Unit
- Oxygen cascade: Oxygen moves from atmospheric air to mitochondria through a series of partial pressure gradients. Each step from inspired air to alveolar gas to arterial blood to tissues involves specific transport mechanisms including ventilation, diffusion, and perfusion.
- Hemoglobin binding: Approximately 98% of oxygen in blood is carried bound to hemoglobin. The oxyhemoglobin dissociation curve illustrates how oxygen affinity changes with pH, temperature, and 2,3-BPG levels, affecting tissue delivery.
- Hypoxemia causes: Four primary mechanisms include hypoventilation (raised PaCO2), diffusion impairment (interstitial lung disease), shunt (blood bypassing alveoli), and ventilation-perfusion mismatch (most common in conditions like COPD and asthma).
- Hypoxia types: Hypoxic hypoxia results from low arterial PO2, anemic hypoxia from reduced hemoglobin, stagnant hypoxia from circulatory failure, and histotoxic hypoxia from impaired cellular utilization (e.g., cyanide poisoning).
- Compensatory responses: Acute hypoxia triggers increased ventilation, heart rate, and pulmonary vasoconstriction. Chronic hypoxia stimulates erythropoietin release and pulmonary hypertension, which may lead to cor pulmonale.
- Oxygen content equation: CaO2 = (Hb x 1.34 x SaO2) + (0.003 x PaO2). This formula shows that hemoglobin concentration and saturation are far more important than dissolved oxygen in determining oxygen delivery.
- Clinical assessment of hypoxemia: Pulse oximetry provides continuous SpO2 monitoring but has limitations: it may be inaccurate in low perfusion, carboxyhemoglobin, or methemoglobin states. Arterial blood gas analysis remains the gold standard for PaO2 and pH assessment.
- Impact of hyperoxia: Excess oxygen can cause absorption atelectasis, oxygen toxicity (CNS and pulmonary), and increased work of breathing in some patients. Management requires setting appropriate FiO2 targets and weaning protocols.
- Oxygen delivery devices: Low flow devices such as nasal cannula provide FiO2 that varies with the patient's minute ventilation and flow rate, typically 24-44% at 1-6 L/min. High flow devices like Venturi masks deliver a precise FiO2 independent of breathing pattern, useful when exact oxygen fraction is critical.
- Indications for oxygen therapy: Acute hypoxemia defined as PaO2 below 60 mmHg or SpO2 below 90% prompts immediate supplemental oxygen. Other indications include acute asthma exacerbations, pneumonia, pulmonary embolism, and post-operative recovery. Chronic home oxygen is prescribed for PaO2 at or below 55 mmHg or SpO2 at or below 88% at rest.
- Target SpO2 ranges: For most acutely ill patients the recommended target is 94-98% to ensure adequate tissue oxygenation without hyperoxia risk. In patients with chronic hypercapnia or at risk of hypercapnic respiratory failure, a target of 88-92% is preferred to avoid suppressing hypoxic ventilatory drive.
Terminology
- PaO2
- Partial pressure of oxygen dissolved in arterial blood, normally 80-100 mmHg.
- SaO2
- Percentage of hemoglobin binding sites occupied by oxygen in arterial blood, normally 95-100%.
- FiO2
- Fraction of inspired oxygen expressed as a decimal or percentage.
- Hypoxemia
- Low oxygen level in arterial blood; distinct from hypoxia which indicates low oxygen at tissue level.
Quick Revision
- The oxygen cascade describes partial pressure gradients from atmosphere to mitochondria.
- Hemoglobin carries most oxygen; the dissociation curve is influenced by several factors.
- Four mechanisms of hypoxemia: hypoventilation, diffusion impairment, shunt, V/Q mismatch.
- Hypoxia is classified into hypoxic, anemic, stagnant, and histotoxic types.
- Oxygen content equation highlights hemoglobin's role in oxygen delivery.
- Pulse oximetry is useful but has limitations; ABG is definitive.
- Hyperoxia poses risks requiring careful FiO2 management.