🫧 Oxygen Therapy Devices Expanded Technical Details · Low‑Flow, High‑Flow, Home O₂ & Hyperbaric Systems

1. Classification of Oxygen Delivery Systems

Oxygen delivery devices are categorized by whether the delivered flow meets or exceeds the patient's inspiratory demand.

2. Low‑Flow Systems: Detailed Analysis

Nasal Cannula

• Flow range: 1–6 L/min (adults). Higher flows (>4 L/min) require humidification to prevent mucosal drying.
• FiO₂ Estimation: Each 1 L/min of O₂ flow adds approximately 3‑4% FiO₂ above 21% (room air).
  Estimated FiO₂ (%) = 21 + (Flow in L/min × 4)
  Example: 3 L/min ≈ 21 + 12 = 33% FiO₂.
• Factors affecting actual FiO₂:
  • Respiratory rate and tidal volume: Higher minute ventilation → more room air entrainment → lower FiO₂.
  • Mouth vs. nose breathing: Mouth breathing reduces FiO₂ (less pharyngeal reservoir).
  • Peak inspiratory flow: Patients in respiratory distress may have inspiratory flows >30‑40 L/min, greatly exceeding delivered O₂ flow → low FiO₂.
• Clinical Pearls: Best for stable patients requiring low‑moderate FiO₂. Cannot reliably deliver FiO₂ >40‑45%. At 6 L/min, actual FiO₂ may be as low as 35‑40% if tachypneic.

Simple Face Mask

• Flow range: 5–10 L/min. Minimum flow 5 L/min to flush exhaled CO₂ from the mask and prevent rebreathing.
• FiO₂ range: ~35–50%. FiO₂ varies with patient effort and mask fit.
• Limitations: Uncomfortable for prolonged use, interferes with eating/drinking, risk of CO₂ rebreathing if flow <5 L/min.

Non‑Rebreather Mask (NRBM) / Reservoir Mask

• Components: Face mask with one‑way valves on exhalation ports (prevent room air entrainment) + reservoir bag with one‑way valve (prevents exhaled gas from entering bag).
• Flow range: 10–15 L/min. Flow must be sufficient to keep reservoir bag at least partially inflated throughout inspiration.
• FiO₂ range: ~60–90%. Never delivers 100% FiO₂ due to mask leak and imperfect valve function.
• Critical Setup: Ensure reservoir bag is inflated before placing on patient. If bag collapses completely during inspiration, increase flow rate.
• Indications: Severe hypoxemia, trauma, pre‑intubation, carbon monoxide poisoning (until HBOT available).
• Warning: Do not use in patients at risk for hypercapnia (COPD) without close monitoring — can deliver dangerously high FiO₂.

Partial Rebreather Mask

• Similar to NRBM but no one‑way valves between mask and reservoir bag. Allows some exhaled gas (rich in O₂ from anatomic dead space) to enter bag.
• FiO₂ ~50–60%. Rarely used clinically; NRBM preferred for high FiO₂ needs.

3. High‑Flow / Fixed‑Performance Systems

Venturi Mask (Air‑Entrainment Mask)

• Mechanism: Uses the Bernoulli principle — high‑velocity O₂ jet passes through a narrow orifice, creating negative pressure that entrains a fixed volume of room air through side ports. Different color‑coded adapters provide specific FiO₂ (24%, 28%, 31%, 35%, 40%, 50%).
• Total gas flow delivered = O₂ flow + entrained air flow. This total flow often exceeds patient's peak inspiratory flow, ensuring stable FiO₂.
• Indications: Gold standard for patients with COPD and other conditions at risk for hypercapnia, where precise FiO₂ control is essential.
• Limitations: Uncomfortable, interferes with eating, FiO₂ limited to ≤50% (higher FiO₂ requires smaller entrainment port → lower total flow → may not meet inspiratory demand).

High‑Flow Nasal Cannula (HFNC)

Advanced Physiology HFNC delivers heated, humidified blended air‑oxygen at flows up to 60 L/min via specialized large‑bore nasal prongs. It provides multiple physiologic benefits beyond simple oxygenation.

Components of HFNC System

• Flow generator (air‑oxygen blender) → Active heated humidifier → Heated inspiratory circuit → Large‑bore nasal cannula.
• Flow rates: 20–60 L/min (adults). FiO₂: 0.21–1.0 (titratable).

Physiologic Mechanisms

Clinical Indications for HFNC

• Acute hypoxemic respiratory failure (pneumonia, ARDS, pulmonary edema)
• Pre‑oxygenation before intubation (apneic oxygenation)
• Post‑extubation support (reduces reintubation rate)
• Do‑not‑intubate patients with hypoxemia
• Bronchoscopy in hypoxemic patients
• Heart failure with pulmonary edema (reduces preload and afterload)

HFNC Weaning Protocol

• Wean FiO₂ first to ≤0.4–0.5 while maintaining SpO₂ target.
• Once FiO₂ is low, gradually reduce flow rate (e.g., by 5–10 L/min every few hours).
• Typical transition: HFNC → conventional nasal cannula when flow ≤20 L/min and FiO₂ ≤0.4.

4. Home Oxygen Therapy: LTOT Criteria & Equipment

Long‑Term Oxygen Therapy (LTOT) Indications (Medicare / ATS/ERS)

• Resting hypoxemia: PaO₂ ≤55 mmHg or SpO₂ ≤88% on room air (at rest, awake).
• PaO₂ 56–59 mmHg or SpO₂ 89% with evidence of cor pulmonale, right heart failure, or hematocrit >55%.
• Exercise‑induced hypoxemia: Desaturation with exertion; ambulatory oxygen improves exercise capacity.
• Nocturnal hypoxemia: Documented on overnight oximetry; may require nocturnal oxygen.

Home Oxygen Delivery Systems

Oxygen‑Conserving Devices (OCDs)

• Purpose: Extend duration of portable O₂ supply by delivering O₂ only during inspiration.
• Types:
  • Pulse‑Dose (Demand) Delivery: Delivers a bolus of O₂ at the beginning of inspiration. Not suitable for patients with weak inspiratory effort, mouth breathing, or high respiratory rates.
  • Reservoir Cannula: Stores O₂ during exhalation in a small reservoir; patient inhales from reservoir.
  • Transtracheal Catheter: Direct tracheal O₂ delivery via small catheter; reduces anatomic dead space, improves efficiency (requires surgical placement and meticulous care).
• Caution: Pulse‑dose devices must be titrated to ensure adequate SpO₂ during activity and sleep; may not provide equivalent FiO₂ to continuous flow in all patients.

5. Hyperbaric Oxygen Therapy (HBOT)

Mechanism of Action

• Patient breathes 100% O₂ at pressures >1 atmosphere absolute (ATA), typically 2–3 ATA.
• Henry's Law: Amount of dissolved gas in liquid is proportional to partial pressure. At 3 ATA, PaO₂ reaches ~2,000 mmHg; dissolved O₂ in plasma alone can meet tissue metabolic demands (independent of hemoglobin).
• Physiologic effects:
  • Reduces gas bubble size (Boyle's Law) — essential for decompression sickness and arterial gas embolism.
  • Enhances neutrophil oxidative killing, inhibits bacterial toxin production (clostridial myonecrosis).
  • Promotes angiogenesis and wound healing (radiation injury, chronic wounds).
  • Displaces CO from hemoglobin and cytochrome oxidase (CO poisoning).

Indications for HBOT (Undersea & Hyperbaric Medical Society)

Contraindications & Complications

• Absolute contraindication: Untreated pneumothorax (risk of tension pneumothorax during decompression).
• Relative contraindications: Severe COPD (air trapping), recent thoracic surgery, uncontrolled seizures, pregnancy (except CO poisoning), claustrophobia.
• Complications: Barotrauma (ear, sinus, lung), oxygen toxicity (CNS: seizures; pulmonary: tracheobronchitis, ↓ DLCO), transient myopia (reversible lens swelling), confinement anxiety.

6. Special Considerations in Oxygen Therapy

Humidification

• Dry O₂ (<5% relative humidity) causes mucosal drying, impaired mucociliary clearance, nasal discomfort, and epistaxis.
• Indications for humidification: Flows >4 L/min via nasal cannula, any flow via tracheostomy, HFNC (always heated/humidified), or patient discomfort.
• Bubble humidifiers (unheated) provide ~30‑40% relative humidity; heated humidifiers (HFNC, ventilators) provide 100% relative humidity at body temperature.

Oxygen Toxicity

• Pulmonary (Lorrain‑Smith effect): Prolonged exposure to FiO₂ >0.6 → tracheobronchitis, impaired mucociliary clearance, absorption atelectasis, and ultimately diffuse alveolar damage (ARDS).
• CNS (Paul Bert effect): Hyperbaric O₂ → seizures (generally self‑limited, no long‑term sequelae).
• Retinopathy of Prematurity (ROP): Hyperoxia in preterm infants → retinal neovascularization, retinal detachment, blindness. Strict SpO₂ targets (typically 90‑95%) in NICU.

Fire Safety

• Oxygen is not flammable, but it vigorously supports combustion. Materials that burn in air will burn explosively in O₂‑enriched environments.
• Safety rules: No smoking, no open flames, keep O₂ equipment ≥6‑10 feet from heat sources, avoid petroleum‑based products (Vaseline) near O₂ (use water‑based lubricants), secure cylinders to prevent tipping.

7. Quick Reference: Oxygen Device Selection

FiO₂ Estimation Reference Table (Low‑Flow Systems)

Note: Actual FiO₂ varies with patient's minute ventilation and inspiratory flow.