As I mentioned in Medical Instrumentation, first 5 chapters and Medical Instrumentation, Chapter 6, I’ve been slogging through one of the potential text books for the circuits course: Medical Instrumentation: Application and Design, 4th Edition. John G. Webster. Publisher: Wiley, 2009. # ISBN-10: 0471676004; # ISBN-13: 978-0471676003. I need to return the book to the library this week (interlibrary loan periods are short), so I’ll probably just skim the book after Chapter 8, to see if there is anything we can use.
BLOOD PRESSURE AND SOUND 293
Robert A. Peura
7.1 Direct Measurements 295
We’re not sticking catheters into people, so this is irrelevant to our class.
7.2 Harmonic Analysis of Blood-Pressure Waveforms 300
A bad presentation of Fourier analysis, giving only amplitude (not phase), but saying “When we compare the original waveform and the waveform reconstructed from the Fourier components, we find that they agree quite well, indicating that the first six harmonics give a fairly good reproduction.” You can’t get a reconstruction like that without the phase information, which they never even mention!
7.3 Dynamic Properties of Pressure-Measurement Systems 301
Uses RLC transmission-line circuits to model pressure in a catheter. The idea of using electrical analogs to model physical systems is an important one, but I’d rather do it with a system that we can actually build and test in the lab. The number of formulas in this section is large enough that probably only a small fraction of students ever read it.
7.4 Measurement of System Response 308
The describe a way to get the step response of a pressure-sensor-catheter system (by using a bursting rubber membrane to get a step decrease in pressure) and sinusoidal response (using an underwater loudspeaker). They do not talk about differentiating the step response to get the impulse response, nor using autocorrelation with random excitation to get impulse response. This section feels like something out of the 1970s.
7.5 Effects of System Parameters on Response 310
Catheter wall stiffness and de-aerating water are important from a fluid dynamics standpoints but are not interesting for a circuits class.
7.6 Bandwidth Requirements for Measuring Blood Pressure 311
Rather trivial discussion of bandwidth, though the mention that looking at the derivative of pressure increases the bandwidth requirement is an important one that students could easily overlook. They claim that the amplitude-vs-frequency characteristic of a catheter-manometer system should be flat (to within 5%) for the first 20 harmonics. For a rapid heart rate of 240 bpm, that means a high frequency of about 80Hz. That is trivial for electronics, but tough for fluid in a narrow tube.
7.7 Typical Pressure-Waveform Distortion 311
Fairly good discussion of the effects of an underdamped and overdamped system, as well as he effects of air bubbles and “catheter whip”. Unfortunately, they don’t talk about what “damping” is and seem to confuse it with “inadequate frequency response”. I would not give this section to someone who didn’t already have a firm grasp of the concepts, as it is likely to lead to serious misunderstandings.
7.8 Systems for Measuring Venous Pressure 313
Medically interesting, perhaps, but nothing new for sensors or circuits.
7.9 Heart Sounds 314
A good description of heart sounds and their correlation to the EKG and blood pressure waveforms. Talks about the rather non-flat frequency characteristics of stethoscopes, and problems with applying the stethoscope and of leakage at the earpieces. They talk about the lack of success of electronic stethoscopes on the market and attribute it physicians’ unfamiliarity with the sounds heard through electronic stethoscopes.
7.10 Phonocardiography 318
One paragraph on recording heart sounds.
7.11 Cardiac Catheterization 318
We’re not sticking catheters into people’s hearts!
7.12 Effects of Potential and Kinetic Energy on Pressure Measurements 323
Standard fluid dynamics stuff (Bernoulli’s equation) applied to blood flow. The stuff about hydrostatic pressure and which way a catheter port points relative to blood flow is undoubtedly important in blood pressure measurements, but is not really relevant to our circuits class.
7.13 Indirect Measurements of Blood Pressure 325
Now we’re finally getting to stuff that could conceivably be useful for the course: non-invasive techniques involving pressure cuffs and listening to the blood flow. The oscillometric method (which senses the pressure in the cuff) is popular in home blood pressure meters. I don’t think that we want to put together the cuff and other mechanical parts of a blood pressure meter, but it would give us an excuse for using a pressure sensor.
7.14 Tonometry 330
Measuring pressure in the eye (and in arteries) by force sensors flattening the object being measured. Not really suitable for our circuits course.
Overall, there is little in Chapter 7 of any use for the circuits class. We could, perhaps, make an electronic stethoscope or oscillometric blood pressure cuff. The notion of modeling fluid pressure using electronic analogs is an important one, but we can’t use the example here in the lab.
MEASUREMENT OF FLOW AND VOLUME OF BLOOD 338
John C. Webster
8.1 Indicator-Dilution Method That Uses Continuous Infusion 338
We’re not sticking catheter in arteries.
8.2 Indicator-Dilution Method That Uses Rapid Injection 341
We’re not sticking catheter in arteries.
8.3 Electromagnetic Flowmeters 338
We’re not cutting people open to stick $500 cuff probes around arteries.
8.4 Ultrasonic Flowmeters 350
Ultrasonic flowmeters using Doppler shift measurements are kind of cool, but I don’t know that we can get decent transducers for a reasonable price. Hmm, there is a 7.2MHz transducer that is 22mm in diameter (a bit big for aiming at a blood vessel!) with wire leads for about $4 each in quantities of 5). I think that the mechanical design problems here could get to be too big for the circuits course. The electronics is a bit tricky also, as they would have to deal with RF (around 7MHz) and detecting the slightly shifted Doppler return (which is modulated by a noise band, not a simple shift) in a huge carrier background. Probably too difficult for this course.
8.5 Thermal-Convection Velocity Sensors 361
We’re not sticking heated probes into people’s blood vessels.
8.6 Chamber Plethysmography 364
A rather bulky apparatus that we have no reason to build.
8.7 Electric-Impedance Plethysmography 366
We could try building the 4-electrode plethysmograph, but it seems like a lot of trouble for a not very interesting result. The authors even say “electrical-impedance plethysmography has been used to measure a wide variety of variables, but in many cases the accuracy of the method is poor or unknown.” We’d probably have to make our own band electrodes also, as the only commercial ones I’ve been able to find are for neonatal EKGs and so are much too small for adults.
8.8 Photoplethysmography 372
This section has the detection of blood flow by shining light through tissue, which I experimented with a bit, but I’ve not found a good way to mount sensors so that they work properly. They do recommend 940nm LEDs and phototransistors, in both transmission and reflection setups. They do mention a problem that I was facing: “large artifacts due to motion saturate the amplifier”. They point out that when patients are in shock, vasoconstriction reduces peripheral blood flow, so these sensors may not be able to detect the pulses. Their suggestion of shining the light through the nasal septum is useful in surgery, no doubt, but probably not appropriate for the circuits course.
I’d still like to get this to work reliably without a lot of mechanical setup, because I really want to have a light-sensor application.
I still don’t think that we can make Medical Instrumentation our main text book, and there isn’t much in Chapters 7 and 8 that we could use. I wish I could get the light-based pulse sensor working without so much hassle—I’ll have to try it again.
One concept worth looking at is the electrical-circuit analog for mechanical devices. It might be worthwhile to set up a lab involving pressure measurements at either end of a long tube and modeling the tube as an electrical circuit. This would give me an excuse for using pressure sensors like the Freescale MPXHZ6250A, which we used in the remotely-operated underwater vehicle as a depth gauge. That gauge has a 1msec response time (10% to 90% of response to a step input), which is fast enough for most of the experiments I can think of for them to do.