Physics and Equipment

Question 1

Which of the following is least likely to cause a microshock to a patient?

1. II (B)
2. I (BF)
3. II (BF)
4. II (CF)
5. III

Answer 1

4. II (CF)

I means the outer casing is earthed.
II means double insulated.
III means has an extra safety voltage source of up to 35V AC or 60V DC

B (body) means maximal leakage current is 500microA, which can cause a microshock

BF (body floating) have an isolating capacitor or transformer, but the same leakage current

CF (cardiac floating) has a maximal leakage current of 50 microamps, making it far less likely to cause a microshock

Question 2

What does LASER stand for?

Answer 2

Light amplification by stimulated emission of radiation

Question 3

How does a laser work?

Answer 3

Energy is applied to lasing medium, such as a flash of light or a voltage, in a process called ‘pumping’

This elevates an electron from ground state (e1) to higher energy shell (e2)

It then falls back to the ground state, and emits this energy as a photon

The wavelength of this photon depends on the lasing medium, and all photons released are the same wavelength

These photons bounce back and forth in the optical resonator, triggering other electrons to jump up to higher energy shells

Once more atoms have ‘pumped’ electrons than ground state electrons, the medium is said to ‘lase’

Laser light is produced and emitted from the lasing chamber

Question 4

What features does laser light have?

Answer 4

It is Monochromatic

• One wavelength and therefore one colour

Coherent

• The photon wavelengths are in phase with one another (meaning the peaks and troughs of the waves are aligned)

Collimated

• The photons form parallel beams that do not diverge or converge

Question 5

What are the components of a laser?

Answer 5

Optical resonator
Lasing chamber with a fully reflective and a partially reflective surface at either end
A lasing medium, which can be solid, liquid or gas
Energy source

Question 6

What wavelength is emitted by a Ruby laser, and what is it used for?

Answer 6

Tatoo removal
694nm - red laser

Question 7

What wavelength is emitted by a CO2 laser, and what is it used for?

Answer 7

Superficial surgery and coagulation
very poor tissue penetration
10600 nm - infra red (far)

Question 8

What does Nd-YAG stand for, and what surgery is it used for?

Answer 8

Nd-YAG (neodynium doped yttrium aluminium garnet)
1060nm - infra red (near)
Good tissue penetration (2-6mm)
Used for cutting and coagulation in surgery and endoscopy
Best for airway surgery

Question 9

What wavelength is emitted by an argon laser, and what is it used for?

Answer 9

480-500 nm - blue green
Used in retinal coagulation and dermatology
Best for eye surgery as it penetrates transparent tissues well

Question 10

What are the safety concerns with regards to lasers?

Answer 10

The main concerns with lasers are their potential to cause burns, particularly to the retina and optic nerve, which depends on:

• How powerful they are
• Whether they stimulate the blink reflex
• This is rated from 1 to 4
  —- Medical = 4 (most dangerous)
  —- CD player = 1 (least powerful and stimulates blink reflex)

Staff and environment safety considerations:
- Goggles
- Block windows
- Lock doors
- Warning signs
- Minimise reflective surfaces
- Good communication
- Fire extinguisher and Saline available

Patient considerations
- Low power when aiming beam
- Fill ET cuff with saline
- Minimise FiO2 <0.3
- Eye protection
- Wet swabs to protect tissue

Question 11

What are the features of a laser-safe endotracheal tube?

Answer 11

Stainless steal flexible spiral coating
Non-combustible
Easy to sterilise and inert in human tissue
Twin distal cuffs are an optional extra
Cuff filled with saline or dye to alert to cuff rupture

Question 12

What is the immediate management of an airway fire?

Answer 12

Turn off the laser
Call for help
Turn down oxygen and stop ventilation if possible
Remove the burnt tube
Irrigate with saline or water
Facemask ventilate and then re-intubate using room air
Bronchoscopy to evaluate degree of injury to airways

Question 13

When positioning a patient, by what three mechanisms can the eye be damaged during anaesthesia?

Answer 13

Direct trauma
Chemical injury
Exposure keratopathy

Question 14

With regards to ultrasound, how is impedance calculated?

Answer 14

Density of the material x velocity of sound in that tissue

Question 15

What determines how much of an ultrasound beam is reflected?

Answer 15

Acoustic impedance mismatch

A bigger difference in impedance causes more reflection. This is why a bone-air interface appears so bright on the screen

Question 16

What is capnography, and how does it differ to capnometry?

Answer 16

• Capnometry is the measure of CO2 concentration in a gas sample
• Capnography is the continous display of a waveform on the monitor

Question 17

How does infrared spectroscopy work for detecting carbon dioxide?

Answer 17

• Molecules with two or more different types of atoms absorb infrared radiation at a specific wavelength
  
  • Absorption is proportional to the concentration of the gas being tested, and the distance the light must travel
  • 4.3 micrometer IR radiation is emitted through a sapphire lens through a reference chamber and a sample chamber
  • Detector receives signal in each chamber and microprocessor subtracts reference signal to remove artefact

Question 18

How else can you measure CO2 in a gas sample?

Answer 18

• Photoacoustic spectroscopy
  
  • Piezoelectric absorption
  • Refractometry
  • Raman scattering
  • Mass Spectrometry

Question 19

Which gases are exhaled during each phase of the capnography waveform?

Answer 19

• Phase 2 - dead space and alveolar gas
  
  • Phase 3 - alveolar gas

Question 20

What information does capnography tell you in a cardiac arrest?

Answer 20

• Confirms airway location and patency
  
  • Respiratory or ventilatory rate
  • Assess quality of CPR
  • Identifying ROSC

Question 21

How can a Severinghaus electrode be used to measure partial pressure of CO2 in the blood?

Answer 21

• CO2 diffuses across semipermeable membrane into buffer solution
  
  • CO2 reacts with water to produce hydrogen ions
  • Hydrogen ions are produced in proportion to the PCO2
  • pH electrode measures potential difference between sample and buffer solution using a H+ sensitive glass membrane

Question 22

How is heat lost from the body in an anaesthetised patient?

Answer 22

Radiation
- 40%
- Heat loss by infrared radiation to the environment, mainly determined by the difference in temperature between patient and environment

Convection
- 30%
- Heat loss to the air in contact with the skin which is then carried away and replaced by cold air

Evaporation
- 15%
- Latent heat of vaporisation from mucous membranes and exposed body surfaces

Respiration
- 10%
- 8% due to the energy required to humidify the air entering the respiratory tract
- 2% to warm the air

Conduction
- 5%
- Loss of heat by direct contact
- IV fluids
- Operating table

Question 23

What are the effects of hypothermia?

Answer 23

Impaired cognition and delirium
Arrhythmias and reduced cardiac myocyte function
Vasoconstriction, increased systemic vascular resistance and hypoperfusion to skin
Delayed drug clearance
Wound infection and delayed healing
Oxygen dissociation curve shifts left
Bleeding and coagulopathy
Increased blood viscosity and DVT risk
Discomfort
Hyperkalaemia and hyperglycaemia
Shivering and increased energy expenditure

Question 24

What methods are there of keeping a patient warm during anaesthesia?

Answer 24

Ambient temperature
- Target ambient temperature is 22°C–24°C and humidity 50-60%

Blankets and mattresses
- Cheap and easy to use
- Rely on patient’s own heat generation

Forced air warmers (FAWs)
- Use convection to prevent and treat hypothermia

Electrical blankets and heated pads
- Can interfere with other monitoring equipment

Water-filled mattresses/blankets

Fluid warming devices
- These should be used if more than 500ml of fluid are to be given, and can be divided into pre-warming and warming during administration

Counter-current heat exchange systems (good for low-flow, e.g. in neonates)

Invasive methods
- Warmed irrigation of cavities (e.g. peritoneal) and organs (e.g. stomach, bladder)
- ECMO and CPB (cardiopulmonary bypass)

Question 25

What physiological factors will influence the accuracy of BIS monitoring?

Answer 25

Hypothermia
Hepatic encephalopathy
Hypovolaemia
Hypotension
Hypoglycaemia
Also if the patient is physiologically asleep