Estimation of Mean Pulmonary Artery Pressure: Simpler Than Expected

Abstract

It is widely admitted that mean pulmonary artery pressure (mPAP) may be accurately estimated by using the standard formula: mPAP = 2/3 dPAP + 1/3 sPAP, where dPAP is diastolic pulmonary artery pressure, and sPAP is systolic pulmonary artery pressure. This rule of thumb implies that knowing the minimal and maximal values of the pressure signal allows a precise estimation of the mean pressure, as also documented at the systemic counterpart. Accordingly, it is admitted that mPAP is twice as sensitive to dPAP as it is to sPAP. The prominent role of dPAP in the formula implies that dPAP may reflect vascular tone more accurately than sPAP, which depends on pulmonary artery compliance, wave reflections, and characteristics of right ventricular ejection. An invasive study by our team1Chemla D Castelain V Humbert M et al.New formula for predicting mean pulmonary artery pressure using systolic pulmonary artery pressure.Chest. 2004; 126: 1313-1317Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar has challenged this model by reporting that sPAP accounts for 98% of mPAP variability in resting humans studied over à range of 10 to 78 mm Hg, and that a new formula (mPAP = 0.61 sPAP + 2) reasonably predicts mPAP, with a 0 ± 2 mm Hg bias (mean ± SD). Our retrospective analysis1Chemla D Castelain V Humbert M et al.New formula for predicting mean pulmonary artery pressure using systolic pulmonary artery pressure.Chest. 2004; 126: 1313-1317Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar of the pressure data previously published by Laskey et al2Laskey WK Ferrari VA Palevsky HI et al.Pulmonary artery hemodynamics in primary pulmonary hypertension.J Am Coll Cardiol. 1993; 21: 406-412Abstract Full Text PDF PubMed Scopus (106) Google Scholar led to similar conclusion. The principle that underlies scientific modeling cautions against favoring the complex models over the simple ones,3Chemla D Castelain V Lecarpentier Y et al.New formula for predicting mean pulmonary artery pressure [letter].Chest. 2005; 128: 467Abstract Full Text Full Text PDF Scopus (2) Google Scholar and this favors the use of sPAP only to estimate mPAP, without the need to include dPAP in the model. Although initially challenged,4Fisher MR Hassoun PM Prediction of pulmonary artery pressure [letter].Chest. 2005; 128: 466-467Abstract Full Text Full Text PDF PubMed Google Scholar our findings have been confirmed in children free of congenital heart diseases,5Friedberg MK Feinstein JA Rosenthal DN A novel echocardiographic Doppler method for estimation of pulmonary arterial pressures.J Am Soc Echocardiogr. 2006; 19: 559-562Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar and have proved useful to estimate mPAP in large-scale studies performed in both adults6Biancari F Kangasniemi OP Luukkonen J et al.EuroSCORE predicts immediate and late outcome after coronary artery bypass surgery.Ann Thorac Surg. 2006; 82: 57-61Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar and children7Calderon-Garciduenas L Vincent R Mora-Tiscareno A et al.Elevated plasma endothelin-1 and pulmonary arterial pressure in children exposed to air pollution.Environ Health Perspect. 2007; 115: 1248-1253Crossref PubMed Scopus (133) Google Scholar for whom only Doppler-derived sPAP data are available. Finally, a linear relationship between mPAP and sPAP has also been documented in pulmonary arterial hypertension patients performing graded, submaximal supine exercise both before and after a 6-week prostacyclin treatment.8Castelain V Chemla D Humbert M et al.Pulmonary artery pressure-flow relations after prostacyclin in primary pulmonary hypertension.Am J Respir Crit Care Med. 2002; 165: 338-340Crossref PubMed Scopus (90) Google Scholar In the present issue of CHEST (see page 633), Syyed et al9Syyed R Reeves JT Welsh D et al.The relationship between the components of pulmonary artery pressure remains constant under all conditions in both health and disease.Chest. 2008; 133: 633-639Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar studied pulmonary hemodynamics in control subjects and in patients with pulmonary hypertension. The authors confirm that mPAP and sPAP are linearly related under resting supine conditions (r2 = 0.98), with the two pressures being related by an empirical formula essentially similar to ours. The newness of their excellent study9Syyed R Reeves JT Welsh D et al.The relationship between the components of pulmonary artery pressure remains constant under all conditions in both health and disease.Chest. 2008; 133: 633-639Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar is the following: (1) the strong mPAP vs sPAP linear relationship is extended to changes in posture and activity; (2) the relationship is documented irrespective of potential differences in heart rate, cardiac output, gender, and age; and (3) the retrospective analysis of data obtained in freely moving rats and horses suggests similar relationship between the two pressures. Because mPAP (invasive) and sPAP (noninvasive) are used interchangeably to define pulmonary hypertension in humans, it is currently accepted that mPAP and sPAP provide an essentially redundant estimate of the state of pulmonary circulation. Although this point may appear counterintuitive from a physiologic point of view, it is now confirmed.1Chemla D Castelain V Humbert M et al.New formula for predicting mean pulmonary artery pressure using systolic pulmonary artery pressure.Chest. 2004; 126: 1313-1317Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar5Friedberg MK Feinstein JA Rosenthal DN A novel echocardiographic Doppler method for estimation of pulmonary arterial pressures.J Am Soc Echocardiogr. 2006; 19: 559-562Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar9Syyed R Reeves JT Welsh D et al.The relationship between the components of pulmonary artery pressure remains constant under all conditions in both health and disease.Chest. 2008; 133: 633-639Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar The limitations of the present study9Syyed R Reeves JT Welsh D et al.The relationship between the components of pulmonary artery pressure remains constant under all conditions in both health and disease.Chest. 2008; 133: 633-639Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar may be discussed. First, data were averaged over several minutes. This may explain a certain amount of scattering and implies caution for the generalizability of the data in patients studied either in the catheterization laboratory or by using Doppler echocardiography. Second, the limited number of chronic pulmonary thromboembolism and idiopathic pulmonary artery hypertension patients prevented any comparison of potential differences in the relationship between the steady and pulsatile component of their pulmonary arterial load, a point that is still under discussion and may have practical implications.10Naeije R Huez S Reflections on wave reflections in chronic thromboembolic pulmonary hypertension.Eur Heart J. 2007; 28: 785-787Crossref PubMed Scopus (32) Google Scholar Third, an important question still awaiting an answer is what is the sensitivity and specificity of various sPAP thresholds to estimate the degree of pulmonary hypertension, as defined by using mPAP? Further large-scale studies are especially needed to answer this practical question. Fourth, the classic formula may be slightly more precise.3Chemla D Castelain V Lecarpentier Y et al.New formula for predicting mean pulmonary artery pressure [letter].Chest. 2005; 128: 467Abstract Full Text Full Text PDF Scopus (2) Google Scholar4Fisher MR Hassoun PM Prediction of pulmonary artery pressure [letter].Chest. 2005; 128: 466-467Abstract Full Text Full Text PDF PubMed Google Scholar However, its “pay off” appears essentially similar to that of our formula, at the price of doubling the number of independent variables, and this is out the norms of standard rules of modeling.3Chemla D Castelain V Lecarpentier Y et al.New formula for predicting mean pulmonary artery pressure [letter].Chest. 2005; 128: 467Abstract Full Text Full Text PDF Scopus (2) Google Scholar Finally, if there were a tight and constant relationship between systolic, diastolic, and mean pulmonary artery pressures,9Syyed R Reeves JT Welsh D et al.The relationship between the components of pulmonary artery pressure remains constant under all conditions in both health and disease.Chest. 2008; 133: 633-639Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar pulmonary circulation could be characterized by any single pressure. This viewpoint may appear questionable because mPAP is less strongly related to dPAP than to sPAP,1Chemla D Castelain V Humbert M et al.New formula for predicting mean pulmonary artery pressure using systolic pulmonary artery pressure.Chest. 2004; 126: 1313-1317Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar9Syyed R Reeves JT Welsh D et al.The relationship between the components of pulmonary artery pressure remains constant under all conditions in both health and disease.Chest. 2008; 133: 633-639Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar and is weakly related to pulse pressure (sPAP − dPAP).1Chemla D Castelain V Humbert M et al.New formula for predicting mean pulmonary artery pressure using systolic pulmonary artery pressure.Chest. 2004; 126: 1313-1317Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar Thus, while mPAP is accurately predicted by the single sPAP, it is likely that two pressures remain necessary to fully characterize pulmonary hemodynamics. What can be inferred from these observations, and does it really matter? For physiologists, the present study9Syyed R Reeves JT Welsh D et al.The relationship between the components of pulmonary artery pressure remains constant under all conditions in both health and disease.Chest. 2008; 133: 633-639Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar appears to confirm the hypothesis that the functional adaptation of the pulmonary circulation to the disease process is rather monotonous because changes in pulmonary artery elasticity mainly depend on mPAP.1Chemla D Castelain V Humbert M et al.New formula for predicting mean pulmonary artery pressure using systolic pulmonary artery pressure.Chest. 2004; 126: 1313-1317Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar10Naeije R Huez S Reflections on wave reflections in chronic thromboembolic pulmonary hypertension.Eur Heart J. 2007; 28: 785-787Crossref PubMed Scopus (32) Google Scholar11Zuckerman BD Orton EC Stenmark KR et al.Alteration of the pulsatile load in the high-altitude calf model of pulmonary hypertension.J Appl Physiol. 1991; 70: 859-868Crossref PubMed Scopus (48) Google Scholar12Lankhaar JW Westerhof N Faes TJ et al.Quantification of right ventricular afterload in patients with and without pulmonary hypertension.Am J Physiol. 2006; 291: H1731-H1737Crossref PubMed Scopus (223) Google Scholar Experimental data have suggested that changes in pulmonary artery compliant properties and pulsatile pressure are primarily due to increases in mPAP under pulmonary hypertension states.11Zuckerman BD Orton EC Stenmark KR et al.Alteration of the pulsatile load in the high-altitude calf model of pulmonary hypertension.J Appl Physiol. 1991; 70: 859-868Crossref PubMed Scopus (48) Google Scholar Furthermore, using both a theoretical model of the pulmonary circulation, and pressure and MRI flow data in humans, is has been reported that the time constant (resistance × compliance) of the large pulmonary arteries remains unchanged in pulmonary hypertension, with pulmonary resistance and compliance being related by an inverse, curvilinear relationship.12Lankhaar JW Westerhof N Faes TJ et al.Quantification of right ventricular afterload in patients with and without pulmonary hypertension.Am J Physiol. 2006; 291: H1731-H1737Crossref PubMed Scopus (223) Google Scholar For the clinician, the estimation of sPAP at rest and with exercise by using Doppler echocardiography has gained increasing importance over standard right-heart catheterization in the diagnosis and prognosis of pulmonary hypertension. Although the applicability of noninvasively estimating mPAP from sPAP may be carefully drawn,1Chemla D Castelain V Humbert M et al.New formula for predicting mean pulmonary artery pressure using systolic pulmonary artery pressure.Chest. 2004; 126: 1313-1317Abstract Full Text Full Text PDF PubMed Scopus (335) Google Scholar3Chemla D Castelain V Lecarpentier Y et al.New formula for predicting mean pulmonary artery pressure [letter].Chest. 2005; 128: 467Abstract Full Text Full Text PDF Scopus (2) Google Scholar4Fisher MR Hassoun PM Prediction of pulmonary artery pressure [letter].Chest. 2005; 128: 466-467Abstract Full Text Full Text PDF PubMed Google Scholar the new formula may be clinically useful for the following: (1) to cross-check the mPAP value estimated by using other Doppler methods; and (2) to improve the noninvasive estimation of pulmonary vascular resistance both before and after treatment, which may have implications for the prognosis and management of pulmonary hypertension.