Medical Gas Systems Vocabulary

Oxygen Therapy Requirements

• Therapy gases must contain at least 20\% \text{O}_2 (ambient air = 21\%).

Oxygen Production & Purity

• Commercial production: fractional distillation of air.
• Alternate methods (less common): molecular sieve, electrolysis, chemical decomposition.
• FDA purity standard for medical \text{O}_2: 99\% (min).

Therapeutic & Specialty Gases

• Helium: valued for very low density → lowers WOB when mixed with \text{O}2 (Heliox must contain \ge 20\% \text{O}2).
• Nitric oxide (NO): inhaled to treat hypoxic respiratory failure/​pulmonary HTN.

Gas Cylinders & Testing

• Material: seamless steel (high-pressure).
• DOT hydrostatic test: every 5\text{ yr} (can be 10\text{ yr} with star mark).
• All cylinders have pressure-relief device on valve stem (rupture disk, fusible plug, spring valve) to prevent rupture.

Cylinder Pressure Interpretation & Monitoring

• Gas cylinders (non-liquid) → pressure is proportional to remaining volume.
  • Example: 2200 → 550 psig ⇒ \frac{550}{2200}=\tfrac14 full.
• Liquid \text{O}_2 cylinders: weigh to determine contents; gauge shows constant pressure while liquid remains.

Estimating Cylinder Duration

Formula (gaseous cylinders):
\text{Time(min)} = \frac{P_{\text{psig}} \times K}{\text{Flow(L/min)}}
where K=0.28 (E) & K=3.14 (H).

• E cylinder @ 800 psig, 3 L/min → \frac{800\times0.28}{3}=74\text{ min}.
• Full E (2200 psig), 3 L/min → \frac{2200\times0.28}{3}\approx205\text{ min}=3\text{ h }25\text{ min}.
• H cylinder @ 2000 psig, 6 L/min → \frac{2000\times3.14}{6}=1047\text{ min}=17\text{ h }27\text{ min}.

Bulk Gas Supply

• Hospital pipeline working pressure: 50\text{ psig} for \text{O}_2, air.

Gas Delivery Devices

• Small diaphragm/turbine compressors can power:
  • small-volume nebulizers
  • IPPB devices
  (large-volume jet neb often needs central gas).
• Bedside outlet → use Thorpe-tube flowmeter (pressure-compensated).
• Always "crack" cylinder briefly before attaching regulator to clear dust/debris.

Types of Hypoxia

• Hypoxic (anoxic): low P{a\,O2} entering blood.
• Circulatory (stagnant): inadequate tissue blood flow.
• Anemic: reduced \text{Hb} concentration/​carrying capacity.
• Histotoxic: cells unable to utilize \text{O}_2 (e.g., cyanide).

Color Coding (U.S.)

• \text{O}_2 – Green
• Air – Yellow
• Helium – Brown
• Heliox ((\text{He}\,/\,\text{O}_2)) – Brown/Green

Safety Connection Systems

• Large cylinders (≥ F) – American Standard Safety System (ASSS).
• Small cylinders (≤ E) – Pin Index Safety System (PISS).
  • Air: pins 1\,\&\,5
  • \text{O}_2: pins 2\,\&\,5
• Low-pressure (< 200 psig) equipment & wall outlets – Diameter Index Safety System (DISS).

Medical Air Quality

• Must be particle-free, oil-free, and dry (meet all three criteria).