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Correspondence |
Hôpital de Bicêtre-Université Paris, Le Kremlin-Bicêtre, France, E-mail: denis.chemla{at}bct.ap-hop-paris.fr
To the Editor:
The letter by Ahn and Lim1 in the November 2005 issue of the Canadian Journal of Anesthesia presents direct arterial blood pressure data in a patient under anesthesia, showing that variations in mean arterial pressure (MAP) may result from differences in pressure waveforms even in the presence of stable systolic and diastolic pressure readings. Quoting a previous letter from our team on the empirical equations for MAP estimation,2 the authors suggest that researchers "sometimes forget the limitations in using these equations".1 We provide the following comments.
A critical evaluation of several empirical equations for estimating MAP has been reported recently.3 As in our previous letter,2 it was stressed that results pertain strictly to the conditions under study, namely patients investigated at rest, under stable hemodynamic conditions, with their pressure being recorded at the aortic root level by using a high-fidelity pressure catheter.3 As previously discussed,2,3 our conclusions do not apply to peripheral pressure recordings, nor to patients with unstable or rapidly varying hemodynamic conditions. We have obtained data in 139 patients, and the mean bias between MAP estimates and true MAP was < 0.5 mmHg with a precision (standard deviation of the bias) < 3 mmHg.3 While there are no guidelines for such comparisons, it must be noted that the Association for the Advancement of Medical Instrumentation recommends that bias < 5 mmHg and precision < 8 mmHg are required for validating the accuracy of a new pressure device when compared to a reference standard.4
Although MAP is essentially similar in the aorta and large peripheral arteries, the pressure wave-form obtained at the aortic root differs significantly with that recorded at the peripheral level. The basic hemodynamic principles explaining such differences have been reviewed.5 We agree with Ahn and Lim1 that acute changes in blood volume, inotropic state, heart rate, vascular tone and arterial stiffness may lead to discrepancies between the actual MAP value and the MAP empirically estimated at the peripheral level. However, we feel that our viewpoint has been misquoted, and we hope that the present comments help to clarify other factors which may influence mean arterial pressure estimation, including the recording site (central vs peripheral), the characteristics of the recording system (high-fidelity vs conventional) and the nature of hemodynamic conditions (stable vs unstable).
Footnotes
Accepted for publication December 12, 2005.
References
1 Ahn W, Lim YJ. Mean arterial blood pressure estimation and its limitation (Letter). Can J Anesth 2005; 52: 1000–1.
2 Chemla D, Hebert JL, Zamani K, Coirault C, Lecarpentier Y. Estimation of mean aortic pressure (Letter). Lancet 1999; 354: 596.[Medline]
3 Chemla D, Antony I, Zamani K, Nitenberg A. Mean aortic pressure is the geometric mean of systolic and diastolic aortic pressure in resting humans. J Appl Physiol 2005; 99: 2278–84.
4 Association for the Advancement of Medical Instrumentation. Electronic or automated sphygmomanometers. ANSI/AAMI SP 10-1992. Arlington (VA): AAMI; 1992. American National Standard. Available from URL; http://www.aami.org/news/2005/022405.bp.html.
5 Nichols WW, O’Rourke MF. McDonald’s Blood Flow in Arteries: Theoretical, Experimental and Clinical Principles, 5th ed. London: Edward Arnold; 2005.
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