Medical Gases & Respiratory Physics Comprehensive Notes

Therapeutic Medical Gases

• Oxygen (O₂)

  • Primary indications: treat hypoxemia, decrease cardiac work, components of institutional “preventive bundles” (STEMI, impending MI – 2 L/min by nasal cannula –, sepsis, etc.)
  • FDA classifies O₂ as a drug ⇒ must document a clinical reason before administration.
  • Not flammable, but vigorously supports combustion.
  • Hospital bulk O₂ is higher-purity than home-care O₂.

• Helium (He) / Heliox (He–O₂ mixtures)

  • Used for airway obstruction because the low-density He molecule “slips” past partial blockages and carries O₂ with it.
  • Decreases work of breathing and turbulent flow in narrowed airways.

• Carbon Dioxide (CO₂)

  • Clinical use: pulmonary-function labs (complete PFT; diffusing-capacity testing).
  • Diffusion rate across the alveolar–capillary (A-C) membrane ≈ 20 × faster than O₂.

• Nitric Oxide (NO)

  • Potent, selective pulmonary vasodilator.
  • Typical loading/therapeutic dose: 20\ \text{ppm}.
  • Routinely used in neonates (≈ ≥ 34 weeks gestation) with severe hypoxemia / PPHN.
  • Contra-indicated if a patent ductus arteriosus (PDA) remains open – vasodilation perpetuates shunting and worsens hypoxemia.

• Nitrous Oxide (N₂O)

  • “Laughing gas”; anesthetic/analgesic, not a routine RT drug.

• Nitrogen (N₂)

  • Inert, non-flammable “space-occupier.”
  • Physiological role: maintains functional residual capacity and alveolar stability.

Gas Cylinder Safety & Central Piping

• Connection systems
  • ASSS – large H/K tanks
  • PISS – small E tanks
  • DISS – threaded, station-outlet (wall) connectors at the flowmeter.
• Cylinder duration (rule-of-thumb example)
  • H-tank in red, 4 L/min ➝ ≈ 6 h remaining.
  • Always apply the duration formula in practice.
• Central O₂ shut-off (zone valves)
  • Decision authority: Fire Department during emergencies, not administrators or charge nurses – a safety/ethical responsibility for RTs.

States & Properties of Matter

• Four classical states: solid, liquid, gas, plasma.
• Temperature is the primary determinant of phase (1st Law of Thermodynamics).

Liquids

• Conform to container shape; exert hydrostatic pressure P = \rho gh (density × depth).
• Clinical ties: blood pressure and flow; urine “specific gravity” (normal 1.005 – 1.030) reflecting hydration – directly affects mucus viscosity.
• Capillary action: liquid moving against gravity via adhesive/cohesive forces. Examples: absorbent dressings, capillary blood-gas (CBG) sampling.

Gases

• Weak intermolecular attraction, expand to fill container.
• Warming ⇒ ↑ kinetic energy, ↑ capacity to carry water vapor.
• Fully saturated tracheobronchial gas at body temperature has an absolute humidity of 43.8 mg H₂O · L⁻¹.

Airway Humidity & Relative Humidity (RH)

• Equation: RH = \dfrac{\text{Content (Absolute)}}{\text{Capacity}} \times 100\%.
• Goal for inhaled gases at the alveolus: \dfrac{43.8}{43.8} = 100\% RH.
• Mechanically ventilated patients require heated humidification to prevent epithelial injury, secretion inspissation, ciliary dysfunction, and infection risk.

Fundamental Gas Laws

• Boyle’s Law: P1 V1 = P2 V2 (pressure and volume inversely proportional). Clinical analogies: pneumothorax, tire puncture.
• Dalton’s Law of Partial Pressures: P{\text{barometric}} = \sum Pi.
  • Example at sea level (~760 mm Hg): P{\text{O}2}\approx 160\ \text{mm Hg}, P{\text{N}2}\approx 593\ \text{mm Hg} ⇒ totals ≈ 760.
• Water vapor also exerts pressure (47 mm Hg at 37 °C); must be subtracted when computing alveolar gas equations.

Heat Transfer & Thermodynamics

• Four clinical modes

  1. Conduction – direct molecular contact (metal spoon in hot soup; skin to cold table).
  2. Convection – heat carried by moving fluid (blood redistributing core heat; microwave heating).
  3. Radiation – infrared transfer without contact (sun warming skin; radiant warmer for neonates).
  4. Evaporation/Vaporization – phase change removes heat (perspiration; humidifier wick).

• Vaporization sub-types

  • Evaporation (below boiling point) – influenced by surface area (shallow pan dries faster).
  • Boiling – at 100^\circ\text{C} at sea level when kinetic energy equals ambient pressure.
  • Critical temperature – immediately above boiling, molecules must leave as gas.

• Altitude effect: ↓ barometric pressure ⇒ ↓ boiling point (water boils sooner in Denver; cooking takes longer).

• Temperature conversion

  • T{\text{F}} = T{\text{C}} \times 1.8 + 32
  • T{\text{C}} = \dfrac{T{\text{F}} - 32}{1.8}

Fluid Dynamics in Respiratory Therapy

Buoyancy (Archimedes’ Principle)

• A body displaces fluid equal to its volume. Pressure beneath > pressure above ⇒ net upward force (why lungs remain partially inflated, why bodies float).

Bernoulli’s Principle

• As the velocity of a fluid increases, its lateral (static) pressure decreases.
• Clinical significance: laminar vs turbulent flow in narrowing airways & capillaries; excessive turbulence raises resistance and risk of barotrauma.

Poiseuille’s Law ("Puce–"/"Poiseuille" in lecture)

• Flow resistance \propto \dfrac{1}{r^4}.
• Halving airway radius ⇒ required driving pressure ↑ by 2^4 = 16 ×.
• Practical: downsizing an ET tube or edema doubling resistance; ventilator must compensate with higher pressures.

Venturi Effect

• High-velocity jet creates sub-atmospheric (negative) lateral pressure, entraining ambient air – basis for air-entrainment (Venturi) masks & large-volume nebulizers (FiO₂ dilution window).

Coandă Effect

• Jet stream adheres to nearby contour; in RT circuits guides condensate away from patient and shapes aerosol pathways.

Viscosity, Surface Tension & Surfactant

• Viscosity – internal friction/opposition to flow (honey > water).
• Surface tension – cohesive force at liquid surface; strong in pure water (H–O bonds).
  • Demonstrations: over-filled water glass; spherical raindrops.
• Laplace’s Law (spheres): P = \dfrac{2T}{r}.
  • Small alveolus (↓r) needs ↑ pressure to stay open; without surfactant, collapse occurs (atelectasis).
• Pulmonary surfactant (Type II cells) lowers surface tension, equalizes opening pressures across varying alveolar sizes.
  • Premature neonates lack surfactant ➝ RDS; exogenous surfactant therapy is lifesaving.

Diffusion & Fick’s Law

• Gas movement across a membrane depends on area, thickness, solubility, and pressure gradient.
• Fick’s simplified lung form:
  \text{Rate}{\text{gas}} = DL \times (P1 - P2)
  where D_L = lung diffusing capacity.
• CO₂ diffuses ≈ 20 × faster than O₂ because of higher solubility.

Key Equations & Numbers to Memorize

• Boyle: P1 V1 = P2 V2
• Dalton: P{\text{total}} = \sum Pi
• Relative humidity: RH = \dfrac{\text{Absolute}}{43.8\ \text{mg·L}^{-1}} \times 100\%
• Poiseuille resistance: R \propto \dfrac{1}{r^4}
• Laplace (sphere): P = \dfrac{2T}{r}
• Temperature: T{F}=1.8T{C}+32, T{C}=\dfrac{T{F}-32}{1.8}
• CO₂ diffusion rate ≈ 20 × O₂.
• Neonatal NO dose: 20\ \text{ppm}.

Ethical & Practical Reminders

• Oxygen is a prescription drug: always confirm indication, flow, and delivery interface.
• During fires or disasters, only the fire department authorizes zone valve shutdown.
• Document tank pressure, calculate duration before transport.
• Ensure humidification to protect airway mucosa; monitor for rain-out, tubing positioning (Coandă).
• Adjust ventilator pressures when airway radius changes (edema, tube size) per Poiseuille.

Suggested Self-Study / Lab Connections

• Watch video on neonatal capillary blood gas sampling (CBG).
• Practice cylinder duration calculations for E, H, and portable liquid units.
• Review complete PFT diffusing-capacity procedure (DLCO) involving CO and CO₂.
• Observe Venturi mask, large-volume nebulizer settings and their actual FiO₂ with an analyzer.