Pulse oximetry and BP monitoring

Question 1

What are 6 forms of non-invasive monitoring used during anaesthesia?

Answer 1

1. ECG
2. Temerature
3. Blood pressure
4. Pulse oximetry
5. End-tidal CO₂
6. Height and weight

Question 2

How is blood pressure measured using an automatic device?

Answer 2

• Using an automated devices cuff
• line marked artery must be placed on anterior part of arm, bladder at front of cuff to push into the front rather than back of arm
• Use correct sized cuff with arm diameter within specied range

Question 3

What is the name of the process allowing non-invasive, automated BP cuffs to work?

Answer 3

oscillometry

Question 4

Explain the 6 steps of oscillometry that allo non-invasive BP to be measured.

Answer 4

1. As pressure rises, there are initial, very small oscillations within the cuff (see bottom line of attached image)
2. The cuff inflates until the oscillations have reduced to a minimum and level out
3. Cuff determines pressure as blood runs under it = systolic pressure
4. The cuff starts to deflate again; as it goes down in a stepwise fashion, the oscillations go away again
5. At maximum oscillations you are close to the mean arterial pressure
6. To find the pressure at diastole, the shape of the envelope of pressures is taken to account and an estimate is made - around where the oscillations reach a minimum again and are small

Question 5

What is meant by the envelope of pressure when talking about oscillations within a BP cuff?

Answer 5

increase then decrease in oscillations, see image

Question 6

What are 2 possible reasons why it could be difficult to work out the envelope of pressure and hence the diastolic pressure when an automated cuff is measuring BP?

Answer 6

1. Atrial fibrillation
2. Surgeon leaning on patient’s arm

Question 7

What are 4 examples causes of error in non-invasive blood pressure?

Answer 7

1. Cuff size and position
2. Calibrations - follow manufacturers instructions
3. Diastolic problem - inaccuracies in measurement e.g. missing pre-eclampsie in obstetrics
4. Atrial fibrillation

Question 8

How do cuff size and position influence errors in non-invasive blood pressure?

Answer 8

1. If cuff is too small, over-read pressure i.e. appears higher
2. Bladder must be at the front

Question 9

What can be done re. the issue of atrial fibrillation producing errors in non-invasive blood pressure measurement?

Answer 9

Use direct arterial monitoring instead

Question 10

What are 2 things that pulse oximetry produces readings for?

Answer 10

1. Pulse rate
2. saturation

Question 11

How does pulse oximetry work, in basic terms?

Answer 11

Use of infra-red light to measure

Question 12

What is the structure of haemoglobin and how does it bind with oxygen?

Answer 12

• Hb has 2 alpha and 2 beta chains, each with its own iron molecule
• Hb binds in a complex shape, with each molecule only carrying 4 molecules of oxygen
• When Hb is fully bound with 4 molecules of oxygen it is 100$ saturation

Question 13

What is the equation used to work out saturation of haemoglobin with oxygen?

Answer 13

Saturation of O₂ (%) = HbO₂ / (HbO₂ + Hb)

Question 14

What is the colour changing property of Hb?

Answer 14

Changes from bright red to bluey-red colour when saturation falls

Question 15

How do the colour changing properties of Hb help pulse oximeters to work?

Answer 15

• Things appear to have colour as they absorb light to a greater or lesser extent and to different colours
• Red objects absorb light at blue end of spectrum (high frequency/ shorter wavelength): therefore red oxyhaemoglobin i.e. more saturated Hb absorbs light at blue end of spectrum
• Haemoglobin of reduced saturation is bluer in colour, so absorbs light at red end of spectrum (longer wavelength, lower frequency), and is near infrared

Question 16

What is the name of the law referring to the fact that when light passes through an absorber, the intensity of light coming out of the absorber will be less than when entering?

Answer 16

Beer-Lambert Law

Question 17

What are 4 variables in the equation illustrating the Beer-Lambert Law?

Answer 17

1. Absorbence (A)
2. Absorption coefficient (a)
3. Path length (l)
4. Concentration we’re trying to measure (c)

The Is in the diagram represent light going into the absorber (I₀) and the light coming out (I₁)

Question 18

How do the different absorptions of haemoglobin (bluer) and oxyhaemoglobin (redder) allow pulse oximetry to work?

Answer 18

Have similar patterns of absorption but at a certain wavelength (650nm) there’s a wide separation between their absorptions of light

At 800nm they cross on the graph i.e. have very similar absorption which is used as the ‘isobestic point, i.e. baseline of total absorption of all haemoglobin

Ratio between these two points will be closely related to saturation

Question 19

What is the name of the form of science that has been used for many use as an earlier form of pulse oximetry?

Answer 19

Spectrophotometry

Question 20

What is spectrophotometry?

Answer 20

• Produces the saturation curve by shining white polychromatic light through a cuvette
• detectors through a filter and a dual wavelength spectrometer produce the curve
• the path length from light to the detectors is fixed and blood inside the cuvette is fully lysed so the cells don’t scatter light
• use many more than 2 wavelenths to look at different types of Hb

Question 21

How does pulse oximetry use different wavelengths to measure oxygen saturation?

Answer 21

• use 2 light sources: one at 660nm - predominantly absorbed by oxyhaemoglobin, and one at the isobestic point of 910nm
• Signal coming out of pulse oximeter: only the top part (pulsatile component) is important
• Light is being absorbed by the arterial end of capillaries

Question 22

What are 10 things that can give false low readings with pulse oximetry?

Answer 22

1. Methaemoglobin (MetHb) reads 85% - only seen in prilocaine poisoning
2. Methylene blue
3. Indocyanine green
4. Venous congestion/ pulsations - tricuspid regurgitation, high airway pressures, Valsalva manoesvre - pushes venous blood into window, afecs pulsatility
5. Fluorescent light
6. Surgical diathermy
7. Shivering
8. Nail varnish
9. Abnormal Hb e.g. foetal and sulph-Hb
10. Bilirubin and very dark skin pigments

Question 23

Below what point are pulse oximeters inaccurate and why?

Answer 23

Few reading below 70% when pulse oximeters were being developed so preciison below his is poor

Question 24

What are 5 things that can give false high readings in pulse oximetry?

Answer 24

1. Carboxyhaemoglobin: can’t differentiate between this and carboxyhaemoglobin; co-oximeter needed to accurately measure oxyhaemoglobin
2. Poor perfusion
3. Anaemia
4. 60s response time: averaging to get around noise, delay between saturation falling and true fall in saturation falling along with pO₂
5. No CO₂ information - will shift curve

Question 25

What is the dissociation curve? Explains its shape.

Answer 25

• Plot of saturation against pO2
• slightly S shaped as first molecule of oxygen is difficult to get on
• rises steeply as molecules 2 and 3 are relatively easy to get on, but 4 is hard so flattens again at the end
• never quite reaches 100%

Question 26

What are 4 things that left shift the dissociation curve (i.e. make it easier for haemoglobin to gain O2 molecules)?

Answer 26

1. Lower hydrogen ions (higher pH)
2. Lower CO₂
3. Lower DPG
4. Lower temperature

Question 27

What are 4 things that right shift the dissociation curve (i.e. make it harder for Hb to pick up oxygen molecules)?

Answer 27

1. Hydrogen ions (lower pH)
2. Rising CO₂: can be so right shifted with high CO₂ that patients desaturation even with normal pO₂
3. Rising DPG
4. Rising temperature

Question 28

What are 3 the goals of developing new pulse oximeters?

Answer 28

1. Reduce impact of movement: Masimo Signal Extraction Technology (SET) pulse oximetry
2. Measuring other Hb types e.g. carboxyhaemoglobin in case of carbon monoxide poisoning, with rainbow pulse oximeters - multi-wavelength range, also methaemoglobin
3. New version can also estimate total Hb

Question 29

What is the difference between capnometry and capnography?

Answer 29

Capnometry produces the CO2 measurements/levels, capnography draws the trace

Question 30

When is capnography used?

Answer 30

Very widely in general anaesthesia, increasingly also ED and critical care; also used with nasal sampling system during sedation

Question 31

What is a potential use for robotic capnography?

Answer 31

Could be used to approach casualties in dangerous situations to detect if the patient is exhaling CO2 or not

Question 32

What can the shape of capnogram tell you?

Answer 32

Tells you a lot about pulmonary disease

Question 33

What are 5 methods of measuring CO2 in expired gas?

Answer 33

1. Infra-red spectrometry
2. Molecular correlation spectrometry
3. Raman spectrometry
4. Mass spectrometry
5. Photoacoustic spectrometry: signal is oscillated at an audible frequency and sensitive microphone used to detect absorption

Question 34

Why can infra-red be used to measure CO2?

Answer 34

CO2 can selectively absorb specific wavelengths of light -4300nm of infra-red; amount of light absorbed is proprtional to concentration of absorbing molecules, so concentration of CO2 can be determined by comparing measured absorbance with absorbance of known standard

Question 35

What are 2 ways that CO2 concentration measured by a monitor can be expressed?

Answer 35

Partial pressure in mmHg or percentage CO2

Question 36

How is expired CO2 concentration measured? 2 ways

Answer 36

1. Sampled from the breathing system after the Y piece - can be returned to the circle system or scavenged to waste - sidestream analyser
2. Cuvette placed after Y piece - mainstream analyser (uncommon)

Question 37

What do the 3 key points on the typical capnograph represent:

1. A-B
2. B-C
3. D?

Answer 37

1. Inspiration
2. Mixed inspired and expired gas coming out of the trachea
3. End-tidal point which, in health, gives a reading close to arterial CO2

Question 38

Does a healthy patient’s capnograph reach a true plateau?

Answer 38

No, but is commonly incorrectly drawn

Question 39

What are 5 important pieces of information that capnography provides?

Answer 39

1. Respiratory rate
2. Estimate of arterial pCO₂ from end-tidal CO₂
3. Disconnection, displacement will cause it to disappear
4. Rebreathing, circuit failures: will cause rise in inspired CO₂, rapidly detected
5. Reduced pulmonary blood flow - e.g. embolus or cardiaic errors will cause big drops in CO₂

Question 40

What are 4 causes of errors in capnography?

Answer 40

1. Poor calibration - easy to zero, just aspirate air; can check it by breathing down it yourself
2. Wrong sample site i.e. patient side of the Y - mustn’t sample just inspired gas
3. Blocked sampling hose
4. Water in hose

Question 41

What is meant by agent analysis?

Answer 41

Anaesthetic agent monitoring, has increased in recent years and there are a number of ways to do it

Question 42

What principle, similar to capnography, can agent analysis use and how?

Answer 42

• Use of the infra-red spectrum
• anaesthetic agents abosrb in the 3-4 and 8-9 micron bandwidths, and have different absorption patterns (see graph of isoflurane, halothane and enflurane)
• devices can now distinguish between different anaesthetic agents themselves

Question 43

Why might agent analysis be confused by what agent to analyse?

Answer 43

Occasionally use more than one agent e.g. one in anaesthetic room then move to threatre and change, so patient will for a while breathe both out

Question 44

What is done to acheve neuromuscular monitoring?

Answer 44

Place electrodes over the ulnar nerve, stimulate it, then the thumb twitches; measure the force produced using a force transducer

Question 45

What must you ensure doesn’t happen when performing neuromuscular monitoring?

Answer 45

Make sure hand doesn’t move - this will affect baseline position and force of twitch measured

Question 46

What are 2 alternative wasy to perform neuromuscular monitoring rather than using the force of the twitch of the thumb?

Answer 46

1. Measuring acceleration instead using an acceleration chip on thumb
2. Piezo crystal in flexible transducer used to measure contractions, produces small voltage when crystal bends (modern method)

Question 47

Is neuromuscular monitoring used on every case?

Answer 47

no, should be available but not used on every case