Medical Gases & Oxygen Therapy – Part 2 (RC121)

Oxygen Therapy: Goals & Assessment

• Primary goals: correct hypoxemia, ↓ work of heart & breathing
• Assess need via: SpO2, SaO2, PaO_2 (ABG = gold standard)
• High-risk conditions: post-op, CO/CN poisoning, shock, trauma, AMI, premature infants
• Key manifestations: tachypnea, tachycardia (first sign), cyanosis, dyspnea, distress

Clinical Signs of Hypoxia

• Respiratory: tachypnea, dyspnea, cyanosis
• Cardiovascular: tachycardia → bradycardia, arrhythmias, hypertension → hypotension, peripheral vasoconstriction
• Neurologic: restlessness, disorientation, H/A, somnolence, blurred vision, impaired judgment → coma
• Other: digital clubbing

Hazards of Oxygen Therapy

• Oxygen toxicity (lungs & CNS)
• Ventilatory depression (COPD hypoxic-drive suppression)
• Retinopathy of prematurity (ROP)
• Absorption atelectasis (N₂ washout, FiO_2 \ge 0.50)
• Fire risk

Oxygen Toxicity Essentials

• Risk ∝ FiO_2 & exposure time
• Rule: limit FiO_2 = 1.0 to < 24 h; ↓ to \le 0.70 by 48 h, \le 0.50 by 5 d
• Symptoms: substernal pain, cough, dyspnea, anxiety, numbness
• Lung damage: pulmonary edema, A/C-membrane thickening, shunt ↑, VC ↓, type-I cell loss

O₂-Induced Hypoventilation (COPD)

• High PaO2 suppresses peripheral chemoreceptors → ↓ drive → PaCO2 ↑

Retinopathy of Prematurity

• Excessive PaO2 → retinal vasoconstriction/necrosis; keep PaO2 < 80\,mmHg

Absorption Atelectasis

• High FiO2 washes out N₂ → alveolar collapse; highest risk with low VT (sedation, pain, CNS issues)

Oxygen Delivery Systems Overview

• Classified by flow & performance:
  • Low-flow (variable) FiO2 \approx 0.24–0.44 • Reservoir (variable) FiO2 \approx 0.24–1.00
  • High-flow (fixed) precise FiO_2, total flow ≥ 40 L/min
  • Enclosures (isolette, hood, tent)

Low-Flow Devices

• Nasal cannula: flow 0.25–6\,L/min; FiO_2 rises \approx 0.04 per L (1 L = 0.24, 6 L = 0.44)
• Transtracheal catheter: flow 0.25–4\,L/min; FiO_2 \approx 0.22–0.35; surgical placement

Reservoir Devices

• Reservoir cannula: flow 0.25–4 L/min (↓ O₂ use)
• Simple mask: flow 5–10 L/min (≥5 to flush CO₂); FiO_2 \approx 0.35–0.55
• Partial rebreather: flow 10–15 L/min; FiO_2 \approx 0.60–0.80
• Non-rebreather: flow 10–15 L/min; FiO_2 \approx 0.80–1.00 (one-way valves)

High-Flow Devices

• Air-entrainment ("Venti") mask: FiO_2 = 0.24–0.50, fixed; jet + entrainment ports
• Air-entrainment nebulizer / blender systems: mix pressurized air & O₂ for precise FiO_2 & high total flow
• High-flow nasal cannula: heated/humidified flow ≤ 60 L/min, precise FiO_2, provides ~3–5 cmH₂O PEEP, washes out dead space

Essential Calculations

• FiO2 = \dfrac{(Air\,flow \times 0.21) + (O2\,flow \times 1.0)}{Total\,flow}
• Air/O₂ ratio: \dfrac{Air}{O2} = \dfrac{100-FiO2\,(\%)}{FiO_2\,(\%) - 21}
  • Common ratios: 24\% = 25:1, 28\% = 10:1, 35\% = 5:1, 40\% = 3:1, 50\% \approx 1.7:1, 60\% = 1:1
• Total flow = (sum of ratio parts) × set O₂ flow; ensure ≥ 40 L/min

Enclosures

• Isolette: flow 8–15 L/min, temp control; FiO_2 variable
• Oxyhood: flow ≥7 L/min, FiO_2 = 0.21–1.00 (fixed)
• Oxygen tent: flow 12–15 L/min, FiO_2 \approx 0.40–0.50 (variable), adds humidity

Oxygen Concentrators

• Molecular-sieve: zeolite removes N₂ → >90\% O₂ (home use)
• Membrane: semipermeable filter → \approx 40\% O₂

Hyperbaric Oxygen Therapy (HBO)

• Pressure >1 ATA (usual 2–3 ATA)
• Effects: bubble reduction, hyperoxygenation, vasoconstriction→edema ↓, immune enhancement, neovascularization
• Indications: decompression sickness, air embolism, CO/CN poisoning, gas gangrene, ischemic grafts, problem wounds, osteomyelitis, etc.
• Complications: barotrauma (ear, sinus, lung), O₂ toxicity (CNS, pulm), fire, visual changes, claustrophobia, ↓ CO

Oxygen Analyzers

• Uses: verify FiO_2 in fixed-performance devices; place sensor near patient
• Types:
  • Physical/paramagnetic (non-continuous, all gases)
  • Electric (thermal conductivity; no flammable gases)
  • Electrochemical – polarographic & galvanic (continuous bedside standard)
• Calibration (two-point): room air 21\% → 100\% O₂ → room air 21\%

Pulse Oximetry Basics

• Measures SpO_2 via spectrophotometry + plethysmography
• Errors: sensor misalignment, ambient light, low perfusion, motion, dark pigment/nail polish, high COHb or MetHb