Anaesthetic machine and vaporizers Flashcards
1
Q
How does a flowmeter work in an anaesthetic machine?
A
- Variable orifice, constant pressure
- The flowmeter consists of a flow control valve, a tapered tube which is wider at the top, and a lightweight rotating bobbin or ball
- The bobbin is held floating within the tube by the flow of gas passing around it - the higher the flow rate, the higher the bobbin rises within the tube
- The upward pressure of the gas flow is countered by the downward pressure of gravity. There is a constant pressure across the bobbin, which will float when the forces are equal. The widening of the tube at the top maintains constant pressure
- At low flow rates, the clearance is longer and narrower, thus acting as a tube, and flow is laminar and a function of gas viscosity
- At high flow rates, the clearance is shorter and wider, thus acting as an orifice, and flow is turbulent and a function of gas density
- The bobbin rotates to prevent it sticking to the sides of the tube
2
Q
Piped gases and Schrader valves
A
- Supplied at a pressure of 4.1Bar
- The changeover valve incorporated in a cylinder bank works on a pneumatic shuttle mechanism - When a cylinder bank starts to run out the pressure will fall, and a passive shuttle will be pushed over by the higher pressure within an adjacent full cylinder bank so that continuous downstream pressure is maintained
- Collars of probes are pipeline specific and non-interchangeable, and pipeline connections also have non-interchangeable screw threads (NIST)
- The Schrader valve contains an internal non-return valve
3
Q
Anaesthetic gas cylinders
A
- Steel alloy cylinders are made of either manganese molybdenum steel, chromium molybdenum steel or nickel chromium molybdenum steel
- Filling ratio is the weight of the cylinder contents divided by the weight of water it could hold. In the UK this is 0.75, but in hotter countries may be 0.67
- Adiabatic expansion of a gas being released at high flow rate will result in the gas cooling and therefore the cylinder will also cool
4
Q
Pneumotachograph
A
- Measures flow rate
- Bidirectional - can measure inspiratory and expiratory flow
- Constant orifice, variable pressure device
- Consists of a tube which widens in order to reduce the velocity of the gas in order to achieve laminar flow
- A mesh within the tube provides resistance to flow, which causes a pressure drop across the mesh
- The drop in pressure is proportional to the flow rate, therefore a differential pressure transducer across the mesh can provide the flow rate of the gas
5
Q
Mapleson A
(Magill)
A
- Afferent - bag stems from inspiratory limb of circuit
- Efficient for spontaneous respiration - no re-breathing will occur if FGF is equal to minute ventilation
- Inefficient for controlled ventilation (positive pressure will open the APL during inspiration, which will vent fresh gas) - FGF must be 2-3x minute ventilation
6
Q
Mapleson B
A
- FGF distal to APL
- Circuit tubing and bag act as reservoir
- Rarely used as very bulky at patient end
- Requires twice the patient’s minute volume for both spontaneous and controlled ventilation
7
Q
Mapleson C
(Water’s circuit)
A
- Very high flow rates required
- Not used to deliver anaesthetic gases
- BVM is a version of Mapleson C
8
Q
Mapleson D
(Bain circuit)
A
- Efferent system - bag stems from the expiratory limb
- FGF enters at patient end
- Inefficient for spontaneous ventilation, FGF must be 2x minute ventilation to prevent rebreathing of alveolar gas
- Efficient for controlled ventilation - FGF should be equal to minute ventilation
- Bain system - ‘coaxial’ Mapleson D. FGF flows down the inner tube, and expired gas down the outer corrugated tubing. Reduces bulk of circuit. Danger of disconnection of internal tube being difficult to notice
9
Q
Mapleson E
(Ayre’s T-piece)
A
- Used in paediatrics as there is minimal dead space and very little resistance (no valves)
- Ineffecient for both spontaneous and controlled ventilation - requires FGF of 2-3x minute ventilation to prevent rebreathing
10
Q
Mapleson F
(Jackson-Rees’ modification of Ayre’s T-piece)
A
- Addition of a reservoir bag with a hole cut in the end to the Mapleson E circuit
- Allows easy visualization of spontaneous ventilation, and addition of PEEP by pinching the bag and manual ventilation as required
