LZ

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