Equipment in Anaesthesia: Part 2

Question 1

What is the significant benefit of high frequency jet ventilators over low frequency manual jet ventilators

Answer 1

Low frequency jet ventilators cause significant movement of the the vocal cords, making airway surgery difficult.
High frequency jet ventilators produce small tidal volumes at high frequencies, producing minimal vocal cord movement.

Question 2

Uses for low frequency jet ventilation

Answer 2

1. short duration airway surgery, especially bronchoscopy
2. prior to the initiation of high frequency jet ventilation
3. rescue ventilation in a can’t intubate, can’t ventilate situation.

Question 3

Disadvantages of low frequency jet ventilation

Answer 3

1. causes significant movement of the vocal cords
2. limited to short periods of time
3. cannot humidify or warm inspired gases
4. TV and FiO2 are variable and impossible to measure
5. risk of baro/volutrauma
6. cannot deliver volatile anaesthetics
7. unable to monitor CO2 accurately

Question 4

What settings are adjustable on high frequency jet ventilators

Answer 4

1. driving pressure: typical starting value in adult 150kPa
2. frequency: typical starting value in adult 100 cycles/minute
3. FiO2: adjusted with oxygen/air blender. Entrains air, so final value hard to predict
4. pause pressure: approximates mean airway pressure. Ideally <20cmH2O

Question 5

How does high frequency jet ventilation theoretically achieve adequate gas exchange, despite very small tidal volumes?

Answer 5

a. convective streaming: high velocity inspiratory jet travels down centre of airway, while exhaled gas simultaneously exits around the edge of the stream
b. simple diffusion: passive oxygenation
c. “pendelluft”: highly compliant alveoli are inflated first, which then empty into less compliant alveoli; this augments ventilation even at the end of inspiration
d. resonance: when the frequency of ventilation is close to the natural frequency of the lung, the amplitude with which air moves in the distal segments of the lung increases
e. cardiogenic oscillations: the vibration of the heartbeats exert a force on the lungs, which may augment gas exchange

Question 6

How does high frequency oscillatory ventilation differ from high frequency jet ventilation

Answer 6

In high frequency oscillatory ventilation:
1. inspiration and expiration are active
2. ventilation takes place in a sealed breathing system, and doe not involve jet airway devices
3. there is no entrainment of air

Question 7

What might be beneficial of oscillatory ventilations compared to standard ventilators (in critical care)

Answer 7

1. causes less alveolar distension, therefore less shearing forces on the small airways
2. minimises leaks through bronchopleural fistulae

Question 8

Settings that can be adjusted on high frequency oscillatory ventilators

Answer 8

MEAN AIRWAY PRESSURE: 3-5cmH2O above mPaw observed during conventional ventilation. Serves same purpose as PEEP. Will vary with other settings’ adjustments.
BIAS FLOW: flow of fresh gas through the ventilator, usually 25-40l/min. Required for adequate removal of CO2.
POWER: controls amplitude of oscillations. Set until adequate “chest wiggle” seen.
INSPIRATORY TIME: 33% of resp cycle
FREQUENCY: 3-7Hz (180-420 breaths per minute!)
AIRWAY PRESSURE ALARM: alarms is mPaw deviates more than e.g. 5cmH20

Question 9

What is the concept of “chest wiggle”

Answer 9

High frequency oscillatory ventilation is noisy, and ventilation cannot be assessed via regular auscultation.
Instead, the clinician looks for clear symmetrical vibration of the chest wall and shoulders, down to the abdomen.

Question 10

Disadvantages of high frequency oscillatory ventilation

Answer 10

1. noisy
2. not portable
3. staff training required
4. vibrations may displace ETT
5. deep sedation required, often with muscle relaxation
6. no clear mortality benefit, and may even exacerbate mortality

Question 11

What are non-interchangeable screw thread connectors

Answer 11

a probe and nut used to connect gas pipelines to an anaesthetic machine, with a gas-specific probe diamete

Question 12

What is a Bodok seal

Answer 12

a neoprene washer surrounded by a steel reinforcing ring;
it provides a gas-tight connection between a gas cylinder and its yoke.

Question 13

Why are rubber seals not used for sealing a gas cylinder and its yoke

Answer 13

frequent changes of cylinders, and heat produced by expansion of gas, create wear and tear.

Question 14

What is a pin index system

Answer 14

pins on the yoke which mate with the holes on the cylinder block; they are gas specific

Question 15

What is the pressure and flow rate of oxygen delivered via the O2 flush

Answer 15

400kPa
35-75l/min

Question 16

Why should one be careful in using the O2 flush

Answer 16

1. may cause baro/volutrauma
2. does not pass through vapourisers, and will dilute any volatile in the system

Question 17

How might one classify different vapourisers

Answer 17

1. mechanism for adding anaesthetic vapour to the fresh gas flow
2. internal resistance of the vapouriser
3. temperature compensation
   [4. other information]

Question 18

What are the mechanisms that vapourisers can use to add anaesthetic vapour to the fresh gas flow

Answer 18

a. variable bypass vapourisers: split FGF into two streams; fully saturate the vapourisation stream, then reunite both streams (e.g. most modern vapourisers)
b. measured flow vapourisers: use a separate heated and pressurised vapour stream, precisely injected into FGF. (e.g. TEC6 desflurane vapouriser)

Question 19

Classify vapourisers according internal resistance

Answer 19

a. draw-over vapourisers have low internal resistance; uses patient’s inspiratory effort to draw fresh gas through the vapouriser; in practice not used in circle systems
b. plenum vapourises have high internal resistance and rely on pressurised gas flow; are used outside the breathing system. (e.g. Tec5, Aladdin casettes, Boyle’s bottle, copper kettle

Question 20

Disadvantages of the Tec5 vapouriser

Answer 20

1. high internal resistance
2. heavy due to heat sink
3. no alarms to indicate low level of liquid anaesthetic
4. temperature compensation only reliable within reasonable ambient temperatures

Question 21

What is the boiling point of Desflurane

Answer 21

23C

Question 22

What temperature is Desflurane heated to by the Tec6 vapouriser

Answer 22

42C

Question 23

What are disadvantages of the Tec6 vapouriser

Answer 23

1. requires electrical power supply
2. requires time to warm up

Question 24

What methods may be used to measure temperature

Answer 24

A. electrical
- resistance thermometer
- thermistor
- thermocouple
- infrared thermometer
B. non-electrical
- liquid expansion thermometer
- gas expansion thermometer
- bimetallic strip
- liquid crystal thermometer

Question 25

What methods are used to measure oxygen in vivo:
1. arterial
2. venous

Answer 25

1. pulse oximeter, transcutaneous
2. fibreoptics

Question 26

What methods are used to measure oxygen in a gas

Answer 26

fuel cell
paramagnetic analyser
mass spectrometer

Question 27

What method is used to measure oxygen in a blood sample

Answer 27

Clark electrode

Question 28

Explain briefly how a fuel cell works

Answer 28

A fuel cell measures the partial pressure of gas entering the breathing system.
Positive anode made of lead;
Negative cathode made of gold.
The electrodes are immersed in a liquid of KOH, separated from oxygen by a membrane that is selectively permeable to oxygen.
A series of chemical reactions causes a flow of electrons (current).
Magnitude of electric current is measured with an ammeter.
The current is directly proportional to the partial pressure of oxygen, which can then be calculated.

Question 29

What do pulse oximeters calculate

Answer 29

The amount of oxyhaemoglobin as a percentage of the total haemoglobin in arterial blood. It measures the saturation of pulsatile blood only.

Question 30

Which two laws are used for the pulse oximeter

Answer 30

1. Beer’s law
2. Lambert’s law

Question 31

State Beer’s law

Answer 31

the absorption of radiation as it passes through a substance increases exponentially as the concentration of the substance increases

Question 32

State Lambert’s law

Answer 32

the absorption of radiation as it passes through a substance increases exponentially as the distance it travels increases

Question 33

What are the wavelengths of the LEDs in a pulse oximeter,
and what form of Hb absorbs them best

Answer 33

940nm: HbO
660nm: Hb

Question 34

Why must pulse oximeters use two waves, rather than just one

Answer 34

If only one wave is used, the cause of a change in reading cannot be interpreted.

Question 35

Uses of arterial blood pressure monitoring

Answer 35

1. the use of inotropic drugs
2. expectations of large fluctuations in intravascular volume or pressure
3. frequent sampling of arterial blood is required
4. non-invasive measurements are likely to be inaccurate, e.g. morbid obesity, atrial fibrillation

Question 36

What is natural frequency

Answer 36

the frequency at which an object oscillates or vibrates after something causes it to move

Question 37

What is resonance

Answer 37

amplification of the amplitude of oscillations if an energy wave applied to an object is equal to the natural frequency of that object

Question 38

What is the minimum natural frequency an invasive arterial blood pressure measuring set should have

Answer 38

40Hz

Question 39

What is the damping coefficient

Answer 39

the property that dictates how much an object resists oscillation;
i.e. it measures the frictional forces in the system

Question 40

What is the damping coefficient in:
critical damping
underdamping
overdamping
optimal damping

Answer 40

1
less than 1
more than one
0.64

Question 41

What is the result of an underdamped system

Answer 41

exaggerated signal detection
over-estimation of SBP

Question 42

What is the result of an overdamped system

Answer 42

does not overshoot zero
misses finer detail
over-estimated DBP, under-estimated SBP