EP29 – Measurement | Anaesthetic Primary Topic | Cardiac Physiology | CT10

Hello guys and welcome back to count to ten. My name is Arnie and in today's pod we're going to be finishing up our cardiac physiology series. Our final topic today is going to be measurement and hopefully this will be a lot more straightforward compared to our previous topics. The key things I want to look at today are the principles behind both non-invasive and invasive blood pressure monitoring and then briefly look at ways to measure cardiac output both as a whole and regional circulation to specific organs. Now after today's episode there'll be a few special episodes released with guests coming on to deliver a topic of interest. I'm going to be recording those talks on Zoom. It's going to have a bit more of a dutactic feel to it and I'm going to release it on our YouTube channel which will be the best place to watch the episodes because they will have a PowerPoint attached with each talk. Those special episodes will be released somewhere at the end of November through December and in January we're going to start our new topic which will be covering general pharmacology. So with that said let's get back to today's topic and finish off cardiac physiology by diving straight in to measurement. The first ending objective is describe the methods of measurement applicable to anesthesia including clinical utility complications and sources of error in particular measurement of pressures including transducers and measurement of blood pressures. This is a very weirdly worded learning objective and in my mind what this learning objective is about is understand how do you measure non-invasive blood pressure and then invasive blood pressure. So looking at non-invasive blood pressure first before we go into different methods of measuring we need to understand what are the parameters that we're measuring. The parameters from blood pressure can be broken down into systolic blood pressure which is the peak pressure generated during systolic contraction, diastolic blood pressure which is the trough pressure generated during diastolic relaxation, pulse pressure which is the difference between systolic minus diastolic and then mean arterial pressure which is the mean pressure during one cardiac cycle and can be calculated using an equation diastolic blood pressure plus one-third of the pulse pressure. Now each of these pressures has its own importance and can tell us both normal physiology and abnormal pathology. Systolic blood pressure can reflect the compliance of the arterial system while diastolic blood pressure is particularly important for coronary blood flow perfusion. Looking at pulse pressure that can be important in identifying different types of shock as well as different types of cardiac pathologies, the classical being aortic regurgitation which has a wide pulse pressure. On the other end of the spectrum a narrow pulse pressure may be a sign of cardiac failure. While the last variable map is one that we really focus on to ensure end organ perfusion. So the reason I bring this up is when you look at a blood pressure reading don't just look at the systolic blood pressure, use all the outputs that you get to make a clinical judgment about the patient. Now getting back to the different ways we can measure on invasive blood pressure, starting off with the simplest which is osculation, this requires a cuff and a stethoscope. The importance with a blood pressure cuff is that it needs to be the right size for the right patient and this is one thing I always forget and have to look up to see how to correctly size a blood pressure cuff. There's a simple 4080 rule by the American Heart Association which relies on first measuring the patient's arm circumference in the mid upper arm. From this it is said that the cuff bladder length should be 80% of the arm circumference while the bladder width should be 40% of the arm circumference. The reason this is really important to get right before you start measuring is that if you overestimate a cuff meaning you put a larger cuff on someone that needs a smaller cuff this leads to underestimation of the patient's blood pressure. If you put too small of a cuff on someone that needs a larger cuff size this leads to overestimation particularly of the systolic blood pressure. That overestimation can be as much as 20 to 30 millimeters of mercury and the underestimation can be as much as 5 to 10 millimeters of mercury and these are insignificant numbers when it comes to accuracy of a patient's blood pressure recording. Now the way you measure the patient's blood pressure using oscultation is that with deflation of the cuff, turbulent blood flow is produced which is audible by the stethoscope. Typically your stethoscope is sitting over the brachial artery and the steps in this method to get a blood pressure include firstly palpating the peripheral artery. Typically you palpate the radial artery and while you're doing this you inflate the cuff. The cuff is inflated until you can no longer feel the radial pulse and then 20 to 30 millimeters above that value. Using your stethoscope then you place your stethoscope at the brachial artery while you slowly deflate the cuff. The cuff is deflated at a rate of 2 to 3 millimeters of mercury per second and what you're listening out for are specific sounds known as crot cough sounds. These have five distinct sounds but the only two sounds you need to know of are the first and the fifth. The first sound you hear is initial snapping tone and this represents systolic blood pressure. This is followed by your second sound which sounds more like a murmur, then your third sound which sounds more like a dumping, then your fourth sound which is more of a muffled sound followed by the fifth sound which is the loss of all sounds and this represents your diastolic blood pressure. So when you're doing a manual blood pressure really all you're interested in is when you can first hear a sound being a systolic blood pressure and then when those sounds go away being your diastolic blood pressure. The advantage of using the osculation method is that it's simple and it's inexpensive. The main disadvantage is it's difficult to osculate those sounds when a patient is hypertensive or has significant peripheral vasoconstriction. Typically with this type of method we say that the systolic blood pressure is more accurate than the diastolic blood pressure because logically it's more easier to know when a sound starts than when a sound finishes. The second method of measuring non-vasible blood pressure is a technique called oscilometri. Oscilometri works on the principle that arterial pulsations cause oscillations in cuff pressure which is then measured by a pressure during stusa within the cuff. The key thing to note with this technique is that not all oscillations are the same. Each oscillation has a different amplitude and we rely on the change in amplitude to help tell us what is systolic, what is mean arterial and what is diastolic blood pressure. The way this is done is that a unique graph is made which depicts on the x axis time and on the y axis the amplitude of the oscillations. This graph is superimposed with a graph of cuff pressure and then these two graphs are used to give us our outputs. So if the setup for an automatic oscilometric non-vasible blood pressure is the same as what you do for an osculation blood pressure. You apply the blood pressure cuff on the mid forearm with the same correct cuff selection as before and because this is an automated blood pressure reading when you press start the cuff is inflated slowly above the patient's blood pressure then deflated at the same ratio of 3 to 5 millimeters per second. While the cuff is being deflated, oscillations are being detected by the transducer. The transducers are recording the amplitude of these oscillations. At the point where the rate of increase in amplitude is at its highest, this represents systolic blood pressure. When you get to the maximum amplitude, this represents mean arterial blood pressure. Then to get diastolic blood pressure, there are several methods of detecting this. The first is the maximum reduction in the rate of amplitude which is just the polar opposite of how we measure systolic blood pressure. The other is using a calculated formula which is equal to 1.5 times the mean arterial pressure minus 0.5 times the systolic blood pressure. The third technique is to note the point when the amplitudes of oscillations are in their fixed portion. This means that the amplitudes aren't increasing or decreasing. Now because of the way we measure each one of these components, we typically say the most accurate measurement is the mean arterial blood pressure. This is followed by the systolic blood pressure, and then lastly by the diastolic blood pressure. The main advantage of using this automated oscillometric non-vacable blood pressure is that it's very easy, it's simple to apply, it's portable, it's versatile, it's what we use for all of our patients for our non-invasive blood pressure recording, not just intraoperatively but postoperatively and on the ward, it's fast and it's most accurate reading is its mean arterial pressure. The main disadvantage of this technique is that the diastolic blood pressure can be inaccurate that if you have movement to the patient, this will alter the oscillations being recorded by the transducer and lead to inaccurate measurement or when you classically see it intraoperatively, leads to the blood pressure recycling again and then waiting an extra minute or 30 seconds to get a new reading. The other main issue with this automated method is that it takes 30 seconds to a minute to get a reading. So therefore the smallest amount of time that you should wait between readings is typically two to three minutes. Now this doesn't always happen, typically after spirals for sezerine sections, we put the reading on one minute and here you just have to remember that the reading might not be as accurate if the whole system it's measuring has not come to complete rest. So these are our two most important techniques for measuring non-invasive blood pressure. I won't go on and talk about other techniques like tonometry because we just don't use that in everyday practice but rather I'll move on and talk about invasive blood pressure monitoring. This is certainly a bit more juicy to understand and can easily be both a viber and an SAQ topic. The best description for invasive blood pressure monitoring, i.e. an arterial line that I've seen anywhere, is a YouTube video by Dr. Ken Hoffman who's an intensiveist at the Alfred Hospital. Now I'm going to try and do that video justice and give you my own explanation of how an arterial line works. To me the most important thing to understand with an arterial line is that it is taking energy in the form of pressure being from the blood and converting it to electrical energy and that electrical energy is going through an analysis which is then giving us an output. Therefore the components we have to get us to that point include a sensor being the transducer and integrator being the module with the analysis is occurring and then the output which we see as both a number and a waveform on our screen. So how do we get to this end result? Well the components you need to set up an arterial line include firstly an intra arterial cannula. This should be short, biologically inert and relatively stiff. Typically it's something that's 20 gauge like a cannula or a wagon set. Then we have fluid filled tubing and the fluid within this tubing should be non compressible and typically that fluid is just normal saline. This column of fluid within the tubing allows for a process called hydraulic coupling to occur. Hydraulic coupling is essentially just the preservation of energy. So it's the preservation of that pressure energy carried to the transducer which is the next step where it can be then converted into an electrical energy. If during that length that pressure energy diminished the electrical energy picked up with the transducer would not be an accurate reflection of the patient's blood pressure. So just like the transducer is a very important part of the setup of an arterial line as equally as important is the fluid filled tubing and the tube itself. That's why arterial lines have their own specific setup and you can't just use a normal IV line to set up an arterial line. Now the next component being the pressure transducer obviously has an important role in converting that arterial pressure energy into electrical energy and we'll talk about how that happens in a little while but just to complete the components of an arterial line setup when you go from your pressure transducer you will see two leads coming out of that. One of those leads is the continuation of the fluid filled line attaching to a pressurized bag of normal saline which is normally pressurized at 300 millimetres of mercury and what that's doing is achieving counter pressure to make sure that arterial blood flow doesn't enter the system. This bag can also be used to flush the line. The other chord coming out of the transducer is the microprocessor chord which is then attached to a separate chord which is part of our module which interprets the signal and gives us our blood pressure reading. So these are the key components of an arterial line and it wouldn't be unreasonable in a short answer question to draw this setup you could also be showing a picture of this setup and then be asked to state the different components seen on this picture. Now by far in a way the most important and the most complex component to understand in this setup is the transducer itself so now we'll look at that in much more detail. This transducer is very cool and a bit nerdy. It has a key part called a diaphragm which moves up and down with each arterial pulsation. Along that diaphragm on either side is a piece of electrical equipment called a strain gauge. A strain gauge is just a thin metal wire which changes shape with a change in pressure. That strain gauge is incorporated into an electrical circuit and when that strain gauge changes its shape it changes the resistance within that electrical circuit. This therefore allows an initial pressure energy to be converted into an electrical energy via a change in resistance. Now normally this strain gauge was part of a bigger electrical circuit called a wheat stone bridge. This may be a term that you've heard before but essentially a wheat stone bridge is incorporated into an electrical circuit to measure an unknown electrical resistance by balancing two legs of a bridge circuit. What a bridge circuit is is that it divides into two separate parts. In between those two separate arms you have a galvanometer measuring the flow between the two arms. Classically a wheat stone bridge incorporated four different types of resistance. Three of them were known and one was unknown. If that one unknown was attached to a strain gauge it was changed its pressure accordingly. The goal of the wheat stone bridge was to balance one side to another and this was picked up when the galvanometer had no flow through it. Now this is probably too complex to know and really you don't need to know this level of detail for the exam. What you do need to know is that the wheat stone bridge is used to measure the resistance of the strain gauge and this improves the accuracy of this measurement. Nowadays a strain gauge is still incorporated into new arterial line models however instead of having three fixed resistance and one variable resistance the new wheat stone circuits have four strain gauges and all this does is improve accuracy even further. So what happens once you do have an electrical signal? Well that electrical signal needs to be manipulated in a way which shows us numbers and a waveform. The way it does this is by going through a microprocessor. The key thing to understand is that an arterial wave isn't just one waveform it's multiple little waves of varying amplitudes and very similar to how a BIS module works an arterial line microprocessor uses a Fourier's analysis to reconstruct those multiple little waves into one waveform that we see. Now what are the common areas that can occur with an arterial line? Well these can be divided into two types of errors. The first type of error is a static error and the second type of error is a dynamic error. Each of these types of errors have two components to it. Within static the first type of error is zeroing. Zeroing refers to having a reference standard to which the arterial line is measured to. The reference standard in this case is atmospheric pressure. So the way we perform this on an arterial line is we turn the arterial line off to patient and we open the chamber to air. This allows the transducer to measure atmospheric pressure and this becomes a reference as zero and when you have a range you need to have an upper limit as well. The upper limit in this case is the pressurized normal saline bag. The second static error is called leveling. This refers to where you place your transducer. The level at which you place your transducer is the level at which arterial blood pressure will read. Classically you should put your transducer at the flabostatic axis which is essentially at the level of the heart. However in different clinical circumstances say you're doing a neuro case you might want to put the transducer at the level of the external acoustic meeters to give you a cerebral perfusion pressure estimate. The key thing to know with leveling is that a change in height can have a huge impact on the output of the measured blood pressure. Classically we say a change in height of 10 centimeters equates to a change in pressure of 7.4 millimeters of mercury. So these are the two static errors and they're pretty straightforward to understand. Now we get into the dynamic errors and these are far more likely to be asked in both SAQs and Vibers. The two dynamic errors are resonance and damping. Resonance is the increase in amplitude of a wave as it approaches its natural frequency. The term natural frequency describes the property of a wave when it will freely oscillate once stimulated and continue to oscillate. Now if you don't quite understand what that means have you ever seen the YouTube videos of the big bridges that just keep swinging once they get a big gust of wind in a storm. Those bridges received an initial energy and then they started swinging at their natural frequency. Here the oscillations continue without further stimulation required. With the naterial line you want the natural frequency of the arterial line system to be well above the natural frequency of arterial pulsations. The maximum natural frequency of arterial pulsations we commonly say is going to be three hertz. That's because the natural frequency of arterial pulsations is based on heart rate. Typically heart rates are between 60 to 100 beats per minute. This is roughly 1 to 1.5 hertz. In extreme cases heart rate can be 180 to 200 beats per minute. This is 3 to 3.5 hertz. So what that means is that the arterial system's natural frequency should be well above that and we say the minimum it should be above is 8 to 10 times the natural frequency of arterial pulsations. Therefore the minimum natural frequency of an arterial system should be at least 24 to 30 hertz. Current arterial systems actually have a natural frequency well above that into the hundreds to 200 hertz range. Why do we care so much about resonance and natural frequency? If the system's natural frequency is low this can lead to an overestimation of systolic blood pressure and an underestimation of diastolic blood pressure. However importantly mean arterial pressure should remain accurate. Now in order to make sure the arterial system's natural frequency is high there's a few things we can manipulate and this is based on the equation to work out natural frequency. Now by no means you have to know this but there are a few important numerators and denominators that we manipulate in order to get a high natural frequency. These involve ensuring that the width of the tubing is no smaller than 1.5 millimeters because smaller width will decrease natural frequency. Another factor is the length of the tubing. Longer tubing decreases the natural frequency however there is a tradeoff with longer tubing and damping which is the next dynamic error. Classically we say that tubing should be less than 1.2 meters because of that fine balance and then the last two components of this natural frequency equation refer to the elasticity of the tubing classically being stiff and the density of the fluid within the tubing classically being non compressible normal say line. All these characteristics essentially ensure that arterial lines that we get from manufacturers have a natural frequency close to 200 hertz. Now the next dynamic error I want to talk about is damping. Damping simply describes how an oscillatory system loses energy. The classic example for this is if you take a tennis ball and you drop it from a height each successive bounce of the tennis ball becomes lower and lower. This is essentially what damping is and when we take this analogy and apply it to the arterial system what we want to do is we don't want to lose the energy of that arterial pulsation as it approaches the transducer. The key difference with damping compared to resonance is that there's a fine balance of either over damping or under damping the system. Both of these things affect the systolic and diacolic blood pressure measurements but don't affect the mean arterial pressure measurement. If our system is overdamped what that means is that it's losing a lot of energy. What this equates to is underestimating systolic blood pressure and overestimating diacolic blood pressure. Therefore arterial line wave trace looks narrower and compressed. On the opposite spectrum, under damping refers to the system oscillating freely. So this will lead to an over-estimation of systolic blood pressure and an under-estimation of diacolic blood pressure so you get exaggerated arterial waveforms. So what we end up doing when talking about damping is referring to this thing called the damping coefficient and more specifically a term called optimal damping. In an arterial line the optimal damping occurs at a damping coefficient of 0.64. This is because if there's too much damping the system loses too much of its energy before it gets to the transducer. If there's too little damping there's too much oscillations occurring and that oscillation will continue even when a new arterial pulsation has arrived. Therefore at optimal damping we get the balance where we don't get too much energy lost due to frictional forces but also we don't get an overshoot of the oscillations and we have an appropriate frequency response time of our system to measure each arterial pulsation accurately. So what are some factors that can affect damping then? Classically we think of factors that lead to over-damping so therefore an increased loss of energy in that system. These involve air bubbles in the lines, long tubing, kinks, narrow tubes, blood clots, arterial spasms. On the other end not many things cause under-damping. Under-damping can occur with really stiff tubing in hypothermic conditions with patients that might have tachycardia or dyseridmias or in patients with excessive movement where the catheter within the artery can have exaggerated waveforms with additional peaks. How do you then tell if your arterial line is over-damped or under-damped? Well what you can do is a test called a fast flush test. This is essentially what it sounds like. You flush the arterial line and then you look at the oscillations subsequent to that flush. In a normal calibrated arterial line there should be two oscillations subsequent to the fast flush followed by the recommencement of the arterial wave trace. If there's less than two oscillations this suggests an over-damped arterial line and if there's more than two oscillations this suggests an under-damped arterial line. The other information you can get from a fast flush test is the natural frequency of the arterial line system. The way you get this is looking at the time between the first and second oscillation after your fast flush test. This time should be less than 30 milliseconds and 30 milliseconds corresponds with the natural frequency of at least 30 hertz. Believe it or not I was actually asked to explain this test in my actual liver on the day. The final thing I want to mention with an arterial line system is that static errors actually change the numbers that we see. They change the systolic, the diastolic and the mean arterial pressure. While dynamic errors change the systolic and diastolic pressures but they don't affect the mean arterial pressure. I think I've exhausted as much as I want to talk about an arterial line for now and I think we should finally move away from an arterial line and look at our next learning objective which is describe the methods of measurement applicable to anesthesia including clinical utility complications and sources of error in particular the measurement of cardiac output. The simplest way to measure cardiac output is by using a transysophageal Doppler ultrasound. This goes back to the principle that cardiac output is equal to heart rate times the stroke volume. The heart rate is easier to figure out and all we need to do now in this equation is figure out stroke volume. The way we can do this is by measuring the cross-sectional area that we get through the transysophageal echo and the mean blood flow velocity going across this cross-sectional area. If we have a velocity and we have an area and we times those two things then we get a volume and this goes back to getting us our stroke volume. So this is a really simple technique apart from the fact that a patient needs to have a transysophageal echo put down to get a cardiac output reading. The other ways we can get cardiac output measurements is using fixed principle. We spoke about fixed principle when we talked about respiratory physiology but we can also apply that to cardiac physiology. Fixed principle works on the basic law of the conservation of mass and if we were to write this out in an equation we could say that an amount of a substance is equal to the concentration of that substance in the arterial blood minus the concentration of that substance in the venous blood times the blood flow. If we rearrange this equation we can say that blood flow is equal to the amount of substance divided by the concentration in the arterial system minus the concentration in the venous system. So now taking this principle and applying it to how we get cardiac output we can use a direct thick method by looking at O2 uptake in the lung to calculate pulmonary blood flow which we know is an approximation of cardiac output. Here we can say that pulmonary blood flow is equal to the oxygen consumed divided by the oxygen content in the arterial blood minus the oxygen content in the venous blood. We know that roughly 250 ml of oxygen is consumed per minute and we know that roughly there's 200 ml per liter of oxygen in the arterial blood and there's 150 ml per liter of oxygen in the venous blood. This means it's 250 divided by 50 which gives us our five liters per minute as our cardiac output. The indirect thick method calculates cardiac output by looking at one blood gas. Typically this is carbon dioxide. Again if we put this into an equation we can say that the cardiac output is equal to the rate of CO2 elimination from the lungs divided by the CO2 concentration within the venous system minus the CO2 concentration in the arterial system. Here it's just flipped because there should be more CO2 within the venous system than the arterial system. If we plug in the numbers that we know which is normally that the rate of CO2 elimination is 200 ml per minute the content within the venous system is roughly 52 ml per liter the content within the arterial system is roughly 48 we get 200 divided by 4 which leaves us to 6 liters per minute. Now you can manipulate this fixed principle to figure out regional circulation blood volumes as well. The classic one being cerebral blood flow. This was developed by Kedi Schmidt and is known as the Kedi Schmidt technique and here an in-net inhaled gas classically nitrous oxide in low concentration was used as a tracer. Blood samples were taken from the jugular bulb catheter and an artery and the nitrous oxide concentration was measured. Again this used a basic principle that an uptake or release of a substance by an organ is the product of blood flow to that organ and the arterial venous concentration difference of that substance. Now the last technique I want to mention for measuring cardiac output which is still sometimes used especially when patients have a Swan Gains catheter in is the thermal dilution technique. This uses two separate channels of the pulmonary artery catheter. In the proximal channel a known bolus of a cold solution typically cold dextrose 5 to 10 ml at 4 degrees is injected into the right atrium. At the second distal channel typically at the pulmonary artery is at the mister measuring the temperature of the blood. As the cold fluid flows from that right atrium towards that the mister in the pulmonary artery the temperature of that fluid should change. The rate of that change of temperature is inversely proportional to the cardiac output. In simple terms if there's a higher cardiac output that cold fluid will change temperature more rapidly. If the cardiac output is slow that cold fluid will stay closer to its initial temperature. Now these measurements are put into a complex equation called the Stuart Hamilton equation which describes this relationship and then spits out a cardiac output. The key thing with this technique is that you shouldn't just do it once. You should do this measurement three times and take the mean of these three values. This is because the thermodilution cardiac output measurement can vary by 10% from measurement to measurement. So that pretty much does it for the learning objectives for this topic. There was an additional learning objective that looked at the ECG but we've already covered this previously so I won't repeat it again here and now we'll move on and look at the past SAQs. The first SAQ to go over was from 2023 and it's been asked twice previously now with a 71% pass rate and this was divided into two parts. The first part being briefly outlined the principles of arterial blood pressure measurement using an automated oscillometric non-basin monitor and the second part being briefly outlined the potential sources of error with this device. Now the examiner report was very straightforward. It just wanted you to go over how this machine works including specifically how systolic, mean and diastolic pressures are measured or calculated. Then talk about the sources of error. The common problems here were that people thought that the non-basid blood pressure transducer was in the cuff itself. There was confusion about the impact of an incorrectly sized cuff or simply people listed errors with no explanation attached to them. Now I do think the easiest way to answer this question is following the ketamine nightmares model answer which breaks the answer down into three components. The device and its components, how the oscillometric non-basid blood pressure works and then the sources of error. So in terms of the device and its components, well you have the cuff with the inflatable bladder, you have the port for air insiplation, you have the connection to the transducer and the transducer being in the module itself and then you have a processor and a display which shows you the systolic, the diastolic and the mean arterial blood pressure. We talked about proper selection of the cuff already and this goes back to the 4080 rule. With the bladder length being ideally 80% of the patient's arm circumference measured in the mid portion of the upper arm and the bladder width being 40% of the arm circumference. The way oscillometry works to quickly recap is it starts off with the cuff being inflated above the patient's systolic blood pressure and this is normally 20 to 30 millimetres above this threshold and then the cuff is slowly released at 3 millimetres of mercury per second. Pulsatile flow is then detected through the cuff as it becomes partially decompressed and this Pulsatile pressure creates oscillations. These oscillations have varying amplitudes and these oscillations and amplitudes are detected by the strain gauge. What this allows the module to do is then create a graph of the different amplitudes of the oscillations as well as a graph of pressure. These three graphs are overlaid over time and the outputs we're looking for come from these two measurements. Sistolic blood pressure correlates to when the rise in the rate of amplitude is at its highest, mean arterial blood pressure correlates to when we reach maximum amplitude and diastolic blood pressure can be measured using several techniques that we discussed previously. The easiest one to remember is when there's the maximum rate of decrease of amplitude. Now in terms of sources of error, I like to think about this as what could go wrong when you're applying the blood pressure cuff, then when the blood pressure is reading, what could go wrong with the module itself, and then finally what's external and causing false readings. When you're applying the blood pressure cuff, errors can happen in wrong application of where you're putting the blood pressure cuff, wrong selection of the cuff itself, and remember we said that if you pick too smaller cuff, it can lead to an overestimation of a blood pressure by up to 20 to 30 millimetres of mercury, and if you pick too bigger of a cuff, it can lead to an underestimation of blood pressure by 5 to 10 millimetres of mercury. Then the errors with the cuff itself is that the diastolic blood pressure is the least accurate measurement, followed by the systolic, and the most accurate is the mean arterial blood pressure. The other error with this machine is that it extremes of blood pressure, so too high and too low, it tends to be less accurate, and you can observe this in the clinical practice. When you get that blood pressure that can't record because the patient's systolic is 220, it just comes up with dash dash and no output value. Finally, in terms of errors that can happen outside the machine, I like to think of this as either happening from the patient, or from external sources, so the easiest one is that the patient can shiver. Now all of a sudden, the oscillations and amplitudes are hard to detect, and therefore an output can't be delivered. The other issue is if someone's leaning against the cuff, or moving the patient's arm. This also leads to the same effect and leads to an inaccurate reading. All in all, this is a very straightforward question, and I think if you just go through those logical steps, you'll cover all the big points easily to get a pass mark. Now the next SAQ is also from 2023, second sitting, which was briefly outlined the principles of measuring cardiac output using Doppler ultrasound. This has a 34% pass rate, and also encompasses some concepts of ultrasound that we haven't previously spoken about. So I'm not going to talk about this question in too much detail, but we'll just go through a brief overview of what the examiner report said. So in terms of the examiner report, the key domains here was discussing how velocities measured using Doppler, and this was possible by reproducing the Doppler equation. And then the second key domain was processing a pulsatile velocity signal to obtain a cardiac output. In other words, this just means calculating an area and a velocity, and times in those two things to get a volume. There was a lot of additional credit you could get as per the examiners, and you could get this by either talking about information about how ultrasound waves are generated, detail about velocity measurement, and illustrating the concepts we've just discussed and describing the clinical measurement techniques. Like I said, we haven't really discussed ultrasound at all previously, and we'll talk about this SEQ a lot more when we eventually get to talking about ultrasounds in our future podcasts, but I didn't want to focus on new information that we haven't covered before in this podcast. Now finally, we get to our last SEQ for today, and this was asked in 2020 and a similar SEQ was asked in 2013. This SEQ was described the principles and sources of error in the measurement of arterial blood pressure using an invasive arterial line and transducer. The variant to this in 2013 was described the effects of resonance and damping on invasive arterial blood pressure tracing. Both of these questions had a pass mark within 30%, so they're a very high likelihood of being asked again, and as for the examiner report, there's a very detailed examiner report from the 2022 sitting. For the first part of the question, being the principles, to achieve a pass mark here, it was expected that you would list the components of the system and describe the function of each of these components. Then talk about the broad principles of the transducer and how this works, and better answers here would discuss how the signal is amplified. In terms of the second part of this question, the major sources of error could be divided into static and dynamic, and again, if these were covered, then you'd have a high chance of passing this question. And in addition to the second part, if you spoke about how these errors are minimized within the arterial system, this would give you even higher marks. My approach to the model answer for this question would not include any diagrams. I would just list out the components to begin with, and for me, this makes most sense going from the patient to the pressure bag. From this, I would then talk about the mechanisms, and to start off again, I would clearly state the role of an arterial line is to convert pressure energy generated from arterial pulsations into electrical energy, displayed as an output, being numbers, and a waveform. Therefore, there's a sensor, an integrator, and an output. Each part of the components has a specific role to play in this pathway. The tubing and the normal saline allows for hydraulic coupling. Therefore, energy is not lost before it gets to the transducer. The transducer has incorporated wheat stone bridges and strain gauges, which change resistance based on change in pressure. This change in resistance is then picked up as a signal within an electrical circuit, and that is transmitted to the microprocessor. Within the microprocessor, these signals are filtered and amplified, and a key thing I would state here is that a arterial waveform is made up of multiple waves. Therefore, a Fourier's analysis is conducted to then reconstruct those waves into one waveform that we see as their output. The outputs we get are the systolic, the diastolic, the mean arterial pressure, and additionally, we could do special outputs like looking at pulse pressure variations or pulse contour analysis. Then moving on to the second part of this question, which is the sources of error, again dividing this into static and dynamic, and we've covered these at nauseam, static errors being zeroing and leveling, and the key thing I would state with static errors, is they can affect systolic, diastolic, and mean arterial pressure. In comparison, dynamic errors affect only systolic and diastolic pressure, but the mean arterial pressure should remain accurate. Within dynamic errors, you have resonance, which is the increase and amplitude of the wave as it approaches its natural frequency, and here I would have a clear definition of natural frequency, which is a tendency of a system to remain in oscillations once an energy is applied. The other key thing to put in here is that the natural frequency of the arterial system is 200 Hz, it's well above the natural frequency of arterial pulsations, which is 3 Hz at its maximum. With this, the way we ensure the natural frequency of the arterial system is so high is by using the natural frequency equation, and by no means do you put this in, but you can put in the factors that affect this, being the tube length, the density of fluid, and the stiffness of the tubing. The key numbers to remember here is that the width of the tubing should be at least greater than 1.5 mm, the length of the tubing should be kept less than 1.2 meters, because there's a tradeoff between resonance and damping that occurs at this mark. Then moving on to damping, you can define damping as the decrease in energy of oscillations with time, and damping can be further divided into overdamped or an underdamped trace. Here we want to achieve a fine balance between overdamping and underdamping, therefore we say the optimal damping occurs at a damping coefficient of 0.64, and then to finish off, you can put in some quick examples of what causes overdamping. These are things like kinks, blood clots, or bubbles within the arterial line. Now that's a lot to put in this answer, but I think it will comfortably give you a past mark if you have those components. That brings us to the end of today's pod, and also to the end of our cardiac physiology series. I really hope you've enjoyed this series. I've now done two full series, the first one being respiratory, and cardiac being the second. I want to thank you guys again for listening to these pods. If you have any feedback, feel free to email me. I'm constantly trying to make these pods better for you guys. I know how hard it is to study for this exam. So keep at it guys, and I'll catch you next time on Count to Ten.