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Why Supine? ABPI

Discussion in 'General Issues and Discussion Forum' started by shellyvortex, Nov 7, 2008.

  1. shellyvortex

    shellyvortex Member

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    Hello all,

    please can anyone help me with an answer??

    when conducting an ABPI, all existing research papers refer to need for the patient to be supine, with only one reference that comes from merriman & tollafield (1995) as to why it should be done this way:

    "ABPI will be 1mmHg higher for ever inch the legs are lower than the heart"-

    Why is this? is this one of those medical facts that 'just is'?

    There appears to be no supporting research for this-and also the agreed method also requires patient to be supine for 10 minutes prior to readings and throughout procedure ...why is it the agreed method? where is the evidence? surely the readings will correlate in ABPI measurements so pre-measurement activity is not relevant?

  2. Johnpod

    Johnpod Active Member

    Hi Shelly,

    That's a very good question and I have deliberately given it a day or two to consider it.

    I suppose the parameters of all tests need to stated. And perhaps the first definitions stick for no more reason than that they were the first described?

    However, I think patient comfort has something to do with 'why supine' in this ABPI case.

    The heart needs to be at the level of the brachial artery site to acheive a meaningful systolic reading. Lying on either side is not then an option. Lying prone compresses the rib cage and makes breathing more difficult. It may also be quite uncomfortable for overweight, paunchy patients. Respiratory distress might cause cardiac distress??

    'Pre-measurement activity is not relevant - surely the readings will correlate...'

    That's a hard one to answer. In an agitated (recently active) person it will be quite difficult to get meaningful readings at all. If the brachial systolic is taken at one definable instant, by the time you have set up to read the ankle systolic, the patient will have further recovered from their recent activity. So you would not be taking correlatable readings. With reduction in the heart rate we would expect to see a reduction in the ankle systolic that is related to relaxation and is in addition to the reduction caused by arterial damage and hence there will be an unrealistic stretching of the very difference we are trying to measure -and distortion of the ratio.

    And how could another clinician repeat the test and have confidence in the history you might present?

    Further, how could you compare your own earlier results when following up, perhaps three months later, under different conditions?

    I believe the answer to your question lies in the fact that we must try to control as many variable parameters as possible in order that we can better understand what our data actually means. In other words, give our data 'validity'.

    Like many tests, ABPI is probably not absolutely ideal. But tight control of the clinical process leads to results that, with the results of other tests (all properly done), can usefully inform a decision (perhaps on whether to amputate, or not?).

    Supporting research might well be difficult to turn up. But the modern obsession with the database must be seen against decades of refinement of clinical skills and understanding which is not written down.

    Intended to help!
  3. shellyvortex

    shellyvortex Member

    hi john,

    thanks for your reply- i appreciate you taking time to deliberate and what you have written makes sense. I'm inclined to agree with you on the current burst for evidence base - sadly much of the health/medical world does not!

    once again thanks,
  4. drsarbes

    drsarbes Well-Known Member

    "The ABI exam, which compares systolic blood pressures obtained at the
    ankles and arms, was traditionally performed with the patient in the supine
    position to prevent error from hydrostatic pressure."

    Johnpod is correct..... in order to have as standardized a test as possible, patient position is one controllable factor that will decrease index variability.

  5. Why is it not done standing up, or sitting down (my G.P. measures brachial with me sitting down)? Since all positions, for example standing erect, on their head, on one leg etc. are repeatable positions, is it that the patient is at greatest "risk" when they are lying flat on their back without the aid of gravity? Hence the test gives a worst case scenario (other than standing on their head;))?

    Or is it just to do with the relationship between height (or depth) and pressure.....
  6. drsarbes

    drsarbes Well-Known Member


    Only you.

  7. Well..... its Monday night and the TV's rubbish.:bash:
  8. Matt Kimball

    Matt Kimball Member

    In the student environment we're expected to, as other people have alluded to, do things because a standard of any evidence (anecdotal, research based or otherwise) says so. "It's like that, and that's the way it is".

    The issues that annoy me the most with the "patient must be supine for 10 minutes" standard for an ABPI are:

    - It takes time! The patient will invariably have lower back pain so we go fetch them a pillow. We do what we can while the patient is supine then do the test, knowing full well that a lot of people (including the patient's GP) don't test their BP this way so validity is compromised anyway.

    - Nowhere can I find evidence as to why it has to be 10 minutes. Does it take that long for the hear rate and blood pressure to reduce to a relaxed state after activity? And as Mr Spooner said, why do they have to be relaxed anyway? I'm sure they're not at any other place they get their BP taken.

    And as a tongue in cheek query to Merriman and Tolliefield's statement on the brachial pressure being 1mmHg higher for every inch higher than the ankle, does this mean taller people have higher ankle systolic pressure measurements when standing than us short folk?

    Sincerely, Matt Kimball.
  9. simonf

    simonf Active Member

    We are not testing BP to determine hypertension like the GP, we are comparing brachial with ankle pressures, so standardising the approach and to aid comparison seems sensible
  10. Admin2

    Admin2 Administrator Staff Member

  11. Johnpod

    Johnpod Active Member

    The whole idea of ABPI is to compare the systolic pressures at two distant sites on the body - at the same level.

    Lying the patient supine puts both sites at the same level - couch top.

    When the readings are taken there will be no differences in pressure due to hydrostatic effects. Makes tall ones and short ones equal, columns of blood and all that!

    Resting systolic pressures can be considerably lower than exercised systolic pressures. (Consider - if you had a patient with an aneurism that was about to explode, would you want him to keep still, or jump about to make it happen?).

    Excercise causes higher systolic pressures (to provide blood in greater volume) as needed in physical exertion. More exertion causes even higher pressures. Resting state is the only possible datum mark.
    Last edited: Nov 11, 2008
  12. Matt Kimball

    Matt Kimball Member

    Thanks Johnpod and Simonf, it all makes sense when it's explained that way!

    Matt Kimball
  13. PodGov

    PodGov Member

    We could also look at the concepts of barometric/atmospheric pressure measurements and relate these to a column of blood (within a vessel) and therefore the effects of gravity. Hence the concept of the effect of hydrostatic (or rather hydrodynamic) pressure and as been mentioned the need to eliminate these variables clinically (the vertical column and effect of gravity, resting the patient, etc.) by allowing the patient to be supine.
  14. andymiles

    andymiles Active Member

    as ABPI is expressed as a ratio i am unsure why this is important.

    a systolic of 100:100 is the same ratio as 120:120
  15. Both resting and post-exercise ABPI are commonly used.

    Vasc Med (2006) 11: 29-33.

    Limitation of the resting ankle-brachial index in symptomatic patients with peripheral arterial disease

    R Stein, I Hriljac, JL Halperin, SM Gustavson, V Teodorescu, JW Olin

    Peripheral arterial disease (PAD) has been demonstrated to be prevalent in the primary care setting. However, it has also been shown to be unrecognized and under-treated. Owing to the association with cardiovascular disease it has been recommended to screen high-risk patients for PAD in the primary care setting using the ankle-brachial index (ABI). ABI has been demonstrated to be highly sensitive and specific in diagnosing PAD in patients with significant stenosis. However, the utility in patients with less severe stenosis and calcified vessels is in question. The aims of this study were to determine the diagnostic utility of measuring the ABI at rest in patients referred to the vascular laboratory for evaluation of suspected PAD, and to assess the added value of pulse volume recordings and post-exercise studies in patients with a normal ABI. A computerized vascular diagnostic laboratory database was queried for symptomatic outpatients referred for measurement of segmental blood pressure, the ABI or pulse volume recordings by physicians not specialized in the evaluation and management of patients with peripheral vascular disease. Of 707 patients undergoing outpatient physiologic arterial evaluations between February 1, 2003 and July 31, 2004, 396 met these inclusion criteria. Data recorded included resting ABI, ABI following treadmill exercise test and the presence of abnormal pulse volume recordings. The study population (n = 396) consisted of equal numbers of men and women (mean age 69 years, range 19-100 years). Among 396 studies, resting ABI values were normal in 183 (46.2%) and abnormal in 159 (40.2%). Of the 138 patients who underwent exercise testing, 84 had normal ABI readings at rest. In the 84 patients who had a normal ABI at rest and underwent exercise testing, the ABI fell below 0.9 after exercise in 26 (31%). Arterial non-compressibility was detected in 54 (13.6%) patients, whose average age was 67 years. Thirteen (24%) of those with non-compressible vessels had abnormal pulse volume recording (PVR) results, compared to five with normal resting ABI who had abnormal PVR findings (2.7%). In conclusion, this study demonstrated that nearly half of patients referred to the outpatient vascular laboratory because of suspected arterial disease had a normal resting ABI. While it is recommended that the ABI be measured at rest in patients at risk of PAD in primary care practice, these findings suggest that patients with symptoms of PAD should be more completely evaluated in a vascular laboratory. Furthermore, when the ABI is normal at rest in patients with symptoms of intermittent claudication, exercise testing is recommended to enhance the sensitivity for detection of PAD.(http://www.bio-computing.org/showab...=yes&terms=post exercise ankle-brachial index)

    See also:


  16. andymiles

    andymiles Active Member

    would the readings that fell below 0.9 after exercise be counted as a false positive?
  17. Depends what you consider as the positive for the test and which test you are talking about; resting or post exercise? If we consider the positive for the resting ABPI to be it's ability to detect an ABPI <0.9, then the resting ABPI results gave a false negative for these individuals, compared to the post exercise test. If however, the positive is considered a result = 1, then the resting did indeed give a false positive, compared to the post exercise test. I hope this makes sense, its a bit of a statistical argument.

    You are really asking about specificity, sensitivity and predictive values here, so read these- they'll probably make more sense than me ;):

    Which test is giving the "real result", i.e. validity, could also become a moot point, but this just makes things even sillier. So I won't go there unless provoked! Moreover, I don't know enough about vascular assessment, so I'll just go with the conclusions of the paper I cited- they seemed to think that the post-exercise gives the "real" result. Interesting thought: how many pods perform a post-exercise test with their treadmills? Probably very very very few. Frightening thought: how many pods have suspected PAD, done an ABPI that came out normal and not performed a post-exercise test or referred to someone who could have carried out further testing because they'd only ever been taught about resting ABPI tests? Of course, the answer is none.
    Last edited: Nov 12, 2008
  18. Evidence for the 10 minutes here:

    "Following exercise, blood pressure rapidly returns to normal. As shown in Figure 1, there is often a transient pressure 'undershoot' caused by a pooling of blood in the dilated, previously exercised muscle beds. This pressure decrement is more pronounced following intense exercise. The baroreceptors work to counter the circulating vasodilatory substances to initially return homeostasis within 10 min following exercise."

    BTW Figure 1 is at the bottom of the page.

    Here's the reference the data is drawn from:

    P.S. Matt, it took me a couple of minutes at most to find this when I went looking for it and it's Dr Spooner, since I have PhD, but you can call me Simon or just Spooner like most everybody else does (to my face). :D

    One final thought on this as it's really not my bag, Shelly:
    This statement really doesn't seem to make any sense since ABPI is a ratio. Let say we performed an ABPI with ankle and brachial pulses horizontal and got an ABPI of 1.0, does this mean that if we repeated it with the feet 1" lower than the heart we'd get an ABPI of 2.0?? I really don't know because it really ain't my bag- but I doubt it very much. Moreover, why would the author mix units like that from metric to imperial within the same sentence- shameless ;-) I think they are saying it may change the ankle reading BTW
    Last edited: Nov 12, 2008
  19. Asher

    Asher Well-Known Member

    I agree Simon.

    We must recognise this limitation of the ABI and report it. Indicate that the values and any conclusions you draw pertain to an "at rest" state. This shows that you are aware of the limitation and it reminds the GP (or whoever) of the limitation and the potential for stress testing if necessary.

  20. Matt Kimball

    Matt Kimball Member

    Thanks Simon, I have learnt to not be such a lazy bones! And I'll pass the data on for the benefit of others around our student clinic. I also agree that an ABPI is only one of the many measurements we have at our disposal. We also need to use our clinical observation and subjective history clues, with the barrage of other objective measurements, in combination to investigate our suspicions of PAD.


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