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