Equipment In Anaesthesia: Part 1 Flashcards
Information from this deck is from the book Equipment in Anaesthesia in Critical Care: A complete guide for the FRCA.
What is a VIE
Vacuum Insulated Evaporator
Storage tank for liquid oxygen
Keep contents below 160 Celsius
Provide piped O2 to most hospitals
Advantages of a VIE
- Storing liquid oxygen is highly space efficient
- Liquid oxygen is stored at much lower pressures than gas at room temperature
- No power source required
- Cheaper option for storage and delivery
Disadvantages of a VIE
- Initial equipment costs higher than manifold
- Backup cylinder manifold or VIE required in case of interruption
- If demand is not continuous, oxygen will be vented and lost.
- SAFETY: VIE must be kept outside due to fire risk.
What is the pressure inside a VIE, and what is the relevance?
7 Bar / 700kPA
It is the saturated vapour pressure of O2 at -160C
What happens to oxygen in a VIE before entering the hospital pipeline
- Superheater brings to ambient temperature
- Pressure regulator brings to 400kPA
Two ways of measuring the oxygen content in a VIE
- Mass: weighing on a tripod scale. Subtract tare weight from measured weight.
- Difference between vapour pressure at the top of VIE and the bottom of the liquid Oxygen. Calculate height of column and deduce volume from cross-sectional area.
What is a cylinder manifold
A pipe with several openings connected to cylinders;
Primary supply for small hospitals OR backup supply for larger hospitals.
Which gases are typically supplied by a cylinder manifold
Oxygen
Nitrous Oxide
Entonox
Size cylinders used in a manifold
J, L
Advantages of cylinder manifold
- Simple and cheap
- Effective backup
- Alarm system avoids running empty
Disadvantage of cylinder manifold
- Limited capacity compared to VIE
Safety considerations for a cylinder manifold
- Keep in a well-ventilated building separate from main building, due to fire and explosion risk;
- Main cylinder store should be in a separate room
Materials used for gas cylinders
a. Chromium molybdenum steel
b. Aluminium
Information found on a gas cylinder
- Label: gas, chemical formula, cylinder size, batch number, maximum safe operating pressure, expiry date, notes on storage, handling, and hazards;
Plastic disk: date cylinder was last tested
Valve block: testing pressure
Cylinder itself (engraved): test pressure, dates tested, tare weight, serial number
What are various cylinder sizes used for:
CD
E
J
CD: transporting patients
E: attached to anaesthetic machine
J: cylinder manifold
Size CD tank:
- oxygen volume at 137 Bar, 15C
- water capacity
- tare weight
- 460 litres
- 2 litres
- 3 kg
Size E tank:
- oxygen volume at 137 Bar, 15C
- water capacity
- tare weight
- 680 litres
- 4.7 litres
- 5.4 kg
Size J tank:
- oxygen volume at 137 Bar, 15C
- water capacity
- tare weight
- 6800 litres
- 47 litres
- 69 kg
Define critical temperature of a gas
Temperature at which the gas turns from liquid into gas,
above the temp it is impossible to compress it into liquid
Critical temperature of:
1. oxygen
2. nitrous oxide
- -116C
- 36.5C
Advantages of gas cylinders
- small cylinders are portable
- variety of connectors exist
- refillable, reusable
Disadvantages of gas cylinders
- big cylinders are heavy to transport
- not all connectors present on all cylinders
- amount of gas limited by volume of cylinder
- no alarm if gas runs out
Safety considerations for gas cylinders
- must be tested every 5-10 years
- filling ratio used (in UK: 0.75 at 15.5C; in warmer climates: 0.67)
Define filling ratio of gas cylinders;
explain its purpose
Weight of the liquid in a full cylinder, divided by the weight of water that would completely fill the same cylinder;
allows for pressure increases if temperature rises, without the risk of explosion.
True/False:
A filling ratio of 0.75 is exactly the same as the cylinder being 75% full
FALSE
Reason: differences between density of water and cylinder contents
Sources of production of medical grade air
- air compressor
- cylinder banks (smaller hospitals)
Pressures and respective uses of medical air
400kPa (4Bar): anaesthetic equipment/ventilators;
700kPa (7Bar): powering surgical equipment
Advantages of air compressor
More cost-effective for larger hospitals than a cylinder bank
Disadvantages of air compressor
Higher initial costs than cylinder bank
Safety considerations: air compressor
- Risk of contamination by air pollution: must be carefully situated and regularly inspected
- Risk of oil mist contamination
- Non-interchangeable Schrader valves to prevent accidental connection of high-pressure air to anaesthetic machine
Function: oxygen concentrator
Produce concentrated oxygen from air;
Inlet: 78% Nitrogen, 21% Oxygen, 1% Argon, variable water vapour;
Outlet: 95% Oxygen, 5% Argon
What is a zeolite
Aluminosilicate;
Forms lattice structure in oxygen concentrator;
molecular sieve;
filters specific molecules while letting others through.
Oxygen concentrator: advantages
- cheap and reliable for home oxygen
- reduce need for commercial deliveries of oxygen
Oxygen concentrator: disadvantages
- Argon accumulates if used at low flows on anaesthetic circle system (leads to hypoxic mixture)
- requires power supply
Oxygen concentrator: safety considerations
Explosion hazard; patient must give up smoking
Safety features of piped medical gas supply
- colour-coded hoses and sockets
- labelled hose and outlet
- schrader probes/connections
- diameter index collar connection
- non-interchangeable screw thread for connection to anaesthetic machine
- emergency shutoff valves
Minimum physical paremeter requirements for a medical vacuum system
Pressure: -400mmHg/-53kPa
Flow: 40l/min
i.e: high pressure, low flow
Medical vacuum system: advantages
- essential for safe anaesthesia
- centralised vacuum supplies are highly reliable
- collection systems are cheap, simple, disposable
Medical vacuum system: disadvantage
- disconnections and leaks are common
- portable units battery life limit use
Use of scavenging systems
reduce environmental anaesthetic gas concentrations by collecting waste gases and venting them outside the building.
Describe active and passive scavenging systems
Active: pump generates negative pressure. Requires open receiving system to prevent transfer of negative pressure to patient.
Passive: uses positive pressure generated by patient’s expiration to transmit gas to atmosphere via closed receiving system
Components of a scavenging system
- Collecting system (connected to APL)
- Transfer system (corrugated hose)
- Receiving system (active or passive)
- Disposal system (modern systems are high flow, low pressure)
How many times per hour should the air in operating theatres be changed
At least 20 times
What is a Cardiff Aldasorber
A simple passive device: canister containing activated charcoal, which absorbs volatile agents.
Used in resource-poor locations.
If heated, agents are vented to atmosphere.
Factors that affect the oxygen delivery by a variable performance device
- mask volume
- respiratory dynamics of patient
- seal of the mask
Examples of fixed performance oxygen devices
- venturi mask
- high flow nasal cannulae
Nasal cannula: advantages
- simple and cheap
- patients can speak, eat, and drink
- good patient compliance
Nasal cannula: disadvantages
- variable FiO2
- drying of nasal mucosa limits flow
Oxygen masks: advantages
- simple, cheap, widely available
- easy to vary oxygen delivered, but not to precise concentrations
- simplest way to deliver high concentrations of oxygen
Oxygen masks: disadvantages
- variable performance device
- precise FiO2 unknown, not suitable for calculations - rebreathing of exhaled CO2 is possible
True/False:
A Venturi mask can deliver higher oxygen concentrations than its labelling
False
State the Bernoulli principle
in order to maintain a constant FLOW,
a fluid must increase its VELOCITY
as it flows through a CONSTRICTION
State the first law of thermodynamics
Energy cannot be created or destroyed, only transferred
True/False:
The apertures of a 60% Venturi mask are smaller than the apertures of a 24% Venturi mask
TRUE
Venturi mask: advantages
- simple, lightweight
- delivers a specific and consistent FiO2 under normal circumstances
- Patient’s respiratory rate and pattern do not alter the FiO2
Venturi masks: disadvantages
- risk of under-delivery if flow set too low
- high flows can dry airways
- less accurate at higher FiO2
- behaves like a variable performance device if patient’s inspiratory flow requirement exceeds the O2 flow.
- droplets from humidifiers may occlude the narrow oxygen inlet
Nasal high-flow devices:
1. max flow deliverable
2. positive airway pressure provided
- 60l/min
- 5 cmH2O
Nasal high-flow: advantages
- better tolerated by some
- fixed performance, permitting up to 100% FiO2
- gas is warmed and humidified
- low level positive airway pressure is possible
Nasal high-flow: disadvantages
- little peer-reviewed evidence so far
- more expensive
- not widely available
Sealing face masks: advantages
- Allows up to 100% FiO2
- Simplest way of applying positive pressure ventilation
Sealing face masks: disadvantages
- seal may be difficult to achieve
- volume within mask is dead space
- patients may be claustrophobic
- masks can cause pressure injuries: skin breakdown, trigeminal nerve injury
Guedel airway: disadvantages
- poorly tolerated by semiconscious patients
- may cause bleeding and oropharyngeal injuries
- incorrect sizing may cause obstruction
Nasopharyngeal airway: advantages
- tolerated in semiconscious patients
- provides a conduit for suctioning
Nasal airways: safety risks
- coagulation disorders
- basal skull fractures
LMA: advantages
- neuromuscular blocking drugs not required
- insertions easier than intubation
- minimal haemodynamic response cf intubation
- emergence is smooth
- part of difficult airway algorithm
LMA: disadvantages
- difficulty obtaining a seal in some patients
- not a definitive airway - risk of aspiration
- may cause laryngospasm
- seals to low airway pressure: 20cmH2O
- position less stable in edentulous patients
Which IV induction agent is better for use with LMA, and why
Propofol;
inhibits airways reflexes to a much greater degree than thiopentone
Bougie: uses
- facilitate tracheal intubation
- airway exchange
- during emergency surgical cricothyroidotomy
Bougie: advantages
- narrow diameter: superior visualisation of airway anatomy
- angled tip to identify tracheal rings
- some shape retention
Bougie: disadvantages
Size 4 ETT minimum
What useful feature is found in an airway exchange catheter
it has an adapter for connection to a standard 15mm connector or jet ventilator. Allows for oxygenation/ventilation during exhange.
What is the correct use for a left-handed laryngoscope blade
In patients with large RIGHT SIDED facial deformities
What is the functional difference between curved and straight laryngoscope blades
Curved blades indirectly lift the epiglottis via pressure on the hyoepiglottic ligament;
Straight blades are placed directly over the epiglottis to lift it directly.
Adjustments to laryngoscopes for patients with large breasts
- blade mounted on the handle at 135 degrees instead of 90 degrees
- stubby handle
- indirect laryngoscopes
Rigid indirect laryngoscopes: advantages
- convert difficult intubations to easy
- reduced lifting force required, therefore reduced haemodynamic response to intubation
- minimal cervical spine movement required
- reduced airway trauma
- easier to teach
Rigid indirect laryngoscopes: disadvantages
- difficulty passing tube despite excellent views
- better mouth opening required than for fibreoptic intubation
- expensive
- blood and secretions significantly impair the view.
Fibreoptic endoscopes for intubation: uses
- awake or asleep intubation in difficult airways
- confirm position of ETT, especially in double lumen tubes
- placement of bronchial blockers
- guiding percutaneous tracheostomy
- suctioning, pulmonary toilet
- diagnosis of upper airway pathology
Define: critical angle of incidence
The angle of incidence that causes the angle of refraction to equal 90 degrees
State Snell’s Law
When light moves from a material with a higher index of refraction to a material with a lower index of refraction, it is bent away from the normal.
Define total internal reflection
Occurs when the angle of refraction is 90 degrees or greater, and is reflected back into the original material. No more light enters the new material.
What should be the inflation pressure of the cuff of an ETT
20-30cmH2O
What are modern ETTs made of
clear polyvinyl chloride
Use of laser ETT
Laser airway surgery, to resist melting and combustion of tube
True/False:
Laser tubes are not necessary for laser surgery away from the airway
True
Safety features of laser ETT
- Metal exterior with a matt surface, defocuses reflected beams to protect healthy tissue
- double cuff to maintain seal if the first one is perforated
What is a microlaryngeal tube
Size 4-6mm ETT with longer lengths that can be used in adult patients.
Uses of microlaryngeal tubes
- microlaryngeal surgery
- patients with pathologically narrow trachea or larynx.
Microlaryngeal tubes: disadvantages
- narrow lumen may become blocked
- high resistance precludes spontaneous ventilation
- surgical access still impaired compared to jet ventilation
