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.  TRUE/ FALSE

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

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

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

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


Patient of the week

Recently I was anaesthetising an adult with a congenital syndrome. I was quite worried about the airway—but 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 dropped to 88, even though she was breathing 100% oxygen. The pleth had a good volume and looked normal.

BT_SQ 1.6BT_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. 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 20µg 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, that she normally has 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 observed a day’s silence in memory of those suffering yesterday, we return to our regular programming with some questions on an old favourite LO 😉

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 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 Dräger 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 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 works?

BT_GS 1.55 Monitoring of neuromuscular blockade

BT_GS 1.55 Describe the concept of depth of neuromuscular blockade and explain the use of neuromuscular monitoring

TRUE/FALSE Response to repeated single twitches at 1Hz is greater than at 0.1Hz

TRUE/FALSE In the late phase of recovery after muscle relaxant admnistration, tetanic stimulation can cause lasting antagonism of neuromuscular blockade

TRUE/FALSE The post tetanic count at which the first twitch on the train of four (TOF) appears is similar for both atracurium and cisatracurium

TRUE/FALSE To completely prevent the bucking response to carinal stimulation the post tetanic count needs to be zero

TRUE/FALSE If no fade is felt with dual burst stimulation the train of four ratio is above 0.7

BT_GS 1.51 Depth of Anaesthesia Measurement

How do we know if a patient is unconscious when given a volatile agent? Depth of anaesthesia is a slippery concept, so it is worth doing some extra reading around the topic.

BT_GS 1.51 Describe the concept of depth of anaesthesia and how this may be monitored

TRUE/FALSE The EEG of a patient when anaesthetised has a smaller amplitude than when they are awake

TRUE/FALSE Anaesthesia with ketamine produces a different pattern of brain EEG compared to anaesthesia with volatile agents

TRUE/FALSE If two patients have the same BIS number, then they are anaesthetised at the same depth of anaesthesia

TRUE/FALSE Administration of a muscle relaxant will reduce the BIS index if EMG is present

TRUE/FALSE The Entropy monitor measures the effect of anaesthetic drugs on the brain by calculating the randomness of the EEG

BT_SQ 1.6 CO2 Analysis

Back to an old favourite LO 😉

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

TRUE/FALSE The 90-95% response time for a CO2 analyser should be less than 150ms
TRUE/FALSE Volatile agents can be distinguished from each other by measuring infrared absorbance at 3.3µm
TRUE/FALSE Collision broadening means that the absorption peak for CO2 at 4.3µm is made wider in the presence of Nitrous Oxide
TRUE/FALSE Infrared analysers measure gas concentration rather than partial pressure
TRUE/FALSE Water is a powerful absorber of infrared light

A couple of follow on questions:

  1. Why is the 90% response time important in a CO2 analyser? The answer is related to what you have learned about requirements for invasive pressure monitoring.
  2. What is the device in the picture below? What is it used for? How does it work?


BT_SQ 1.6 : measurement of blood pressure

And another guest post (yesterday’s post by this guest accidentally came up on 13 December – scroll back for more on resonance and damping if you missed it) :

Once again I was with my registrar with the expert knowledge of physics waiting for the cardiothoracic registrar to take down the mammary.  It was as though time stood still.   We were both looking at the clock, and reminiscing on the wonderful mechanics of (non digital) clocks.  She commented to me in passing “I do miss the slow natural frequency of the pendulum of a grandfather clock” which made me consider the fast swinging pendulum of a cuckoo clock.   Needless to say, my mind turned to the concept of natural resonance frequencies in invasive pressure monitoring systems, and I thought back to the days of my music lessons…

TRUE/FALSE  The natural resonant frequency of a system is proportional to the stiffness or tension in the system, and inversely proportional to the mass.

TRUE/FALSE  As in tightening a violin or guitar string, increasing the stiffness or tension will lead to an increase in the natural resonant frequency (a higher note on the instrument)

TRUE/FALSE  Like the pendulum of a grandfather clock being slower than that of a cuckoo clock on the wall, the pulmonary artery tracing on the monitor is not as good as the arterial system, as the pulmonary artery system has a much longer system and as such more mass and a lower natural frequency

TRUE/FALSE  The ideal system for an arterial monitoring system has a large length and very stiff tubing to ensure that its natural frequency is close to the frequency of the system being monitored

TRUE/FALSE  The ideal frequency for a pressure monitoring system is determined by the pressure range being measured, rather than by the frequency of the system.

Dynamic airway closure

It seems as though nerve conduction is not a very popular topic – it is a bit dry.

Perhaps today’s topic, dynamic airway closure, will be of more interest as it is one that candidates really struggle with in vivas.

BT_SQ1.6 Describe the methods of measurement applicable to anaesthesia, including clinical utility, complications and sources of error in particular:
· 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

West describes the topic well in his book, where the following diagram is taken from. The red circle shows the net pressure gradient between the intrapleural space and the airway.


Dynamic airways closure may occur during normal tidal breathing TRUE/FALSE

Dynamic airway closure accounts for the effort dependent portion of the expiratory limb of the flow-volume loop TRUE/FALSE

During forced expiration, positive pressure generated will be transmitted equally across the respiratory system TRUE/FALSE

The trachea is never subject to dynamic airway closure TRUE/FALSE

During the effort independent part of an expiratory flow volume loop, maximum air flow rate is determined by lung volume TRUE/FALSE

BT_SQ 1.6 : measurement of blood pressure

A contribution from an ex chair of the primary exam :



I was working in theatre with a registrar who had a previous degree in physics, and we were discussing the issues of accuracy with arterial monitors, in particular the concept of damping… Together we determined that this diagram probably illustrates some important concepts.

TRUE/FALSE  System A is an example of a system where the natural resonance frequency of the system is similar to that being measured.

TRUE/FALSE  System B is the most ideal for a blood pressure measurement response because it provides the most accurate reading.

TRUE/FALSE  System C is an example of optimal damping as it provides a rapid response with minimal sacrificing of accuracy

TRUE/FALSE  In all systems the eventual measured pressure will be accurate

TRUE/FALSE  Damping is a reduction in the amplitude of an oscillation as a result of energy being drained from the system to overcome frictional or other resistive forces.

Oxygen analysis BT_SQ 1.6

Let’s stay on this topic for another day…

TRUE/FALSE  Operating room paramagnetic analysers use 100% oxygen as the reference gas

TRUE/FALSE  Gas sampled from the patient wye will allow measurement of both inspired and exhaled gas tension

TRUE/FALSE  A fuel cell is a battery

TRUE/FALSE  A fuel cell requires temperature compensation

TRUE/FALSE  A fuel cell lifespan can be increased by exposing it to air instead of oxygen at the end of the list