Medical Gases: Properties, Storage, Safety, and Duration Calculations
Oxygen basics and medical gas context
In hospital settings, there is piped in compressed air and medical gas supply in addition to oxygen. There are also smaller portable air compressors for hospital or home use. Some devices used at home for breathing treatments include PulmoAide/pulmonary aids; these are essentially clean compressors.
Oxygen is colorless, transparent, odorless, tasteless, and nonflammable, but it supports combustion. It cannot be ignited but will accelerate combustion when present with fuel.
Oxygen properties mentioned in lecture:
Oxygen is colorless, transparent, tasteless, nonflammable, but supports combustion.
Oxygen is slightly heavier than air (implied by “1.1 times heavier than water” note in lecture; see also later notes about density and barometric effects).
Oxygen solubility: only about of dissolves in 1 liter of water at standard temperature and pressure (STP).
Liquid O₂ can be liquefied at very low temperatures: (≈ ).
Liquid O₂ remains liquid while the temperature stays below its boiling point; it is pale blue when liquid.
Diatomic oxygen: forms when two oxygen atoms share two electrons.
Oxygen’s color (as a gas) is colorless; in liquid form it is pale blue.
FiO₂ and PiO₂ concepts:
Fraction of inspired oxygen: . In ambient air, (about 21%).
Partial pressure of inspired oxygen (Po₂) depends on barometric pressure: , where is barometric pressure.
At sea level, ; thus, with , .
The lecture notes that FiO₂ remains constant with altitude, but what changes is barometric pressure; thus Po₂ changes with altitude due to Pb.
Altitude and Po₂ discussion (Dalton’s considerations):
As barometric pressure drops with altitude, Po₂ changes accordingly even if FiO₂ stays the same.
The lecture references Dalton’s idea of partial pressures of inspired oxygen in different pressure environments.
A quick aside from the lecture example tying to depth: for every 33 ft underwater, the atmospheric pressure increases by about 1 atm. Examples given:
33 ft down: ~2 atm total
66 ft: ~3 atm
99 ft: ~4 atm
132 ft: ~5 atm
The speaker emphasizes memorizing the concept “for every atmosphere we go down, we go 33 ft deeper.”
Oxygen delivery systems and bulk supply context:
Large hospital oxygen is produced by fractional distillation of air and stored in bulk oxygen systems. In hospitals you’ll see large white tanks (often labeled Maxair or similar) on the facility grounds; these supply oxygen via bulk systems.
The oxygen company fills hospital storage tanks by tanker deliveries.
Medical gas storage and distribution: cylinder types and regulation
Cylinders used in clinical settings include large “H” cylinders and smaller “E” cylinders.
The Department of Transportation (DOT) regulates cylinder manufacturing, testing, transport, and hazardous materials including compressed gases and cryogenic liquids. Cylinders are periodically tested (every 5–10 years per DOT).
Cylinder markings, color coding, and labels are used to prevent wrong gases from being delivered to patients. The DOT insignia is stamped on cylinders with service pressures and last test dates.
Typical full cylinder service pressure referenced: (they also mention 2200 psi as a value used in calculations; some examples use 2200 psi as full). The important practical point is to know the full-tank pressure used in calculations, which is often for an aqueous full cylinder in practice.
Gas cylinder colors and identification (US color coding mentioned):
Oxygen: green
Air: yellow
Nitrogen: black
Carbon dioxide (CO₂): gray
Helium (He): brown
Heliox (He + O₂): brown and green
Nitrous oxide (N₂O, “laughing gas”): blue
Nitric oxide (NO): teal (cyan-green)
Note: The lecturer emphasizes memorizing the colors listed, not necessarily every possible color in every system.
Gas cylinder safety systems and connectors
Three safety systems exist to ensure the correct gas is delivered:
American Standard Safety System (ASSS) for larger tanks (H tanks). Abbreviation used in lecture: ASSS.
Pin Index Safety System (PISS) for smaller cylinders (E cylinders). Abbreviation used: PISS.
Diameter Index Safety System (DISS) for low-pressure connections (50 psi working pressure). Abbreviation used: DISS.
Pin index specifics:
Oxygen pin pattern: two pins, at positions 2 and 5 (2-5).
Nitrous oxide pin pattern: 3 and 5 (3-5).
There is a regulator with corresponding pins that must align with the cylinder’s pin holes.
Yoke vs. threaded connections:
E cylinders (smaller) use a “Yoke” type system with pins (PISS).
H cylinders (larger) use threaded connections with American Standard safety connections (ASSS).
The bigger cylinders (>200 psi inside system) typically use ASSS (H tanks) and their regulators connect via threaded interfaces.
Diameter Index Safety System (DISS):
Used for low-pressure connectors, designed to prevent interchange of gases at pressures around 50 psi.
Ensures that the gas flow system (e.g., a wall outlet or flowmeter) is compatible with the gas and the outlet’s pressure.
Borden gauge (regulator/gauge):
This gauge takes the tank pressure (high pressure) and reduces it to the working pressure used by regulators/flow meters (typically 50 psi).
Flow meters and outlet pressures:
Medical gas equipment requires working pressure around .
Flow meters are connected to wall outlets that provide 50 psi; not all flow meters are interchangeable with all gases because of different connectors (e.g., oxygen outlets vs. air outlets) and different color codes.
Practical note from the lecture: even if a hose/tubing isn’t color-coded, the safest practice is to ensure color-coded green for O₂ lines; sometimes tubing is white.
When changing regulators or using different gases (e.g., O₂ vs NO or Heliox), different pin indices and regulators must be used; you cannot mix one gas’s regulator with another gas’s cylinder without the proper interface.
Oxygen supply formats and types of storage
Medical gases can be stored in gaseous cylinders, liquefied gas cylinders, or bulk liquid systems feeding pipelines.
The two cylinders most commonly encountered in teaching and clinical labs are the H and E cylinders.
Bulk oxygen systems exist for hospitals and include stand tanks or doers (liquid oxygen vessels); these bulk systems can be backed up by stand-alone gas tanks and backflow arrangements to maintain oxygen supply during outages.
Flow duration calculations (duration of gas flow)
Practical need: determine how long the oxygen supply will last when transporting a patient or during a trip.
The calculation uses three key inputs:
Gas flow rate to patient (in L/min)
Cylinder size (type: E or H)
Cylinder pressure (psig) at the start of therapy
Core formula:
Cylinder factors (conversion factors from cylinder pressure to liters, given in lecture):
E cylinder:
H cylinder:
Contents conversion concept:
For gas-filled cylinders, the volume of gas is roughly proportional to pressure; contents are often tabulated, but the calculation uses the cylinder factor to convert the pressure in psi to usable liters.
Example calculations from the lecture:
Example 1 (E cylinder, full at 2200 psi, flow = 4 L/min):
(the instructor showed 2 h 33 m in a rough mental math walkthrough; exact value with 154 min is 2 h 34 m).
Example 2 (H cylinder, full at 2200 psi, flow = 4 L/min):
Practical tips:
If the tank is full, use 2200 psi as the starting pressure for calculations (the lecturer noted 2200 psi as the typical full-tank reference, occasionally noting 2015 psi as service pressure).
For exams or quizzes, you’ll often be asked to choose the closest answer; numerical rounding should be considered.
Partial credit is typically limited in board-style exams; ensure you show the steps, but scoring may be binary correct/incorrect for the final answer.
Medical air vs ambient air and ventilatory considerations
Medical air (often produced by a medical-grade compressor) is a clean, toxin-free source of compressed air used in hospitals; it is not the same as ambient room air from outdoors but is a controlled supply with similar composition (roughly 78% nitrogen and 21% oxygen in ambient air).
The hospital ventilatory systems use a mix of air and oxygen delivered either via walls-outlet flow meters or via ventilators; some ventilators can use Heliox (He + O₂) when needed.
Heliox usage:
Heliox is a mixture of helium and oxygen used for airway-related resistance issues (e.g., asthma); it is delivered through a ventilator setup that can accommodate a Venturi system to blend the gases.
The lecture mentions Heliox: a slight metallic odor; nonflammable; does not support life by itself; helium-oxygen mixture used in specific respiratory conditions.
Nitric oxide (NO):
NO is FDA-approved for use in infants with hypoxic respiratory failure; NO acts as a smooth muscle relaxer and vasodilator.
High concentrations of NO alone can cause methylhemoglobinemia, which decreases oxygen delivery to tissues.
NO is toxic at high concentrations; it is used in infants with hypoxic pulmonary hypertension and other studies in adults.
Nitrous oxide (N₂O):
Commonly known as laughing gas; blue color coding in some systems; used in dentistry (dentist office).
Ethical and practical implications
Ensuring patient safety through proper gas identification and avoiding interchange errors is critical; misconnecting a gas outlet or using the wrong regulator can lead to serious harm.
Hospitals must have documented contingency plans for oxygen supply disruption; the speaker highlights the importance of backups and backflow strategies to maintain continuous oxygen delivery.
Proper training in gas cylinder handling, lifting precautions, and safe storage reduces risk of accidents and injuries among healthcare staff.
Summary
Medical gases involve a well-defined system of storage, color-coding, safety interfaces, and calculation methods to ensure safe and reliable patient care. Mastery of FiO₂, PiO₂ concepts, cylinder safety systems (PISS/ASS/S), working pressures, cylinder factors, and duration calculations is essential for clinical practice and exam performance.