Medical Gases and Oxygen Therapy – Part I

Classification of Medical Gases

• Laboratory: calibration/diagnostics – N2, He, CO2
• Therapeutic: treat hypoxemia – Air, O2, Heliox \left(He/O2\right), CO2/O2, NO
• Anesthetic: with O2 for surgery – N2O

Fire-Risk Categories

• Non-flammable
• Oxidizing: support combustion (e.g., O_2, Air)
• Flammable

Air (Medical)

• Composition: 78\%\,N2, 21\%\,O2, 1\% trace
• Density =1.29\,g/L @ STPD
• Produced by filtering & compressing ambient air

Oxygen (Medical)

• Atmospheric fraction =20.9\% ; partial pressure =P_B\times0.21 (≈ 159\,mmHg at 760\,mmHg)
• Density =1.429\,g/L @ STPD; accelerates combustion
• Liquid below -184^\circ C

Production

• Fractional distillation (↓ cost, 99.5\% pure)
• Physical separation: molecular sieve (>$90\%$), membrane (~40\%)

Carbon Dioxide

• Non-flammable; therapeutic use rare; 99\% purity

Helium / Heliox

• Density 0.1785\,g/L ; cannot sustain life alone
• Therapeutic ratios: 80/20, 70/30, 60/40 \left(He/O_2\right)
• Flow-conversion factors: 1.8, 1.6, 1.4 respectively
  • Actual flow =\text{factor}\times\text{meter setting}
  • Set flow =\dfrac{\text{desired}}{\text{factor}}

Nitric Oxide

• Potent pulmonary vasodilator; treats PPHN & ARDS at low ppm
• High levels → NO_2 formation, chemical pneumonitis, methemoglobinemia

Nitrous Oxide

• CNS depressant anesthetic; chronic exposure → neuropathy, fetal risk

Gas Cylinders

• DOT specs: 3A (steel), 3AA (high-strength), 3AL (aluminum)
• Common sizes:
  • H: 6900\,L; factor 3.14
  • G: 5260\,L; factor 2.41
  • E: 622\,L; factor 0.28
• Full pressure =2200\,psig ; change at 500\,psig
• Color codes: O2–green, CO2–gray, He–brown, Air–yellow, Heliox–brown shoulder/green body, N_2O–light blue

Duration Formula

\text{Time (min)}=\dfrac{\text{psig}\times\text{factor}}{\text{flow (L/min)}}

Bulk Oxygen & LOX

• Bulk supply >20000\,ft^3 ; piped at 50\,psig
• Liquid O$_2$ critical temp -181.4^\circ F
  • 1\,L LOX =860\,L gas
  • 1\,lb LOX =344\,L gas
• Amount remaining in LOX determined by weight, not pressure

Connector Safety Systems

• ASSS: large cylinders (>E), thread differences
• DISS: low-pressure (<200\,psig) outlets & equipment
• PISS: small cylinders (E or smaller); O$_2$ pins 2\,\&\,5
• Quick-connect: wall outlets (variation of DISS)

Pressure & Flow Devices

• Reducing valve: drops pressure to 50\,psig
• Flowmeter types:
  • Flow restrictor: fixed orifice, preset flow
  • Bourdon gauge: fixed orifice/variable pressure; good for transport; reads high with back-pressure
  • Thorpe tube: variable orifice/constant pressure; accurate, vertical use; pressure-compensated standard
• Regulator = reducing valve + flowmeter

Cylinder Safety

• Crack before regulator attachment; bleed after use
• Secure tanks upright; use carts/stands

Hypoxemia vs. Hypoxia

• Hypoxemia: low PaO_2
• Hypoxia: inadequate tissue O_2 (tachycardia = first sign)

Types of Hypoxia

• Hypoxic: low PaO_2 (e.g., high altitude, V/Q mismatch, shunt)
• Anemic: normal PaO_2, low Hb or abnormal Hb
• Circulatory: low perfusion (stagnant/AV shunt)
• Histotoxic: tissues can’t use O_2 (e.g., cyanide)