Chapter 41 Notes – Storage & Delivery of Medical Gases

Learning Objectives

• Production pathways, clinical applications, storage forms, flow-time calculations, handling practices, supply systems, safety mechanisms, and delivery devices for medical gases
• Individual competencies include the ability to:
  • Describe industrial/bed-side generation of O$_2$ and other gases
  • Distinguish gaseous vs. cryogenic storage
  • Compute remaining therapy time for both compressed and liquid cylinders
  • Identify/operate bulk supply, pipeline, and bedside regulation equipment
  • Apply ASSS, PISS, DISS, and quick-connect safety indexing
  • Troubleshoot failures or malfunctions at any point in the supply chain

Classification & Clinical Roles of Medical Gases

• Laboratory gases → calibration & diagnostics (e.g., CO$_2$ mixtures for blood-gas analyzers)
• Therapeutic gases → symptom relief & oxygenation (O$_2$, Air, Heliox, NO)
• Anesthetic gases → combined with O$2$ to induce/maintain surgical anesthesia (N$2$O ± volatile agents)

Physical & Combustion Properties (STPD unless stated)

• Oxygen
  • Colorless/odorless; density 1.429\,\text{g·L}^{-1} (≈10 % heavier than air)
  • Only 3.3\,\text{mL} dissolve in 100\,\text{mL} H$_2$O at 1 atm → low solubility
  • Non-flammable yet vigorously accelerates combustion; burning velocity ↑ with either ↑\,%\text{O}_2 or ↑total pressure
• Air
  • 20.95 % O$2$, 78.1 % N$2$, ≈ 1 % trace gases; density 1.29\,\text{g·L}^{-1}
  • Medical grade: filtered & compressed; water & oil removed
• Carbon Dioxide (CO$_2$)
  • Specific gravity 1.52 (≈1.5× air); extinguishes flame, purity ≥ 99 %
  • Generated by thermal reaction: limestone + H$_2$O
  • Key uses: analyzer calibration, lab diagnostics, certain surgical insufflations
• Helium (He)
  • Density 0.1785\,\text{g·L}^{-1} (≈1/7 density of air) → very low Reynolds number
  • Always blended with ≥20 % O$_2$ (Heliox) to avoid hypoxia
  • Clinical: severe airway obstruction; ↓work of breathing via laminarization of flow
• Nitric Oxide (NO)
  • Toxic, supports combustion; high [ ] causes methemoglobinemia → tissue hypoxia
  • FDA-approved for term/near-term neonates with hypoxic respiratory failure
• Nitrous Oxide (N$_2$O)
  • Slightly sweet odor/taste; potent analgesic–anesthetic
  • Must be given with O$_2$ to avert hypoxemia
  • Produced by thermal decomposition of ammonium nitrate; occupational hazards: neuropathy, fetal anomalies, spontaneous abortion on chronic exposure

Industrial / Bedside Production of O$_2$

• Small-scale chemical: Electrolysis of H$2$O or decomposition of NaClO$3$
• Fractional distillation of air (dominant, least costly)
  1. Air filtered (pollutants, H$2$O, CO$2$ removed)
  2. Compressed & cooled → liquefaction (Joule–Thomson effect)
  3. Slow warming → N$2$ boils off first; remaining liquid ≈ 99 % O$2$
• Physical separation
  • Molecular sieve: zeolite absorbs N$2$, trace gases; outputs ≥90 % O$2$
  • Membrane concentrator: semipermeable polymer preferentially allows O$_2$ diffusion

Cylinder Construction & Identification

• Seamless steel; DOT type 3A (carbon steel) or 3AA (heat-treated alloy)
• Shoulder stamping → size, service pressure, serial #, manufacturer, re-test dates
• Color coding (U.S.): O$2$ = green; Air = yellow; CO$2$ = gray; He = brown; Heliox = brown/green; N$_2$O = blue

Periodic Hydrostatic Testing

• Every 5 or 10 yrs; pressurized to \tfrac{5}{3} service pressure
• Inspect leakage, elastic expansion, wall integrity; results re-stamped

Cylinder Safety Relief Devices

• Frangible disk (pressure-activated rupture)
• Fusible plug (melts at set °C)
• Spring-loaded pop-off (reseats after venting)

Charging Specifications

• Compressed gases → filled to labeled service pressure (70 °F); permissible overfill = +10 %
• Liquefied gases (CO$2$, N$2$O)
  • Filled by filling density = \dfrac{\text{mass\,(liquid gas)}}{\text{mass\,(H}_2\text{O that would fill cylinder)}}

Gauging Contents

• Gas cylinders: pressure ∝ volume (Boyle’s law)
• Liquid cylinders: pressure reflects vapor–liquid equilibrium, NOT remaining content → weigh for inventory

Remaining-Time Calculations (Compressed O$_2$)

• Cylinder conversion factor
  \text{Factor\,(L·psig}^{-1}) = \dfrac{\text{cubic ft (full)}}{\text{pressure (full psig)}} \times 28.3
• Duration of flow
  \text{Time\,(min)} = \dfrac{P_{\text{current\,(psig)}} \times \text{Factor}}{\text{Flow\,(L·min}^{-1})}
• Remember to reserve ≈ 200 psig for safety

Remaining-Time Calculations (Liquid O$_2$)

• Density relationship: 1 L liquid O$_2$ weighs 2.5 lb and expands to 860 L gas
• Gas available
  \text{Liters} = \dfrac{\text{Weight\,(lb)} \times 860}{2.5}
• Duration
  \text{Time\,(min)} = \dfrac{\text{Liters available}}{\text{Flow\,(L·min}^{-1})}

Cylinder Handling: Storage → Transport → Bedside Use

• Storage
  • Rack or chain; <125 °F (52 °C); away from combustibles & heat sources
  • Segregate full/empty; cap in place when idle Flammables stored separately; liquid O$_2$ cool, ventilated area (continuous venting)
• Transport
  • Use cart with securing strap; valve cap on; avoid impact, drag, roll; only labeled cylinders
• Clinical use
  • Secure to wall/cart; “crack” valve 1st (no one in front)
  • No oil/grease; replace damaged valves; respect color/markings; no heat sources nearby
  • Connection standards: ASSS (H/G), PISS (E); post “No Smoking” where O$_2$ in use

Bulk Oxygen Systems

• Provide ≥20,000 ft$^3$ supply; gas or liquid form
• Advantages: lower long-term cost, fewer change-overs, space-efficient, operates at low P (safer), centralized regulation
• Common configurations
  1. Cylinder manifold (alternating): two banks of H/K cylinders; auto-switch when primary P drops
  2. Cylinder supply with reserve: primary → secondary → reserve hierarchy
  3. Bulk liquid with reserve (most hospitals): vacuum-insulated evaporator stores cryogenic O$_2$; backup cylinders/stand tank mandatory
• Contingency: staff must quickly deploy cylinders / bag-mask when bulk failure alarm sounds

Central Pipeline Distribution

• Reduced to working 50\,\text{psig} at source
• Main alarm for pressure loss; zone valves for maintenance/fire isolation
• Terminal outlets: DISS or quick-connect (gas-specific geometries)

Indexed Connector Safety Systems

• ASSS → large cylinders; thread size, pitch, nipple groove pattern unique per gas
• PISS → E-size & smaller;
  • O$_2$: pin positions 2–5
  • Air: positions 1–5
• DISS → ≤200 psig low-pressure

Regulators & Flow Control Devices

• High-pressure reducing valve: single- or multi-stage; preset or adjustable; drops cylinder pressure → 50 psig
• Regulator = integrated reducing valve + flowmeter

Three Categories of Low-Pressure Flowmeters

1. Flow Restrictor
   • Fixed orifice; cheapest; delivers specific flow when upstream P_{\text{in}} constant
   • Governing relation R = \dfrac{P1 - P2}{V} \;\Rightarrow\; V = \dfrac{P1 - P2}{R}
   • Not adjustable once installed; inaccurate if supply P fluctuates
2. Bourdon Gauge
   • Fixed orifice + variable spring‐driven pressure gauge
   • Paired with adjustable pressure reducer; indicates (approximates) flow
   • Advantage: not gravity-dependent → ideal for transport
   • Over-reads when downstream obstruction elevates back-pressure
3. Thorpe Tube (variable orifice, constant pressure)
   • Tapered glass tube + float; supplied at 50 psig
   • Flow ↑ as needle valve enlarges inlet orifice; float rises until drag = weight
   • Types:
     • Pressure-compensated: needle valve distal; calibrated at 50 psig; accurate despite downstream resistance; gravity-dependent (vertical)
     • Uncompensated: needle valve proximal; calibrated at atmospheric P; reads low when resistance distal increases

Integrated O$_2$ Cylinders

• All-in-one units (valve + regulator + flowmeter); eliminate keys/wrenches
• Audible/visual alert at <15 min remaining

Ethical / Safety Implications

• Patient reliance on continuous O$_2$ or ventilator support demands redundant systems & rapid emergency protocols
• Long-term occupational exposure to waste anesthetic (N$_2$O) or high NO levels requires scavenging & monitoring; ethical duty to protect staff pregnancies and neurologic health
• Accurate labeling, color coding, and indexing directly protect against fatal misconnections (e.g., O$2$ vs. N$2$ lines)

Connections to Prior Knowledge / Practice

• Gas laws (Boyle, Dalton, Joule–Thomson) underpin pressure-volume behavior, distillation, and cylinder calculations
• Fluid dynamics: Reynolds number and density dependent laminar vs. turbulent flow → rationale for Heliox
• Infection-control parallels: proper storage/handling prevents not microbes but fire/explosion hazards

Practical Tips & Common Pitfalls

• Always leave >200 psig “cushion” in cylinders; replace before empty to avoid debris entrainment
• Never rely on pressure gauge to estimate liquid CO$2$ or N$2$O content—use weight
• When Bourdon gauge indicates flow yet bagging feels difficult, suspect downstream occlusion & over-registration
• In transport, convert Thorpe tube to Bourdon gauge