central vs peripheral 1

Monitoring of a patient in severe septic shock and with high dose pressor infusions. We happened to have two intra-arterial blood pressures, shown as the red tracings and numbers on the monitor. A central pressure from a PiCCO line in the femoral artery (top tracing), and a peripheral pressure from an arterial cannula in the arm (bottom tracing). You’ll also see atrial fibrillation and a no-good pulse-ox trace. The interesting part is the difference between the central and peripheral art line readings.

Degrees of hypotension
In the image above, the central tracing shows hypotension, but the peripheral tracing shows pretty extreme hypotension. Pressure isn’t flow. But pressure is what we measure, and also, we still need a minimum of pressure to keep things going. In extreme situations, with clamped down peripheral circulation, pressures will be different in different parts of the circulation.

central vs peripheral 2
One more image showing the same relationship. Patient’s pressure is slightly better, but still severely hypotensive judged by the peripheral trace. Adequate blood pressure judged by the central trace. And the central trace would be the one that interests you the most.

Distal pulse amplification
In a normal patient with no major disturbance of the circulation, you might see a peripheral arterial pressure with a higher systolic and lower diastolic component than the central pressure due to distal pulse amplification. The MAP will usually be comparable, though. This can all change in the critically ill. But there’s been some debate to the phenomenon of distal areterial pressures being different from central pressures.

ICU research paradox
I think this might be due to the ICU research paradox: To get larger patient numbers in your ICU research, you need open up your inclusion criteria. You’ll then end up with a heterogenic cohort where you can’t really prove anything. And on the way often excluding the really critical patients. This might be the problem with this paper, that finds no difference between distal and central arterial pressures; I think the group studied was the wrong population – too heterogenous, not ‘critical’ enough.

The ‘right’ critical patient
To find a possible difference, you’ll need to select the few patients you have in severe shock with shut-down peripheral circulation, and possibly on high dose pressors to keep them afloat. Then you’ll find a difference. Like they did in the study below. That study found numbers comparable to the images in this post. The peripheral pressure was lower than the central pressure. The result was that they could slightly lower the noradrenaline infusion in some of these patients and still have an adequat blood pressure measured by central catheters.

The arterial pressure difference in my patient was an interesting observation, useful to keep in mind when dealing with severe shock and impaired peripheral circulation.

Radial artery pressure monitoring underestimates central arterial pressure during vasopressor therapy in critically ill surgical patients, Crit Care Med, 1998.

You might also want to read our post on COMPARING NIBP AND ABP

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  1. Matthieu Komorowski says:

    Thanks for this interesting post and your overall great blog! Any idea of what the NIBP was? What I usually do when in doubt with the peripheral invasive BP is checking it with the cuff, which quite often is found to be normal. I usually come to the conclusion that there is something wrong with the catheter (clot, twisting…) but I realised after reading this post that I might be wrong! Thanks.

    • Thomas D says:

      Thanks for your comment. There was no NIBP measures here, unfortunately. The catheter in the arm was put in just before these images, flushed fine and was fine for drawing blood samples. SO it seemed to work OK, and we related the problems to the impaired circulation. But I agree, usually with readings like this, it’s some sort of catheter problem and a NIBP will unmask it.

  2. This is immensely interesting. Thank you for sharing.

  3. nfkb says:

    as we say in France : “dans mes bras” ! i’m thinking exactly the same thing as you write here.

    thank you for this clever post

  4. nfkb says:

    P.S. and maybe when you lower the noradrenaline infusion you may alleviate some kind of bad vasocontriction that impede venous return or microcirculation, the less Nad the better !

    • Thomas D says:

      Thanks for your reply! That’s what we were thinking too. And in the second image, we had been able to improve circulation a little, and lower NA a little bit. But despite aggressive fluid loading, we were unable to get to more acceptable NA levels in this patient. PiCCO was OK, echocardiography showed an impaired, but decent ejection fraction that improved slightly with dobutamine, but unfortunately the patient still needed high NA to keep afloat.

  5. nfkb says:

    it’s off topic but i’m sure you’ll like this comment : http://www.nejm.org/doi/full/10.1056/NEJMc1306658?af=R&rss=currentIssue

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