Oxygen analysis BT_SQ 1.6

Moving to measuring oxygen in its gaseous form…

TRUE/FALSE An operating room paramagnetic analyser incorporates a pressure transducer

TRUE/FALSE  Pressure exerted on the side of a tube decreases as flow rate increases

TRUE/FALSE  Nitric oxide at clinically used concentrations will falsely increase oxygen concentration in a paramagnetic analyser used in theatre

TRUE/FALSE  Paramagnetic analysis degrades oxygen molecules into free radicals so the gas cannot be returned to the circuit

TRUE/FALSE  Oxygen tension can also be measured with infrared analysis

Oximetry BT_SQ 1.6

Since you’ve already been reading about pulse oximeters we’ll stay on this topic.

TRUE/FALSE  Response time is faster when the oximeter is on the earlobe cf the finger

TRUE/FALSE  Bilirubinaemia can result in a falsely low oxygen saturation with pulse oximetry

TRUE/FALSE  The percentage of the signal which is pulsatile in finger pulse oximetry is approximately 80%

TRUE/FALSE  Anaemia may cause under-reading of oxygen saturations with pulse oximetry

TRUE/FALSE  A pulse oximeter will detect a drop in oxygen tension from 600mmHg to 200mmHg

Oximetry BT_SQ 1.6

This LO is massive, and covers essentially an entire textbook. For this reason we will return to it frequently.

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.

· Measurement of blood pressure

· Measurement of cardiac output

· Measurement of temperature

· Oximetry

· Gas analysis, including capnography

· Methods used to measure respiratory function, including:

– Forced expiratory volume

–  Peak expiratory flow rate

–  Vital capacity

–  Flow-volume loops

–  Functional residual capacity and residual volume

TRUE/FALSE A pulse oximeter radiating 2 wavelengths of light can only differentiate 2 different forms of Hb.

TRUE/FALSE Oxygenated haemoglobin absorbs light at a wavelength of 660nm.

TRUE/FALSE The y axis on the plethysmograph is an estimate of arterial calibre and thus sympathetic tone.

TRUE/FALSE Methaemoglobin is strongly absorbed at 660 and 940nm

TRUE/FALSE An adult pulse oximeter cannot accurately read oxygen saturations when foetal haemoglobin is present.

Study Tip: Drug Structures

I sometimes get asked how much you need to know about drawing structures of drugs. When I sat for the primary, I didn’t know the answer to this question either, so I just learned everything. This strategy was good memory training, but I don’t think it is the most productive use of your study time.

The only reason you would be asked about a drug structure, is so that you can discuss how the structure affects its activity. The two drug classes where SARs are specifically called out in the LOs are:
Inhalational Agents
Sedative Hypnotics: The one in particular we are interested in is Midazolam

Other drugs where SARs are commonly asked are:
Local Anaesthetics

It is not necessary to be able to draw the structures of these drugs. You may be shown a structure of one of the more common drugs, however, and asked if you recognise it. The question would then go on to SARs.

You may also be asked to describe some of the important features of the structures of muscle relaxants or opioids, particularly morphine derivatives. Being able to recognise the shape of the morphine molecule will help you to explain why related compounds have different actions.

Gas Connectors BT_SQ 1.10

Being the weekend, I will take a break from physiology and show a few pictures. How these devices work is examinable. The legal points are not, but I have put them in here to give you some understanding of the issues involved in trying to standardise equipment in your department.

BT_SQ 1.10 Describe the supply of medical gases (bulk supply and cylinder) and features to ensure supply safety including pressure valves and regulators and connection systems.


Figure 1.sis-oxygen


What is the name of this connection?

How is it made gas specific?











Figure 2.


What is the name of this connector?

How is it made gas specific?

Is it legal to use this in Australia?

Would its use make the device non TGA compliant?

Does this comply with the Australian Standard?


Figure 3. Fitting from CO2 insufflator.


What general kind of fitting is this?

Do you think this fitting is gas specific?

If you ignore the rust, would this fitting comply with the Australian Standard?


Next week I am hoping to do some equipment and measurement to spice things up!


V/Q mismatch measurement BT_PO 1.30

BT_PO 1.30 Outline methods used to measure ventilation-perfusion inequalities


TRUE/FALSE In a healthy conscious resting subject, there is no significant difference between PCO2 of end-expiratory gas and arterial blood

TRUE/FALSE The Bohr equation is a reliable estimate od dead space in both the exercising and resting subject

TRUE/FALSE Position of the neck and jaw alters the anatomical dead space by more than 30mls in an adult

TRUE/FALSE The neck flexed-chin depressed position causes more anatomical dead space than the neck extended-jaw protuded position

TRUE/FALSE The arterial/end-expiratory PCO2 difference can be used to assess the magnitude of anatomical dead space

Airway Resistance BT_PO 1.16

BT_PO 1.16 Describe the factors affecting airway resistance and how airway resistance may be measured

TRUE/FALSE An increase in airway resistance will cause a decrease in FEV1

TRUE/FALSE Airway resistance measurement with an oesophageal balloon is inaccurate in severe airways disease

TRUE/FALSE Airway resistance is highest in the terminal bronchioles

TRUE/FALSE Flow in medium sized airways is laminar

TRUE/FALSE Airway conductance is linearly related to lung volume

Shunt BT_PO 1.28

BT_PO 1.28 Describe the shunt equation

TRUE/FALSE shunt refers to the proportion of cardiac output which does not participate in gas exchange

TRUE/FALSE An increased PaCO2 is generally not caused by venous admixture

TRUE/FALSE The haemoglobin concentration needs to be known in order to calculate pulmonary shunt

TRUE/FALSE Mixed venous PO2 can be measured using blood taken from the CVP lumen of a central line

TRUE/FALSE a healthy patient under general anaesthesia usually has a pulmonary shunt fraction of  10%


This week I will try sticking to a single topic.

BT_PO 1.35 Discuss the physiological consequences of intermittent positive pressure ventilation and positive end-expiratory pressure

TRUE/FALSE Very high levels of PEEP may decrease SaO2

TRUE/FALSE 15-20% of lung volume may be atelectatic during an anaesthetic where IPPV is used

TRUE/FALSE Oxygenation is improved more when hypovolaemic patients are given PEEP compared to normovolaemic patients

TRUE/FALSE Venous admixture may increase to 10% of cardiac output with IPPV and anaesthesia

TRUE/FALSE An increased BMI decreases atelectasis by increasing splinting of the chest wall