VivaCast 13 – Ultrasound, Cardiac Output Monitoring, Diathermy

1. Ultrasound Physics and Applications

• Question: What do you use ultrasound for in anaesthetic practice?
• Answer:
• Ultrasound is used for imaging tissues to guide:
  • Vascular access (e.g., central venous cannulation)
  • Regional anaesthesia (nerve blocks)
  • Echocardiography
  • Assessing cardiac function
  • Imaging of other structures (lung)

• Question: How does an ultrasound machine work?
• Answer:
• Utilises piezoelectric crystals emitting sound waves (via reverse piezoelectric effect). Reflected waves return to the probe, creating electrical signals processed into images.
• Key assumptions:
  • Sound travels in straight lines
  • Uniform speed of sound in tissues
  • Uniform attenuation

• Question: What is the Doppler effect?
• Answer:
• Describes frequency shifts of sound waves reflected from moving objects (e.g. red blood cells).
  • Blood moving towards probe → increased frequency.
  • Blood moving away → decreased frequency.

Utilised in assessing blood flow direction and velocity. The optimal angle of ~ 60° across the aorta.

2. Cardiac Output Monitoring

• Question: How might we assess cardiac output?
• Answer:
  • Clinical examination:
    • Pulse
    • Blood pressure
    • Capillary refill
    • Peripheral perfusion
    • Urine output
    • Mental status
  • Non-invasive techniques:
    • Echocardiography
    • Plethysmography (pulse oximetry waveform)
    • Capnography
  • Invasive techniques:
    • Arterial waveform analysis
    • Transoesophageal Doppler/echo
    • Thermodilution methods (LIDCO, PICO)
    • Pulmonary artery flotation catheter (Swan-Ganz)

• Question: Describe a pulmonary artery flotation catheter.
• Answer:
• Inserted via a proximal vein, advanced through:
  • Right atrium
  • Right ventricle
  • Pulmonary arteries

Measures:

• Pulmonary artery pressures
• Pulmonary capillary wedge pressure (PCWP) as a surrogate for left atrial pressure.
• Cardiac output via thermodilution (injecting cold fluid and measuring temperature changes).

3. Diathermy Principles and Safety

• Question: How does diathermy work?
• Answer:
• Uses high-frequency electrical currents for cutting/coagulating tissues.
  • Monopolar diathermy:
    • Current flows from small tip (high current density) through patient to return electrode pad.
  • Bipolar diathermy:
    • Current passes between forceps tips, no return pad required.

• Question: Safety considerations?
• Answer:
  • Risks:
    • Burns
    • Arrhythmias
    • Interference with pacemakers/ICDs
    • Fire hazards
  • Use bipolar diathermy in patients with implantable devices, turn the devices off, or disable their clever sensing modes,
  • if you’ve got to use monopolar, don’t put the plate in a position where current must cross a device to get to it.
  • ECG interference mitigated via:
    • Software filtering
    • Common-mode rejection: Cancels out identical signals transmitted along both wires, removing noise.

Key Takeaways

• Ultrasound is essential for vascular access, regional anaesthesia, and cardiac imaging.
• Cardiac output monitoring should progress from non-invasive to invasive techniques, balancing accuracy with risk.
• Diathermy requires careful consideration of safety, particularly in patients with implantable cardiac devices.

Debrief

So, let’s quickly recap what we covered and what you can take away from this viva session.

First off, I really want to emphasise the approach to answering viva questions efficiently. We structured the cardiac output discussion, from the bedside assessment through to the non-invasive and invasive monitoring techniques.

Always start with simple techniques: pulse, blood pressure, perfusion, mental status, urine output, these demonstrate you are a clinician not just a monitoring robot. Only after that do we escalate to things like echocardiography, Doppler, or LIDCO. It shows a systematic, safe approach, and examiners want you to remember you have hands that can check peripheral warmth.

It’s easy to dive straight into advanced methods, jumping straight to thermodilution or LIDCO, because you’re stressed.

We had a detailed chat about pulmonary artery flotation catheters (Swan-Ganz catheters). I know these aren’t something we see every day anymore, but they’re still fair game for exam questions. The key point here is understanding the pressures as the catheter moves through the heart, right atrium, right ventricle, pulmonary artery, and finally when the balloon is wedged to reflect left atrial pressures. It’s tempting to try to memorise specific numbers, but instead, think physiologically, what pressures would logically be happening in each chamber? That’ll help you reason through the question if your memory fails you under pressure.

Finally, on diathermy safety, remember it’s not just about knowing how it works but also understanding the potential for burns, interference with cardiac devices, arrhythmias, and fire hazards if you’re using Ether…. or sloshing chlorhexidine about. We touched on practical steps like preferring bipolar diathermy in certain patients, and how things like software filtering and common-mode rejection help reduce ECG interference.

Hopefully, listening to this helped you frame your answers, structure your thoughts, and feel more confident going into the real thing.

References & Further Reading

• FRCA Primary Exam Syllabus
• Principles of Anaesthesia Equipment – Smith & Aitkenhead
• Physics, Pharmacology and Physiology for Anaesthetists – Stocks & Murphy
• Oxford Handbook of Anaesthesia – Allman & Wilson

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Podcast Information

Transcript GasGasGas Episode 013: Physics, Clinical Measurement and Equipment Safety

Introduction and Episode Overview

00:00-01:30

Please listen carefully. Hello, and welcome to Gas, Gas, Gas, the podcast that covers the FRCA primary exam. We’re going to fit into your day and give you as much of your life back as you could possibly imagine. I’m here to make your studying easier – listen to us on your commute, in the gym, in the shower, or when you’re ironing your scrubs. Expect facts, concepts, model answers and the odd tangent. Check out the show notes for all the detail, and remember to follow the show so that you never miss an episode. Let’s get on with it.

Hello and welcome. It’s nine o’clock in the evening, and you’re still in and almost 95% alive. Are you ready for your physics, clinical measurement and equipment safety viva? Yeah, let’s go. Well, train hard, fight easy. If you can answer these questions at 9pm, you will be able to do it in Russell Square, bright-eyed and bushy-tailed. Are you ready? Okay. Start that timer.

Ultrasound Principles and Applications

01:30-06:15

Summary: Tom explains ultrasound applications, physics principles, and the Doppler effect.

So, Tom, what do you use ultrasound for?

Ultrasound is a technique used for imaging body tissues deep to the skin to give diagnostic information to aid in carrying out medical procedures, and occasionally I use it for imaging the inside of the human body for intravenous or vascular access, echocardiography and assessment of cardiac function.

And what do you use it for?

For central venous access.

For what purpose? Any other uses for this in the anaesthetic room before an operation? For example, on orthopaedics?

Always, we also use it to aid in providing regional anaesthetic techniques and imaging nerves for that purpose.

Okay. But how does the ultrasound machine work?

An ultrasound probe works essentially using the equation v = fλ. It sends sound waves into body tissues, and when they are reflected back, it uses a time delay to map the depth at which they were reflected, and then to map this into an image on the screen.

The way it does this is through an array of piezoelectric crystals. Voltage is applied to these and via the reverse piezoelectric effect, they vibrate at high frequency, so significantly over 20,000 hertz – hence ultrasound, because it’s beyond our audible sound waves for human ears.

The sound waves then travel into the body tissues. Some are reflected back and the piezoelectric array – those same crystals – are vibrated and through the piezoelectric effect create potential difference which can be detected and used to map those responses into an image.

The ultrasound itself makes several assumptions about sound waves. It assumes that they travel in straight lines and reflect off perpendicular surfaces back to the probe. It assumes that the speed of sound through body tissues is uniform. And it assumes that the attenuation of signal is uniform as the signal travels from superficial to deep tissues. All of these things aren’t quite true and can result in artefacts on the ultrasound image.

I think you’ve got that there, but what is the Doppler effect?

The Doppler effect is an effect by which a sound wave reflected off a moving object can have its apparent frequency changed by the movement of that object. So, the common example given of this is when a police car or an ambulance with a siren goes past you. As it approaches you, the sound sounds higher pitched and as it moves away from you, it sounds lower pitched. And that’s because the sound waves are in effect stretched as that object moves away from you.

This effect can be utilised in imaging the body to dictate flow of blood primarily in one direction or another. So as ultrasound waves bounce off red blood cells back to the probe, if the cells are moving away from them, it will lead to a reduction in the frequency of the detected signal. And if they’re moving towards the transducer, it will lead to an increase in the frequency of the detected signal.

This will only happen if the signal is not exactly perpendicular to the movement of the red blood cells, so there’s normally an angle of around just below 60 degrees to the direction of flow of blood. And the equation that helps describe this is the, I think, called the Doppler equation.

Okay, that’s your five minutes. Moving on.

Cardiac Output Assessment Methods

06:15-11:45

Summary: Comprehensive discussion of clinical, non-invasive, and invasive methods for assessing cardiac output.

Well done, Tom. How might we approach assessing cardiac output in our patients?

So cardiac output can be assessed through clinical examination, through non-invasive techniques and through invasive techniques. Looking at patients’ blood pressure, heart rate, peripheral perfusion, capillary refill time can give us important information about their cardiac function as well as the clinical history and the patient’s medical history.

Tell me about non-invasive approaches.

So non-invasive approaches of assessing cardiac function include echocardiography, so we can directly image the heart, look at things like stroke volume, and look at the left ventricular function of the heart. We can also use plethysmography and oxygen saturations to give us an idea of cardiac output and we can use capnography in order to look at whether or not someone has cardiac output, and that can be done invasively or non-invasively.

Moving on to invasive techniques, we’ve already mentioned capnography. Arterial waveform analysis and assessing area under the curve of arterial waveforms can give us an idea of cardiac output and some other indications of cardiac function, such as position of dicrotic notch, which gives an idea of afterload of the heart.

We can also look at transoesophageal Doppler to give us a slightly different imaging of the heart and give us a better idea of function than transthoracic echocardiography in some situations. And we can then use dilutional techniques, so LiDCO – injecting lithium ions via peripheral site and then looking at their dilution when they reach the heart in order to assess cardiac output.

You can use temperature change methods to perform a similar function by injecting cooled fluid and looking at temperature differences at sensors in the heart to try and assess cardiac function, or sensors at least in the central venous system. And then we can use Swan-Ganz catheters and look directly at pulmonary artery wedge pressures and almost directly measure cardiac output by temperature dilutional measurement techniques.

Pulmonary Artery Flotation Catheter

11:45-15:00

Summary: Detailed explanation of Swan-Ganz catheter insertion, function, and wedge pressure significance.

Talk me through a pulmonary artery flotation catheter.

So, a pulmonary artery flotation catheter is a peripherally inserted catheter, usually done via a proximal vein. It’s then moved through the right atrium into the right ventricle and through to the pulmonary arteries, where it measures pressure directly at that point, which will be taken as being almost the same as the pressure in the vascular bed of the pulmonary vessels.

And then you mentioned wedge pressure. What is that?

The wedge pressure is the pressure reached at the pressure transducer, as it’s placed as distally as it will reach in the pulmonary arteries – i.e., when it’s wedged into them as they descend in size.

And what’s the significance of that measurement?

So the significance of that measurement is that through the low pressure system in the pulmonary vascular bed, there is very little difference between that wedge pressure and pressures within the left atrium, so it can give a surrogate measurement of left atrial pressure.

Excellent. That is your five minutes there, Tom.

Diathermy: Principles and Safety

15:00-21:30

Summary: Tom explains monopolar vs bipolar diathermy, mechanisms of action, and comprehensive safety considerations.

So final question here, Tom. We have five minutes. Diathermy is used in surgical procedures. How does it work?

Diathermy is a device that uses electrical energy for cutting and dissecting and coagulating tissues within the surgical field. It does this through use of high-frequency electrical signals and can do so via two different modalities: monopolar diathermy or bipolar diathermy.

Both of these use high-frequency currents to coagulate tissues, but if you look at the signal produced by each one, the monopolar diathermy causes constant high-frequency discharge of alternating current, whereas the bipolar diathermy uses bursts with gaps with less or no electrical transduction in those gaps.

The monopolar diathermy uses a conducting pad placed on the patient and the electrical current passes through the patient but over a large area, therefore causing low energy per unit area and not causing any tissue damage at the point where the pad is placed on the patient.

The pad is large. What’s the difference between that and the tip of your diathermy?

So the surface area at the tip of the diathermy is very small, so the concentration – the current density – is much higher at the tip of the diathermy, which is what allows it to cut tissues and coagulate and things like that.

The bipolar diathermy doesn’t use a connector on the patient at all. It uses, if you imagine, a pair of forceps where the current passes between the two tips. This means that there’s reduced risk associated with the travel of current through the patient.

What are the safety considerations when using diathermy?

So, diathermy is a source of heat and can be a source of fire, so that has to be considered in the particular surgical site it’s used on, the tissues it’s being used on and the materials that are used around it.

There is a consideration of the electrical signals and the damage they can cause. So if they’re travelling through the patient, either deliberately in monopolar diathermy or by accident, they can cause burns and electrical damage to the patient. They can cause arrhythmias if current travels through the cardiac conduction system, and they can interfere with medical devices such as pacemakers, implantable cardiac defibrillators, and vagal nerve stimulators.

The type of device needs to be taken carefully into consideration, as well as the indication for that device when deciding whether or not diathermy can be used with patients. Generally speaking, if there is an implantable device that can suffer from interference from diathermy, it’s preferable to use bipolar diathermy and to use it as minimally as possible.

ECG Interference and Common Mode Rejection

21:30-24:00

Summary: Discussion of ECG interference from diathermy and mechanisms to reduce it.

So I’m sure you’ve noticed when diathermy is being used that the ECG trace goes awry. What mechanisms are present to try and reduce that interference?

When we see aberrations in ECG trace when diathermy is being used, surgical techniques that can reduce that interference – so not using the diathermy constantly – gives us some window through which to see an accurate heart trace.

The machinery itself has some filtering software, so there are clever ways in which repetitive and high-frequency signals can be filtered out – ones that are reliably non-biological. So using a filter through the software of the ECG monitor can reduce interference as well.

So what do you know of common mode rejection?

I believe common mode rejection takes a signal coming via two different signal sources, and if the exact signal is transmitted along both wires or both sensors, then that signal itself is rejected. The most common way of doing this is through a twisted pair, which is commonly done in audio devices, and any signal which is identically transmitted to both is assumed not to come from the source and is therefore electronically removed from the signal.

Excellent, lovely. Thank you. That is your five minutes.

Post-Viva Discussion and Feedback

24:00-28:45

Summary: Tom and James discuss the viva performance and approach to structuring cardiac output assessment.

How’d you feel that went?

Yeah, okay, not too bad again. I was thinking back, I think, to the other two sessions when I was about to answer, but I think that particular fifteen minute section went a bit better.

Yeah, I like the fact that when I asked you about cardiac output, you didn’t just jump into LiDCO. You showed clinical assessment, monitoring in a non-invasive versus increasingly invasive approach. That’s cool.

Just to get a little bit of a vision out of you, what would your approach be to describing cardiac output monitoring? How would you structure it or go at it?

Yeah, so reasonably similarly to you. So, clinical assessment at bedside of examinable findings. So you said, what’s the pulse like? Skin colour, skin temperature, capillary refill time, the respiratory rate. And then are they confused? Are they urinating? So evidence of end organs. Are they confused? Are they urinating? Do they have raised JVP? Do they have evidence of pump failure? Just think about your septic patient or your septic laparotomy type patient.

And then I would open with monitoring and progressively more invasive. So you would open with non-invasive blood pressure monitoring, pulse oximetry, your ECG to see what their rhythm is.

Is there a snappy phrase to talk about how pulse oximetry works there? Would you say the waveform gives a qualitative evidence of cardiac output?

And you could simply say: if I’m struggling to get a sats trace, I know that peripheral perfusion is poor. Yeah, okay.

And then graded up in levels of invasiveness from arterial lines to transoesophageal Doppler, transoesophageal echocardiography, and your LiDCO PiCCO stuff. And then I would max out with – and something that is not terribly commonly used nowadays – is the pulmonary artery flotation catheter, which, as it can get exceptionally close to the heart, because it transits the heart, yields the most accurate information.

You could also frame across this that clinical assessment to Swan-Ganz is progressively closer to the heart, more accurate, but more invasive. It’s that thing about: the further you are away, the less accurate you are; the closer you are, the more accurate but the more dangerous it is.

Host’s Comprehensive PAFC Teaching Section

28:45-35:30

Summary: James provides detailed explanation of pulmonary artery flotation catheter mechanics, insertion, and clinical considerations.

So everyone, I think this is a prime opportunity just to quickly go over pulmonary artery flotation catheters.

PAFC Structure and Function

So a pulmonary artery flotation catheter, sometimes called a Swan-Ganz catheter, is introduced to the venous system via an introducer, which is a large sheath, which is sometimes sited purely on its own for exceedingly rapid infusion of blood in exsanguinating haemorrhage. It’s a specialised intravascular catheter with a number of features above and beyond that of your standard central venous catheter for infusion and maybe central venous pressure monitoring.

So the PAFC has a balloon at its distal end which facilitates occlusion of a branch of your pulmonary arteries. It also has a proximal lumen which is used for delivering a bolus of cold fluid, and then a distal thermistor in order to measure temperature change. This can be used for cardiac output monitoring. The other features of these lumens mean that you can measure pressure in a number of different points in the heart.

Insertion Technique

How is it introduced? So it’s introduced via this catheter, and the tip is transduced using standard transduction equipment. It is advanced down the superior vena cava, into the right atrium, into the right ventricle, and subsequently into the pulmonary artery. Ideally you site the tip in a position that facilitates occlusion of a vessel.

Wedge Pressure Mechanism

What does occlusion of a vessel achieve? Well, when you inflate the distal tip balloon and occlude a pulmonary artery, you subsequently end up with a column of blood that is interfacing with the left atrium. This means you can measure left atrial pressures, although you’re doing so indirectly, so it is called a pulmonary capillary wedge pressure – i.e. wedged in the lung.

The waveform here will reflect the left atrial pressures and also look a little bit like your jugular venous pressure waveform because you’re observing the left atrial pressure waves.

Current Clinical Use

It is not used anywhere near as much as once upon a time it was used, because now there are perhaps less invasive techniques that give sufficient information in order to not have to float a balloon through someone’s heart and into their pulmonary artery and then occlude a vessel, which could lead to rupture and all sorts of badness.

It may get sited in cardiac surgery. It may get sited in a very, very ill person on intensive care in a specialist centre. You wouldn’t see it in a DGH ITU.

Understanding PAFC Pressures

The way to imagine the flotation of this catheter and the pressures you associate with it: don’t try and memorise the pressures that the catheter experiences. Just remember the different pressures in the heart, because you know that central venous pressure is low, and you know that the right atrial pressure ties in with the central venous pressure. And then you know that the right ventricular pressure obviously has to have a systole and a diastole that reflects the fact that there’s a valve in the way.

You can then think, well, and then you get to the pulmonary artery, and the systolic pressure in the pulmonary artery is going to be the same as the right ventricular systolic pressure, but because you’ve got a valve that occludes flow back from pulmonary artery to right ventricle, your diastolic pressure is going to be higher.

Your pulmonary capillary wedge pressure is going to reflect left atrial pressures – ten to twelve millimetres of mercury – and boom, you’ve done it without ever having to worry about what the tip does per se, just frame it differently in your mind.

Practical Discussion

You’ve not done one then, James. I thought you might have done one.

No, I’ve seen one in organ donation. So, yeah, it’s difficult when it’s something you’ve never seen, but yeah, it’s thermodilutional technique to give you an idea of cardiac output. But then I’m just trying to think – the reason I was slowing down during the viva is that I could remember that it gave you left atrial pressure as near as damn it, but I couldn’t remember why you cared.

They’re more likely to ask you about the pressures that the catheter would read as you journey and float through. And you just need to imagine that right atrial pressure is like low, like five to six millimetres of mercury, and then you’ll see a right ventricle that jumps up to 25 and then drops back down to five or so, like your right atrium.

And then, once you’re in the pulmonary artery, you’ll see a similar systolic peak as the right ventricle contraction. But because there’s now a valve that is still semi-competent, even though you’ve thrown it, the pressure won’t drop down to five, it’ll drop down to like 10 or 12. And then when you occlude, then you’re going to measure the pressure distal to that balloon and get the left atrium.

How long do they occlude for? And is that done manually?

Well, I don’t know. I mean, there’s probably technology that does it automatically. But not for very long, just enough to get a measurement. And I can imagine if you’re doing it with a syringe and then you couldn’t deflate or something – you’d feel pretty bad. And that’s the risk. You’d essentially just give someone a PE. Well, technically a segmental PE because you’re not occluding any of the main branches of the pulmonary artery, you’re getting a little bit further out in the arterial tree of the lungs.

Closing

35:30-36:00

Thanks for listening to that episode, guys. If you found it useful or awful, please like and subscribe and rate the show. Definitely go check out the show notes on gasgasgas.uk. We all know that this is a bucket of content. I want you to take some time for yourself and don’t overcook it. Don’t freak out. Keep studying.

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