Medical / Engineering Question

[Ivor the Engineer]

I'm interested in finding out about continuous blood pressure measurement techniques. I've done a bit of searching on the web and have found a few different methods described, but unfortunately they are not quite detailed enough.

Specifically, can anyone explain the technique described here (http://hrf.jsc.nasa.gov/hardware/cbpd.asp):

The CBPD is a non-invasive continuous blood pressure measurement device that uses plethysmography and measures blood pressure at the finger according to the technique of Penaz. One (or two) finger cuff(s), which have a small bladder and an infrared spectral measurement transmitter/receiver, are placed around the finger(s). A basic three-lead electrocardiogram (ECG) amplifier is also provided for measurement of ECG from the subject.

Also, what method is used to monitor the blood pressure of a patient during an operation?

[Deetee]

I'm interested in finding out about continuous blood pressure measurement techniques. I've done a bit of searching on the web and have found a few different methods described, but unfortunately they are not quite detailed enough.

Specifically, can anyone explain the technique described here (http://hrf.jsc.nasa.gov/hardware/cbpd.asp):

Also, what method is used to monitor the blood pressure of a patient during an operation?
Not quite sure what there is to explain. I am not familiar with the CPBD, which appears to be an electronic digital method of measuring systolic and diastolic pressure, presumably displaying this as a continual read out or graph of some kind. Here (http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/9639/30462/01403675.pdf?arnumber=1403675) is one method which describes the method/principles involved.

Currently, anaesthetists will monitor BP during surgery using a digital spygmomanometer with a cuff around the upper arm which can be preset to automatically take readings at short intervals (eg every 5 minutes). I guess if CBPD is properly validated/accurate and inexpensive enough that we would expect to see it being used in these situations.

In places like ITU, patients usually have an invasive device, an intra-arterial line, inserted. This enables both BP measurement as well as providing a source of arterial blood for sampling for checking blood gas values like oxygenation levels.

[Ivor the Engineer]

Thanks Deetee for the info.

I'm finding medical electronics and instrumentation to be a fascinating subject.

[Ivor the Engineer]

For those interested (probably just me), here's an interesting report on different methods of measuring blood pressure.

http://circ.ahajournals.org/cgi/content/full/111/5/697

Seems there can be quite a few errors in the measurement of blood pressure.

[Dr. Imago]

Ah! Right up my alley! :)

We do routinely use continuous invasive blood pressure monitoring. This is via insertion of a catheter into an artery, typically the radial artery in the wrist. The patient is (usually) already anesthetized when we do this, because it can be quite painful to do it awake (like hitting a sensitive nerve in a tooth).

The device then uses the principle of a continuous fluid column and a transducer that measures pressure against that fluid column using the principle of impedance plethysmography. Essentially, the tranducer is "zeroed" at the height of interest (in this case the level of the heart) and the pulsations against the continuous fluid column generate an electrical signal that can be measured.

The "fluid column" is a continuous solution of usually normal saline (0.9% NaCl in sterile water) mixed with a little bit of heparin. It flows through a line connected to the catheter inserted in the artery. The reason for this is two-fold: (1) it is to ensure that there is a continuous column of fluid (e.g., small air bubbles will "dampen" the readings) and, (2) to make sure the tip of the catheter doesn't clot-off, as it's sitting in a pulsating artery.

The pulsations generated from the heart travel as fluid wave down the vessels, and because this catheter is sitting in a pulsatile vessel, is further transmitted up the fluid wave in the saline/heparin column to the transducer. The minute variations of pulsation against the transducer generate the electrical signal, as described above, and this is interpreted in the computer as a blood "pressure".

Because of computer sampling/measurement time, the read out "refreshes" roughly every 2-3 seconds.

Hope that helps!

-Dr. Imago

[Ivor the Engineer]

Thanks, Dr. Imago.

[Graham Jackman]

I'm interested in finding out about continuous blood pressure measurement techniques. I've done a bit of searching on the web and have found a few different methods described, but unfortunately they are not quite detailed enough.

Specifically, can anyone explain the technique described here (http://hrf.jsc.nasa.gov/hardware/cbpd.asp):



Also, what method is used to monitor the blood pressure of a patient during an operation?

This sounds like an earlier device called the Finapres, which measured the bp continuously through a finger cuff. The most common non-invasive method is known as oscillometry and relies on the fact that if you inflate a cuff around a limb to greater than the systolic pressure and allow it to gradually deflate, pulsations from the blood can be detected in the cuff. These suddenly increase as you reach systolic pressure, are maximum at mean blood pressure and fade away below diastolic. Typically these sytems read 5-10mm below what is recorded by the manual sphygmoanometer origianlly used in a doctor's surgery. Since the recommendations on blood pressyre are derived from the sphygmomanometer readings, it is normal to adjust the automated measurements, when evaluating the readings. The Finapres used this technique to make the initial estimate of blood pressure, then adjusted the cuff to mean pressure and either followed the absorbance of light through the finger to track blood flow and thus pressure changes, or it tried to maintain the reaing constant by adjusting the cuff pressure. However it was done, this gave a reading that followed the real time variation in blood pressure, whereas the oscillometric techniques can only give you sbp, mean and dbp every 30 seconds or so.
Whichever method you use, you need to be aware that bp varies considerably from beat to beat, person to person and within individuals. Continuous monitoring is useful for following changes with time that can be related to stimuli or drug treatment.

[Ivor the Engineer]

This sounds like an earlier device called the Finapres, which measured the bp continuously through a finger cuff. The most common non-invasive method is known as oscillometry and relies on the fact that if you inflate a cuff around a limb to greater than the systolic pressure and allow it to gradually deflate, pulsations from the blood can be detected in the cuff. These suddenly increase as you reach systolic pressure, are maximum at mean blood pressure and fade away below diastolic. Typically these sytems read 5-10mm below what is recorded by the manual sphygmoanometer origianlly used in a doctor's surgery. Since the recommendations on blood pressyre are derived from the sphygmomanometer readings, it is normal to adjust the automated measurements, when evaluating the readings. The Finapres used this technique to make the initial estimate of blood pressure, then adjusted the cuff to mean pressure and either followed the absorbance of light through the finger to track blood flow and thus pressure changes, or it tried to maintain the reaing constant by adjusting the cuff pressure. However it was done, this gave a reading that followed the real time variation in blood pressure, whereas the oscillometric techniques can only give you sbp, mean and dbp every 30 seconds or so.
Whichever method you use, you need to be aware that bp varies considerably from beat to beat, person to person and within individuals. Continuous monitoring is useful for following changes with time that can be related to stimuli or drug treatment.

Thanks for the info. on the Finapres, Graham. I've been studying the oscillometric method and other non-invasive techniques. After reading the AHA article it sounds like the change in BP is more clinically significant during continuous monitoring than the absolute accuracy (within limits).

Also, after reading about the "white-coat effect" (the presence of a physician causing your blood pressure to rise*), it makes you wonder how many people are on medication for high bp that do not need to be.

I'm hoping to get into the design of medical electronics and instrumentation (as opposed to borehole electronics and instrumentation:)).

*I know of one physician on this forum that has that effect on me:)

[Southwind17]

Also, after reading about the "white-coat effect" (the presence of a physician causing your blood pressure to rise*), it makes you wonder how many people are on medication for high bp that do not need to be.

*I know of one physician on this forum that has that effect on me:)

Even when she's ignoring you?! ;)

p.s. You're no doubt familiar with the similarly symptomatic 'bikini effect' too?!

[Ivor the Engineer]

Even when she's ignoring you?! ;)

Perhaps it's just my sense of British fair-play, but if you're going to debate or discuss an issue then you should be prepared to listen to the point of view of everyone who is involved in the discussion. If you're not then don't get involved!

Personally, I will never put anyone on ignore and I always try to respond to people who ask me questions or comment on my posts, even if just to take the p.

p.s. You're no doubt familiar with the similarly symptomatic 'bikini effect' too?!

Oh yes. I imagine physicians in bikinis could be fatal:D

[Dr. Imago]

We also use a device called the "T-line" which is placed over the radial artery externally. When properly positioned, it can detect the pulsations of the radial artery and give a very accurate, real-time measurement of beat-to-beat blood pressure.

-Dr. Imago

[T.A.M.]

Late to the party, but Dr. Imago covered it well.

Love the disclaimer at the bottom...lol

TAM:)

[Graham Jackman]

Thanks for the info. on the Finapres, Graham. I've been studying the oscillometric method and other non-invasive techniques. After reading the AHA article it sounds like the change in BP is more clinically significant during continuous monitoring than the absolute accuracy (within limits).

Also, after reading about the "white-coat effect" (the presence of a physician causing your blood pressure to rise*), it makes you wonder how many people are on medication for high bp that do not need to be.

I'm hoping to get into the design of medical electronics and instrumentation (as opposed to borehole electronics and instrumentation:)).

*I know of one physician on this forum that has that effect on me:)

You're right about the changes in BP often being of more significance and that is one reason we use ambulatory BP monitors that record the BP every 30 minutes or so. They are also very useful in detecting white coat hypertension.

[T.A.M.]

Actually there is a growing voice in the medical community that feels that "white coat" hypertension may actually be significant enough to monitor regularly, and perhaps even treat...the theory being that the BP, if healthy enough, should not elevate significantly (high enough to be called hypertensive) even in the presence of a physician. Further research is needed.


The widespread assumption that it is not necessary to treat office hypertension in patients with "normal" ambulatory blood pressures, although attractive, is an unwarranted, untested assumption that amounts to wishful thinking.

http://www.cmaj.ca/cgi/content/full/161/3/275

TAM:)

[Ivor the Engineer]

We also use a device called the "T-line" which is placed over the radial artery externally. When properly positioned, it can detect the pulsations of the radial artery and give a very accurate, real-time measurement of beat-to-beat blood pressure.

-Dr. Imago

I've looked up on the web how that device works. For those interested:

http://electronicdesign.com/Articles/ArticleID/11520/11520.html

[Soapy Sam]

Why would constant monitoring of blood pressure be different from monitoring any other fluid pressure?

[Graham Jackman]

Why would constant monitoring of blood pressure be different from monitoring any other fluid pressure?

The greatest difficulty is accessing the arteries. Not too many people volunteer to have an arterial catheter inserted. That is why it is so common to use indirect methods using inflatable cuffs.

[Ivor the Engineer]

Here's an interesting way of monitoring blood pressure, using the Pulse Wave Transit Time (PWTT):

http://2005.iccas.org/submission/paper/upload/ICCAS2005_jgy.pdf