Finding an LVAD MAP: What the Books Don’t Tell You

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Andrew Phillips
Andrew Phillips

Chair, AAEM Critical Care Medicine Section
Staff Intensivist, Washington Hospital, Fremont, CA
@warejko
Phillips.EM.CCM@gmail.com

The Pre-brief

Real case: A middle aged female with a left ventricular assist device (LVAD) presents by ambulance to the ED. No blood pressure or doppler MAP can be obtained, and the patient appears pale and cool so norepinephrine is empirically started. An arterial catheter is emergently placed and shows a blood pressure of 160/147. The norepinephrine is stopped. The paleness and coolness are found to be secondary to severe hypoxemia and hypothermia.

What went wrong? How could a systolic blood pressure of 160 mmHg be missed?

When auscultating for noninvasive blood pressure, it is common to place the stethoscope chestpiece over the brachial artery, inflate the cuff, and then listen for the Korotkoff sounds. This is fine as the stethoscope chestpiece is large enough and the sounds are loud enough that the chestpiece just has to be in the general area of the brachial artery. 

HOWEVER, obtaining a doppler “MAP” for a patient with an LVAD is a different procedure altogether and must be approached differently. LVAD patients typically do not have a palpable or audible pulse, so instead the cuff is inflated to complete compression, and the pressure at which the first doppler signal is detected is considered the “MAP.” (It is really the systolic blood pressure–sort of–but this is the convention because you won’t lose doppler signal as the cuff is deflated; no diastolic pressure can be measured.) The doppler pen must be directly over the artery to catch the signal, so you can’t listen blindly. You must FIRST find the signal, THEN inflate the cuff, eliminating the signal, and THEN slowly deflate the cuff and record the pressure at which the signal returns. 

Sound silly? Not as silly as not detecting a systolic blood pressure of 160 peripherally. It is important to observe how your team is gathering this vital sign.

The Debrief

  • The cuff pressure at which doppler is first detected as the cuff pressure is reduced is considered the MAP for LVAD patients because they often do not have a detectable pulse pressure on noninvasive blood pressure measurements.
  • It is important to find the location of the artery with doppler first, THEN inflate the cuff to the point of no longer observing doppler signal, THEN deflate the cuff and mark the pressure.

References

  1. Phillips AW. Things I wish someone had told me. Journal of Anecdotal Medicine. Perpetually edited.
  2. Bennett MK, Adatya S. Blood pressure management in mechanical circulatory support. J Thorac Dis. 2015;7(12):2125-2128. doi:10.3978/j.issn.2072-1439.2015.11.05. PMID: 26793332.

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