Hypoxemia

Shunt: perfusion without ventilation (V/Q = 0). No ↑ pCO₂ until shunt > 50% (chemoreceptor compensation). Dead space: ventilation without perfusion (V/Q = ∞). ↑ pCO₂.

Alveolar gas equation: PAO₂ = FiO₂ (Patm − PH₂O) − (PaCO₂ / 0.8) = 0.21 × (760 − 47) − (40 / 0.8) ≈ 100 mmHg (room air)

A-a gradient: P(A-a)O₂ = PAO₂ − PaO₂

Normal A-a: - < 10 mmHg at FiO₂ 0.21 - < 60 mmHg at FiO₂ 1.0 - (age / 4) + 4 - a/A ratio > 0.75

Normal PaO₂: 103 − (age / 3)

1. Low inspired O₂: - Altitude (normal FiO₂, ↓ barometric pressure) - Hypoxic gas mixture (crossed gas lines, pipeline failure)

2. Hypoventilation: - Drugs (opioids, benzos, barbiturates) - Chest wall damage (rib fx splinting) - Neuromuscular disease (residual NMB, GBS, ALS) - Obstruction (OSA, mass) - Very responsive to supplemental O₂ — high FiO₂ swamps the PaCO₂/0.8 term

3. Diffusion impairment: - ↑ pathway: pulmonary edema, fibrosis - ↓ surface area: emphysema, pneumonectomy - ↓ O₂-Hb association rate: high CO, anemia, PE

4. R-to-L shunt (does NOT correct with O₂): - Congenital (TOF, TA, ASD/VSD with Eisenmenger) - AVM - Pulmonary fluid (pneumonia, CHF, ARDS, NPPE, TACO, TRALI) - Atelectasis (mucus plug, GA) - Endobronchial intubation (mainstem)

5. V/Q mismatch: - COPD, ILD - Dead space (PE, surgical clamping) - ↓ CO (MI, CHF)

Often mixed — example: COPD + opioid + pneumothorax + anemia + LV dysfunction.

Trace a path from the alveoli back to the machine.

1. Listen to the lungs: - Atelectasis (rales) - Pulmonary edema (rales, ↓ BS) - Bronchospasm (wheeze, shark-fin EtCO₂, ↑ PIP, ↓ TV) - Mucus plug (↑ PAP, ↓ TV, mucus in ETT, rhonchi) - Right mainstem (SpO₂ ~90%, ↑ PAP, ↓ TV, unilateral BS — common with insufflation, repositioning) - Pneumothorax (unilateral BS, ↑ PAP, ↓ TV, HD instability, tracheal deviation) - Esophageal intubation (no EtCO₂, gastric BS)

2. Check ETT: cuff deflation, kink, bite, dislodgement (head turned 180°).

3. Check circuit: disconnect at machine, at ETT, gas-sampling line.

4. Check machine: inspiratory/expiratory valves, bellows, minute ventilation, FiO₂, pipeline & cylinder pressures.

5. Check monitors: confirm with pulse-ox waveform, look at the patient (cyanosis, mottling), check gas analyzer.

Assuming SpO₂ is accurate:

• 100% FiO₂, high flow
• Manual ventilation — assess compliance, leaks, listen
• Recruitment maneuver if atelectasis suspected and hemodynamics tolerate (sustained 40 cmH₂O × 30 s, or step-wise PEEP)
• Auscultate — confirm bilateral BS, ETT position
• Bronchodilators if bronchospasm (albuterol, epi)
• Fiberoptic bronchoscopy to assess ETT position, suction
• Suction airway and ETT
• Consider cardiovascular causes (low CO → ↓ mixed venous O₂)
• Restore volume, RBCs, cardiac output
• Send ABG/VBG
• Consider CXR