ 100. ≥ 2 = high risk.
- SIRS: temp <36 or >38, HR >90, RR >20 or PaCO₂ <4.3, WCC <4 or >12. ≥ 2 = SIRS; older definition.
Sepsis 6 (deliver within 1 hour):
| Give | Take |
|---|---|
| O₂ (SpO₂ 94–98%) | Blood cultures (before antibiotics if possible) |
| IV broad-spectrum antibiotics | Serum lactate |
| IV crystalloid (30 ml/kg) | Hourly urine output |
👩⚕️ Warm vs cold sepsis. Early sepsis is "warm" — vasodilated peripheries despite hypotension. Late sepsis turns "cold" when myocardial depression sets in. Warm + hypotensive in any stem = distributive.
Anaphylactic shock
IgE-mediated mast cell degranulation → histamine → vasodilation, capillary leak, bronchoconstriction. Onset within minutes.
➡ Adrenaline 500 micrograms (0.5 ml of 1:1000) IM anterolateral thigh, repeat every 5 min if no improvement.
➡ High-flow O₂, lie flat with legs up, IV crystalloid bolus.
➡ Adjuncts after adrenaline: chlorphenamine, hydrocortisone, salbutamol if wheezy.
👩⚕️ Dose trap: IM 1:1000 in anaphylaxis. IV 1:10 000 in cardiac arrest. IV 1:1000 adrenaline is fatal.
Neurogenic shock
Loss of descending sympathetic outflow after cord injury above T6 (sympathetics arise T1–L2). Unopposed vagal tone → bradycardia, hypotension, vasodilation and warm peripheries after trauma. Do not confuse with spinal shock — the transient flaccid areflexia distal to a cord lesion, a neurological phenomenon, not a haemodynamic one. Treat with volume, noradrenaline, ± atropine.
Preload, afterload, contractility
Every haemodynamic intervention pulls one of three levers:
➡ Preload (end-diastolic stretch) — raised by fluids; reduced by haemorrhage, venodilators, tamponade.
➡ Afterload (resistance the ventricle pumps against) — raised by vasoconstrictors; reduced by vasodilators, sepsis.
➡ Contractility — raised by inotropes (dobutamine, adrenaline, milrinone); reduced by ischaemia, acidosis, hypoxia, β-blockers.
Frank–Starling explains why fluids work in hypovolaemia — until the ventricle reaches the descending limb, where more fluid drops CO.
Vasopressors & inotropes
| Drug | Receptors | Effect | First-line in |
|---|---|---|---|
| Noradrenaline | α1 >> β1 | ↑↑ SVR, modest ↑ contractility | Septic shock, most distributive shock |
| Adrenaline | α1, β1, β2 | ↑ HR, ↑ contractility, ↑ SVR | Anaphylaxis, arrest, refractory shock |
| Dobutamine | β1 >> β2 | ↑ contractility, ↑ CO, slight ↓ SVR | Cardiogenic shock (with adequate BP) |
| Vasopressin | V1 | Pure non-adrenergic vasoconstriction | Adjunct in refractory sepsis |
| Phenylephrine | Pure α1 | ↑ SVR, reflex bradycardia | Spinal-anaesthesia hypotension |
| Milrinone | PDE-3 inhibitor | Inodilator (↑ cAMP) | Cardiogenic shock with pulmonary HTN |
👩⚕️ Pick the drug from the deficit. Low SVR with adequate CO → vasoconstrictor (noradrenaline). Low CO with adequate SVR → inotrope (dobutamine). Both low → adrenaline. Septic shock ladder: noradrenaline → vasopressin → adrenaline → hydrocortisone. Vasoactive infusions should run through a central line — peripheral extravasation of noradrenaline causes tissue necrosis.
Crystalloids, colloids and blood
| Fluid | Na⁺ | K⁺ | Cl⁻ | Other | Tonicity |
|---|---|---|---|---|---|
| 0.9% saline | 154 | 0 | 154 | — | Isotonic |
| Hartmann's | 131 | 5 | 111 | Ca²⁺ 2, lactate 29 | Isotonic |
| Plasma-Lyte | 140 | 5 | 98 | Mg, acetate, gluconate | Isotonic |
| 5% dextrose | 0 | 0 | 0 | Glucose 50 g/L | Hypotonic in vivo |
| Dextrose-saline (4%/0.18%) | 30 | 0 | 30 | Glucose 40 g/L | Hypotonic |
| Gelofusine | 154 | — | 120 | Gelatin colloid | Iso-oncotic |
➡ Crystalloid first in almost every shocked patient. Hartmann's for trauma and surgery; 0.9% saline if K⁺ is high or in TBI.
➡ Avoid Hartmann's in hyperkalaemia and severe renal failure (5 mmol/L K⁺), and through the same line as blood (Ca²⁺ chelates citrate → clots).
➡ Switch to blood once estimated loss exceeds ~1.5 L (Class III) or in ongoing bleeding. Major haemorrhage protocols use 1:1:1 RBC : FFP : platelets.
➡ Colloids confer no mortality benefit; starches (HES) are contraindicated in sepsis (AKI risk).
👩⚕️ "Normal saline is not normal." Its 154 mmol/L chloride (plasma 98–106) causes hyperchloraemic metabolic acidosis with a normal anion gap — a favourite biochemistry SBA.
Maintenance fluids — the 4-2-1 rule
For a fasting patient, replace water, sodium, potassium and glucose:
➡ Water 25–30 ml/kg/day (~2.0–2.5 L for 70 kg)
➡ Na⁺ 1 mmol/kg/day, K⁺ 1 mmol/kg/day
➡ Glucose 50–100 g/day (prevents starvation ketosis)
4-2-1 hourly: 4 ml/kg/hr for first 10 kg, 2 ml/kg/hr for next 10 kg, 1 ml/kg/hr thereafter. A 70 kg adult = (40 + 20 + 50) = 110 ml/hr ≈ 2.6 L/day.
A typical 24-hour prescription: 1 L Hartmann's + 2 L 5% dextrose with 20 mmol KCl in each bag.
Resuscitation endpoints
Stop asking "what's the BP?" — start asking "is this patient perfusing?"
➡ MAP ≥ 65 mmHg (renal and cerebral autoregulation floor)
➡ Urine output ≥ 0.5 ml/kg/hr adult, 1 in a child, 2 in an infant
➡ Falling lactate — the single best marker of restored perfusion; normalise within 6 h
➡ CRT < 2 s, warm peripheries, clear mentation, improving base deficit
👩⚕️ Sudden zero urine output in a catheterised patient = blocked catheter until proven otherwise. Flush before you escalate.
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Test yourself
A patient presents with bradycardia, low CVP, low SVR, and low cardiac output. What is the most likely type of shock?
- ((Hypovolaemic shock::Tachycardia, not bradycardia — sympathetic drive raises HR and SVR.))
- ((Cardiogenic shock::CVP and SVR are both raised; HR is up, not down.))
- ((Septic shock::Tachycardic and vasodilated, but never bradycardic.))
- ((Neurogenic shock::☑️ The only shock with bradycardia — loss of sympathetic outflow above T6.))
- ((Anaphylactic shock::Low SVR but driven by histamine, with tachycardia not bradycardia.))
👩⚕️ Neurogenic = the only shock with bradycardia. "No sympathetics, no tachycardia."
A patient is hit in the precordium. He is conscious but has a raised JVP and narrow pulse pressure. What type of shock?
- ((Hypovolaemic shock::JVP would be low — circulating volume is reduced.))
- ((Cardiogenic shock::Pump failure can raise JVP, but the mechanism here is mechanical compression.))
- ((Obstructive shock::☑️ Traumatic tamponade — Beck's triad of raised JVP, muffled sounds, hypotension.))
- ((Tension pneumothorax::Also obstructive but expect tracheal deviation, absent breath sounds, hyperresonance.))
- ((Neurogenic shock::JVP is low, and there is no spinal injury in the stem.))
👩⚕️ Narrow pulse pressure + raised JVP after blunt chest trauma = tamponade.
In which type of shock is SVR raised?
- ((Hypovolaemic shock::☑️ Compensatory sympathetic vasoconstriction raises SVR to defend perfusion pressure.))
- ((Septic shock::Inflammatory vasodilation drops SVR — the hallmark of distributive shock.))
- ((Neurogenic shock::Loss of sympathetic tone — SVR falls.))
- ((Anaphylactic shock::Histamine-driven vasodilation — SVR falls.))
- ((Early septic shock::Even "warm" sepsis has a low SVR.))
👩⚕️ SVR high in hypovolaemic, cardiogenic, obstructive; low in every distributive cause.
A 55-year-old collapses with warm peripheries, HR 124, BP 60/30, RR 34. Most likely diagnosis?
- ((Cardiac failure::Cool peripheries from compensatory vasoconstriction.))
- ((Haemorrhage::Cool, clammy, vasoconstricted — not warm.))
- ((Hypovolaemia::Cold peripheries and raised SVR.))
- ((Sepsis::☑️ Warm peripheries + profound hypotension = distributive shock from sepsis.))
- ((Pulmonary embolism::Hypoxia and pleuritic chest pain dominate, not warm peripheries.))
👩⚕️ Warm + hypotensive = distributive (sepsis or anaphylaxis).
A postoperative patient becomes restless. Urine output over 5 hours: 80, 80, 80, 0, 0, 0 ml. Most likely cause?
- ((Hypovolaemia::Causes a gradual decline, not an abrupt cliff edge.))
- ((Acute kidney injury::Typically progressive oliguria, not a sudden cessation.))
- ((Blocked catheter::☑️ Abrupt fall from 80 to 0 ml/hr in a catheterised patient — mechanical until proven otherwise.))
- ((Sepsis::Would expect fever, tachycardia and gradual oliguria.))
- ((Urinary retention::Cannot retain past an in-situ catheter unless it is blocked.))
👩⚕️ Flush the catheter before you call the renal team.
A 58-year-old post-appendicectomy develops sepsis. Expected HR, SVR, CO?
- ((↓HR, ↓SVR, ↑CO::Sepsis causes tachycardia, not bradycardia.))
- ((↑HR, ↑SVR, ↓CO::This is hypovolaemic or cardiogenic — sepsis vasodilates.))
- ((↑HR, ↓SVR, ↑CO::☑️ Early "warm" sepsis: low SVR, tachycardia and high output compensate.))
- ((↓HR, ↑SVR, ↓CO::Late cardiogenic or neurogenic patterns — not sepsis.))
- ((↑HR, ↓SVR, ↓CO::Late "cold" sepsis with myocardial depression — not the early profile.))
👩⚕️ Warm shock first, cold shock when the heart gives up.
A trauma patient is restless, thirsty, with cool peripheries, oliguria and hypotension. Profile of hypovolaemic shock?
- ((↑HR, ↓SVR, ↑CO::Profile of early septic shock — hypovolaemia raises SVR.))
- ((↓HR, ↓SVR, ↓CO::Neurogenic shock pattern.))
- ((↑HR, ↑SVR, ↓CO::☑️ Tachycardia, vasoconstriction, low CO from inadequate preload.))
- ((↑HR, ↑SVR, ↑CO::CO cannot rise without circulating volume.))
- ((↓HR, ↑SVR, ↑CO::Sympathetic response drives tachycardia, not bradycardia.))
👩⚕️ Cool peripheries and prolonged CRT are bedside clues to a high SVR.
First-line vasopressor for septic shock with low SVR?
- ((Adrenaline::Second-line — added when noradrenaline alone fails; risks tachyarrhythmia.))
- ((Dobutamine::An inotrope, not a vasoconstrictor — may worsen hypotension.))
- ((Noradrenaline::☑️ α1 agonist — first-line per Surviving Sepsis Campaign.))
- ((Vasopressin::Adjunct in refractory cases, not monotherapy.))
- ((Phenylephrine::Pure α1; may reduce CO and is not first-line in sepsis.))
👩⚕️ Septic shock ladder: noradrenaline → vasopressin → adrenaline → steroids.
What defines hypotension in an adult (ATLS)?
- ((Systolic BP < 100 mmHg::Pre-shock threshold for qSOFA, not ATLS.))
- ((MAP < 65 mmHg::Defines inadequate perfusion in ITU, not the ATLS cut-off.))
- ((Systolic BP < 90 mmHg::☑️ Standard ATLS and MRCS definition of hypotension.))
- ((Systolic BP < 80 mmHg::Severe hypotension, but the threshold is 90.))
- ((Diastolic BP < 60 mmHg::Diastolic alone is not used to define hypotension.))
Target urine output in a resuscitated adult?
- ((> 2 ml/kg/hr::Infant target, not adult.))
- ((> 1 ml/kg/hr::Paediatric target.))
- ((0.5–1 ml/kg/hr::☑️ Adult target — roughly 30–50 ml/hr in a 70 kg patient.))
- ((> 100 ml/hr::Supranormal — not a standard target.))
- ((0.2 ml/kg/hr::This is oliguria — inadequate resuscitation.))
👩⚕️ 0.5 ml/kg/hr adult, 1 ml/kg/hr child, 2 ml/kg/hr infant.
What percentage of body weight is water in an adult male?
- ((40%::That's intracellular fluid alone.))
- ((50%::Closer to elderly or obese values.))
- ((60%::☑️ The "60-40-20" rule — 60% TBW, 40% ICF, 20% ECF.))
- ((70%::Applies to infants.))
- ((75%::Overestimates adult TBW.))
👩⚕️ Women and the elderly hold less — more fat, less water.
Daily maintenance fluid for a 70 kg adult?
- ((1500 ml::Inadequate — under 25 ml/kg/day.))
- ((2000 ml::Slightly under the 25–30 ml/kg/day target.))
- ((2500 ml::☑️ 25–30 ml/kg/day; plus 1 mmol/kg Na⁺, 1 mmol/kg K⁺, 50–100 g glucose.))
- ((3500 ml::Excess — risks iatrogenic overload.))
- ((4000 ml::Frankly excessive maintenance prescription.))
👩⚕️ 4-2-1 rule per hour ≈ 110 ml/hr for a 70 kg adult.
A 70 kg woman with 45% TBSA burns. First 8 hours of Hartmann's by Parkland?
- ((3150 ml::That's a quarter of total — Parkland gives half in the first 8 hours.))
- ((6300 ml::☑️ 4 × 70 × 45 = 12 600 ml total; half (6300 ml) in the first 8 h from time of burn.))
- ((12 600 ml::That's the full 24-hour volume, not the first 8 hours.))
- ((4200 ml::Uses an incorrect multiplier of 3, not 4.))
- ((8400 ml::Incorrect two-thirds split — Parkland is half/half.))
👩⚕️ Parkland = 4 × kg × %TBSA, half in first 8 h from time of burn. Hartmann's, not saline.
Na⁺ content of 0.9% saline?
- ((140 mmol/L::That's plasma sodium, not saline.))
- ((131 mmol/L::That's Hartmann's.))
- ((154 mmol/L::☑️ 154 Na⁺ and 154 Cl⁻ — supraphysiological chloride causes hyperchloraemic acidosis.))
- ((120 mmol/L::Hyponatraemic and not a standard fluid.))
- ((170 mmol/L::Exceeds any standard isotonic crystalloid.))
👩⚕️ "Normal" saline isn't physiological — its chloride is 50% above plasma.
Na⁺ content of Hartmann's solution?
- ((154 mmol/L::Normal saline value.))
- ((131 mmol/L::☑️ Closer to plasma — the "balanced" crystalloid.))
- ((140 mmol/L::Plasma sodium, not Hartmann's.))
- ((120 mmol/L::Too low for Hartmann's.))
- ((145 mmol/L::Upper plasma reference, not Hartmann's.))
K⁺ content of Hartmann's solution?
- ((0 mmol/L::That's normal saline.))
- ((2 mmol/L::Underestimates Hartmann's K⁺.))
- ((5 mmol/L::☑️ 5 mmol/L K⁺ — avoid in hyperkalaemia and renal failure.))
- ((10 mmol/L::Dangerously high; not Hartmann's.))
- ((20 mmol/L::Concentration in K⁺ replacement bags, not Hartmann's.))
👩⚕️ Hartmann's K⁺ = 5 — switch to saline if K⁺ is high or in established renal failure.
Which IV fluid is isotonic and closest in composition to plasma?
- ((0.9% saline::Isotonic but no K⁺, no Ca²⁺, no buffer — and excess chloride.))
- ((5% dextrose::Isotonic in the bag, hypotonic in vivo once glucose is metabolised.))
- ((Hartmann's solution::☑️ Na 131, K 5, Ca 2, Cl 111, lactate 29 — the "plasma-lite" crystalloid.))
- ((Dextrose-saline::Hypotonic with only 30 mmol/L Na⁺.))
- ((Gelofusine::A colloid plasma expander, not a balanced crystalloid.))
👩⚕️ Hartmann's is the balanced default for surgery and trauma; saline for hyperkalaemia and TBI.
Revision summary
➡ Shock = inadequate tissue O₂ delivery. DO₂ = CO × Hb × SaO₂; CO = HR × SV; MAP = CO × SVR.
➡ Four buckets: hypovolaemic, cardiogenic, obstructive, distributive. Distributive = the only low-SVR group.
➡ Neurogenic = the only shock with bradycardia. Cord injury above T6.
➡ ATLS I–IV: 15 / 30 / 40 / >40% blood loss. Systolic BP drops only in Class III. Pulse pressure narrows in Class II.
➡ Tamponade: Beck's triad — raised JVP, muffled sounds, hypotension. Pulsus paradoxus.
➡ Sepsis 6 within 1 hour: O₂, IV antibiotics, IV fluids, blood cultures, lactate, urine output.
➡ Anaphylaxis: adrenaline 0.5 mg IM (1:1000), repeat every 5 min. Not IV.
➡ Vasopressors: noradrenaline first-line in sepsis; dobutamine in cardiogenic; adrenaline in anaphylaxis/arrest; vasopressin as adjunct.
➡ Hartmann's = balanced (Na 131, K 5). 0.9% saline = Na/Cl 154 → hyperchloraemic acidosis.
➡ Avoid Hartmann's in hyperkalaemia, renal failure, and through the same line as blood.
➡ Maintenance: 25–30 ml/kg/day water, 1 mmol/kg Na⁺ and K⁺, 50–100 g glucose. 4-2-1 hourly.
➡ Parkland: 4 × kg × %TBSA, half in first 8 h from time of burn.
➡ Endpoints: MAP ≥ 65, urine ≥ 0.5 ml/kg/hr, falling lactate, warm peripheries.