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聽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.


Slightly off topic, 聽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 1.5渭m, 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聽Describe the methods of measurement applicable to anaesthesia, including clinical utility, complications and sources of error in particular:

  • Oximetry
  • Plus heaps of others 馃槈


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

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

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

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

Red nail polish is likely to cause inaccurate SpO2 readings 聽 聽 聽 聽 聽 聽 聽 聽 聽 聽 聽 聽 聽 聽 聽 聽 聽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聽Nasopharyngeal temperature probes use thermocouples

T/F聽Thermistors demonstrate hysteresis

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

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

T/F聽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?


BT_PO 1.38聽Define humidity and outline the importance of humidification


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

T/F聽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:

路 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聽 T/F

Burst suppression is seen in physiological sleep聽 T/F

Propofol can cause an isoelectric EEG聽 T/F

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

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

BIS Monitoring

BT GS 1.52 Explain the principles involved in the electronic monitoring of depth of anaesthesia,聽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聽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 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聽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聽 T/F

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

There is a clear dose-response relationship between BIS values and plasma propofol concentrations聽 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


BT_PO 1.114 Methods for assessing coagulation, platelet function and fibrinolysis

These questions relate to ROTEM which seems to be the favourite viscoelastic assay in use at present. Even my humble little hospital has one of these things.

Q. It takes about an hour to provide clinically useful information.聽 TRUE/ FALSE

Q. Will reliably detect platelet dysfunction as a result of clopidogrel therapy.聽 TRUE/ FALSE

Q. Reliably detects the presence of heparin therapy.聽 TRUE/ FALSE

Q. Can detect primary and delayed fibrinolysis.聽 TRUE/ FALSE

Q. Will be abnormal if the patient is hypothermic.聽 TRUE/ FALSE


Patient of the week

Recently I was anaesthetising an adult with a congenital syndrome. I was quite worried about the airway鈥攂ut in the end it wasn’t that which caught me out. She had no congenital heart disease, but had a pericardial effusion drained a few years previously. I was quite sparing with the induction agents as I wanted to maintain spontaneous respiration, but nonetheless…

About 5 minutes after induction, I noticed the blood pressure was 54/28.

BT_SQ 1.6

T/F At low levels of blood pressure, the NIBP tends to give spuriously low values.

T/F The most accurate component of the NIBP is the mean.

At the same time, her saturation聽dropped to 88, even though she was breathing 100% oxygen. The pleth had a good volume and looked normal.

BT_SQ 1.6,聽BT_PO 1.29

T/F The fall in SpO2 was most likely to be artifactual.

I gave three doses of 1mg metaraminol, but, although the saturation improved, the blood pressure remained in the low 70s. Heart rate was in the 40s.聽Worried that I might see another fall in saturation I decided to run a noradrenaline infusion.

BT_PO 1.52

In such a situation, the most appropriate vasoactive agent would be:

a) Ephedrine

b) Metaraminol

c) Adrenaline

d) Noradrenaline

e) Isoprenaline

After a 20碌g bolus dose of noradrenaline, the heart rate dropped to 28.

T/F The most likely cause of the fall in heart rate is alpha 1 receptor agonism in the SA node.

I found out, after the (otherwise uneventful) operation,聽that she normally has聽quite a low blood pressure. A good reminder that, when having trouble with anaesthesia, one should first look to the proximal end of the needle.

BT_SQ 1.6 Flow Measurement

Having聽observed a day’s silence in memory of those suffering yesterday, we return to our regular programming with some questions on聽an old favourite LO 馃槈

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

路 SI units

路聽Measurement of volumes, flows, and pressures, including transducers.聽

路 Measurement of blood pressure

路 Measurement of cardiac output

路 Measurement of temperature

路 Oximetry

路 Gas analysis, including capnography

路 Methods used to measure respiratory function, including:

– Forced expiratory volume

–聽聽Peak expiratory flow rate

–聽聽Vital capacity

–聽聽Flow-volume loops

–聽聽Functional residual capacity and residual volume

TRUE/FALSE If exhaled gas is not warmed to patient temperature in a pneumotachograph, volume will be underestimated
TRUE/FALSE Volume is the area under a flow/time curve

TRUE/FALSE A pneumotachograph calculates flow from a known resistance and a measured pressure difference
TRUE/FALSE A pneumotachograph uses the hydraulic version of Ohms Law

TRUE/FALSE A pneumotachograph measures flow accurately only when it is turbulent

This is a Dr盲ger flow sensor from one of the limbs of a circle circuit. If you look closely you can see a fine wire between the top two prongs. The wire between the lower two prongs isn’t shown so clearly. What principle is this flow sensor using?


Here are two different views of聽the sensor that modern GE machines use for measuring flow in a circle. The two tubes are hollow, and you can see on the photo on the left that they are open to the circuit. What principle is this flow sensor using? How do you think that it聽works?