Chapter_041

Learning Objectives

  • Explain production methods, clinical applications, and storage formats for medical gases & gas mixtures.
  • Perform run-time calculations for compressed and liquid O$_2$ cylinders.
  • Apply proper procedures for storing, transporting, and using cylinders.
  • Differentiate bulk supply and central piping systems; outline emergency actions if bulk O$_2$ fails.
  • Match pressure-regulating / flow-control devices to clinical situations; assemble, test, and troubleshoot them.
  • Identify and apply all relevant connector safety systems (ASSS, PISS, DISS, quick-connect).

Classification & General Uses of Medical Gases

  • Laboratory gases ⇒ calibration / diagnostics.
  • Therapeutic gases ⇒ relieve symptoms, improve oxygenation in hypoxemia.
  • Anesthetic gases ⇒ combined with O$_2$ during surgery.

Physical & Chemical Characteristics of Individual Gases

• O$_2$

  • Colorless, odorless, tasteless, transparent.
  • STPD density = 1.429\;\text{g·L}^{-1} (heavier than air).
  • Poor water solubility (3.3 mL per 100 mL).
  • Non-flammable yet accelerates combustion; burn rate ↑ with ↑ partial pressure or ↑ concentration.
    • Air
  • 20.95 % O$2$, 78.1 % N$2$, ≈1 % trace gases; density = 1.29\;\text{g·L}^{-1}.
  • Medical-grade → filtered & compressed.
    • CO$_2$
  • Colorless, odorless, specific gravity = 1.52 (≈1.5× air).
  • Non-combustible; produced by heating limestone + H$_2$O; FDA purity ≥99 %.
  • Uses: blood-gas analyzer calibration, lab diagnostics.
    • Helium
  • Inert, density = 0.1785\;\text{g·L}^{-1} (≈1/7 of air).
  • Obtained from natural-gas liquefaction → purity ≥99 %.
  • Must contain ≥20 % O$_2$; Heliox (e.g.
    80 : 20) ↓ airway resistance, promotes laminar flow, ↓ work of breathing in severe obstruction.
    • Nitric Oxide (NO)
  • Colorless, toxic, supports combustion.
  • High doses → methemoglobinemia → tissue hypoxia.
  • FDA-approved for term / near-term neonates with hypoxic respiratory failure.
    • Nitrous Oxide (N$_2$O)
  • Colorless, slightly sweet odor/taste; anesthetic.
  • Always delivered with O$_2$.
  • Made by thermal decomposition of ammonium nitrate.
  • Chronic exposure risks: neuropathy, fetal disorders, spontaneous abortion.

Oxygen Production Methods

• Chemical (small-scale)

  • Electrolysis of H$_2$O.
  • Thermal decomposition of NaClO$_3$.
    • Fractional Distillation (large-scale, cheapest)
  • Filter ambient air (remove pollutants, H$2$O, CO$2$).
  • Compress & rapidly expand (Joule-Thomson) → liquefaction.
  • Slow warming → N$2$ boils off first, leaving liquid O$2$.
    • Physical Separation (small-scale)
  • Molecular sieve concentrators (zeolite removes N$2$, H$2$O, trace gases).
  • Membrane concentrators (semipermeable plastic; N$2$ diffuses faster than O$2$).

Storage of Medical Gases – Cylinders

• Construction & Regulation

  • Seamless steel; DOT 3A (carbon steel) or 3AA (heat-treated alloy).
  • Color coding + shoulder stamp → size, service pressure, serial #, manufacturer, owner.
  • Hydrostatic test q5–10 yr @ \frac{5}{3} service pressure; measures leakage, expansion, wall stress (results re-stamped).
    • Relief Devices
  • Frangible disk (ruptures at set pressure).
  • Fusible plug (melts at specific temperature).
  • Spring-loaded valve (opens @ preset pressure).
    • Filling Rules
  • Compressed gases → filled to stamped service pressure @ 70 °F; may overfill by 10 % if DOT-approved.
  • Liquefied gases (CO$2$, N$