Hypoxia

Hypoxia is a state of inadequate oxygen supply to tissues relative to metabolic demand. It is distinct from hypoxemia (low arterial PO₂), though hypoxemia is one of its most common causes.

Classification & Causes

Hypoxia is classically divided into four types (Harrison's, p. 272):

Type	Mechanism	Common Causes
Hypoxic hypoxia (hypoxemia)	Low PaO₂ due to reduced alveolar O₂	High altitude, hypoventilation, V/Q mismatch, diffusion impairment, right-to-left shunt
Anemic hypoxia	Reduced O₂-carrying capacity	Anemia, CO poisoning, methemoglobinemia
Stagnant (circulatory) hypoxia	Impaired O₂ delivery due to low blood flow	Heart failure, shock, peripheral vascular disease
Histotoxic hypoxia	Cells cannot utilize O₂	Cyanide poisoning, H₂S toxicity

Pathophysiology

Oxygen delivery (DO₂) = Cardiac Output × Arterial O₂ Content

Arterial O₂ content = (Hb × 1.34 × SaO₂) + (PaO₂ × 0.003)

When DO₂ falls below tissue oxygen consumption (VO₂), cells shift from aerobic to anaerobic metabolism, producing lactate and leading to cellular dysfunction and, ultimately, death.

CNS Effects (Harrison's, p. 1140)

The CNS — especially higher cortical centers — is highly sensitive to hypoxia:

Mild/acute: Impaired judgment, motor incoordination (resembling alcohol intoxication)
High altitude: Headache (cerebral vasodilation), nausea, dizziness, insomnia, fatigue, somnolence
HAPE (High-Altitude Pulmonary Edema): Pulmonary arterial/venous constriction → capillary leakage → worsening hypoxia (a vicious cycle)
HACE (High-Altitude Cerebral Edema): Severe headache, papilledema, coma (rare)
Severe: Brainstem regulatory failure → respiratory arrest → death

Hypoxia CNS pathophysiology diagram showing direct neuronal metabolic stress and compensatory cardiorespiratory feedback loops

Adaptation to Hypoxia

The body mounts several compensatory responses:

Timeframe	Response
Immediate	Hyperventilation (hypoxic ventilatory response via carotid bodies), tachycardia, increased cardiac output
Hours–days	Cerebral & coronary vasodilation; pulmonary vasoconstriction (HPV — Euler-Liljestrand reflex)
Days–weeks	Erythropoietin (EPO) release → polycythemia; increased 2,3-DPG (rightward shift of O₂-Hb curve)
Molecular	HIF-1α (Hypoxia-Inducible Factor) activation → upregulates VEGF, glycolytic enzymes, EPO

Clinical Features

System	Manifestations
Respiratory	Dyspnea, tachypnea, use of accessory muscles
Cardiovascular	Tachycardia, hypertension (early), hypotension/bradycardia (late/severe)
CNS	Agitation, confusion, coma, seizures
Skin/Mucosa	Central cyanosis (SaO₂ < ~85%), mottling
Metabolic	Lactic acidosis, elevated lactate

Diagnosis

Pulse oximetry (SpO₂): Quick, non-invasive; normal ≥ 95% at sea level. Note: unreliable in CO poisoning (falsely normal).
Arterial Blood Gas (ABG): Gold standard — measures PaO₂, PaCO₂, pH, SaO₂, and allows calculation of A-a gradient.
A-a gradient = PAO₂ − PaO₂ (normal < 10–15 mmHg in young adults)

A-a Gradient	Likely Mechanism
Normal	Hypoventilation, low inspired FiO₂
Elevated	V/Q mismatch, diffusion impairment, right-to-left shunt

Lactate: Elevated (>2 mmol/L) indicates inadequate tissue O₂ utilization
CBC, COHb, MetHb to exclude anemic/toxic causes
Imaging (CXR, CT) to identify pulmonary etiology

Management

Treatment is directed at the underlying cause plus supportive oxygenation:

Supplemental O₂: Titrate to SpO₂ 94–98% (88–92% in hypercapnic COPD)
Airway management: Non-rebreather mask → high-flow nasal cannula (HFNC) → NIV → mechanical ventilation as severity escalates
Treat cause:
- Pneumonia → antibiotics
- PE → anticoagulation/thrombolysis
- Heart failure → diuresis, afterload reduction
- CO poisoning → 100% O₂ or hyperbaric O₂
- Cyanide poisoning → hydroxocobalamin
HAPE: Descent, supplemental O₂, nifedipine or PDE-5 inhibitors, portable hyperbaric chambers
Positioning: Prone positioning improves V/Q matching in ARDS

Hypoventilation as a Cause (Harrison's, p. 2202)

Hypoventilation elevates PaCO₂ and reduces PaO₂ by the alveolar gas equation. Causes include:

Central hypoventilation syndrome
Obesity hypoventilation syndrome
Neuromuscular disease, sedative drug overdose

Treatment targets the primary cause; NIV (BiPAP) is a mainstay for chronic hypoventilatory hypoxia.

Key distinction: Hypoxia ≠ hypoxemia. Hypoxemia always causes hypoxia (if severe enough), but hypoxia can occur with normal PaO₂ (e.g., anemia, cyanide poisoning). Clinical assessment must integrate SpO₂, hemoglobin, cardiac output, and tissue perfusion together.

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