02. Measurement & Monitoring

Question 1

In a pulse oximeter, what saturation does methaemoglobinaemia approach?

Answer 1

85% because it absorbs light equally at 660nm and 940nm.

Question 2

What does Beers Law state in relation to pulse oximetry? Lamberts Law different in that?

Answer 2

Relates the absorption of transmitted light to the CONCENTRATION of the substance.

Lamberts law relates absorption of transmitted light to LENGTH of pathway.

Question 3

Pulse oximetry:

The AC component (pulsation) signal is approximately what percentage of the total absorption?

Answer 3

2%

Question 4

What type of molecules absorb infra-red light?

Answer 4

Small molecules that have asymmetric bonds so that vibration can occur.

Question 5

Motion artefacts produce a high AC : DC signal ratio.

T/F

Answer 5

True- motion will produce an increase in AC components however this is noise.

The signal to noise ratio will be lower.

Question 6

Fuel cells can be affected by the presence of N2O? T/F

Answer 6

True - some can be affected.

Question 7

Which is faster paramagnetic analysers or Clark electrodes?

Answer 7

Paramagnetic is faster.

Question 8

What technique is used to measure O2 in blood gas analyser?

Answer 8

Polarographic technique - Clark.

Question 9

What does Co-oximeter measure?

Answer 9

Measures the concentration of different haemoglobins and calculated O2 saturation’s from the relevant percentage.

Question 10

What does a blood gas analyser directly measure?

Answer 10

Hydrogen ion (concentration using a glass electrode.)

Oxygen tension (polarographic)

[carbon dioxide- indirectly through modification of the glass electrode]

Question 11

ABG: how does storage at room temperature affect pH?

Answer 11

Samples become acidic with time, so pH falls.

Question 12

ABG: effect of heparin on pH?

Answer 12

Heparin is acidic so will reduce the pH.

Question 13

ABG: how will PO2 readings be affected in hypothermic patients if temperature correction is not applied?

Answer 13

Inaccurately high PO2

Question 14

ABG: how does the storage of ABG sample at room temperature affect PO2?

Answer 14

Fall in PO2 as the white cells metabolise oxygen.

Question 15

Clark electrode incorporates what type of metal electrodes?

What solution?

Answer 15

silver anode, platinum cathode

Potassium chloride solution

Question 16

What metals are used in fuel cell?

Answer 16

Lead anode and a gold mesh cathode

Question 17

Is a fuel cell faster or slower than a polarographic electrode?

Answer 17

Fuel cell slower because it is an electrochemical process.

Question 18

What can affect fuel cell function?

Answer 18

Temperature and nitrous oxide in the mixture can react with lead anode and is broken down to nitrogen.

Question 19

Where does the oxygen consumption reaction take place in the Clark electrode?

Answer 19

At the cathode where there is a gain of electrons (reduction).

Question 20

Clark electrode can be used in gases or liquids?

Answer 20

Can be used in both.

Question 21

Severinghaus electrode incorporates which type of solution?

Answer 21

KCL and NaHCO3

Question 22

Which molecules have infra-red absorption spectra which overlaps with carbon dioxide?

Answer 22

water vapour, sevoflurane, nitrous oxide.

Oxygen can interfere even though it doesn’t absorb infrared light.

Question 23

What is the isobestic point for oxy and deoxyHb.

Answer 23

805 mm

Question 24

At what wavelength is there the maximum difference in absorption of the two forms of haemoglobin?

Answer 24

650 mm

Question 25

Refractometers:
How does it work?
How is used in vaporisers?

Answer 25

Used to measure vapour concentrations in gas mixtures by measuring the bending of light waves due to the change in gas composition.

Influenced by water vapour.

Used to calibrate vapouriser and can measure vapour concentrations indirectly.

Question 26

How can storage of ABG sample at 4 degrees C affect PO2 value?

Answer 26

False low because of O2 metabolism by white cells or false high due to air bubbles.

Question 27

ABG: how does too much heparinised saline affect readings?

Answer 27

Falsely low reading PCO2 - either due to dilutional effects (low PO2/CO2) or due to the heparin itself (high potassium measurement) is

Question 28

What wavelength is visible light?

Answer 28

400 - 700 nm

Question 29

What is the peak absorption of CO2 and volatile agents?

Answer 29

CO2 - 4.3 micrometers

Volatiles- 3.3 micrometers

Question 30

What absorption can methane interrupt gas analysis?

Answer 30

Can absorb in the 3-4 micrometers range (so between CO2 and volatiles) but volatiles also have another peak at 9-11 that is not affected by methane.

Methane can build up in a circle system.

Question 31

In a mass spec:
1. The analysing chamber is operated at what pressure?

2. What causes ion acceleration?
3. How many electromagnets does a quadruple mass spec?

Answer 31

1. The analysing chamber operates at near vacuum. Requires a pump to maintain this pressure.
2. Ions are accelerated by the cathode.
3. 4 large electromagnets that produce ion gate.

Question 32

In a mass spec:

1. Can the gas be returned to the breathing circuit?
2. Is it more or less accurate than infra-red absorption spectrophotometer?

Answer 32

1. Samples cannot be returned to the breathing circuit- they are ionised.
2. Mass spec is the GOLD STANDARD of gas analysers.

Question 33

T/F: fuel cells can be used for breath to breath analysis of O2 conc.

Answer 33

False, Clark and fuel cells are too slow for this.

Question 34

T/F: damping coefficient is inversely proportional to the amplitude ratio in invasive BP monitoring?

Answer 34

True- high amplitude ratio suggests little damping, therefore low damping coefficient.

Question 35

The natural frequency of a mass string dynamic oscillator is inversely proportional/ proportional to…?

Answer 35

Inversely proportional to the square root of its mass.

Question 36

What is Fourier Transformation?

Answer 36

It’s a mathematical operation that deconstructs a complex signal into its constituents frequencies.