How do we monitor anaesthesia? Pulse Oximetry

Question 1

1. Is pulse oximetry the gold standard for measuring blood O2?
2. What info can be gained from pulse oximetry?

Answer 1

1. No blood gas analysis is. But using pulse ox is easy, quick and non-invasive.
2. • Hb O2 saturation levels (%).
     - Heart / pulse rate.
     - Wave form (some machines).

Question 2

1. What is a limitation of pulse oximetry?
2. How can PaO2 be calculated?

Answer 2

1. Only see changes as go lower on the oxyhaemoglobin dissociation curve.
2. Multiply inspired fraction of O2 by 5.

Question 3

1. Benefits of a handheld pulse oximeter.
2. Drawbacks of handheld pulse oximeter.

Answer 3

1. Versatile – can be used anywhere.
2. Susceptible to damage i.e. can be dropped.
   Rely on batteries which need regular changing.

Question 4

1. Benefits of pulse oximeter on multiparameter machine.
2. Drawbacks of pulse oximeter on multiparameter machine.

Answer 4

1. Sturdy and less likely to be dropped and damaged.
2. More expensive initially.

Question 5

How does the pu.se oximeter work?

Answer 5

• Light absorption.
• Differentiation of wavelengths of light.
• Probe consists of light emission from one side and light detector on the other side.
• Light must contend w/ muscle, tendons, ligaments, skin, bone, giving a background consistent absorption of light.
• Oxyhaemoglobin absorbs greater amounts of near-infrared light and lower red light.
• Deoxyhaemoglobin absorbs more red light than oxyhaemoglobin.
• Red light = 660nm.
• Infrared light = 940nm.
• Ability to detect only arterial blood due to blood volume fluctuations during cardiac cycle.

Question 6

1. What is oxyhaemoglobin?
2. What is deoxyhaemoglobin?

Answer 6

1. 4 subunits of haemoglobin are all saturated w/ O2 molecules.
2. Haemoglobin that is not carrying O2.

Question 7

What laws of physics cover the workings for pulse oximetry and what do they state?

Answer 7

• Beers Law – States that the amount of light absorbed increases OR the light transmitted decreases as the concentration of the substance increases.
• Lamberts Law – States that the intensity of transmitted light decreased exponentially as the distance travelled through the substance increases.

Question 8

How to set up and use pulse ox.

Answer 8

• Check the unit visually for damages etc.
• Ensure batteries present / unit plugged in.
• Ensure probe connected.
• Test probe.

Question 9

Where on the patient is it best to place the probe?

Answer 9

• Best on the tongue as unpigmented and damp and bald.
• If cannot get access to mouth then others such as inter-digital space, ear, prepuce, vulva. Must ensure non-pigmented and hairless.

Question 10

What are we looking for when measuring w/ pulse ox?

Answer 10

100% is best!
95-100% is ok/good.
90-95% is causing for more attention to patient and equipment.
<90% is a worry.

Question 11

1. Name the wave form produced by the pulse ox.
2. What does this wave form mimic? – what should it match?
3. What does the upward wave represent?
4. What does the downward wave represent?

Answer 11

1. Plethysmograph.
2. Arterial BP trace. – should match the HR.
3. Systole (cardiac contraction).
4. Diastole (cardiac relaxation and refilling).

Question 12

1. What is the dicrotic notch representing?

Answer 12

1. On the downward wave when the aortic valve close and the distended aorta contracts, creating a brief change in pressure into the arterial circulation.

Question 13

Benefits of pulse oximetry.

Answer 13

• Non-invasive.
• Available in almost all settings.
• Reasonably priced.
• Non-painful.
• Quick and easy to set up and use..
• Gives a clear reading.
• Can be used on conscious and unconscious patients.

Question 14

Limitations of pulse oximetry.

Answer 14

• False readings.
• Susceptible to damage (esp. handheld).
• Doesn’t work well in anaemia.
• Can cause tissue compression in SAs..
• Won’t work well on pigmented skin.
• Patient movement.
• Poor perfusion states can alter readings (e.g. hypothermia, shock, vasoconstriction).
• Too thin tissue can cause poor readings (e.g. cat ear).
• Interference.
• Carboxyhaemoglobin cannot be distinguished from oxyhaemoglobin so may see falsely elevated levels.

Question 15

How can we troubles shoot for the temperamental nature of the pulse oximeter?

Answer 15

• Re-position the probe e.g. to different part of the tongue or completely different location altogether.
• Wet the area.
• Ensure it is working and test it on own finger.
• Change the batteries in the pulse oximeter.
• Check the probe for damage.
• Check the patient to see if the numbers displayed are true to their state.