From air to mitochondria
"Resuscitation is, at its core, the restoration of oxygen delivery to the tissues. Every shock state is a failure somewhere along the chain from inspired air to the mitochondrion — and each link (content, flow, extraction) is separately measurable and separately fixable."
Summarised from Irwin & Rippe's Intensive Care Medicine; with Marino's The ICU Book and Oh's Intensive Care Manual.Oxygen moves down a pressure cascade: inspired air (~150 mmHg) → alveolus (~100) → arterial blood (~90) → capillary/tissue → mitochondrion (~4–20 mmHg). At each step PO₂ falls; critical illness widens the drops.
How much oxygen the blood carries
Almost all oxygen is bound to haemoglobin; the dissolved part (0.003 × PaO₂) is trivial (~1.5%). Two consequences follow:
- Anaemia hits content harder than hypoxaemia. Halving Hb halves CaO₂; halving PaO₂ (90→45) drops CaO₂ by only ~20%.
- The oxyhaemoglobin dissociation curve links SaO₂ to PaO₂ (P50 ≈ 27 mmHg). A right shift (↑ temp, ↑ CO₂/H⁺ — Bohr, ↑ 2,3-DPG) unloads O₂ to tissues; a left shift holds onto it.
The oxygen supplied to the body per minute
Three — and only three — levers set delivery: cardiac output, haemoglobin, and arterial saturation. Every resuscitation manipulates one of these.
- Cardiac output — preload (fluids), contractility (inotropes), rate/rhythm, afterload.
- Haemoglobin — transfusion (usually threshold Hb 7 g/dL; higher in active ischaemia).
- SaO₂ — oxygen, PEEP/recruitment, treat the lung.
What the tissues actually use
This is the Fick principle: consumption equals blood flow times the arterio-venous oxygen difference. Normally the body extracts about a quarter of what is delivered:
Because DO₂ (~1000) far exceeds VO₂ (~250), there is a large reserve: when delivery falls, the tissues simply extract more (SvO₂ falls) and consumption is preserved — up to a point.
Supply-independent until it isn't
Supply-independent zone
As DO₂ falls, extraction rises to keep VO₂ constant. Consumption is independent of delivery — the normal, safe state.
Critical DO₂
At a threshold delivery, extraction is maximal and can rise no further.
Supply-dependent zone
Below critical DO₂, VO₂ falls with DO₂ — the tissues can't get enough O₂ → anaerobic metabolism, rising lactate, oxygen debt and organ failure.
The goal of resuscitation is to keep DO₂ above the critical threshold — but supranormal, protocolised "boosting" of DO₂ to fixed targets did not improve outcomes and can harm. Resuscitate to perfusion (lactate clearance, MAP, organ function), not to a magic number.
Is delivery meeting demand?
| Marker | Meaning | Concern when… |
|---|---|---|
| Lactate | Anaerobic metabolism / hypoperfusion (also adrenergic) | Elevated or failing to clear |
| SvO₂ (mixed venous, PA catheter) | Global balance of delivery vs consumption (normal ~65–75%) | Low = extraction maxed (↓ DO₂ or ↑ VO₂) |
| ScvO₂ (central venous) | Surrogate for SvO₂ (runs a few % higher) | Low = inadequate delivery |
| Veno-arterial CO₂ gap | Adequacy of flow to clear CO₂ | >6 mmHg suggests low flow |
| Clinical perfusion | Mentation, urine output, capillary refill, mottling | Any hypoperfusion sign |
In sepsis and cyanide/mitochondrial poisoning, tissues cannot extract O₂ — SvO₂ is high despite tissue hypoxia (impaired extraction). Interpret SvO₂ with lactate and context.
Four ways tissues go short of oxygen
| Type | Problem | Example |
|---|---|---|
| Hypoxaemic | Low SaO₂/PaO₂ → low content | Respiratory failure, high altitude |
| Anaemic | Low Hb (or dysfunctional Hb) → low content | Haemorrhage; CO & methaemoglobinaemia |
| Stagnant / circulatory | Low flow → low delivery | Shock, cardiac arrest, local ischaemia |
| Histotoxic | Cells can't use O₂ (impaired extraction) | Cyanide, mitochondrial poisons, sepsis |
Pull the right lever
- Flow: restore preload with fluids (only if fluid-responsive), add inotropes for pump failure, control tachy-/brady-arrhythmia, reduce excess afterload.
- Haemoglobin: transfuse to a threshold ~7 g/dL (restrictive); higher only for active bleeding/ischaemia.
- SaO₂: oxygen and lung support to full saturation — but avoid needless hyperoxia.
- Reduce demand (VO₂): treat fever/agitation/shivering, provide analgo-sedation, and ventilate the exhausted patient to offload respiratory muscle work.
- Endpoint: improving lactate/ScvO₂ and organ perfusion — not a fixed supranormal DO₂.
Common mistakes
Content is mostly haemoglobin — a high PaO₂ won't rescue an anaemic, low-output patient.
Protocolised delivery "boosting" to fixed high targets does not improve outcomes and can harm. Resuscitate to perfusion.
In sepsis/histotoxic hypoxia, a high SvO₂ with high lactate signals impaired extraction, not adequacy.
Fever, shivering and agitation raise VO₂ — treating them is as valid as raising DO₂.
A restrictive threshold (~7 g/dL) is as safe or safer than liberal transfusion in most ICU patients.
Exam pearls
Q: Write the CaO₂, DO₂ and VO₂ equations.
CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂); DO₂ = CO × CaO₂ × 10 (~1000 mL/min); VO₂ = CO × (CaO₂ − CvO₂) × 10 (~250 mL/min).
Q: Normal oxygen extraction ratio?
≈25% (VO₂/DO₂) — leaving a large reserve; SvO₂ ~65–75%.
Q: Anaemia vs hypoxaemia — which hurts content more?
Anaemia. Halving Hb halves CaO₂; halving PaO₂ drops it only ~20% (most O₂ is Hb-bound).
Q: What is critical DO₂?
The delivery below which extraction is maximal and VO₂ becomes supply-dependent → lactate rises (oxygen debt).
Q: Name the four types of tissue hypoxia.
Hypoxaemic, anaemic, stagnant (circulatory) and histotoxic.
All the comparisons in one place
The distinctions that anchor the physiology and the viva — gathered at the end.
DO₂ vs VO₂
| Feature | DO₂ (delivery) | VO₂ (consumption) |
|---|---|---|
| Definition | O₂ supplied per minute | O₂ used per minute |
| Formula | CO × CaO₂ × 10 | CO × (CaO₂ − CvO₂) × 10 |
| Normal | ~1000 mL/min | ~250 mL/min |
| Set by | CO, Hb, SaO₂ | Metabolic demand + extraction |
Hypoxaemia vs Anaemia (effect on CaO₂)
| Feature | Hypoxaemia | Anaemia |
|---|---|---|
| Defect | Low SaO₂/PaO₂ | Low Hb |
| Impact on content | Modest (curve is flat up high) | Large (linear with Hb) |
| Fix | O₂, ventilation | Transfusion |
Supply-independent vs Supply-dependent VO₂
| Feature | Supply-independent | Supply-dependent |
|---|---|---|
| DO₂ | Above critical threshold | Below critical threshold |
| Extraction | Rises to compensate | Already maximal |
| Lactate | Normal | Rises (anaerobic) |
SvO₂ vs ScvO₂
| Feature | SvO₂ (mixed venous) | ScvO₂ (central venous) |
|---|---|---|
| Site | Pulmonary artery | SVC / right atrium |
| Access | PA catheter | Central line (easier) |
| Value | ~65–75% | Runs a few % higher; tracks trends |
References
- Irwin RS, Lilly CM, Mayo PH, Rippe JM (eds). Irwin & Rippe's Intensive Care Medicine. 9th ed. Wolters Kluwer; 2023.
- Marino PL. Marino's The ICU Book. 5th ed. Wolters Kluwer; 2025.
- Bersten AD, Handy JM (eds). Oh's Intensive Care Manual. Elsevier; 2026.
- Vincent JL, De Backer D. Circulatory shock. N Engl J Med. 2013;369:1726–1734.
- Hébert PC, Wells G, Blajchman MA, et al. (TRICC). A multicenter, randomized, controlled clinical trial of transfusion requirements in critical care. N Engl J Med. 1999;340:409–417.