Monitoring and measurement

Question 1

What range of voltage and current does a nerve stimulator give for transcutaneous testing of neuromuscular blockade?

Answer 1

I = V/R

Battery power 3 to 9 volts.

Resistance of dry skin 1000 ohms, wet 100 ohms

Therefore current with dry skin 1 to 10 mA, wet skin 100mA.

Question 2

What response is seen when you use the nerve stimulator on the ulnar nerve?

Answer 2

The hypothenar muscles contract with strong flexion of the little finger due to contraction of the flexor digitorum brevis, the lubrical and interossei.

No flexion of the middle and index fingers due to the interossei muscles only innervated by the ulnar nerve to these two fingers.

The thumb adducts due to contraction of adductor pollucis brevis - one of the thenar muscles.

Question 3

Where does the ulnar nerve lie?

Answer 3

Between palmaris longus and flexor capir ulnaris.

Question 4

What nerves other than the ulnar nerve can be used to stimulate the peripheral nerve?

Answer 4

• Facial nerve
  
  • as it leaves the styloid foramen by the mastoid bone or as it passes through the parotid gland
  • one in front of tragus and other superior to this
  • facial nerves are less sensitive to NM blockade (less accurate as may result in direct muscle contraction)
• Common peroneal/lateral popliteal nerve at head of fibula
  
  • place electrodes over lateral aspect of fibula neck
  • results in dorsiflexion of foot
• posterior tibial nerve behind medial malleolus

Question 5

What is a supramaximal stimulus?

Answer 5

The current is sufficiently high to ensure that all the otor nerves are depolarised.

6 - 60 mA (if an electrode gel is used to reduce skin resistance, current is always >50 mA)

Duration is usually 0.2 -1 ms

Question 6

What % of receptors must be occupied by a competitive blocker before depression of the twitch?

Answer 6

75%

Question 7

WHat is the muscle repsonse to a nerve stimulator in a partial depolarising block?

Answer 7

Reduced magnitude of twitch response to tetanus or TOF stimulus

Question 8

What is post-tetanic count?

Answer 8

Tetanus increases the amount of ACh available so will enhance the next response.

The post tetanic count is the no. of twitches that are visible following a tetanic burst.

Tetanic burst is given at 50Hz for 5s. Following this stimuli are administered at 1Hz (1 per second)

Question 9

When is post-tetanic count used?

Answer 9

To monitor profound neuromuscular blockade

< 5 indicates deep neuromuscular block

>15 indicates that reversal of block may be possible

Question 10

What are the train of four twitches?

Answer 10

4 supramaximal stimuli at 2Hz 0.5s apart.

3 twitches means only 25% recovery - reversal can be given.

2 twitches = 20% recovery

1 twitch = 10% recovery

Question 11

What is double burst stimulation?

Answer 11

3 stimuli at 50Hz then 0.75 s later another 3 at 50Hz.

Easier to compare visually (without accellerometer) than TOF.

Question 12

What would be the characteristics of an ideal nerve stimulator?

Answer 12

• battery operated
• portable
• rechargeable
• leads that can be sterilised
• optimum pulse duration 0.25 ms
• range of pulses
  
  • monophasic
  • rectangular
  • square wave
  • variable current output 20-50 mA

Question 13

When would you use a peripheral nerve stimulator?

Answer 13

During anaesthesia following administration of muscle relaxants.

Can be used to assess depth of block and suitability for reversal, and guide subsequent doses of neuromuscular blocking agents.

Question 14

What is absolute humidity?

Answer 14

The mass of water vapour present in a given volume of gas at a given temperature and pressure

Question 15

What is relative humidity?

Answer 15

The mass of water vapour present in a given volume of gas divided by the mass of water vapour required to fully saturate that volume of gas at the same temperature and pressure.

Question 16

What are the units for absolute humidity?

Answer 16

g/m³

(or mg/L)

Question 17

What are normal values for absolute humidity at room temp, in the trachea and alveoli?

Answer 17

Room temp - 17 g/m³

Trachea - 34 g/m³

Alveoli - 44 g/m³

Question 18

What is this apparatus?

Answer 18

Wet and dry bulb hygrometer.

1 mercury bulb at ambient temp and reads the true air temp.

The bulb of the other thermometer is surrounded by a wet wick, this reads a lower temp due to the cooling effect due to latent heat of vaporisation.

The rate of evaporation from the wick depends on ambient humidity. The temp difference between the 2 thermometers can be looked at on a table to obtain predetermined value for relative humidity.

Question 19

What other methods are there for measuring humidity?

Answer 19

• hair hygrometers
• regnault’s hygrometer
• mass spectrometry
• humidity transducer (measures changes in resistance/capacitance)
• UV light absorption (uncommon)

Question 20

What is the dew point?

Answer 20

Temp at which the air is 100% saturated, and so water vapour condenses to form water droplets (dew).

This is how Regnault’s hygrometer works and relative humidity can be calculated based on the dew point.

Question 21

What is latent heat?

Answer 21

The amount of heat energy required to convert a substance from one physical phase to another without a change in temperature, for instance, solid to liquid, liquid to vapour.

Expressed in Joules.

Question 22

What is the specific latent heat of vaporisation?

Answer 22

The amount of energy required to convert a given mass of liquid to vapour at a given temperature without a change in temperature.

Expressed in Joules/kg.

Question 23

Why do we humidify gases in anaesthesia?

Answer 23

• reduce heat loss to maintain normothermia
• provides a moist physiological surface for gas exchange at an alveolar level and helps with mucociliary clearance
• absence of humidification can lead to:
  
  • keratinisation
  • ulceration of airway
  • thicker secretions
  • mucous plugging
  • drying of the airway
  • poor ciliary function

Question 24

What ways can gas be humidified?

Answer 24

• water baths - cold or heated
• heat and moisture exchangers
• heated nebulisers
• ultrasonic nebulisers

Question 25

How does an HME work?

Answer 25

• uses principles of latent heat
• consist of paper or sponge gauze coated in a hygroscopic gel such as lithium chloride
• as warm expired air passes over the HME, water vapour condenses + the element is heated by the latent heat of condensation
• during inspiration, the gases are warmed and humidified as they pass over the now moist and warm filter

Question 26

What are the disadvantages of using water baths to humidify gases?

Answer 26

• cold water baths are cheap but not efficient
• hot water baths run the risk of infections, burns to the airway, excessive moisture in the airway, electronic malfunction and overheating
• they can both lead to water gathering in airway circuits and thus affect ventilation if present in sufficient amounts

Question 27

How does a hair hygrometer work?

Answer 27

Relies on the fact that as humidity increases, hair lengthens and moves a pointer displayed on a scale to indicate the relative humidity

Question 28

How does Regnault’s hygrometer work?

Answer 28

Consists of a silver or foil-wrapped tube containing ether.

Air is bubbled through the ether until water droplets start to condense on the outside of the tube.

This is the dew point.

Question 29

What is the easiest way of telling if an ECG shows right or left axis deviation?

Answer 29

Look at leads I, II and III.

In normal axis - all 3 show positive deflection.

In right axis - I becomes negative (downwards) and III becomes positive (upwards)

In left axis, the QRS negative in III (but not significant until QRS deflection is negative in II as well)