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MAP vs. BP

Question# 744

Knowing that monitoring devices, (LP15/Zoll), use oscillometry to accurately detect MAP and not SIBP, have the MOH and BHs made any strides in reviewing and changing all medical directives to reflect MAP as the accurate measure of hypo-perfusion and not “SBP >90mmhg”?

Answer:

In short, at the current state, there is no intention of changing the medical directives to reflect MAP instead of BP.

As you stated, monitoring devices use the oscillometric measurement technique to estimate BP. The oscillometric technique does not use Korotkoff sounds to determine blood pressure; rather, it monitors the changes in pressure pulses that are caused by the flow of blood through the artery. The NIBP monitor inflates the cuff around the patient’s arm to a value that occludes the artery, and then deflates the cuff in steps. When blood starts to flow through the artery, the increasing blood flow causes the amplitude of the pressure pulses in the cuff to increase. As the NIBP monitor steps the pressure down, the pulses reach a peak amplitude and then start to decrease. The rising and falling amplitude values form a curve that is analyzed to yield systolic pressure, diastolic pressure, and mean arterial pressure (MAP).

As with any non-invasive oscillometric blood pressure monitor, clinical conditions, such as the patient’s physiological condition, can affect the accuracy of the measurements obtained. For example, shock may result in a blood pressure waveform that has a low amplitude, making it difficult for the monitor to accurately determine the systolic and diastolic pressures, and in turn, the MAP. This is why it’s sometimes preferred to obtain a manual blood pressure.

MAP is traditionally calculated via an invasive arterial catheter, which allows for targeted vasopressor support and fluid resuscitation. For obvious reasons, this is not done in the prehospital world.

The monitoring devices thus employ a mathematical equation based on the obtained systolic and diastolic pressure to calculate MAP [MAP = DP + 1/3(SP – DP)], or (DBP+DBP+SBP\3). As you can appreciate, if the blood pressure in inaccurate, so too will the MAP be.

This equation takes time to manually do, and is not user friendly, especially in the presence of an acutely ill patient; but some machines do calculate it for you. It’s important to note that provincially, and even regionally, not all paramedic services turn this feature on, nor is this part of the standard education paramedics receive.

Blood pressure is just one measure of shock states, and the entire patient presentation should be considered as well (i.e. change in mental status, heart rate, tachypnea, skin condition, acidosis on EtCO2, O2 sats, etc.). These are a few of the reasons why there is no current appetite to modify the directives to reflect MAP.

That doesn’t mean we can’t consider utilizing MAP as part of a robust patient assessment however.

Mean arterial pressure (MAP) is the average arterial pressure throughout one cardiac cycle, systole, and diastole. To perfuse vital organs requires the maintenance of a minimum MAP of 60 mmHg. If MAP drops below this point for an extended period, end-organ manifestations such as ischemia and infarction can occur. If the MAP drops significantly, blood will not be able to perfuse cerebral tissues, there will be a loss of consciousness, and neuronal death will quickly ensue; so it is valuable to measure. While the optimal end-organ perfusion pressure is unclear, in general, it’s suggested maintaining the mean arterial pressure between 65 to 70 mmHg.

Lastly, in the context of a trauma patient, there is a growing body of literature out there suggesting that the shock index (SI) may be a more sensitive indicator of shock than blood pressure alone. Unlike MAP, shock index is easy to calculate; it’s simply HR divided by SBP. The normal range is <0.5-0.7, and anything greater than 0.9 is associated with increased likelihood of transfusion and increased mortality. To simplify: if your heart rate is above your SBP, you’re SI will be high.

We hope this helps clarify, but please reach out if you have any further questions. Thank you for advocating for you patients, and for change. Stay safe out there.

References

  • Stryker LP 15 Manual
  • DeMers D, Wachs D. Physiology, Mean Arterial Pressure. [Updated 2023 Apr 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538226/#
  • UpToDate: Approach to shock in the adult trauma patient
  • UpToDate: Evaluation of and initial approach to the adult patient with undifferentiated hypotension and shock
  • Koch E, Lovett S, Nghiem T, Riggs RA, Rech MA. Shock index in the emergency department: utility and limitations. Open Access Emerg Med. 2019 Aug 14;11:179-199. doi: 10.2147/OAEM.S178358. PMID: 31616192; PMCID: PMC6698590.
  • https://reference.medscape.com/calculator/599/shock-index

Published

16 November 2023

ALSPCS Version

5.2

Views

574

Please reference the MOST RECENT ALS PCS for updates and changes to these directives.