Oxygenation

Oxygenation is the delivery of oxygen from inhaled gas into arterial blood. It depends on inspired oxygen concentration, alveolar gas exchange, ventilation-perfusion matching, and hemoglobin’s capacity to bind and transport oxygen.

Oxygenation is monitored at the bedside primarily through pulse oximetry (SpO2), which estimates the percentage of hemoglobin saturated with oxygen. Normal SpO2 is typically 94-100% on room air. Values below 90% indicate significant hypoxemia and warrant immediate intervention — supplemental oxygen, airway repositioning, or escalation to positive-pressure support.

The critical clinical principle is that oxygenation and ventilation are independent processes. A patient receiving supplemental oxygen may maintain normal SpO2 even while ventilation fails — CO2 rises, respiratory acidosis develops, and the patient’s mental status deteriorates, all while the pulse oximeter reads reassuringly. This is why capnography (exhaled CO2 monitoring) is essential alongside pulse oximetry: SpO2 tells you about oxygen in; capnography tells you about CO2 out.

Oxygenation failure can result from:

• Hypoventilation — not enough fresh gas reaching the alveoli (opioid toxicity, neuromuscular weakness)
• Ventilation-perfusion mismatch — areas of lung that are ventilated but not perfused, or perfused but not ventilated (pulmonary embolism, pneumonia)
• Shunt — blood passing through the lung without encountering ventilated alveoli (atelectasis, ARDS)
• Diffusion impairment — thickened or damaged alveolar-capillary membrane (pulmonary fibrosis)

Each mechanism responds differently to supplemental oxygen. Shunt physiology, in particular, may not improve significantly with increased FiO2 alone — a distinction that matters when oxygenation fails to improve despite high-flow oxygen.

Related terms

• Ventilation — the companion process that must be assessed independently
• Preoxygenation — maximizing oxygen reserves before airway intervention
• Capnography — monitoring that detects ventilation failure when oxygenation is maintained