Download New Formula for Predicting Mean Pulmonary Artery Pressure

estimates of SPAP,3 we caution against the use of this technique
to diagnose pulmonary hypertension.
Micah R. Fisher, MD
Paul M. Hassoun, MD
Johns Hopkins University
Baltimore, MD
Reproduction of this article is prohibited without written permission
from the American College of Chest Physicians (www.chestjournal.
org/misc/reprints.shtml).
Correspondence to: Micah R. Fisher, MD, Division of Pulmonary
and Critical Care, Johns Hopkins University, 1830 E. Monument
St, 5th Floor, Baltimore, MD 21205; e-mail: [email protected]
References
1 Chemla D, Castelain V, Humbert M, et al. New formula for
predicting mean pulmonary artery pressure using systolic
pulmonary artery pressure. Chest 2004; 126:1313–1317
2 Bland JM, Altman DG. Statistical methods for assessing
agreement between two methods of clinical measurement.
Lancet 1986; 1:307–310
3 Arcasoy SM, Christie JD, Ferrari VA, et al. Echocardiographic assessment of pulmonary hypertension in patients
with advanced lung disease. Am J Respir Crit Care Med 2003;
167:735–740
New Formula for Predicting Mean
Pulmonary Artery Pressure
To the Editor:
We appreciate the interest of Drs. Fisher and Hassoun in our
recent study (October 2004).1 In their first comment, they
suggest that a more appropriate evaluation of our formula would
have been obtained by determining the mean difference between
the calculated mean pulmonary artery pressure (mPAP) and the
measured mPAP (bias). This important methodologic issue was in
fact taken into account in our article, where the bias value was
presented in the validation sample.1 We feel that obtaining a
predictive formula in the test sample and calculating the bias in
the validation sample only is appropriate from a statistical point of
view. Nevertheless, in an attempt to comply with their comment,
we have calculated the bias in the overall population from raw
pressure data. We obtain a – 0.2 mm Hg bias (95% limits of
agreement from – 5.6 to ⫹ 5.2 mm Hg), and the slight discrepancies between their calculations and ours may be explained as
follows. In our study, high-fidelity pressures were automatically
calculated with a 0.1 mm Hg precision, and values were averaged
out over ⬎ 10 consecutive cycles.1 However, for sake of simplicity
for the reader, the pressures presented in both text and tables
have been corrected to the nearest whole number, which makes
more clinical sense. Finally, as far as the validity of our new
formula is concerned, namely mPAP ⫽ 0.61 sPAP ⫹ 2 mm Hg
(where sPAP is systolic PAP), our retrospective analysis of the
data by Laskey et al2 led to an essentially similar relationship.
In their further comment, Drs. Fisher and Hassoun have
calculated mPAP by using the standard formula (mPAP ⫽ 2/3
dPAP ⫹ 1/3 sPAP; where dPAP is diastolic PAP), and we obtain
roughly similar results (bias ⫽ ⫺ 1.4 mm Hg; 95% limits of
agreement, – 4.1 to 1.3 mm Hg). They conclude that “the
standard formula is easier and arguably more accurate,” and our
reply will discriminate practical from physiologic considerations.
From a practical point of view, there is no need of any formula in
the catheterization laboratory, as mPAP is automatically integrated over time by the computer system. Conversely, we feel
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that our formula is easier to use than the standard one in the
echo-Doppler laboratory, where precise measurements of dPAP
are not always available, whereas more reliable measurements of
sPAP are often obtained. Furthermore, our formula was less
biased (and thus more accurate) than the empirical formula,
which was, however, more precise as evidenced by the narrower
95% limits of agreement.
From a physiologic point of view, the principle that underlies
scientific modeling caution against favoring the complex models
over the simple ones. This logical principle is attributed to the
medieval philosopher William of Occam (1285 to 1349) who
stated that plurality should not be posited without necessity
(Occam’s razor). The standard formula (two-pressure model)
implies that mPAP is twice as sensitive to dPAP as it is to sPAP.
Although we agree that the standard formula is accurate and
precise, we suspect that this model is essentially redundant, as
sPAP explained 98% of the mPAP variance in our study1 and 99%
in our retrospective analysis of the study by Laskey et al.2 This
strengthens the validity of an alternative, single-pressure model
in which sPAP reasonably predicts mPAP.
In their final comment, Drs Fisher and Hassoun caution
against the use of our formula to diagnose pulmonary hypertension from Doppler-derived sPAP estimates given the “significant
inaccuracy” of sPAP estimates. This was not the aim of our study
to test this point, and we were very cautious in our conclusions.
Although we are fully aware of studies reporting discrepancies
between invasive and noninvasive sPAP,3 we feel that both the
careful estimation of right atrial pressure and the correction for
confounding factors (eg, demographic ones) may help improve
the reliability of sPAP Doppler measurements, especially when
performed by experienced teams. Finally, further technical improvements of Doppler devices may well help obtain even more
reliable sPAP estimates in the near future.
Denis Chemla, MD
Vincent Castelain, MD
Yves Lecarpentier, MD
CHU de Bicêtre
Le Kremlin-Bicêtre, France
Marc Humbert, MD
Gérald Simonneau, MD
Hôpital Antoine Béclère
Clamart, France
Philippe Hervé, MD
Hôpital Marie Lannelongue
le Plessis-Robinson, France
Reproduction of this article is prohibited without written permission
from the American College of Chest Physicians (www.chestjournal.
org/misc/reprints.shtml).
Correspondence to: Prof. Denis Chemla, MD, Service EFCR,
Broca 7, Hôpital de Bicêtre, 78 rue du Général Leclerc, 94 275 Le
Kremlin Bicêtre, France; e-mail: [email protected]
References
1 Chemla D, Castelain V, Humbert M, et al. New formula for
predicting mean pulmonary artery pressure using systolic
pulmonary artery pressure. Chest 2004; 126:1313–1317
2 Laskey WK, Ferrari VA, Palevsky HI, et al. Pulmonary artery
hemodynamics in primary pulmonary hypertension. J Am
Coll Cardiol 1993; 21:406 – 412
3 Chemla D, Castelain V, Hervé P, et al. Haemodynamic
evaluation of pulmonary hypertension. Eur Respir J 2002;
20:1314 –1331
CHEST / 128 / 1 / JULY, 2005
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