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Mean arterial blood pressure estimation and its limitation

Seoul National University Hospital, Seoul, South Korea, E-mail: aws{at}snu.ac.kr

To the Editor:

Mean arterial blood pressure (MAP) has clinical and physiologic significance in both the representation of perfusion pressure and its utilization in the calculation of hemodynamic variables. The accurate MAP is defined as the zero frequency (direct current) component following Fourier analysis of the arterial waveform,¹ or as the time-weighted integral of the instantaneous pressures derived from the area under the curve of the pressure-time.² However, clinicians have yet to find an easier way that may be applied in clinical practice. Some researchers continue to investigate more accurate MAP estimation methods.^2–4

Even though we sometimes forget the limitations in using these equations,⁵ we should keep in mind that they will not have the same MAP if they have different arterial pressure waveform morphologies, no matter what are the causes of the differences.¹ The following is an example.

We analyzed a patient’s direct arterial blood pressure data, which was continuously checked by an anesthesia monitor, sampled at 250 Hz, and stored in a computer through analogue output. The data length was about one hour. We divided the data in two-second intervals, so the total data sections numbered around 1,800. In each section, we calculated the mean using the Fourier transformations by a computer software program (Matlab®, Natick, MA, USA). The maximum difference of MAP within the same systolic arterial pressure and diastolic arterial pressure pairs was sought. We then compared the two waveforms with the maximum MAP difference. The MAP differences were originated mainly from, in my case, an early reflected wave (Figure). However, there may be other sources of waveform differences, for example, the existence of a dicrotic notch, high inotropic pressure pulse,⁶ and a double dicrotic notch.

As clinicians, we need an easily calculable determinant of MAP, but we should understand its limitations as applied to clinical practice, especially where the arterial waveform could be different from the one used to derive the simplified equation. Factors which may alter arterial waveform, and hence MAP determination include increasing age, atherosclerosis, and changes in blood volume status.

View larger version (26K): [in this window] [in a new window]   FIGURE Different mean arterial pressures (MAP) of one patient. The X-axis is arbitrary time; the Y-axis is blood pressure in mmHg. Both readings have the same systolic and diastolic pressures, but have a 10 mmHg difference in MAP.

1 Poullis M. New formula to calculate mean aortic pressure? (Letter). Lancet 1999; 353: 2075.

2 Meaney E, Alva F, Moguel R, Meaney A, Alva J, Webel R. Formula and nomogram for the sphygmomanometric calculation of the mean arterial pressure. Heart 2000; 84: 64.[Free Full Text]

3 Chemla D, Hebert JL, Zamani K, Coirault C, Lecarpentier Y. A new formula for estimating mean aortic pressure. Lancet 1999; 353: 1069–70.[Medline]

4 Razminia M, Trivedi A, Molnar J, et al. Validation of a new formula for mean arterial pressure calculation: the new formula is superior to the standard formula. Catheter Cardiovasc Interv 2004; 63: 419–25.[Medline]

5 Chemla D, Hebert JL, Zamani K, Coirault C, Lecarpentier Y. Estimation of mean aortic pressure (Letter). Lancet 1999; 354: 596.

6 Murray, WB, Gorven, AM. Invasive v. non-invasive blood pressure measurements -- the influence of the pressure contour. S Afr Med J 1991; 79: 134–9.[Medline]

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				D. Chemla Factors which may influence mean arterial pressure measurement. Can J Anesth, April 1, 2006; 53(4): 421 - 422. [Full Text] [PDF]

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