Instagram Live – Pressure Transducers And Arterial Line Waveforms

Pressure transducers are a cornerstone for measuring arterial, central venous, pulmonary artery, and even intracranial pressures. These transducers operate through a Wheatstone bridge, a circuit with an arrangement of resistors of known resistance except for one. This unknown resistor is a strain gauge which is coupled to your pressure of interest (A-line, CVP, etc.) via incompressible tubing usually filled with (heparinized) saline. As the force on the gauge changes due to fluctuations in pressure, the intrinsic resistance within the gauge also changes causing a shift in the current flowing through the Wheatstone circuit. An algorithm is used to interpret this flux in current as a change in the pressure magnitude – this is the number we ultimately read on the monitor. 🙂

The waveform produced by the monitor is dependent on what is actually being measured. For example, the morphology of a CVP tracing looks very different than that of an arterial line. For the purposes of this post, I’ll talk about arterial lines since they’re fairly common in the intensive care unit (ICU) and operating room (OR).

Normal arterial line waveform

First of all, unlike an automated blood pressure cuff, an arterial line (A-line) directly measures the systolic pressure, diastolic pressure, and mean arterial pressure. In the waveform above, the nadir corresponds to the diastolic blood pressure (DBP). The upstroke immediately after represents ventricular contraction and culminates at the systolic peak which represents the systolic blood pressure (SBP).

As the ventricle relaxes, its pressure falls below that of the aorta resulting in closure of the aortic valve which creates a reflected pressure wave represented by the dicrotic notch. As one moves distally in the body (think about an A-line in the foot versus in the radial artery), the dicrotic notch will appear more delayed. Also, in severely vasodilated patients (septic shock, neurogenic shock), the dicrotic notch will occur at a much lower pressure. In this case, you may want to consider afterload enhancement with agents like norepinephrine and/or vasopressin.

Now how does the overall morphology of the arterial line change as you move more distally? Again, let’s consider a patient who has two arterial lines: one in the radial artery and one in the dorsalis pedis artery (foot). The mean arterial pressure (MAP) between the two should be similar, but the more distal foot A-line will have a higher SBP and lower DBP compared to the radial A-line. This is a consequence of reflected pressure waves at all the branch points blood encounters as it exits the heart and barrels down the arterial tree.

Drop me a line with questions below! I’ll be addending this post with more information in the coming days.

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  1. Ruky says

    Hey! I have had issues with trouble shooting art lines. If I am having a hard time drawing blood off the art line what are things I can do for trouble shooting purposes.

    1. Rishi says

      We’re often called for arterial lines that “transduce but don’t draw back”, and honestly, if the patient still warrants having an arterial line, I’ll just replace it either by rewiring the existing line or placing an entirely new one. Usually this is due to a kink in the catheter, but can also be from vasospasm/vasoconstriction of the vessel more proximally.

  2. Tibo K., RN says

    Could you please elaborate on reflected pressure waves and why is distal SBP is higher and DBP is lower than proximal readings from radial a-line. Thank you!

    1. Rishi says

      Great question! With each heart beat, a pressure wave is transmitted down the arterial tree with the ejected stroke volume. As blood travels further from the heart, it encounters more branch points and narrower blood vessels. This translates to more resistance (especially in older patients with “stiff” arteries) causing some of the pressure wave to be reflected backwards (a pathway with less resistance).

      Now why does the systolic increase? I think it’s partly due to the acceleration of pressure waves through the narrower vessels. The diastolic pressure decreases because a component of the initial pressure wave is now reflected retrograde and essentially subtracted from the diastolic pressure you would expect.

      Fluid dynamics. 😯

      1. Tibo says

        Thank you for the reply!
        So in terms of numbers I would see 120/80 in R a-line and 140/60 in fem a-line and that essentially would be the same since MAP should be equal. And that would prompt me not to chase higher SBP in fem line considering the fluid dynamics? ?

        1. Rishi says

          So the problem is yes, from a purely academic standpoint, we should probably focus more on MAP rather than SBP. However, a lot of physicians write for blood pressure goals and PRN anti-hypertensives in terms of SBP which is affected by where the arterial line is located. I’ve tried explaining this to colleagues with varying levels of success.

  3. Allie R., RN says

    You mention there is no need to replace the vented stopcock caps with non-vented ones. I’m wondering why that is, as I believe it is taught to always replace with caps without the holes (I’ve always assumed for infection prevention?). Thanks for all you do!

    1. Rishi says

      Hey Allie! Maybe it’s just an institutional thing (and depends on the setting). In the OR, I never changed the stopcock caps to non-vented ones, but it sounds like in some ICUs, this is the norm? Goes to show the variability in even the smallest things from place to place, haha.