Gas supplies Flashcards
Define critical pressure
The minimum pressure required to liquefy a gas at its critical temperature
How is O2 supplied to a hospital?
- VIE
- Cylinder maifolds
- Portable cylinders
- O2 concentrators
What are the components of a VIE O2 tank?
- Double skinned steal tank
- Vacuum of 0.16-0.3 exists between skins
-Temp -160 - -180
-O2 is below critical temp (-118) so in liquid form with a saturated vapour at 10bar - O2 vapour drawn out of the top
- Latent heat of evaporation required to vaporise O2 to equilibrium
How does a low demand/warm weather VIE work?
- Reduces the cooling effect of the vaporising process
- Pressure increases
- Safety valve opens at 15bar
How does a high demand/cold weather VIE work?
- Insufficient energy to sustain adequate evaporation to meet demand
- Liquid O2 can be trapped at the bottom of the system
What is the critical temp of NO2? What is its form at room temp?
36.4
Liquid & gas
In what form is NO2 inhaled & exhaled?
Inhaled: Vapour
Exhaled: Fixed gas
What are the curves of constant temperature called?
Isotherms
Describe the isotherm curve at 40 degrees for NO2
- Above critical temp
- Fixed gas
- Relationship to Boyle’s law
Describe the isotherm curve at 36.4 degrees for NO2
- At critical temp
- Sharp inflexion at 73 where liquefaction occurs (critical pressure)
- Sharp pressure rise with vol decrease as liquids are incompressible
Describe the isotherm curve at 20 degrees for NO2
- Room temp line
- Steady linear increase as vapour is compressed
- Plateau in pressure as vol reduction causes liquefaction
- Sharp rise once all vapour liquefied
How are NO2 cylinders filled?
- Cylinders filled to 75% of the weight of water due to risk of explosion
- Filling ratio based on weight not vol due to differing densities
- Cannot use Bourdon pressure gauges
Describe the characteristics of Entonox
- 50:50 NO2 and O2
- Components separate below pseudo-critical temp of -5.5 degrees
- To reverse store cylinder above temp to allow diffusion into vapour
- Store horizontal
What are the characteristics of medical cylinders?
- Checked every 5 years
- Made of molybdenum
- Can withstand pressures 65-70% above working pressure
What Mapleson circuits are used in paeds?
E & F
What are the key differences in each Mapleson circuit?
Where APL, fresh gas inlet & reservoir bag are placed
Describe the Mapleson A circuit
- Most efficient in spont respiration
- Example of Magill & Lack system
- Dead space too great to use in kids
- Re-breathing dead space gas & fresh gas at 70%
- At expiration fresh gas fills bag & dead space gas enters system
- At pause pressure rises in system to valve opens & alveolar gas leaves
- At inspiration dead space gas is inhaled followed by fresh gas
Why is Mapleson A inefficient in controlled ventilation?
- APL valve partially closed
- At beginning of expiration reservoir bag almost empty due to high positive pressure when squeezing the bag
Describe the Mapleson D circuit
- Least efficient for spont breathing
- In expiration dead space, alveolar & fresh gas all reach bag
- In pause fresh gas pushes out alveolar gas through valve
- In inspiration initially fresh gas but if not enough then dead space & alveolar gas (including CO2) from the bag inhaled
- Increasing length of tubing does not affect performance
What are the advantages of a circle breathing system?
- Minimal equipment dead space
- Conservation of heat & humidity
- Ability to remove CO2 & recycle gases
- Reduction in pollution
- Minimize fresh gas consumption
What are the disadvantages of a circle breathing system?
- Multiple components to maintain & test
- Potential for leaks
- If inadequate fresh gas flow then circuit can empty
What is the reaction that occurs in the soda lime?
Exothermic
Produces 1 mole H2O for every mole CO2 removed
Soda made of calcium hydroxide
What is the chemical equation for the reaction in the soda lime canister?
- CO2 + H20 –> H2CO3
- H2CO3 + NaOH. –> H2O + NaHCO3
- NaHCO3 + Ca(OH)2 –> H2O + NaOH + CaHCO3
