Medical Gases Lecture Notes

Chapter 41: Medical Gases

Introduction to Medical Gases

• Classification of Medical Gases:

  • Laboratory Gases: Used for equipment calibration and diagnostic testing.

  • Therapeutic Gases: Relieve symptoms and improve oxygenation in patients with hypoxemia.

  • Anesthetic Gases: Used in combination with oxygen (O2) to provide anesthesia during surgery.

Characteristics of Medical Gases

Oxygen (O2)

• Physical Properties:

  • Colorless, odorless, transparent, and tasteless.

  • Density at standard temperature and pressure (STPD): $1.429 ext{ g/L}$.

  • Slightly heavier than air: $1.29 ext{ g/L}$.

  • Not soluble in water.

• Combustion Properties:

  • Nonflammable but greatly enhances combustion.

  • Increased burning speed with higher O2 concentration at fixed pressure or increased total pressure at constant gas concentration.

Oxygen Production

• Methods of Production:

  • Chemical Methods:

  • Electrolysis of water.

  • Decomposition of sodium chlorate ($ext{NaClO}_3$).

  • Fractional Distillation:

  • Common method for large quantities.

  • Involves separating air components using heat.

  • Atmospheric air is filtered to remove pollutants, water, and CO2.

  • Air is liquefied by compression and cooled, then nitrogen boils off, leaving concentrated O2.

Characteristics of Other Medical Gases

Air

• Physical Properties:

  • Colorless, odorless gas mixture.

  • Composition:

  • 20.95% (21%) O2

  • 78.1% Nitrogen

  • About 1% trace gases.

• Density at STPD: $1.29 ext{ g/L}$.

• Production of Medical-Grade Air: Filtered and compressed atmospheric air.

Carbon Dioxide (CO2)

• Physical Properties:

  • Colorless and odorless gas at STPD.

  • Specific gravity: $1.52$ (approximately 1.5 times heavier than air).

• Combustion Characteristics:

  • Does not support combustion.

• Common Uses:

  • Calibration of blood gas analyzers.

  • Diagnostic purposes in clinical laboratories.

Helium (He)

• Physical Properties:

  • Odorless, tasteless, and nonflammable.

  • Chemically and physiologically inert; non-toxic, non-anesthetic properties.

  • Density: $0.1785 ext{ g/L}$; much lighter than air.

• Production: Produced from natural gas through liquefaction to purity standards of at least 99%.

• Regulation with Oxygen: Always mixed with a minimum of 20% oxygen.

• Therapeutic Use:

  • Heliox: Mixture of O2 and helium.

  • Used in managing severe airway obstruction.

  • Decreases work of breathing due to lower density, enhancing laminar gas flow.

Nitric Oxide (NO)

• Physical Characteristics:

  • Colorless, nonflammable, toxic gas that supports combustion.

• Health Risks:

  • High concentrations can cause methemoglobinemia, impairing oxygen transport in blood.

• FDA Approval: Used for treating term and near-term infants with hypoxic respiratory failure.

• Clinical Use: Used as a vasodilator.

Nitrous Oxide (N2O)

• Physical Characteristics:

  • Colorless gas with a slightly sweet odor.

  • Not flammable but supports combustion.

• Clinical Use: Commonly used as an anesthetic agent.

• Production: Generated through thermal decomposition of ammonium nitrate.

• Risks:

  • Long-term exposure risks include neuropathy, fetal disorders, and spontaneous abortion.

Storage of Medical Gases

Gas Cylinders

• Use and Regulation:

  • Used for storing and shipping compressed or liquid medical gases.

  • Subject to industrial standards and federal regulations.

  • Constructed from seamless steel.

• DOT Classification:

  • Type 3A: Carbon steel.

  • Type 3AA: Steel alloy tempered for strength.

Markings and Identification

• Cylinders Marked with:

  • Color coding for gas identification.

  • Stamping indicating size, filling pressure, serial number, ownership, and manufacturing method.

• Safety Testing: Performed every 5 to 10 years; pressures tested and results stamped on the cylinder.

Color Codes

• Specific Colors for Identification:

  • O2: Green

  • CO2: Gray

  • N2O: Blue

  • He: Brown

  • He-O2: Brown/Green

  • Air: Yellow

  • N2: Black

Cylinder Safety Relief Valves

• Designed to release gas to the atmosphere to prevent excessive pressure when heated.

Cylinder Filling and Measuring Flow

• Filling Cylinders:

  • Compressed or liquefied.

  • Liquefied gases like CO2 and N2O filled based on specified filling density.

• Measuring Cylinder Contents:

  • Gas-filled cylinders: Volume related to pressure.

  • Liquid gas cylinders: Pressure does not indicate the remaining liquid amount; weight must be used.

Estimating Cylinder Gas Flow Duration

• Factors That Affect Duration:

  • Gas flow (liters delivered).

  • Cylinder size.

  • Initial cylinder pressure (PSI).

• Formula:

  • $ext{Duration of flow} = rac{ ext{Content}}{ ext{Flow}}$

  • $ext{Duration of flow (min)} = rac{ ext{Pressure (psig)} imes ext{Cylinder factor}}{ ext{Flow (L/min)}}$

Example Calculation for Duration:

• E Cylinder with 1000 psi at 3L/min:

  • $ext{Calculation: } 1000 imes 0.28 / 3 = 93.33 ext{ minutes}$

  • Converted to hours: $93.33 / 60 = 1.55 ext{ hours}$

  • Final answer: 1 hour and 33 minutes.

• G Cylinder with 800 psi at 8L/min:

  • $800 imes 2.41 / 8 = 241 ext{ minutes}$

  • Converted: 241/60 = 4 hours.

Safety in Storage and Use of Cylinders

• **Storage: **

  • Cylinders must be stored securely in racks or chains.

  • Keep away from combustibles and heat sources.

  • Flammable gases must be stored separately from oxidizers.

• Using Cylinders:

  • Secure in patient areas.

  • Avoid flammable materials on equipment (e.g., oils).

  • "Crack" cylinder valve to remove dust before connecting regulators.

  • Use "NO SMOKING" signs when O2 is in use and exercise caution in MRI areas.

Bulk Oxygen Systems

• Usage in Healthcare:

  • Bulk oxygen systems effectively meet substantial oxygen requirements for facilities, holding at least 20,000 cubic feet of gas.

  • Can be stored in gas or liquid form.

• Advantages:

  • Cost-effective in the long run.

  • Reduces inconvenience and hazard associated with multiple cylinders.

  • Eliminates the need for individual pressure-reducing valves at every outlet.

  • Operates at low pressures, enhancing safety.

• Safety Precautions:

  • Essential for patients requiring O2 during therapy.

  • Backup systems should be in place, such as a secondary smaller liquid tank or gas manifold.

  • Staff must be prepared to rapidly transition affected patients to backup oxygen sources.

Distribution and Regulation

• Primary Function: Deliver O2/air to bedside at usable pressure.

• Central Piping Systems:

  • Delivers compressed gas throughout the hospital.

  • Pressure reduced to 50 psi at storage locations with alarm notifications for pressure drops or flow interruptions.

  • Zone valves for maintenance and fire safety adjustments.

Safety Indexed Connector Systems

• Types:

  • American Standard Safety System (ASSS):

  • For large cylinders, prevents accidental misconnections.

  • Pin-Index Safety System (PISS):

  • For small cylinders up to size E; prevents misconnections via unique pin configurations.

  • Diameter-Index Safety System (DISS):

  • For low-pressure connectors found at outlets from pressure regulators and central systems.

Regulating Gas Pressure and Flow

• Flowmeters:

  • Used for flow control to patients.

• Regulators:

  • Control both the pressure and flow of gases.

• Categories of Flowmeters:

  • Flow Restrictor:

  • Simplest type, fixed orifice for specific flow at consistent pressure based on flow resistance.

• Bourdon Gauge:

  • Operates under variable pressure in combination with a pressure-reducing valve; not gravity dependent, ideal for transport.

• Thorpe Tube:

  • Measures true flow, must be upright; available in designs preventing downstream resistance changes.