section 1

RCP Section 1: Medical Gas Supply Equipment

Kinetic Theory of Gases

  • Gases Composition: Gases are comprised of discrete molecules in random motion.

  • Molecular Behavior:

    • Collisions between molecules are elastic - they bounce off one another.

    • Their activity is temperature-dependent.

    • No physical attraction exists between molecules.

Gas Pressure

  • Definition: Gas pressure is the force exerted by a gas per unit area on an object.

  • Influencing Factors:

    • Results from molecular collisions with each other and the walls of their container.

    • Temperature affects the velocity of gas molecules, with a rise in temperature leading to increased kinetic energy.

Pascal's Law

  • Principle: The pressure exerted by a confined fluid is undiminished and transmitted equally in all directions.

  • This principle explains that pressure against the wall of a container acts uniformly throughout.

Instruments Measuring Gas Pressure

cmH2O

  • Definition: Unit of measurement for gas pressure, equivalent to centimeters of water pressure.

Mercury Barometer

  • Description: Measures air pressure changes using a glass tube partially filled with mercury.

  • Function: The mercury column opposes atmospheric pressure to determine atmospheric pressure levels.

Aneroid Barometer

  • Description: Changes in air pressure are measured without liquids, utilizing a metal container, a spring, and a pointer.

Bourdon Gauge

  • Design: A dial that shows oxygen flow in liters per minute.

  • Construction: Consists of a hollow coiled metal tube with an elliptical cross-section, commonly found on medical gas cylinders.

Gas Flow Principles

Gas Flow

  • Definition: Gas flow occurs due to pressure differences between two points, moving from high to low pressure.

  • Factors Influencing Flow Velocity:

    • Pressure gradient

    • Size of the opening

Bernoulli's Principle

  • As gas flows through a tube, an increase in the velocity results in a decrease in lateral pressure, maintaining constant total energy.

Viscous Shearing

  • Definition: A high-velocity jet injected into a stationary gas creating shearing forces.

Vorticity

  • Refers to the decrease in jet gas velocity due to the swirling mixture of two gases.

Ducted Ejectors

  • Utilize a nozzle, viscous shearing, and vorticity to maintain total flow efficiently without changing tube diameter.

  • Used in nebulizers for high velocity and stable pressure, less affected by downstream pressure changes.

Venturi's Principle

  • An extension of Bernoulli's principle where a tube with a larger radius (less than 15°) draws ambient air into a high-velocity gas flow.

Oxygen Delivery Devices

HAFOE (High Air Flow with Oxygen Enrichment) Mask

  • Also known as the Venturi mask.

  • Increased downstream pressure decreases ambient air entrainments, resulting in higher fractional inspired oxygen (FiO2).

Venturi Mask

  • Designed for oxygen delivery, featuring escape holes for exhaled air and adjustable color-coded entrainment ports for oxygen concentration.

Flow Dynamics and Gas Behavior

Choked Flow

  • Occurs when actual pressure ratio is below the critical ratio, restricting flow.

  • Defined as when gas velocity stabilizes at a specific pressure, for example, at 26 psi driving pressure in nebulizers.

Reynolds Number

  • Determines gas flow type (laminar vs. turbulent).

    • Laminar Flow: Smooth and less energy intensive (Re < 2000).

    • Turbulent Flow: Erratic and more energy-consuming (Re > 2000).

    • Formula: R = (velocity x density x diameter) / viscosity.

Poiseuille's Law

  • Describes the relationship between flow rate, pressure gradient, vessel radius, and viscosity.

  • As the tube radius is halved, resistance to flow increases by a factor of 16.

STP (Standard Temperature and Pressure)

  • STP involves conditions of 0.00°C and 1 atm (760 mmHg).

  • Conversion from Celsius to Kelvin: K = C° + 273.

Gas Laws

Boyle's Law

  • Formula: P1V1 = P2V2.

  • Describes the relationship between pressure and volume at constant temperature.

Charles' Law

  • Formula: V1/T1 = V2/T2.

  • States that volume increases with temperature at constant pressure.

Gay-Lussac's Law

  • Formula: P1/T1 = P2/T2.

  • Describes pressure changes with temperature when volume is constant.

Combined Gas Law

  • Formula: P1V1/T1 = P2V2/T2.

  • Integrates pressure, volume, and temperature relationships for a fixed amount of gas.

Dalton's Law of Partial Pressures

  • The total pressure of a gas mixture equals the sum of the partial pressures of individual gases in the mixture.

Fick's Law of Diffusion

  • The net diffusion rate of a gas across a fluid membrane is proportional to the difference in partial pressure and inversely proportional to the membrane's thickness.

Henry's Law

  • States gas solubility in liquid is directly proportional to its partial pressure above the liquid.

Graham's Law

  • The rate of diffusion of a gas is inversely proportional to the square root of its molar mass.

Medical Equipment and Properties

Water Content in Gas

  • At 37°C (body temperature), gas can hold 44 mg of water per liter; at 760 mmHg (1 atm), 47 mmHg is attributed to water vapor pressure.

FiO2 (Fraction of Inspired Oxygen)

  • Represents the percentage of oxygen in inhaled gas.

Compressors

  • Provide compressed air for equipment while mixing with O2; must be oil-free to avoid fire hazards.

  • Operate under 50 psi for safe use.

  • Types:

    • Piston

    • Diaphragm

    • Centrifugal

Piston Compressor

  • Utilizes a motor-driven piston to compress atmospheric air.

Diaphragm Compressor

  • Employs a flexible diaphragm driven by an electric motor suitable for powering small nebulizers.

Centrifugal Compressor

  • A dynamic compressor that uses an electrically powered impeller to generate large volumes of air suitable for medical facilities.

Oxygen Production Methods

Fractional Distillation (O2)

  • Air is liquefied, cooled, and subsequently heated.

  • Nitrogen and trace gases are separated due to lower boiling points, resulting in 99.5% pure oxygen.

Physical Separation (O2)

  • Involves molecular sieves and semi-permeable plastic membranes for oxygen extraction.

Concentrators

  • Medical devices that separate nitrogen from ambient air, allowing for breathability of up to 95% pure oxygen.

Molecular Sieve

  • A material featuring small, uniform holes that selective allow gas molecules of a certain size to pass while blocking larger molecules.

Membrane Enricher

  • Uses a semipermeable polymer to allow O2 molecules to pass based on size differences, designed for oxygen enrichment.

Medical Gas Storage and Supply

Bulk Supply Systems

  • Liquid reservoir systems containing large amounts of gas; 1 ft³ of liquid O2 is equivalent to 861 ft³ gas.

Portable Reservoirs

  • Liquid reservoir systems with liquid duration based on weight measurement, accommodating both stationary home models and ambulatory versions.

Supply (Piping Systems)

  • Includes manifolds, bulk liquid systems, or both, with a required reserve supply tested to 1.5 times working pressure.

Safety Features and Cylinder Information

Safety Features (Piping Systems)

  • Alarms to alert drops in system pressure.

  • Zone valves provided for emergency shutoff (e.g., fire).

  • Pressure sensors ensure maintained levels at 50 psi.

Cylinder Designations

  • E Cylinder: Holds 625L of oxygen, with a full pressure of 2200 psi.

  • H Cylinder: Large stationary oxygen tank holding 6900L of oxygen.

Cylinder Construction

  • Regulated by DOT, using chrome-molybdenum steel, aluminum alloy, and lightweight aluminum with carbon fiber wrap.

  • Fill to 3,000 psi with cylinder markings indicating important manufacturing and test information.

Color-Coded Gas Cylinders

  • Green: Oxygen (O2)

  • Gray: Carbon Dioxide (CO2)

  • Light Blue: Nitrous Oxide (N2O)

  • Orange: Cyclopropane (C3H6), very reactive.

  • Brown: Helium (He)

  • Yellow: Air

  • Brown and Green: Heliox

  • Gray and Green: CO2 + O2

Cylinder and Valve Specifications

Tank Factors

  • Calculation: Size (ft³) x 28.3L/ft³ / Pressure when full (psi).

  • Table for E and H cylinders detailing tank transport safety factors.

Cylinder Valves Overview

  • Direct Acting Cylinder Valve: Needle valve with two washers; opens directly for gases like oxygen and helium.

  • Diaphragm Cylinder Valve: An indirect acting system utilized in larger tanks (H cylinder).

Safety Systems

  • American Standard Safety System (ASSS): Prevents the attachment of incorrect equipment through unique threading.

  • Pin Index Safety System (PISS): Sorts connections using different pin placements to avoid incorrect gases.

Safety Mechanisms and Reducing Valves

Safety Mechanisms

  • Frangible Disk: A thin metal disk that ruptures excessively high pressure.

  • Fusible Plug: Melts at temperatures exceeding 208-220°F.

Reducing Valves

  • Reduce pressure to a working pressure of 50 psi, with various types suited for different applications, particularly medical ventilators.

  • Single Stage Reducing Valve: Operates with internal spring tension and is responsive to gas pressure variations.

  • Multistage Reducing Valve: Provides more precision and flow by combining multiple stages.

Regulators and Flow Meters

Regulators

  • A combination of a reducing valve and a flowmeter.

  • Types of flowmeters include Bourdon types and Thorpe tubes.

Oxygen Flowmeters

  • Include fixed orifice flowmeters that manage gas flow based on the orifice size, common in E cylinder applications.

  • Bourdon-Gauge Flowmeter: Combines pressure reading with flow measurement but can misrepresent flow under back pressure.

Main Supply Shut-Off Valve

  • Controls gas flow from the main source to all dispensing locations, centrally shutting off gas supply when needed.

Flowmeter Types

  • Uncompensated Thorpe Tube Flowmeter: Features needle valves proximal to the flow tube and isn't compensated for back pressures.

  • Compensated Thorpe Tube Flowmeter: Needle valves distal to the flow tube allow the ball to jump without affecting pressure.

Flowmeter Ranges and Proportioners

Flowmeter Ranges

  • Typical ranges from 0-15 LPM; lower ranges (0-3 LPM) are for pediatric/neonatal use or COPD patients to match smaller airways.

  • High flowmeter ranges can reach up to 75 LPM, suitable for CPAP or Vapotherm.

Proportioners (Air-Oxygen Blenders)

  • Facilitate precise mixing of air and oxygen, providing 50 psi with concentration control via proportioning valves and built-in alarms for pressure drops.

Demand Pulse Flow Delivery

Demand Pulse Flow O2 Delivery Devices

  • Deliver oxygen only during inspiration, minimizing the unwanted oxygenation of anatomical dead space.

  • Known as Oxygen Conserving Devices (OCD), these often require exercise oximetry studies for appropriate use.

Diameter Index Safety System (DISS)

  • Ensures non-interchangeable, gas-specific connections to the pipeline, preventing incorrect gas delivery to patients.