💨 Chapter 18: Acute Oxygen Therapy

Hypoxia vs hypoxaemia · Oxygen delivery systems (low-flow vs high-flow) · Target saturations · Venturi masks · Complications of hyperoxia · Monitoring oxygen therapy

🔍 Core Concepts: Acute Oxygen Therapy

🫁 Hypoxia vs Hypoxaemia
Hypoxia = reduced O2 at tissue level. Hypoxaemia = low arterial PaO2. Causes: V/Q mismatch, shunt, diffusion limitation, hypoventilation, low FiO2. First three cause ↑A-a gradient.
🎯 Target Oxygen Saturations
Type I failure / critically ill: 94–98%. Type II (hypercapnic risk): 88–92% to avoid worsening CO2 retention. CO poisoning: 100% O2 via reservoir mask.
📦 Low-flow systems
Nasal cannula (0.5–4 L/min, FiO2 22–40%), simple mask (5–10 L/min, 40–60%), reservoir mask (10–15 L/min, 60–90%). FiO2 depends on patient's minute ventilation.
💨 High-flow systems
Venturi mask (24–50% fixed FiO2 via colour-coded valves), high-flow nasal cannula (heated/humidified, up to 100% FiO2, provides some CPAP effect).
⚠️ Complications of high-concentration O2
Absorptive atelectasis, oxidant lung injury, retinopathy of prematurity (newborns). Also may worsen hypercapnia in COPD due to Haldane effect and loss of hypoxic drive.
📊 Monitoring
Pulse oximetry (fifth vital sign). Factors affecting accuracy: hypotension, anaemia, dyshaemoglobins, nail polish. Co-oximetry for CO poisoning or methaemoglobinemia.

🩺 Stepwise Approach: Acute Oxygen Therapy in Children

1
Assess and monitor
Check SpO2, work of breathing, sensorium. Identify risk for hypercapnia (COPD, neuromuscular disease, obesity hypoventilation). Obtain ABG if deterioration or need for precise PaCO2.
2
Choose target saturation
Most patients: 94–98%. At risk of hypercapnic failure: 88–92% (use Venturi mask). Carbon monoxide poisoning: 100% via non-rebreather.
3
Select delivery system
Mild hypoxaemia: nasal cannula 1-2 L/min. Moderate: simple mask 5-10 L/min (minimum 5 L to washout CO2). Severe hypoxaemia: reservoir mask 10-15 L/min. For precise FiO2: Venturi mask.
4
High-flow nasal cannula (HFNC) indications
Moderate distress, post-extubation, bronchiolitis. Provides heated/humidified gas, reduces dead space, possible low-level CPAP. Contraindicated in severe ARDS or apnoea.
5
Reassessment & escalation
If SpO2 remains below target despite optimal FiO2 → consider NIV or intubation. Falling SpO2, rising work of breathing, altered mental status → invasive ventilation.
6
Avoid complications
Wean O2 as tolerated. Avoid prolonged 100% O2 unless necessary. Humidify high-flow gases. Monitor for absorptive atelectasis (develops after 12-24h high FiO2).