Shock

 Shock = flow + content

=> Shock = inadequate FLOW × inadequate CONTENT → tissue hypoxia

In shock: First open pressure -> Then restore flow -> Then optimise content -> Then trust microcirculation — not the monitor. 

=> SHOCK = FLOW × CONTENT FAILURE => Pressure opens the gate. Flow saves organs

What shock is NOT

• Not just MAP <65

• Not just hypotension

• Not just vasopressor deficiency

What shock ACTUALLY is

• Failure of oxygen delivery to meet tissue demand

• Mathematically: DO₂ = Cardiac Output × CaO₂

• So shock = FLOW problem ± CONTENT problem

DO₂ = CO × (Hb × SaO₂ × 1.34)

• Lungs don’t deliver oxygen.

• Blood flow delivers oxygen. Blood + flow fixes this.

• PaO2 is not that significant because the multiplication factor is too low: Actual CaO2 = (Hb x SaO2 x 1.34) + (PaO2 x 0.003)

Tissue hypoxia determinants (in order of importance) are: 

• 1. Cardiac output

  2. Hemoglobin

  3. Oxygen saturation

  4. PaO₂ (almost irrelevant)

 Shock exists if ANYONE is present:

1. Lactate ≥ 2 or rising

2. CRT > 3 sec

3. Cold/mottled peripheries

4. Altered mentation

5. UOP < 0.5 ml/kg/hr

=> Normal BP does NOT exclude shock

Management

Phase 1 (fix pressure and flow) – MACRO PERFUSION (FIRST 30–60 min): 

Goal: Push oxygen to organs at any cost

Targets:

1. MAP ≥ 65 (only as an entry point)

2. SBP ≥ 90

3. Improving mental status

4. Improving urine output trend

5. Capillary refill < 3 sec

Tools:

1. Fluids

2. Vasopressors

3. Inotropes

4. Early blood if needed

MAP is a gatekeeper, not the endpoint

Phase 2 – MICRO PERFUSION (REAL RESUSCITATION)

Once MAP is “acceptable”: check hands, knees, and colour – CRT, temperature, mottling.

If they’re abnormal, the shock is not over, whatever the MAP says

Targets:

1. CRT (index finger pulp or anterior knee) normalisation - main tool. Prolonged CRT (finger 3s, knee 4s) = poor microcirculatory flow, even if MAP is fine. Serial trend every 30–60 min in shock.

Target: CRT normalisation as a resuscitation endpoint.

2. Lactate clearance

3. Skin temperature (warm vs cold shock). Ask three questions with your hands:

   1. 1. Are the hands/feet cold compared to your own? Cold, clammy extremities = vasoconstriction, poor peripheral flow.

      2. Forearm–finger gradient: Feel mid-forearm → fingertip. Normal: roughly the same temperature. Abnormal: clearly colder fingers = bad peripheral perfusion.

      3. Central–peripheral gradient: Warm chest/abdomen but cold hands/feet → classic ongoing shock pattern.

4. Mottling: Mottling – knee is your window. Look around the knees for patchy, purple, net-like discoloration. Grade it (simple version): 0 – no mottling, 1 – small patch around patella, 2 – up to mid-thigh, 3+ – extensive mottling above mid-thigh, More extension = worse perfusion and worse prognosis. Use in practice: “Mottling score” at admission → then after fluids/pressors. Goal: mottling reduced or gone as shock improves.

5. Urine output trend (not instant)

6. Device-based peripheral marker: Peripheral Perfusion Index (PPI) from pulse ox (Low PPI = high vasoconstriction / poor peripheral perfusion. <1–1.4 → impaired peripheral flow. Trend is more important than any one number. Use like CRT: document and trend with resuscitation.

Tools:

1. Inotrope optimisation

2. Flow optimisation (CO)

3. Hb correction

4. Acid–base correction (Acidosis converts shock from a flow problem to cellular failure, even if MAP is “normal”): 

   1. 1. 1. • Acidosis = capillary shutdown

            • Metabolic acidosis ↓ vascular smooth muscle response to catecholamines

            • Acidic pH stiffens the RBC membrane. RBCs can’t pass narrow capillaries → functional shunt

            • Mitochondrial inhibition. Low pH inhibits oxidative enzymes. Oxygen reaches the tissue,  but cannot be used. This is cytopathic hypoxia

            • Acid–base effect ladder

              1. 1. pH < 7.25

                    • Catecholamines fail

                    • Microperfusion collapses

                 2. pH < 7.15

                    • Myocardium underperforms

                    • Vasopressor resistance

                 3. pH < 7.10

                    • ATP generation impaired

                    • Oxygen utilisation fails

 If lactate is not clearing, shock is still ongoing, MAP be damned.

FLOW > PRESSURE (MOST IMPORTANT SHIFT)

FLOW = CARDIAC OUTPUT

CO = HR × Stroke Volume

Stroke volume depends on:

• Preload (fluids)

• Contractility (inotropes)

Afterload (vasopressors matter here)

Pressors raise MAP

Inotropes raise FLOW

When MAP is okay, but shock persists, Think:

• Low CO

• High SVR

• Poor microcirculation

Classic mistake: adding norepi endlessly

Correct move: add dobutamine/epi/milrinone

CONTENT = CaO₂ (OXYGEN CARGO)

Order of importance (MEMORISE THIS)

1. Hemoglobin

2. SaO₂

3. PaO₂ (least important)

Transfuse if:

• Hb <7 routinely

• Hb <8 if:

  • 1. Shock ongoing

    2. Rising lactate

    3. Myocardial ischemia

    4. TBI / stroke

    5. Persistent tachycardia

Strategies:

STEP 1: Confirm shock

STEP 2: Fix MACRO

STEP 3: Classify shock phenotype

STEP 4: Fix FLOW

STEP 5: Fix CONTENT

STEP 6: MICRO endpoints (MOST IMPORTANT)

Strategies:

Shock is not low pressure.

Shock is low flow × low content.

Resuscitate flow first, then content, then microcirculation.

STEP 1: Confirm shock;  

• Hypotension or

• Lactate ≥2 or

• Poor perfusion despite normal BP

STEP 2: Fix MACRO

• Fluids (guided, not blind)

• Norepinephrine → MAP ≥65

STEP 3: Classify shock phenotype

• Cold shock => High SVR, low CO, cold extremities => Inotrope

• Warm shock => Low SVR, adequate CO => Vasopressor

• Mixed shock => Both => Pressor + inotrope

STEP 4: Fix FLOW

• Is CO adequate?

• Is HR appropriate?

• Is SVR too high?

STEP 5: Fix CONTENT

• Correct anemia

• Ensure SaO₂ >90%

• Avoid hyperoxia unless needed

STEP 6: MICRO endpoints (MOST IMPORTANT)

Aim for:

• CRT < 3 sec

• Lactate clearance ≥10–20% / 2–4 hrs

• Warm peripheries

• Improving UOP

MAP chasing after this point is wrong.

VENTILATOR WEANING FAILS IN SHOCK because

• Metabolic alkalosis ↓ respiratory drive

• Low DO₂ → lactic acidosis

• Poor tissue perfusion keeps patient tachypneic

Fix flow + content, not ventilator numbers.

Cardiac output

Cardiac Output 

Monitoring Priority

1. Clinical perfusion

2. Lactate trend

3. Echo (VTI, LV function)

4. SCvO₂

5. CVP trend

6. Advanced monitors

Clinical Indicators of CO 

1. Mental status (brain is flow-sensitive)

2. Urine output trend (delayed but reliable)

3. Skin temperature & gradient

4. Capillary refill time (CRT)

5. CRT >3 sec = low flow until proven otherwise

6. Pulse pressure

   1. • Narrow → low SV

      • Wide → good SV / vasodilatory state

 If these improve → CO improving

If MAP is okay, but these are bad → CO failure

FLOW FAILURE — FOUR MECHANISMS

1. Preload failure → fluids (stop if no perfusion response)

2. Contractility failure → inotrope

3. Excess afterload → reduce pressor + add inotrope

4. Rhythm failure → fix rhythm first

How do we know afterload is excessive? 

• THE CORE CLUE -> MAP is high or “okay”, but perfusion is worsening (Cold, clammy extremities, Prolonged CRT (>3 sec), Oliguria or falling UOP, & Rising lactate).  - That is afterload shock until proven otherwise.

• Narrow pulse pressure = low stroke volume

PP < 25–30 mmHg → SV suppressed (Often due to high SVR)

=> Pressors raise MAP mainly by increasing diastolic pressure, narrowing PP.

• Echo pattern of excess afterload: 

  • • • • LV looks tense, not empty

        • Walls contract, but cavity empties poorly

        • No LV dilatation (unlike pure cardiogenic shock)

        • Sometimes EF looks “okay”, but the flow is poor

        • Classic mistake: “EF is 55%, so heart is fine” => No — the heart is pushing against a clamp.

• ScvO₂ behaviour (functional proof))

Key clue: ScvO₂ falls as norepinephrine dose increases (MAP improves, ScvO₂ worsens). This means: CO fell due to afterload rise (Tissues extracting more oxygen)

It's Pressor-induced low flow.

• Lactate paradox

In excess afterload: Lactate rises despite MAP ≥65 Or lactate stops clearing after initial improvement

Interpretation: You opened the gate, then closed the pipes.

• Time-course clue (very reliable)

Afterload excess typically appears: 6–24 hours into septic shock

After repeated NE escalation. When clinicians chase MAP instead of perfusion

• • • Early sepsis → vasodilation

    • Late sepsis → pressor overcorrection

Excess afterload is 

• Not hypovolemia (IVC not collapsing, fluids don’t help)

• Not primary cardiogenic shock (LV not grossly failing)

• Not “refractory sepsis”

• It is iatrogenic physiology

How to confirm excess after load?

Ask these 4 questions:

1. MAP >75?

2. Pulse pressure narrow?

3. Extremities cold + CRT prolonged?

4. Lactate rising or static?

If YES to ≥3 → afterload excess

 Correct move: 

1. Reduce vasopressor dose slightly

2. Add inotrope (dobutamine / low-dose epinephrine)

3. Aim to improve forward flow, not MAP

Cold patient + good MAP = suspect afterload excess.

Excess afterload is diagnosed by good pressure + poor perfusion

The treatment is less clamp, more push

Echo: “Is CO low?” and “Why is it low?” It checks

1. LV contractility

   1. • Eyeballing EF is enough in shock

      • Hyperdynamic vs depressed heart

2. Stroke volume (LVOT VTI)

   1. • Low VTI (<18 cm) = low CO

      • Rising VTI = improving CO

3.  IVC dynamics (supportive, not absolute)

   1. • Collapsible → preload responsive

      • Plethoric → congestion / RV failure

Summary 1:

1. Fix flow first

   1. • Adequate preload

      • Appropriate inotrope if CO is low

      • Avoid vasoconstrictor overkill

2. Fix the cause of acidosis

   1. • Control sepsis

      • Stop hypoxia

      • Improve perfusion

3. Ventilate intelligently

   1. • Avoid respiratory alkalosis

      • PaCO₂ ~35–40 unless ICP issue

4. Track microperfusion

   1. • CRT

      • Lactate clearance

      • Urine output

      • Mottling score

In shock: First open pressure -> Then restore flow -> Then optimise content ->Then trust microcirculation — not the monitor.

Do NOT chase ABG numbers blindly

Do NOT treat MAP alone. MAP opens arteries. pH decides whether capillaries live or die.

Do NOT over-ventilate shock patients

MAP tells you the pressure.

CO tells you the flow.

Perfusion tells you the truth.

SpO2 and SaO2:

1. SpO₂ = estimate. Peripheral oxygen saturation. estimated by pulse oxi. its a surrogate, not a measurement

2. SaO₂ = truth. arterial oxygen saturation. measured by ABG + co-oximetry. Gold standard

Why they differ => 

• SpO₂ assumes normal physiology & SaO₂ does not assume — it measures. Cold Shock (peripheral constriction, and motion artefact) make SpO₂ LIE

• Pulse oximeter cannot differentiate Carboxy-Hb & Met-Hb. ABG with co-oximetry: Gives real SaO₂. => CO poisoning → SpO₂ normal, SaO₂ low

SpO₂ is enough in patients with good perfusion

When SaO₂ is mandatory in 

• Shock

• ARDS

• CO poisoning

• Methemoglobinemia

• Low Hb / high lactate mismatch

• Any patient where decisions depend on CaO₂

SpO₂ is for monitoring. SaO₂ is for decision-making.

Summary 2

1) STEP 1 - Shock exists if ANYONE is present: Lactate ≥ 2 or rising, CRT > 3 sec, Cold/mottled peripheries, Altered mentation, UOP < 0.5 ml/kg/hr (remember Normal BP does NOT exclude shock)

2) STEP 2 - OPEN THE PRESSURE GATE (Once): Target (entry only, not destination): MAP ≈ 65 mmHg (or SBP ≈ 90), Tool: Norepinephrine ± fluids (Do NOT chase MAP beyond this point)

3) STEP 3 — THINK FLOW (CORE STEP): Ask 3 Questions. Is cardiac output adequate? 2. If low — WHY? 3. Will fluids help or harm?

4) STEP 4 - A. PRELOAD problem? 

• Low CVP / dynamic response → fluid challenge (250–500 ml), 

• Reassess CRT, pulse pressure, and echo

• Stop fluids if: CVP rising, pulmonary congestion, or no perfusion improvement

B. CONTRACTILITY problem? (commonly missed)

• Suspect when: MAP ok, but shock persists, Cold extremities, Narrow pulse pressure, Poor LV on echo / low VTI, or Lactate rising

• Add INOTROPE: Dobutamine → low CO, high SVR. Low-dose epinephrine → septic myocardial depression, Milrinone → RV failure / PHTN (Escalating norepinephrine alone worsens flow)

C. AFTERLOAD excess?: (High MAP + cold peripheries + low pulse pressure) -> Reduce pressor + Add inotrope => Restore organ flow

D. RHYTHM problem?: AF fast / VT / severe brady => Fix rhythm (You cannot resuscitate shock with a bad rhythm).

5) STEP 5 — FIX CONTENT (CaO₂ = OXYGEN CARGO)

Order of importance (NON-NEGOTIABLE): 1st. Hb, 2nd.SaO₂, 3rd. PaO₂ (least important)

Practical transfusion logic: 

• Hb < 7 → transfuse

• Hb < 8 with shock/lactate/MI/TBI → transfuse

Improving Hb improves DO₂ more than increasing FiO₂

6) STEP 6 — MICRO-PERFUSION = FINAL JUDGE

Success = CRT < 3 sec, Lactate ↓ 10–20% in 2–4 hr, Warm extremities, Improving mentation & UOP

 MAP normal, but lactate rising = shock ongoing

7) Be cautious about common errors

• “MAP is 72, shock-treated” 

• “More fluids despite CVP 18 + oliguria” 

• “Pressor escalation without inotrope” 

• “Perfect SpO₂ but Hb 6 ignored” 

Rule: Open pressure → restore FLOW → correct CONTENT → trust MICRO-perfusion.

Pitfalls in shock management

• Treating SpO₂ without checking Hb

• Ignoring lactate because “SpO₂ is 99”

• Trusting SpO₂ in cold shock

• Missing CO poisoning

Microcirculation Tools;

MAP tells you pressure. Microcirculation tells you whether blood actually reaches tissue. 

1. TEMPERATURE GRADIENTS (HAND-BASED, ZERO COST)

Physiology

In shock: Sympathetic vasoconstriction shunts blood to heart/brain -> Skin perfusion drops early -> The temperature difference appears before BP falls => So temperature gradient = peripheral flow meter

How to examine; Use your own hands — not thermometers.

A. Peripheral–central gradient; Compare chest/abdomen vs hands/feet

Normal → similar warmth

Abnormal → warm core, cold extremities

=> Classic ongoing shock

B. Forearm–finger gradient (VERY sensitive)

Feel mid-forearm → fingertip

Normal → same temperature

bnormal → fingers clearly colder

=>Early microcirculatory failure

C. Bilateral comparison

One limb colder → think vascular obstruction / arterial line artefact

Interpretation

Note; Cold hands + MAP 70 = shock

2️. MOTTLING SCORE (KNEE WINDOW)

Physiology

Mottling reflects: Patchy vasoconstriction, Heterogeneous capillary flow, Severe microcirculatory shutdown -> It correlates directly with mortality in septic shock.

Where to look: Around the knees (best window), Also thighs, sometimes abdomen

Knees are reliable because: Thin skin, Poor collateral flow, & Early vasoconstriction zone

How to score (simple bedside version)

0 No mottling

1 Small patch around patella

2 Extends to mid-thigh

3 Above mid-thigh

4–5 Extensive, bilateral

Higher = worse perfusion + worse prognosis

How to use it clinically; Document at admission

Recheck after fluids/pressors/inotropes

Goal: mottling reduces or disappears

Persistent or worsening mottling = resuscitation failure. Even if MAP and labs look “better”

3️. PPI — Peripheral Perfusion Index (PULSE OX DERIVED)

PPI = ratio of: pulsatile blood flow ÷ non-pulsatile flow

In simple words: How strong is the arterial pulse at the periphery

(yes, it's a fancy CRT)

Physiology

Vasoconstriction → pulsatile flow drops → PPI falls

Vasodilation / improved flow → PPI rises

Typical values (approximate)

PPI Interpretation

> 2 Good peripheral perfusion

1–2 Borderline

< 1–1.4 Poor peripheral perfusion

Trend matters more than absolute number

How to use PPI in shock-> Record baseline. Recheck after: Fluids, Inotropes, & Pressor adjustment

PPI rises → microflow improving

MAP ↑ but PPI ↓ → afterload excess

Microperfusion tools tell you

1. Temperature gradient. Is blood reaching skin?

2. Mottling . Is microflow heterogeneous / failing?

3. PPI. Is pulsatile flow adequate?

Summary 3

1️. OPEN PRESSURE — ENTRY ONLY, NOT DESTINATION

Key truths: MAP allows perfusion; it does not guarantee perfusion

Different organs need different pressures: Brain / kidney usually fine at MAP ~65Chronic          HTN may need 70–75 (individualise)

Rule: Use the lowest MAP that improves micro-perfusion

If CRT, urine, and mentation improve at MAP 62–65 → stop escalating.

Caution: More pressor → ↑ SVR → ↓ CO → worse microflow

2. RESTORE FLOW — THIS IS WHERE SHOCK IS WON OR LOST

FLOW FAILS IN 4 COMMON WAYS 

1. Underfilled (Preload failure)

   • • Flat IVC

     • Tachycardia responds to fluid (Fluid challenge → reassess)

Caution: Stop once perfusion doesn’t improve

2. Weak pump (Contractility failure)

• Septic myocardial depression; Cardiogenic components in sepsis (very common after 6–12 hrs)

Clue: Cold periphery + narrow pulse pressure + MAP ok => think CO failure

Rule: Add inotrope

Caution: Don’t keep increasing norepi

3. Excess clamp (Afterload failure)

• Pressor-induced shock (MAP 80–90 with worsening perfusion)

Rule: Reduce vasopressor & Add inotrope (Flow improves → lactate falls)

4. Rhythm chaos: AF fast most common

Rule: Rate/rhythm first

Caution: You cannot resuscitate through an arrhythmia

3️. CORRECT CONTENT — OXYGEN CARGO MATH (CLINICALLY)

CaO₂ priorities (never debate this again)

1st: Hb

2nd: SaO₂

3rd: PaO₂

Truths: Hb 7 → CaO₂ ~9, 

          Hb 10 → CaO₂ ~13 (40% jump!)

A unit of blood often gives more DO₂ than turning FiO₂ to 100%

Watch-outs: Hyperoxia → vasoconstriction → ↓ microflow

SpO₂ 92–96% is enough unless in special situations

DO₂ is estimable indirectly at the bedside.  DO₂ is not measured by machines; it is judged by tissue response. 

DO₂ = Cardiac Output × CaO₂

DO2 = Flow (CO) x Content (CaO₂)

DO₂ (ml/min) = CO × Hb × SaO₂ × 1.34 × 10. * (×10 because Hb is per dL)

DO2 monitoring:  If DO₂ < VO₂ → lactate (best marker) rises. Clearing lactate → DO₂ restored

DO2: You don’t measure oxygen going in — You judge oxygen being used. The following (microperfusions ) reflect effective DO₂: 1) CRT 2) Skin temperature 3) Mentation, 4) Urine output trend

Oxygen extraction ratio => DO2-VO2 curve.  (DO2 = oxygen delivery, VO2 = oxygen consumption, not delivery = SaO2-SvO2. or VO₂ = CO × (CaO₂ − CvO₂). VO₂ depends on blood flow and how much oxygen tissues extract). So DO₂ = oxygen "sent". VO₂ = oxygen "burned". Normal adult VO₂ ≈ 250 mL O₂/min. VO2 ↑ with sepsis and ↓ with sedation.

You cannot measure VO₂ directly. You infer it using: ↓ VO₂ clues (Rising lactate/Organ dysfunction/Poor capillary refill). ↑ VO₂ strategies: Treat fever, Control agitation, Sedate when appropriate, Treat pain, Stop shivering

Reducing VO₂ is as important as improving DO₂ in shock. In shock, survival depends on improving DO₂ and reducing VO₂. Oxygen-delivery keeps organs alive, & Oxygen-consumption makes cells live

DO₂–VO₂ curve; (You don’t draw the curve. You perform a physiologic challenge): There are two zones. 

1. Supply independent, where DO₂ is adequate, VO₂ is constant (Tissues extract what they need -> Lactate normal & CRT normal) = > Increasing DO₂ does nothing. 

2. Supply dependent (this is shock); VO₂ falls when DO₂ falls; Anaerobic metabolism -> Lactate rises -> Organ dysfunction begins. => Here Increasing DO₂ improves VO₂ and lactate. 

The bedside physiological challenges to test the DO2 curve are (Response = proof of DO₂ dependency)

1. Fluid bolus: Lactate ↓ / CRT improves → patient was DO₂-dependent

2. Inotrope escalation; Warm peripheries + better urine → flow was limiting

3. Blood transfusion; Tachycardia settles + lactate improves → content (CaO₂) was limiting

4. Lactate; Normal (supply independent), Rising (entering supply dependent), falling (back towards safe zone)

Note; CRT and lactate beat numbers

If DO₂ were normal, lactate would fall, CRT would normalise, and urine would flow. If they don’t — DO₂ is still inadequate.

The DO2 checklist

1. Is CO adequate?

2. Is Hb enough?

3. Is SaO₂ reasonable (90–96%)?

4. Is lactate clearing?

5. Is CRT improving?

4️. TRUST MICRO-PERFUSION — THE FINAL DECISION MAKER

Bedside micro-perfusion > labs

1. CRT: Best early perfusion marker

2. Temperature gradient: Flow vs clamp

3. Skin mottling: Shock severity

4. Lactate trend: Global DO₂ balance

5. UOP trend: Renal flow recovery

Rule: If lactate is falling and CRT is normalising — you’re winning, ignore MAP anxiety

5️. TIMING PEARLS 

• First 1 hour: Pressure + early flow + early content correction

• Next 6 hours: Myocardial depression often appears (Need inotrope reassessment)

• After 12–24 hours: Vasoplegia + high output low extraction states are common (High ScvO₂ can be misleading)

6️. LACTATE — USE IT RIGHT

Lactate rise ≠ hypoxia only

Caused by: 

• Low CO 

• Anemia 

• β-agonists

• Mito dysfunction

Target: Downtrend > absolute number

If pressure is good but the patient is cold, lactate rising, urine poor — you are treating numbers, not shock. Add flow, not pressure.

Open pressure → restore FLOW → correct CONTENT → trust MICRO-perfusion.

Pressure permits life, flow sustains life, micro-perfusion proves it.

DO₂–VO₂ MISMATCH — CLINICAL CASES 

CASE 1 (Extraction failure case): Septic shock, MAP 70, SpO₂ 99, lactate 6

What juniors think: oxygenation is fine. Reality: DO₂ ≠ VO₂ match

• Mechanism: CO may be okay, CaO₂ acceptable, but Microcirculatory shunting + Mitochondrial dysfunction. Oxygen delivered but not extracted => high lactate = extraction failure

• Action: Stop pressor escalation, Optimise flow (gentle inotrope), Reduce VO₂ (sedation, antipyretics), & Source control

CASE 2 (content failure case): Trauma + Hb 6.5, SpO₂ 100%, lactate rising

Mechanism: CaO₂ is critically low. Even normal CO can’t compensate. VO₂ maintained initially → then crashes

Action: Blood first, Not more oxygen or Not more fluids

CASE 3 (flow failure case from excess afterload): Cardiogenic shock, MAP 75 on high norepi

Mechanism: High SVR → low stroke volume -> CO falls -> VO₂ becomes DO₂-dependent

Action; Reduce pressor, Add inotrope, Improve forward flow

WHY FEVER KILLS SHOCK PATIENTS

• For every 1°C increase → VO₂ ↑ by ~10–13%

What that means in shock

• DO₂ already limited

• Fever raises metabolic demand

• Patient crosses critical DO₂ threshold faster

  • • Lactate rises

    • Myocardial ischemia

    • AKI worsens

Truth; Fever is not benign in shock.

Actions; Early paracetamol, Active cooling if needed, Treat infection source

Don’t “observe” fever in shock; You’re not treating temperature — you’re protecting VO₂ margin.

WHY SEDATION SOMETIMES SAVES KIDNEYS

Sedation does ALL of these:

• ↓ VO₂ (less metabolic demand)

• ↓ Sympathetic tone

• ↓ Tachycardia

• ↓ Afterload

• ↓ Work of breathing

Result:

• Better CO

• Better renal perfusion

• ↓ lactate

Classic scenario; Septic patient, Agitated, tachypneic, high work of breathing, Rising creatinine => Intubate + sedate → UOP improves in 6–12 hrs. Why? => You reduced VO₂ enough to rebalance DO₂–VO₂

Sedation is not “comfort care” in shock. It is metabolic resuscitation.

 VO₂ MANIPULATION IN ICU — THE MISSING HALF OF RESUSCITATION

Most doctors only push DO₂. Smart ICU work also pulls VO₂ down.

Ways to REDUCE VO₂ (HIGH VALUE)

• Fever control - Biggest VO₂ lever

• Treat agitation & pain - Pain spikes catecholamines → ↑ VO₂

•  Control work of breathing - RR 35 = huge oxygen cost. Ventilation offloads the respiratory muscles

• Hypothermia / normothermia - Used intentionally post-cardiac arrest

Fluids/inotrope/blood => ↑ DO₂

Sedation/cooling => ↓ VO₂

Best outcomes come from doing BOTH

ONE MASTER ICU CONCEPT 

Shock is not just failure to deliver oxygen. It’s a failure to match delivery with demand.

You can resuscitate by increasing DO₂ OR decreasing VO₂.

• DO₂–VO₂ mismatch causes lactate and organ failure

• Fever worsens shock by raising VO₂

• Sedation helps shock by lowering VO₂ and improving flow

• ICU resuscitation = DO₂ up + VO₂ down

Birth of modern hemodynamics

Adolf Eugen Fick bridged physics + physiology. He studied

1. Diffusion of gases through membranes (Fick’s Law of Diffusion)

2. Oxygen transport in blood

3. The relationship between blood flow and oxygen uptake → Fick’s Principle

He said, "Blood flow to an organ (body) can be calculated if you know how much a substance (oxygen) is taken up by the organ and the concentration difference across it." - This is the foundation for cardiac output measurement. 

• Flow = (Oxygen consumption) ÷ (Arterial – Venous oxygen content difference)

For the whole body: Cardiac Output (CO) = VO₂ ÷ (CaO₂ – CvO₂)

How to Calculate Oxygen Consumption (VO₂): Exact VO₂ (Gold standard) requires: Inspired O₂, Expired O₂, and Minute ventilation. But we rarely have this.  This is the direct calculation from Fick, which needs complex equipment to measure that data. Indirect calculation (Fick) often substitutes values more easily obtained, and will be a little inaccurate but practical. A common substitution is oxygen consumption. another sub -> peripheral blood to measure SaO2 (instead of left atrial sample) and ScvO2 (instead of pulm artery sample. ScvO2 is also replaced with SvO2) => take ABG & VBG together

Practical ICU Approximation of VO₂ ≈ 3 × body weight (kg). 

another equation is VO2 = 125 x BSA  (or 110 x BSA if age > 70 years) 

VO2 changes in hypo/hyperdynamic consumption: In shock, sedation, fever, ventilator demands 

Calculate CO (= flow) Using Fick:

You only need 3 things: 1. amount of O2 taken up by the body (VO2), 2. O2 conc in arterial blood (CaO2), and 3. O2 conc in venous blood (CvO2)

Step 1: Calculate CaO₂ = 1.34 × Hb × SaO₂ + 0.003 × PaO₂ (negligible)

Example: Hb 10, SaO₂ 98%, PaO₂ 80

CaO₂ = (1.34 × 10 × 0.98) + (0.003 × 80)  => CaO₂ ≈ 13.1 + 0.24  => CaO₂ ≈ 13.3 mL O₂/dL

Step 2: Calculate CvO₂ using ScvO₂. CvO₂ = 1.34 × Hb × ScvO₂ + 0.003 × PvO₂ (negligible)

Example: ScvO₂ 60% (0.60), Hb 10

CvO₂ = 1.34 × 10 × 0.6 => CvO₂ ≈ 8.0 mL/dL

Step 3: Apply Fick: CO = VO₂ ÷ (CaO₂ – CvO₂)

Difference = 13.3 – 8.0 = 5.3 mL/dL

Convert to mL/L → multiply by 10 → 53 mL/L

Assume VO₂ = 250 mL/min.

CO = 250 ÷ 53 ≈ 4.7 L/min

This is the patient’s global cardiac output.

Before fick method, calculating CO was based on description, CO = stroke volume x HR. normal stroke volume is 65-80 ml. 

Why Measure CO by Indirect Fick in Sepsis?

1. MAP 65 tells you nothing about cardiac output. High MAP can coexist with low CO (vasoconstricted, underfilled heart). Calculate CO -> Stop treating blind

2. It exposes DO₂–VO₂ mismatch. This identifies oxygen debt far earlier than lactate. For example:

• Low CO + normal MAP + ScvO₂ 70% = distributive + pump failure mix

Interpretation: Why is this “mixed distributive + cardiogenic physiology”? in real pure cardiogenic shock: CO ↓ , High SVR -> ScvO₂ is always LOW (≤55–60%). Here, MAP is being preserved -> The only way to have normal MAP with low CO is -> high SVR. This is by Either: The patient already has high SVR from septic vasoconstriction recovery, or you are giving norepinephrine, which raises MAP but may drop CO.

Normally: Low CO → tissues extract more oxygen → ScvO₂ should drop (<60%). But here it stays 70%, meaning. Extraction is NOT increasing. Why because microcirculation remains distributively deranged. In sepsis: 1. Capillary shunting, 2. Mitochondrial dysfunction, 3. Maldistributed flow) → Tissues cannot extract oxygen even though delivery is low. This is classic “distributive shock physiology.” So you have: 1. Pump failure (low CO) & 2. Distributive failure (poor extraction, ScvO₂ falsely normal)- that is the mix. In real pure cardiogenic shock: Low CO, High SVR, & ScvO₂ is always LOW (≤55–60%)

WHY NOT PURE DISTRIBUTIVE? In pure distributive shock, CO is usually high (due to vasodilation and sympathetic drive)

Final verdict: Low CO = pump failure component, Normal MAP = high SVR (either endogenous or due to vasopressors), ScvO₂ 70% = extraction failure (distributive physiology)

These three cannot coexist in any single shock type → it must be mixed. 

This patient typically needs: Reduce NE if CO is suppressed, consider dobutamine (if echo supports low stroke volume), Optimise preload, avoid more fluids if no response, and Target microcirculation markers (CRT, lactate, mottling)

• High CO + very low ScvO₂ = aggressive extraction → impending collapse

  • • Reason: Because distribution at the microcirculatory level is failing, even though global flow is high.

      • • ScvO₂ reflects the balance between DO₂ and VO₂.

here, ScvO₂↓⇒VO₂>DO₂

So in this case: CO is high → DO₂ should be high

Yet ScvO₂ is very low (<60%, often <50%)

This means:  Despite high global flow, effective tissue delivery is insufficient.

That is not pump failure.

• Why low ScvO₂ with high CO is NOT cardiogenic => In cardiogenic shock:

CO is low, Extraction increases, & ScvO₂ falls

ScvO₂ = balance between delivery (DO₂) and consumption (VO₂)

ScVO2 = either ↓ DO₂ or ↑ VO₂ or both

And, DO2 = CO x CaO2

So ScvO₂ does NOT diagnose shock type by itself. It only tells you whether delivery is failing relative to demand.

In Cardiogenic shock (flow failure): ↓ CO -> ↓ DO₂-> Tissues compensate by ↑ extraction -> ScvO₂ ↓ (often <60%)

low ScvO₂ is driven by LOW FLOW, not by excess extraction alone.

=> Low ScvO₂ + low CO = cardiogenic until proven otherwise

if ScvO₂ is low despite high CO (remember cardiogenic shock cannot produce high CO), the only possibilities are: 1. Extreme oxygen extraction (Severe hypoxia at tissue level, Mitochondria demanding more oxygen than delivery despite flow), 2. Markedly increased VO₂ (Fever, Shivering, Agitation, Seizures, Severe work of breathing), 3. Low CaO₂ (Severe anemia, Hypoxemia. CO high × content low = DO₂ still inadequate)

• ↓ ScvO₂ + ↓ CO Low-flow shock (cardiogenic / hypovolemic / obstructive)

• ↑/normal ScvO₂ + ↑ CO Distributive shock (early sepsis)

• ↓ ScvO₂ + ↑ CO High demand or low content problem, NOT cardiogenic

Fick's contributions: 

1. Fick's law of diffusion: Explains how gases move across the membrane. it describes

   1. 1. • Alveolar gas exchange

         • Tissue oxygen diffusion 

         • Capillary–cell oxygen delivery

2. Fick's principle: applied mass conservation. 

   1. 1. • 'Whatever goes into a system − whatever comes out' = what the system used.

         • Step 1: Pick a substance; Fick chose oxygen, because: Blood carries it clearly, Tissues consume it continuously, and it’s measurable.

         • Step 2: Ask a simple question; “If I know how much oxygen the body uses per minute,

and how much oxygen blood loses while passing through, can I calculate how much blood must be flowing?” -> Yes.

Oxygen consumption (VO₂)=Blood flow (CO)×(Arterial − Venous O₂)

Before Fick: Circulation was descriptive (Pulse, colour, warmth — its subjective)

After Fick: Circulation became quantitative, Flow could be "calculated", not guessed => This is the birth of modern hemodynamics.

Fick would say: “A pressure number without flow and extraction means nothing.”

The concepts derived from Fick's idea.

• SvO₂ (Oxygen extraction)

• DO₂ (Flow × content)

• Lactate (Failed utilisation)

• Shock (DO₂ < VO₂)

Clinical applications from this logic:

• Hb low → CaO₂ ↓

• CO normal → still hypoxic

• SvO₂ low → over-extraction

• Lactate rising → utilisation failure

• MAP may look “fine”. But the Patient is still dying.

Fick taught us that life depends on flow and content — not pressure.

Fick: Flow and Extraction (the foundation)

1. 1. • What tissue consumes = blood flow x extraction

VO2 = CO x (Ca-Cv) O2

This means: Life depends on FLOW. Blood pressure is secondary. Oxygen delivery is not optional

Rewriting it: DO2 = CO x CaO2

Shock = DO2 < VO2

Extraction markers (how tissues scream for help)

↓ SvO₂ High extraction → low flow/content

↑ Lactate Anaerobic metabolism / mitochondrial failure

CRT cold Peripheral vasoconstriction

Low urine Renal hypoxia 

Guyton

Fick answers “why perfusion fails”

Guyton answers, “what limits cardiac output”

Guyton’s venous return concept: The heart pumps only what it receives

Venous return depends on: 

1. Mean systemic filling pressure (volume + tone)

2. Right atrial pressure

3. Venous resistance

Caution: High CVP ≠ good preload

=> CVP that rises with fluids and ↑ CO = responsive

Fick v/s Guyton

• Fick answers “why perfusion fails”

• Guyton answers, “what limits cardiac output”

• Is oxygen reaching tissues? Fick

• Can the heart increase flow? Guyton

• Should I give fluids or pressors? Guyton → then Fick

• Is MAP enough? Neither cares about MAP alone

DO₂–VO₂ RELATIONSHIP (THIS IS WHERE SHOCK LIVES)

Normal state

1. DO₂ >> VO₂

2. Extraction ~ 25%

3. SvO₂ ~ 70–75%

4. Lactate normal

Early / Compensated shock

1. CO ↓ or Hb ↓

2. Extraction increases

3. SvO₂ falls

4. Lactate may still be normal

=>This is salvageable

Critical DO₂ (the cliff): Once DO₂ drops below a threshold:

1. VO₂ becomes DO₂-dependent

2. Anaerobic metabolism starts

3. Lactate rises steeply

=> This is true shock

Late / Septic shock (the trap)

1. CO may be normal or high

2. SvO₂ normal or high

3. Lactate high

=> Reason; Extraction failure

• Microcirculatory shunting

• Mitochondrial dysfunction

=> This is why high SvO₂ can mean death

MAP mythology

MAP = Pressure

Perfusion = Flow + Content + Microcirculation

Scenario 1: (hypoxic at the cellular level)

MAP 75 + CO low + Hb low + Lactate high

Fick would say: “Pressure without flow is meaningless.”

If you manage shock by MAP alone:

• You will miss hypoxia

• You will misread SvO₂

• You will chase numbers, not perfusion

Fick → physiology

Guyton → mechanics

Microcirculation → reality

Blood pressure keeps monitors calm. Oxygen delivery keeps patients alive.

Why EF is NOT cardiac output

• EF = % of LV emptying

• CO = SV × HR

EF ignores: Heart rate, LV size, Preload, & Afterload

A patient can have: Normal EF + low CO

Low EF + high CO

Real examples

• Septic shock: EF 30%, HR 130 → CO high

• LVH / small stiff ventricle: EF 60%, HR 70 → CO low

So estimate CO, not EF alone. Practical bedside CO surrogates

1. LVOT VTI (best, if echo skilled)

2. Echo: “visually hyperdynamic vs poor forward flow”

3. ScvO₂ trend

4. Pulse pressure + clinical perfusion

EF only tells contractility. CO tells flow. Shock cares about flow.

WHY LACTATE RISES EVEN WITH NORMAL CO

Three reasons (not just hypoxia):

1. True hypoxia → Low DO₂

2. β-adrenergic drive (sepsis, adrenaline) → Aerobic glycolysis

3.  Mitochondrial failure → Oxygen present, unusable

=> Lactate ≠ always low flow

  Trend > single value

Again: Fick + microcirculation

FLOW-CONTENT-MICRO Algorithm (Modern tools: echo, lactate, CRT)

STEP 1 — OPEN PRESSURE

• MAP ≥ 65 (means nothing beyond this)

• Fluids ± norepi

STEP 2 — RESTORE FLOW (CORE STEP)

Ask:

• Is CO adequate?

• Is preload responsive?

Tools:

• Echo (LVOT VTI)

• Pulse pressure variation

• Passive leg raise

STEP 3 — FIX CONTENT

• Hb ≥ critical threshold?

• SaO₂ adequate?

• Think CO poisoning, MetHb

STEP 4 — TRUST MICRO-PERFUSION

Use:

• SvO₂ / ScvO₂

• Lactate trend

• CRT

• Urine output

• Mentation

SHOCK PHENOTYPES — FICK-BASED (NOT MAP-BASED)

1. COLD SHOCK (Low flow state) => Problem: CO ↓

Physiology (Fick)

• DO₂ ↓

• Extraction ↑

• SvO₂ ↓

• Lactate ↑ (late)

Causes: Cardiogenic shock, Hypovolemia, Massive PE, Late septic shock (pump failure)

Bedside signs: Cold extremities, Prolonged CRT, Narrow pulse pressure, Oliguria

What helps: Fluids (if preload responsive), Inotropes (dobutamine, milrinone), Fix Hb, oxygenation

Caution: Pure vasoconstrictors worsen perfusion

2. WARM SHOCK (Extraction failure) => Problem: Microcirculation / mitochondria

Physiology

• CO normal or high

• DO₂ adequate

• SvO₂ normal / high

• Lactate ↑

Causes: Early septic shock, Cytokine storm, Severe acidosis

Bedside signs: Warm extremities, Bounding pulses, Wide pulse pressure

What helps: Source control, Time + sepsis care, Avoid over-pressing MAP, Lactate clearance

Cation: Do not chase CO blindly

3. HYPOXIC SHOCK (Content failure) => Problem: CaO₂ ↓

Common misses

• Severe anemia

• CO poisoning

• Methemoglobinemia

• ARDS despite high PaO₂

Physiology: 

• CO may be normal

• Extraction maxed

• SvO₂ ↓

• Lactate ↑

Fix: Hb transfusion (this is where blood saves lives), FiO₂ / ventilation, Remove toxin

HOW TO THINK AT THE BEDSIDE

Shock is not low blood pressure. Shock is a failure of oxygen delivery relative to demand. (this explains: Sepsis, Cardiogenic shock, Anaemia shock, CO poisoning, Mitochondrial dysfunction). Forget “septic/cardiogenic” for a moment. Think FLOW + EXTRACTION.

Stepwise shock logic

1️⃣ Open pressure (initial fluids ± vasopressor)

2️⃣ Restore FLOW (CO, preload, contractility)

3️⃣ Fix CONTENT (Hb, SaO₂, CaO₂)

4️⃣ Trust MICRO-perfusion (SvO₂, lactate, CRT, urine)

If step 2 fails, step 1 is meaningless.

Shock is not low blood pressure. Shock is a failure of oxygen delivery relative to demand.

Most ICUs over-treat pressure and under-treat flow and content.

That’s why:

• Lactate doesn’t clear

• Kidneys fail despite MAP 75

• Patients “mysteriously” crash

If MAP OK + Lactate high → TRUST MICRO

If MAP low + SvO₂ low → FIX FLOW

If SvO₂ high + Lactate high → EXTRACTION FAILURE

CO + ScvO₂ + Lactate → SHOCK DECODER:

FLOW → EXTRACTION → METABOLISM

STEP 1 — Look at Cardiac Output (FLOW)

CO LOW; You are in low-flow shock until proven otherwise → cardiogenic/hypovolemic / obstructive

CO NORMAL or HIGH; Heart is not the primary problem → distributive or demand/content problem

STEP 2 — Add ScvO₂ (Extraction status)

ScvO₂ < 65% (Difference > 35–40%) → High extraction (low delivery)

→ Delivery is insufficient

→ Low-flow states

ScvO₂ 65–75% (Difference ≈ 25–30%) → Balanced (or falsely reassuring)

ScvO₂ > 75% (Difference < 20%) → Poor extraction / shunting

  → Distributive shock / mitochondrial failure

It’s the gap that matters, not ScvO₂ alone

Low ScvO₂ with low SaO₂ = content problem, not flow problem.

STEP 3 — Add Lactate (Metabolic stress/microperfusion)

Normal lactate → compensation working

High lactate → anaerobic metabolism / mitochondrial failure / microcirculatory failure

Lactate rises when: DO₂ < VO₂ Or mitochondria can’t use oxygen

What matters

Step 1 — Is CO adequate? Not EF. Forward flow (EF = contractility, not flow. CO = what perfuses organs. EF is a cardiology number. CO is a resuscitation number.)

• Tachycardia + narrow pulse pressure + oliguria → low CO 

• LVOT VTI < ~18 cm → low flow (if available)

EF answers only one question: How well is the ventricle contracting relative to its size?

Shock asks a different question: Is enough blood reaching the tissues per minute? => That’s CO.

However, at the bedside, you can replace EF with CO. Stop asking, “What is the EF?”, Start asking “Is forward flow adequate?”. CO ≈ (visual stroke volume) × HR.  Visual stroke from echo; Is the ventricle ejecting meaningfully, or just squeezing? Cavity empties visibly → CO likely OK (Good forward flow). LV squeezes but cavity hardly empties→ CO likely LOW (Poor forward flow)

Pulse pressure cross-check (cheap CO proxy)

• Narrow PP (<30 mmHg) → low stroke volume → low CO

• Wide PP (>40–50 mmHg) → high SV → high CO

 (This is extremely reliable in shock.)

In Echo, looking for answert to ONE QUESTION ONLY: Is forward cardiac output enough for this patient right now? 

• Is the myocardium hyperdynamic or sluggish? (visual judgement)

  • • • Hyperdynamic + small LV → hypovolemia → Low preload => Fluids 

      • Hyperdynamic + normal LV → distributive / high VO₂ → High CO -> Distributive / VO₂ problem => Vasopressors / sedation

      • Sluggish walls → low contractility → Low CO => Inotrope ± preload

• Is there meaningful cavity emptying?

  • • • Good flow → cavity size clearly reduces in systole

      • Poor flow → walls move, but cavity barely empties

(however, Wall motion ≠ flow)

if cavity is small? Is SV small? -> Check: pulse pressure => If narrow + lactate rising → low CO despite normal EF

At the bedside, EF is only useful if it explains CO. If it doesn’t, ignore EF and treat flow.

Echo view points: Hyperdynamic + small LV → hypovolemia

Hyperdynamic + normal LV → distributive / high VO₂

Sluggish walls → low contractility

(Non-echo surrogates for CO ); 

ScvO2

Pulse pressure

CRT

Urine

Mentation

Think of ScvO₂ as CO × content adequacy sensor.

• Low ScvO₂ (<60%) → CO likely insufficient for demand

• Rising ScvO₂ after intervention → CO improved

If EF “looks okay” but ScvO₂ is crashing → CO is not okay

Echo does not diagnose shock. Echo tells you whether FLOW is the problem

If echo + ScvO₂ + pulse pressure agree → act decisively.

If they disagree → trust perfusion and trends, not numbers.

Step 2 — Is oxygen content adequate?

• Hb & SaO₂

Note: Low EF with preserved content may still meet DO₂.

Step 3 — Is VO₂ excessive?

Fever/Shivering/Work of breathing

Step 4 — Is the extraction working? (ScvO₂–SaO₂ gap = extraction status)

ScvO₂ high with lactate → sepsis physiology

ScvO2 trend:

1. ScvO₂ rising after intervention (Flow or content improved) => Extraction demand met

Example: ScvO₂ 55% → fluids → 68%

Good response

2. ScvO₂ falling despite MAP support (Pressors raised SVR + Flow worsened)

Classic:

• NE ↑

• MAP okay

• ScvO₂ drops from 65 → 55

You strangled the flow

3. High ScvO₂ with rising lactate

Extraction failure => Sepsis / mitochondrial dysfunction

Stop chasing flow numbers

Shock is a flow–content–demand problem.

The summary

1️⃣ CO ↓ + ScvO₂ ↓ + Lactate ↑ = TRUE LOW-FLOW SHOCK

Seen in:

• Cardiogenic shock

• Hypovolemic shock

• Obstructive shock

• Late septic cardiomyopathy

Management focus

• Improve FLOW

• Fluids (if preload issue)

• Inotropes (dobutamine / milrinone)

Fix obstruction:  Pressors alone won’t fix this

2️⃣ CO ↑/N + ScvO₂ ↑ + Lactate ↑ = CLASSIC EARLY SEPTIC (DISTRIBUTIVE) SHOCK

What’s happening:

• Flow is high

• Oxygen is delivered

• Extraction fails (microcirculatory shunt + mitochondrial dysfunction)

Management focus

• Vasopressors (NE)

• Source control

• Antibiotics

• NOT more inotropes: 

High ScvO₂ here is BAD, not good

3️⃣ CO ↑ + ScvO₂ ↓ + Lactate ↑ (THIS IS THE CONFUSION ZONE)

NOT cardiogenic; Means-> Flow is adequate (If CO is high, cardiogenic shock is OFF the table.)

Demand or content problem dominates. Causes:

• Fever

• Shivering

• Severe work of breathing

• Agitation / seizures

• Severe anemia

• Hypoxemia

Management focus

• Reduce VO₂ (sedation, antipyretics, ventilatory support)

• Improve CaO₂ (oxygen, Hb)

 Giving inotropes here is physiological malpractice

4️⃣ CO ↓ + ScvO₂ N/↑ + Lactate ↑ = SEPSIS WITH CARDIAC DEPRESSION

Mechanism:

• Sepsis initially distributive

• Now secondary myocardial dysfunction

• Extraction still impaired

Management

• Vasopressor + inotrope combo

• Avoid fluid overload

• Echo-guided decision

5️⃣ CO N + ScvO₂ N + Lactate ↑ = OCCULT SHOCK / MICROFAILURE

Seen in:

• Early sepsis

• Post-ROSC

• Liver failure

• Metformin / toxins

Management

• Treat cause

• Lactate trend > single value

Pressors fix pressure

Inotropes fix flow

Sedation fixes VO₂

Blood + oxygen fix content

If you don’t know which one to choose → CO + ScvO₂ will tell you

Low CO decides shock type

ScvO₂ tells you who’s losing

Lactate tells you how bad the loss is

Shock is failure of oxygen delivery relative to demand.

First open pressure, then restore flow, then correct content, and finally judge success by microperfusion — not by MAP

Q: Why is ScvO₂ high in early sepsis?

A: Extraction failure due to microcirculatory shunting and mitochondrial down-regulation.

Q: Why does ScvO₂ fall when norepinephrine is increased?

A: Excess afterload reduces stroke volume and CO.

Q: Why does sedation improve urine output?

A: VO₂ reduction restores DO₂–VO₂ balance.

Q: When is blood better than oxygen?

A: When Hb is low, Hb determines CaO₂.

Q: When does increasing DO₂ stop helping?

A: In late metabolic failure with irreversible mitochondrial damage.

"Low CO decides shock type & ScvO₂ tells you who’s losing.”  Decoding

PART 1 — Low CO decides shock type- Why?

Because shock categories are defined by FLOW, not pressure, not lactate.

Cardiac Output (CO) = how much blood reaches tissues per minute. If CO is low, the problem is upstream in the circulation.

So what does LOW CO mean? It immediately puts you into LOW-FLOW SHOCK territory:

• Cardiogenic

• Hypovolemic

• Obstructive

• Late septic cardiomyopathy

These are mechanical/flow failures. => That’s why low CO decides shock type. It answers:

Is this a pump/volume/obstruction problem or not?

If CO is high, cardiogenic shock is OFF the table.

PART 2 — ScvO₂ tells you who’s losing

Now assume CO is whatever it is (low or high). ScvO₂ answers a different question:

Is oxygen delivery (DO₂) keeping up with oxygen demand (VO₂)?

What ScvO₂ really represents?  ScvO₂=DO₂ – VO₂ balance

It does NOT tell you shock type. However, it tells you which side of the equation is failing.

Low ScvO₂ (<60%) => Tissues are extracting aggressively.

This means: Delivery is insufficient for the current demand

So delivery is losing.

Delivery can fail because:

• CO is low

• Hb is low

• SaO₂ is low
  
  

The circulation is losing the race

High ScvO₂ (>75%)

Tissues are not extracting oxygen. This means: Oxygen is delivered but not used

🔋 So utilisation is losing.

Causes:

• Sepsis (microcirculatory shunt)

• Mitochondrial dysfunction

• Late metabolic failure
  
  

🔋 The cells are losing the race

1️⃣ Low CO + Low ScvO₂

Flow is low

Extraction is high

👉 True low-flow shock

(cardiogenic / hypovolemic / obstructive)

Pump/circuit is losing

2️⃣ High CO + High ScvO₂

Flow is fine

Extraction fails

👉 Early septic shock (utilisation disorder)

Mitochondria are losing

3️⃣ High CO + Low ScvO₂

Flow is fine

Demand is excessive, or the content is low

👉 High VO₂ or content failure

(fever, shivering, anaemia, hypoxia)

Demand is losing

4️⃣ Low CO + High ScvO₂

Flow is low

Extraction still fails

👉 Sepsis + cardiomyopathy (mixed shock)

Both sides are losing

Most clinicians ask:

“What type of shock is this?”

That’s the wrong first question.

The correct sequence is:

Is flow low? → CO

If yes, what kind of low-flow shock?

If no, why is oxygen still failing? → ScvO₂

Summary:

CO answers: What kind of shock?

ScvO₂ answers: Who is failing right now—delivery or utilisation?

Final one-line reframe

CO classifies the shock.

ScvO₂ tells you which side of the oxygen equation is collapsing.

Sample ICU Cases

CO tells you what kind of shock you’re in.

ScvO₂ tells you which side of the oxygen balance is currently failing.

Ask CO first

Low → low-flow shock (decides type)

High/normal → not cardiogenic

Then read ScvO₂

Low → delivery losing (flow/content)

High → utilisation losing (sepsis/mitochondria)

Use lactate to judge severity and trend

CASE 1 — Septic shock, MAP looks fine

Data

MAP 74 on norepinephrine

CO (echo): low

ScvO₂: 56%

Lactate: 5.1

Cold extremities, narrow pulse pressure

Read it

Low CO → low-flow shock phenotype

Low ScvO₂ → delivery is losing

Diagnosis 👉 Sepsis with cardiomyopathy (flow failure)

Correct move. 

Add inotrope, 

Reduce pressor if afterload is excessive

Reassess CRT/lactate

Common ICU error

“MAP is okay, continue norepi”

CASE 2 — Early sepsis, scary lactate

Data

MAP 68 (minimal pressor)

CO: high

ScvO₂: 82%

Lactate: 3.9

Warm hands, wide pulse pressure

Read it

CO high → not cardiogenic

ScvO₂ high → utilisation is losing

Diagnosis 👉 Early septic utilisation disorder

Correct move

Source control, antibiotics

VO₂ reduction (antipyretics, analgesia)

Don’t chase CO/inotropes

CASE 3 — “Refractory” shock on high norepi

Data

MAP 82 on high norepinephrine

CO: falling

ScvO₂: drops 68 → 55%

Lactate: plateauing

PP narrow, hands cold

Read it

Low CO → low-flow shock

ScvO₂ falling → delivery losing due to clamp

Diagnosis 👉 Afterload excess (pressor-induced low flow)

Correct move

Reduce norepi

Add inotrope

Watch PP widen, ScvO₂ rise

CASE 4 — ARDS + sepsis

Data

MAP 70

CO: normal

ScvO₂: 60%

Lactate: 4.6

Hb 7.1, SpO₂ 92% on high PEEP

High RR, agitated

Read it

CO is not the issue

ScvO₂ low → delivery losing vs demand

Content + VO₂ problem

Diagnosis 👉 High VO₂ + borderline content

Correct move

Sedation/ventilation to reduce VO₂

Consider transfusion

Avoid inotropes

CASE 5 — Massive PE

Data

MAP 60 on norepi

CO: low

ScvO₂: 52%

Lactate: 6.8

RV dilated, LV compressed

Read it

Low CO → obstructive shock

Low ScvO₂ → delivery losing

Diagnosis 👉 Obstructive low-flow shock

Correct move

Thrombolysis/embolectomy

Pressor only as bridge

CASE 6 — GI bleed, misleading saturation

Data

MAP 72

CO: normal

ScvO₂: 58%

Lactate: 4.0

Hb 6.3, SpO₂ 99%

Read it

CO okay → not cardiogenic

Low ScvO₂ → delivery losing

Content is the limiter

Diagnosis 👉 Content failure (anaemia)

Correct move

Blood transfusion

Oxygen alone won’t fix DO₂

CASE 7 — Late septic shock

Data

MAP 68 on NE + vaso

CO: low

ScvO₂: 84%

Lactate: 9.2, pH 7.08

Mottling, anuric

Read it

CO low, but ScvO₂ high

Neither delivery nor extraction is working

Diagnosis 👉 Late metabolic/mitochondrial failure

Correct move

Supportive care, goals discussion

Escalation won’t clear lactate

CASE 8 — Post-intubation improvement

Data (before)

CO high, ScvO₂ 57%, lactate 5.0

RR 38, febrile, agitated

After intubation/sedation

ScvO₂ 57 → 70%

Lactate starts falling

Read it

Delivery unchanged

Demand was losing, now corrected

Diagnosis 👉 VO₂-driven shock

Correct move

Continue VO₂ control

Avoid unnecessary inotropes

TRAP CASE 1 — Septic + cardiogenic (false reassurance by MAP)

Data

Septic shock, day 2

MAP 72 on norepinephrine

Echo: EF 35%

CO: low

ScvO₂: 68%

Lactate: 4.5

Cold extremities, narrow PP

Where people go wrong

“ScvO₂ is normal, so CO is okay.”

Correct reading

Low CO decides shock type → cardiogenic component present

ScvO₂ falsely normal → distributive extraction failure masking low flow

Diagnosis 👉 Mixed distributive + cardiogenic shock

Correct action

Reduce afterload

Add inotrope

Stop chasing MAP

📌 Normal ScvO₂ does NOT exclude low CO in sepsis

TRAP CASE 2 — High CO but patient worsening

Data

Septic patient, febrile (40.5°C)

CO: high

ScvO₂: 55%

Lactate: 5.2

RR 36, agitated

Where people go wrong

“Low ScvO₂ → give inotrope”

Correct reading

CO high → not cardiogenic

ScvO₂ low → demand (VO₂) is losing

Diagnosis👉 High-VO₂ shock, not flow failure

Correct action

Sedation

Intubation if needed

Antipyretics

📌 Inotropes here worsen lactate

TRAP CASE 3 — Vasopressor overdose masquerading as refractory shock

Data

MAP 85 on high norepinephrine

CO: falling

ScvO₂: 52%

Lactate: rising

Cold hands, PP 18

Where people go wrong

“Shock refractory → add vasopressin”

Correct reading

Low CO → low-flow shock

Low ScvO₂ → delivery losing

Cause = excess afterload

Correct action

Reduce norepi

Add inotrope

Accept lower MAP

📌 Refractory shock is often iatrogenic

TRAP CASE 4 — Late sepsis misread as ongoing hypoperfusion

Data

MAP 68

CO: low-normal

ScvO₂: 84%

Lactate: 9.0, pH 7.06

Mottling persists

Where people go wrong

“Increase flow further”

Correct reading

ScvO₂ high → extraction failure

Lactate vis ery high → metabolic failure

Diagnosis 👉 Late mitochondrial failure

Correct action

Avoid escalation

Supportive care

Prognosis discussion

📌 High ScvO₂ + high lactate = biology has failed

TRAP CASE 5 — Anemia hidden by good numbers

Data

MAP 70

CO: normal

ScvO₂: 58%

Lactate: 4.1

Hb 6.8, SpO₂ 99%

Where people go wrong

“Flow issue → inotrope”

Correct reading

CO normal → not cardiogenic

Low ScvO₂ → delivery losing

Cause = content failure

Correct action

Blood transfusion

📌 Hb beats FiO₂ every time

DAILY ICU ROUND CHECKLIST (FLOW–ScvO₂–LACTATE)

Use this every morning and after every intervention.

STEP 1 — FLOW (CO)

Ask only this: Is forward flow adequate? 

Answer using: 

• Echo (hyperdynamic vs poor emptying)

• Pulse pressure

• Clinical perfusion

• If CO low → shock type is low-flow (cardiogenic/hypovolemic/obstructive/late sepsis)

STEP 2 — ScvO₂ (WHO IS LOSING?)

Ask: Is delivery or utilisation failing?

ScvO₂ <60% → delivery losing

ScvO₂ >75% → utilisation losing

Trend more important than value

STEP 3 — CONTENT

Ask: 

Hb adequate?

SaO₂ reasonable (92–96%)?

If not → fix the content before the drugs

STEP 4 — LACTATE (SEVERITY + TRAJECTORY)

Rising → shock ongoing

Plateau → wrong intervention

Falling → winning

📌 MAP does not enter this decision

STEP 5 — MICROPERFUSION

Check:

1. CRT

2. Temperature gradient

3. Mottling

4. Urine output trend

If bad → shock persists regardless of numbers

STEP 6 — VO₂ LOAD

Always ask: 

1. Fever? 

2. Pain? 

3. Agitation?

4. Work of breathing?

Reducing VO₂ is often the missing intervention