SAQ 2017.2 Question 4

Describe the generation and features of a normal awake EEG (15)

Briefly discuss the processing performed by EEG monitors (BIS/Entropy) to produce a single dimensionless number from the EEG (10)

This material is adequately covered in a couple of the books on the recommended reading list – Magee & Tooley, and Davis & Kenny. There are some better review articles around, and a mob of FANZCAs in Cairns paralysed each other sans anaesthesia using BIS monitoring and published it (British Journal of Anaesthesia, Volume 115, Issue suppl_1, 1 July 2015, Pages i95–i103).

The EEG measures action potentials     TRUE/FALSE

As a patient becomes more deeply anaesthetised their EEG drops in amplitude and frequency    TRUE/FALSE

Burst suppression becomes more pronounced with deeper levels of anaesthesia    TRUE/FALSE

Phase coherence becomes more pronounced with deeper levels of anaesthesia    TRUE/FALSE

Frowning will increase RE (response entropy) more than SE (state entropy)    TRUE/FALSE

Aviation 1 – Introduction

Today we start a periodic series of posts with an aviation theme.

I am sure you have heard tropes comparing aviation and anaesthesia: human factors, crisis management, check lists, take off, cruise and landing to name a few.

You may not be aware that many physiological principles relevant to anaesthesia are also relevant in aviation and space travel.

The environmental stressors experienced by pilots and astronauts are similar to those experienced by patients, but magnitude of these stressors and the physiological effects can be more extreme.

Thinking about these effects can test and hopefully extend your understanding of the underlying principles.

There is only one primary exam learning outcome (BT_PO 1.37) that specifically mentions altitude and this has been the subject of previous posts.  There is also one in the final exam curriculum (SS_IC 1.102).

However there are many others that have relevance to aviation physiology.

BT_SQ 1.6 Describe the methods of measurement applicable to anaesthesia, including clinical utility, complications and sources of error in particular: 

  • SI units 
  • Measurement of volumes, flows, and pressures, including transducers

Pressure is measured in different ways throughout anaesthesia and a working knowledge of conversion factors between various units is important.

TRUE/FALSE  1 atmosphere (ATM) = 760 mmHg

TRUE/FALSE  29 psi = 200 kPa

TRUE/FALSE  30 cmH2O = 22 mmHg

TRUE/FALSE  5 kPa = 40 mmHg

Some harder ones:

TRUE/FALSE In aviation, altitude is measured in feet not metres

TRUE/FALSE There is an exponential decline in pressure with increasing altitude

TRUE/FALSE There is a linear decline in temperature with increasing altitude

Oximetry

BT_SQ 1.6 Describe the methods of measurement applicable to anaesthesia, including clinical utility, complications and sources of error in particular:

· Oximetry

 

Don’t know what happened to my carefully written questions. I will have to come up with some more!

 

TRUE/FALSE The constant in the Beer-Lambert law is called the extinction coefficient.

TRUE/FALSE When calculating the R value, the AC values are divided by the DC values to account for the fact that the absolute absorbance is different at the two wavelengths

TRUE/FALSE The accuracy of a pulse oximeter at a given range is given as a 95% confidence interval.

TRUE/FALSE Motion artefact make an oximeter read 85%

TRUE/FALSE Oximeters are affected by ambient light sources flickering at the mains frequency.

 

ETCO2

BT_SQ 1.6 Describe methods of measurement (including) … gas analysis, including capnography
BT_PO 1.29 Discuss regional ventilation-perfusion inequalities

Each of the following T/F statements applies to a cardiac arrest with CPR being performed

T / F  an ETCO2 of 15 mmHg would indicate good CPR

T / F  the PaCO2 will correlate with the ETCO2

T / F  the lower ETCO2 is due to a reduced venous CO2 content

T / F  the lower ETCO2 is due to absent aerobic cellular respiration

T / F  the lower ETCO2 is due to increased alveolar dead space

Can you provide a rationale for the correct statements above, from first principles? Hint… this blog relates to learning outcome BT_PO 1.29

 

BT_SQ 1.6 Oximetry

There have been a couple of posts on this topic before, but as we were talking about oxygen, I thought it might be worth revisiting.

Here is a comprehensive review of the sources of error with pulse oximetry, following a discussion of the principles. See the link to the CinderHK page below for more information on good references.

OLYMPUS DIGITAL CAMERA

Slightly off topic,  though I think fascinating, here is a picture of the absorption spectrum of different contaminants of glass. The glass used in long distance telecommunication fibre optic cables is ultra pure. The signal is transmitted in the IR spectrum, at about 1.5μm, such that the attenuation of the signal is only 0.2dB per km! I took this photo at the Hong Kong Science Museum.

BT_SQ 1.6 Describe the methods of measurement applicable to anaesthesia, including clinical utility, complications and sources of error in particular:

  • Oximetry
  • Plus heaps of others 😉

 

Pulsatile venous flow may cause an over estimation of SpO2                              TRUE/FALSE

The red:IR absorption modulation ratio (R) equals 1 at SpO2 85%                      TRUE/FALSE

Intravenous injection of indocyanine green causes a transient reduction in SpO2   TRUE/FALSE

The accuracy of SpO2 in humans has not been calibrated below 70%               TRUE/FALSE

Red nail polish is likely to cause inaccurate SpO2 readings                                  TRUE/FALSE

Temperature Measurement

BT_GS 1.69a Describe how a patient’s temperature is monitored and discuss the indications for temperature monitoring with the advantages and disadvantages of particular sites and methods (also refer to monitors and monitoring standards, which is covered in the Safety and quality in anaesthetic practice clinical fundamental)

This is also covered by BT_SQ 1.6, but I didn’t want BT_GA 1.69a to feel left out.

There are plenty of posts so far on the effects of temperature, but how do you measure it?

T/F Nasopharyngeal temperature probes use thermocouples

T/F Thermistors demonstrate hysteresis

T/F A smaller thermistor will have a faster response time

T/F An infrared thermometer works by emitting IR radiation and measuring the amount reflected by the tympanic membrane

T/F A Swan Ganz catheter incorporates a thermistor

 

Supplementary Questions:

Temp Probe.jpgWhat kind of device is this?

How does its output vary with temperature?

What temperature ranges is it accurate over?

Humidity

BT_PO 1.38 Define humidity and outline the importance of humidification

RainOnGlass_2

Not a riveting topic, but giving dry gas to a patient can cause problems, so you need to know something about it.

T/F at a relative humidity of 100%, a litre of gas at 21 degrees Celcius will contain the same amount of water as a litre of gas at 37 degrees Celcius

T/F In an awake person, during quiet nasal breathing, humidification of inhaled air occurs in the upper airway

T/F Airway lining fluid acts as heat and moisture exchanger during respiration

T/F Relative humidity levels need to be monitored and controlled in the operating theatre

T/F Administration of dry gases may impair surfactant activity

Nitrogen Analysis

BT_SQ 1.6 Describe the methods of measurement applicable to anaesthesia, including clinical utility, complications and sources of error in particular:

· Gas analysis, including capnography

 

I thought I would do a special post on Nitrogen, because it is a gas we use in most of our anaesthetics, but rarely monitor. It obviously is possible to measure it in real time, because nitrogen washout one of the methods used in the determination of FRC.

T/F Nitrogen can be measured using a paramagnetic analyser

T/F Nitrogen can be measured using a infra-red analyser

T/F Nitrogen can be measured using a gas discharge tube

T/F Nitrogen can be measured using a rapid response Clark electrode

T/F Nitrogen can be only be measured using a mass spectrometer

Depth of anaesthesia monitoring…again

BT_GS 1.52 Explain the principles involved in the electronic monitoring of depth of sedation and anaesthesia, including the use of EEG analysis

Again, the comments relate to the beloved BIS again.

BIS has been validated as a depth of anaesthesia monitor in large clinical trials  T/F

Burst suppression is seen in physiological sleep  T/F

Propofol can cause an isoelectric EEG  T/F

Volatile anaesthetics can suppress the EEG but not cause an isoelectric EEG  T/F

Spindle formation on the EEG is only seen in slow wave sleep  T/F

BIS Monitoring

BT GS 1.52 Explain the principles involved in the electronic monitoring of depth of anaesthesia, including the use of EEG analysis.

The statements relate to BIS as this is the most commonly used ‘depth of anaesthesia’ monitor in Australasia. Entropy is pretty similar. A plea from a pragmatic anaesthetist- if you are going to use BIS then please make sure you are displaying the EEG trace on your monitor (and make sure you’ve turned the filter off). The dimensionless number by itself is close to useless. You should be able to interpret a raw EEG trace as well as you can an arterial waveform or ECG trace. I highly commend the ICETAP.org website as an educational resource. It is an excellent place to learn how to interpret the EEG and get the most clinically useful information from your processed EEG monitor. If you don’t know the answer to the third statement below then can I suggest you look at the paper by Whitlock et al in Anesthesiology 2011; 115: 1209-18. Figure 4 should astonish you if you haven’t already seen it.

The algorithm by which the BIS value is calculated has been made known to clinicians T/F

Ketamine can elevate the BIS reading because of its effect on the beta ratio  T/F

There is a clear dose-response relationship between BIS values and end-tidal volatile concentrations  T/F

There is a clear dose-response relationship between BIS values and plasma propofol concentrations  T/F

SR denotes the Suppression ratio which is the percentage of time in the preceding 63s that the EEG has been suppressed. T/F