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Commentary |

Systolic and Mean Pulmonary Artery PressuresSystolic and Mean Pulmonary Artery Pressures: Are They Interchangeable in Patients With Pulmonary Hypertension? FREE TO VIEW

Denis Chemla, MD; Marc Humbert, MD; Olivier Sitbon, MD; David Montani, MD; Philippe Hervé, MD
Author and Funding Information

From the Faculté de Médecine, Universite Paris-Sud, Le Kremlin Bicêtre; AP-HP, Services des Explorations Fonctionnelles et de Pneumologie, Hôpital Bicêtre, Le Kremlin Bicêtre; and Inserm UMR_S999, Centre Chirurgical Marie Lannelongue, Le Plessis-Robinson, France.

CORRESPONDENCE TO: Denis Chemla, MD, Service des Explorations Fonctionnelles, Broca 7, Hôpital de Bicêtre, 78 rue du Général Leclerc, 94 275 Le Kremlin Bicêtre, France; e-mail: denis.chemla@bct.aphp.fr


Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details.


Chest. 2015;147(4):943-950. doi:10.1378/chest.14-1755
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Pulmonary hypertension (PH) is a common complication of numerous diseases, including left-sided heart diseases and chronic lung diseases and/or hypoxia, where PH is associated with exercise limitation and a worse prognosis. Other forms of PH include pulmonary arterial hypertension (PAH), chronic thromboembolic PH (CTEPH), and PH with unclear multifactorial mechanisms. Over the past decade, it has been documented that systolic pulmonary artery pressure (sPAP) may help estimate mean pulmonary artery pressure (mPAP) in adults with high accuracy and reasonably good precision (mPAP = 0.61 sPAP + 2 mm Hg). This strong linear relationship between sPAP and mPAP was unexpected from a classic physiologic point of view. Consistent results have been obtained from independent teams using either high-fidelity micromanometer-tipped PA catheters or fluid-filled catheters. Overall, the strong link between sPAP and mPAP has been documented over a wide range of PAPs, heart rate, cardiac output, wedge pressure, and causes of PH, during changes in posture and activity, and irrespective of patient’s sex, age, and BMI. A review of available invasive data confirms that patients with CTEPH and idiopathic PAH matched for their mPAP exhibit essentially similar sPAP. Pressure redundancy may be explained by the dependence of PA compliance upon mPAP. The 25 mm Hg threshold used to define PH accurately corresponds to an sPAP of 38 mm Hg. Although the limits of the echocardiographic estimation of sPAP are widely documented, results from invasive studies may furnish an evidence-based sPAP-derived mPAP value, potentially useful in the multiparameter echocardiographic approach currently used to diagnose and follow patients with PH.

Pulmonary hypertension (PH) is defined on right-sided heart catheterization as resting mean pulmonary artery pressure (mPAP) ≥ 25 mm Hg. In adults, PH is a common complication of numerous diseases, including left-sided heart diseases and chronic lung diseases and/or hypoxia, where PH is associated with exercise limitation and a worse prognosis.1,2 Other forms of PH include pulmonary arterial hypertension (PAH), chronic thromboembolic PH (CTEPH), and PH with unclear multifactorial mechanisms.3 Thus, there is an urgent need to noninvasively diagnose PH in the majority of patients. In this respect, the point/counterpoint editorials by Rudski4 and Rich5 published in CHEST on this topic is especially welcomed. Both Rudski4 and Rich5 must be congratulated for having elegantly supported the “yes” and “no” answers, respectively. Although a multiparameter echocardiographic approach has been rightly advocated by numerous authors to rule in or rule out PH diagnosis,48 the accurate estimation of systolic pulmonary artery pressure (sPAP) remains at the center of the debate.4,5

sPAP is currently estimated from both the continuous Doppler maximum velocity of tricuspid regurgitation and the estimated right atrial pressure, assuming that sPAP and right ventricular peak systolic pressure are equal. Because PH is defined by using mPAP, Rudski4 discussed the corresponding sPAP threshold. This appears to be a difficult task. It was first indicated that if mPAP is two-thirds diastolic pressure plus one-third systolic pressure, and if one assumes an mPAP of 25 mm Hg and a pulmonary artery diastolic pressure of 15 mm Hg, this would correlate with an sPAP of about 45 mm Hg.4 It was acknowledged that many authors use the previously proposed upper limit of normal sPAP of > 30 to 35 mm Hg.9 Two consensus documents were also quoted, indicating that “In the absence of other potential etiologies of PH, such as left heart disease or advanced lung disease, an estimated RV systolic pressure of greater than 40 mm Hg generally warrants further evaluation in the patient with unexplained dyspnea”6 and that “If the estimated SPAP is > 35 to 40 mm Hg, stronger scrutiny may be warranted to determine if PH is present, factoring in other clinical information.”7

The cutoff values applied to large populations must be evidence based. In this respect, this commentary has two aims: (1) to summarize the main recent findings favoring that mPAP and sPAP are strongly related and provide essentially redundant estimates of pulmonary circulation, and (2) to discuss the potential implications for PH pathophysiology and diagnosis.

In 2004, our group demonstrated that mPAP and sPAP were related through a strong linear relationship in adult patients prospectively studied by using a high-fidelity micromanometer-tipped pulmonary artery (PA) catheter,10 according to the following equation:

mPAP=0.61sPAP+2mmHg 

Results were obtained in 31 subjects, namely nine control subjects, nine subjects with PAH, seven subjects with CTEPH, and six patients with postcapillary PH. This observation has been further confirmed by using both micromanometer-tipped1113 and fluid-filled14,15 PA catheters (Table 1).

Table Graphic Jump Location
TABLE 1 ]  Empirical Formulas Describing the Linear Relationship Between mPAP and sPAP

C = control subjects; CTEPH = chronic thromboembolism pulmonary hypertension (group 4); F = female; HPREF = chronic heart failure of reduced left ventricular ejection fraction; M = male; mPAP = mean pulmonary artery pressure; NA = not indicated or impossible to extrapolate from the subgroup data; No PH = mean pulmonary artery pressure < 25 mm Hg; PA = pulmonary artery; PAH = pulmonary arterial hypertension (group 1); PH = pulmonary hypertension (ie, mPAP ≥ 25 mm Hg); PH-HD = pulmonary hypertension due to heart diseases (group 2); PH-LD/H = pulmonary hypertension due to lung diseases/hypoxia (group 3); sPAP = systolic pulmonary artery pressure.

a 

Indicates a value estimated from the corresponding figure in the article.

A strong linear relationship between mPAP and sPAP has been documented by Syyed et al11 in 65 subjects (of whom 47 had PH of various causes) who were retrospectively studied by micromanometer-tipped PA catheters. A high degree of accuracy was maintained following changes in posture and activity.11 A retrospective analysis of all studies having documented pulmonary artery pressure (PAP) by using micromanometer-tipped PA catheter measurements involved a total of 166 individuals, of whom 58% had PH caused by many different conditions. It confirmed high accuracy (0 mm Hg mean bias) and reasonably good precision (3 mm Hg SD of the bias) of our empirical mPAP estimate, with sPAP explaining 98% of mPAP variance.12 The strong linear relationship between mPAP and sPAP has also been documented in patients with precapillary PH performing moderate to vigorous supine cycling.13

Two large-scale studies using fluid-filled PA catheters have documented the mPAP vs sPAP empirical relationship in patients with left-sided heart disease.14,15 The corresponding equation was identical14 or remarkably similar15 to ours10 (Table 1), although signal distortions are known to be unavoidable when using conventional catheters for pulsatile pressure analysis. Overall, the strong link between mPAP and sPAP has been documented over a wide range of mPAPs, diastolic PAP (dPAP), heart rate, cardiac output, PA wedge pressure, and causes of PH and irrespective of patient’s sex, age, and BMI.1015

Thus, in an attempt to predict mPAP, one pressure is enough, namely sPAP (single-pressure model). Thus, one does not necessarily need to know the dPAP value and then apply either the classic rule of thumb (two-thirds dPAP + one-third sPAP) or the proposed geometric mean of sPAP and dPAP12 (dual-pressure models). From a classic physiologic point of view, this link between the steady and the pulsatile components of PA pressure was unexpected, and such a link is less marked as compared with the systemic counterpart.10 The strong linear relationship between mPAP and sPAP may be viewed as a new property of pulmonary circulation. Before discussing the clinical and pathophysiological implications, some special patient populations must be individualized.

A fluid-filled pressure catheter study has suggested that our empirical equation applies in distal (nonoperable) CTEPH, whereas it has to be adapted in proximal (operable) CTEPH given higher sPAP than that predicted at a given mPAP.16 However, the reanalysis of the only two previous studies having documented individual PAP values with micromanometer-tipped pressure catheters in CTEPH10,11 does not confirm this new proposal.17 Furthermore, if one applies our equation to the largest hemodynamic database of 1,000 patients with CTEPH undergoing pulmonary endarterectomy18 and whose mPAP is 46.1 ± 11.4 mm Hg and sPAP is 75.7 ± 18.8 mm Hg, the mPAP is accurately predicted by the empirical equation (48.2 mm Hg), with a 2.1-mm Hg mean bias (4.5%), which seems clinically acceptable. The alternative formula proposed16 performed less well (mean bias of 3.8 mm Hg, 8.2%). Although the possibility that some patients with markedly increased wave reflection may significantly deviate from our model cannot be excluded, especially when fluid-filled PA catheters are used,17 it may be concluded from the above arguments that the empirical equation applies reasonably well in CTEPH. A similar conclusion was reached by Syyed et al,11 who have compared the sPAP vs mPAP relationship between patients with and without CTEPH and concluded that “it was clear that in qualitative terms there was little difference.”

Applying the same empirical formula to all forms of PH, including CTEPH, may appear at variance with the widely admitted notion that PAP pulsatility19,20 and wave reflection21,22 are increased in CTEPH, especially as compared with patients with idiopathic PAH (iPAH). Because the PA pressure pulsatility necessarily increases with mPAP because of the pressure-dependent decreases in PA compliance, sPAP must be compared in PH patient groups having similar mPAP. This hampers the rationale analysis of previous hemodynamic studies with marked mPAP differences between patient groups.19,20 To our knowledge, four fluid-filled pressure catheter studies performed in patients with CTEPH and iPAH allow comparing their sPAP at similar mPAP levels.16,2325 The four studies have documented similar sPAP in the two groups (Table 2). The same was also confirmed by our reanalysis of the only two micromanometer-tipped PA catheter studies available (Table 2). Thus, against general belief, it may be concluded that patients with CTEPH and iPAH matched for mPAP exhibit essentially the same sPAP. When observed, a higher PA pulse pressure in CTEPH may be mainly explained by lower dPAP, a point that deserves further confirmatory studies.

Table Graphic Jump Location
TABLE 2 ]  Hemodynamic Data From Studies Having Used Fluid-Filled PA Catheters and High-Fidelity Pulmonary Artery Pressure Catheters in CTEPH and in iPAH

Fluid-filled PA catheter data and statistics are as published. High-fidelity PA catheter data and statistics have been calculated based on the published individual PAP values, and only patients with mPAP ≥ 25 mm Hg were included in the analysis. PAPs are mean ± SD. iPAH = idiopathic pulmonary arterial hypertension; NS = not significant. See Table 1 legend for expansion of other abbreviations.

In patients with acute pulmonary embolism, PH is often associated with a “ventricularization” of the PAP curve (ie, disproportionately low dPAP), especially in cases of massive proximal pulmonary embolism.26,27 This is also observed in patients with pulmonary valve insufficiency. It has been suggested that differences in the dPAP level may well impact the accuracy of our empirical formula, especially when pulmonary blood flow conditions vary.28 However, if one reanalyzes the hemodynamic database of individual sPAP and mPAP values of patients with acute massive pulmonary embolism published in the seminal article by Miller and Sutton,29 mean bias between the mPAP predicted from sPAP (25.4 ± 4.3 mm Hg) and the mPAP measured (26.6 ± 4.7 mm Hg) was −1.2 ± 2.7 mm Hg. This suggests accurate sPAP estimation in patients with acute pulmonary embolism, a point that deserves to be confirmed in further studies.

The strong linear link between sPAP and mPAP has been confirmed in young subjects with congenital heart diseases studied by using fluid-filled pressure catheters.30,31 Although the empirical formula was accurate (mean bias = 0 mm Hg), its low precision (SD of the bias = 6 mm Hg) favors the use of empirical formulas relying on sPAP and dPAP (two-pressure model) in children with congenital heart diseases.31

Overall, what can be inferred from these observations? To quote the French philosopher and scientist Blaise Pascal (1623-1662): “When we do not know the truth of a thing, it is of advantage that there should exist a common error which determines the mind of man.”

Up to now, in Doppler studies, the sPAP threshold used as an estimate of an mPAP of 25 mm Hg is variable, as discussed in the introduction. Because the cutoff values applied to large populations must be evidence based, results from invasive studies performed over the past decade may be helpful. Using our empirical equation,10 and the slightly modified equation proposed by Syyed et al11 as well, an mPAP of 25 mm Hg corresponds to an sPAP of 38 mm Hg with 0 mm Hg mean bias, as previously discussed.10,32

Our results must be carefully used to rule in or rule out PH diagnosis with Doppler echocardiography, as a fixed sPAP threshold with a “yes” or “no” answer certainly does not apply.4,5 It is outside the scope of the present article to comment on the other aspects of sPAP Doppler calculation, advantages, and drawbacks.4,5 The accuracy of the sPAP-derived mPAP estimate will mathematically depend on the accuracy of the sPAP estimate, and we share the various remarks made by Rudski4 and Rich5 concerning the widely documented limits and uncertainty of echocardiographic sPAP estimates. A multiparameter echocardiographic approach must be favored,48 which includes sPAP/mPAP estimation, the analysis of RV wall thickness, cavity dimension and fractional area change, tricuspid annulus plane systolic excursion, curvature of the interventricular septum, and mean velocity and acceleration time of PA flow, among others.

Finally, the redundancy between sPAP and mPAP has been shown useful to cross-check the Doppler PAP database and to improve the estimation of pulmonary vascular resistance in adults.33,34 The mPAP has been estimated from Doppler-derived sPAP obtained both at rest and on exercise.35

The mPAP reflects the steady component of the circuit and the functional status of the distal (resistive) pulmonary vasculature.3638 On the other hand, for a given mPAP, sPAP relates to the pulsatile component of the circuit, which includes the characteristics of right ventricular ejection and the characteristics of the proximal (elastic) pulmonary arteries and wave reflections.21,38,39 Therefore, the fact that mPAP and sPAP may be used interchangeably to study pulmonary circulation is not intuitive. From a phenomenological point of view, it has been proposed that the PAP curve may slightly deviate from a perfect parabolic pressure curve, where mean pressure value is 0.67 peak value.12 As far as physiologic modeling is concerned, it is admitted that the steady and pulsatile components of PA load are necessarily coupled, as PA compliance (1/PAstiffness) decreases when the mPAP increases.21,36,40 In chronically pulmonary hypertensive calves, major changes in elasticity and right ventricular pulsatile load are primarily due to an increase in mPAP, whereas the increases in PA wall thickness do not separately produce any measurable changes in arterial distensibility.40 This may suggest that the prevailing mPAP has a greater influence on PA compliance/stiffness than the disease-related changes in vascular wall properties.10,11,22

Pulmonary circulation may be characterized using a two-element windkessel model composed of pulmonary vascular resistance (R) and total pulmonary arterial compliance (C) disposed in parallel, with C being most often estimated by using the stroke volume over PA pulse pressure ratio.39 It has been suggested that the time constant of the pulmonary arterial windkessel (RC = R × C product) was constant in health and diseases.39 It has been hypothesized that the empirical equation relating sPAP and mPAP may result from the RC constancy.39,41,42 However, studies have challenged the RC constancy paradigm.4346 Patients with high PA wedge pressure exhibit shorted RC values.43,44 Because our empirical equation is observed irrespective of PA wedge pressure,1014 this may well imply that it applies irrespective of the RC value. Further studies are needed to solve this issue.

Mean PAP is a flow-dependent variable. Despite well-known limitations of the model,3638,47 the mPAP-cardiac output linear relationship is the most popular way to describe pulmonary circulation. Studies from our group have documented major redundancy between mPAP on the one hand and sPAP,10,12 PA mean ejection pressure (Pej = 1.25 mPAP),48 and PA end-ejection pressure or notch pressure (Pnotch = mPAP)49 on the other hand. Such pressure redundancy has been also documented on exercise11,13,48 and during the Valsalva maneuver.49 Thus, it may be hypothesized that a single pressure-powered function is reflected in any of these four pressures (mPAP, sPAP, Pej, Pnotch), all reflecting the steady component of arterial load. As a result, it may be kept in mind that there exists a family of pressure-flow relationships using any of these four pressures. Conceptually, each representation is similar, in that the abscissa is mean cardiac output and the ordinate is some representative measure of the steady pressure component opposing to flow (mPAP, sPAP, Pej, Pnotch). This approach may be useful to describe the steady-state, time-independent coupling between the ventricle and its load.50 Both dPAP and PA pulse are also linearly related to mPAP,1013 with mPAP explaining approximately 90% of variance of the two pressures. However, the linear relationship appears slightly less strong, such that the small remaining part of dPAP and PA pulse pressure variance may be explained by mechanisms related to the pulsatile component of PA load.

Right-sided heart catheter is the gold standard technique for measuring mPAP and confirming PH.51 However, it must be kept in mind that mPAP may change spontaneously, with the amount of variation (coefficient of variation) in mPAP averaging 8% over 6 h in patients with PH.52 A small number of patients may deviate from our model, and thus there is also a need to increase the precision of the sPAP-derived mPAP estimate (ie, to decrease the SD of the bias). In this respect, it must be noted that both sPAP and mPAP values slightly depend upon the patient’s age,9,53,54 and the role of BMI has also been discussed.9,54 Although the accuracy of our empirical equation has been documented irrespective of taking into account patient’s clinical characteristics,1014 further studies are needed to test the hypothesis that refined empirical equations taking into account patient’s age and BMI may increase the precision of the mPAP estimate. Finally, the sPAP vs mPAP relationship remains to be documented in patients with PH with unclear multifactorial mechanisms (group 5).3

The current recommendations mandate that confirmation of PH be made by right-sided heart catheterization. Over the past decade, numerous invasive studies have documented that sPAP and mPAP may be used interchangeably to study pulmonary circulation in adults (mPAP = 0.61 sPAP + 2 mm Hg), with high accuracy and reasonably good precision in patients with PH, including CTEPH (Table 3). Physiologic hypotheses to explain this relationship may involve the major dependence of PA compliance upon mPAP. The 25 mm Hg threshold used to define PH corresponds to an sPAP of 38 mm Hg. Further invasive studies are needed to improve the precision of the sPAP-derived mPAP estimate.

Table Graphic Jump Location
TABLE 3 ]  Salient Points

dPAP = diastolic pulmonary artery pressure. See Table 1 and 2 legends for expansion of other abbreviations.

Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

CTEPH

chronic thromboembolic pulmonary hypertension

dPAP

diastolic pulmonary artery pressure

iPAH

idiopathic pulmonary arterial hypertension

mPAP

mean pulmonary artery pressure

PA

pulmonary artery

PAH

pulmonary arterial hypertension

PAP

pulmonary artery pressure

Pej

pulmonary artery mean ejection pressure

PH

pulmonary hypertension

Pnotch

pulmonary artery notch pressure

sPAP

systolic pulmonary artery pressure

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Tedford RJ, Hassoun PM, Mathai SC, et al. Pulmonary capillary wedge pressure augments right ventricular pulsatile loading. Circulation. 2012;125(2):289-297. [CrossRef] [PubMed]
 
Tedford RJ. Determinants of right ventricular afterload (2013 Grover Conference series). Pulm Circ. 2014;4(2):211-219. [CrossRef] [PubMed]
 
Chemla D, Creuzé N, Hoette S, et al. Relationship between the pulsatile and steady component of right ventricular aftertload in patients with precapillary pulmonary hypertension. A1899. Paper presented at: American Thoracic Society International Conference; May 16-21, 2014; San Diego, CA.
 
Hadinnapola C, Pepke-Zaba J, Toshner M. The time constant of the pulmonary circulation is not so constant. A4717. Paper presented at: American Thoracic Society International Conference; 2014; May 16-21, San Diego, CA.
 
Naeije R. Pulmonary vascular resistance. A meaningless variable? Intensive Care Med. 2003;29(4):526-529. [PubMed]
 
Chemla D, Castelain V, Zhu K, et al. Estimating right ventricular stroke work and the pulsatile work fraction in pulmonary hypertension. Chest. 2013;143(5):1343-1350. [CrossRef] [PubMed]
 
Chemla D, Hébert JL, Coirault C, Salmeron S, Zamani K, Lecarpentier Y. Matching dicrotic notch and mean pulmonary artery pressures: implications for effective arterial elastance. Am J Physiol. 1996;271(4 pt 2):H1287-H1295. [PubMed]
 
Schroff SG, Weber KT, Janicki JS. Coupling of the left ventricle with the systemic arterial circulation.. In:Nichols WW, O’Rourke MF., eds. McDonald’s Blood Flow in Arteries. London, England: Arnold; 1990:343-359.
 
Hoeper MM, Bogaard HJ, Condliffe R, et al. Definitions and diagnosis of pulmonary hypertension. J Am Coll Cardiol. 2013;62(suppl 25):D42-D50. [CrossRef] [PubMed]
 
Rich S, D’Alonzo GE, Dantzker DR, Levy PS. Magnitude and implications of spontaneous hemodynamic variability in primary pulmonary hypertension. Am J Cardiol. 1985;55(1):159-163. [CrossRef] [PubMed]
 
Kovacs G, Berghold A, Scheidl S, Olschewski H. Pulmonary arterial pressure during rest and exercise in healthy subjects: a systematic review. Eur Respir J. 2009;34(4):888-894. [CrossRef] [PubMed]
 
McQuillan BM, Picard MH, Leavitt M, Weyman AE. Clinical correlates and reference intervals for pulmonary artery systolic pressure among echocardiographically normal subjects. Circulation. 2001;104(23):2797-2802. [CrossRef] [PubMed]
 

Figures

Tables

Table Graphic Jump Location
TABLE 1 ]  Empirical Formulas Describing the Linear Relationship Between mPAP and sPAP

C = control subjects; CTEPH = chronic thromboembolism pulmonary hypertension (group 4); F = female; HPREF = chronic heart failure of reduced left ventricular ejection fraction; M = male; mPAP = mean pulmonary artery pressure; NA = not indicated or impossible to extrapolate from the subgroup data; No PH = mean pulmonary artery pressure < 25 mm Hg; PA = pulmonary artery; PAH = pulmonary arterial hypertension (group 1); PH = pulmonary hypertension (ie, mPAP ≥ 25 mm Hg); PH-HD = pulmonary hypertension due to heart diseases (group 2); PH-LD/H = pulmonary hypertension due to lung diseases/hypoxia (group 3); sPAP = systolic pulmonary artery pressure.

a 

Indicates a value estimated from the corresponding figure in the article.

Table Graphic Jump Location
TABLE 2 ]  Hemodynamic Data From Studies Having Used Fluid-Filled PA Catheters and High-Fidelity Pulmonary Artery Pressure Catheters in CTEPH and in iPAH

Fluid-filled PA catheter data and statistics are as published. High-fidelity PA catheter data and statistics have been calculated based on the published individual PAP values, and only patients with mPAP ≥ 25 mm Hg were included in the analysis. PAPs are mean ± SD. iPAH = idiopathic pulmonary arterial hypertension; NS = not significant. See Table 1 legend for expansion of other abbreviations.

Table Graphic Jump Location
TABLE 3 ]  Salient Points

dPAP = diastolic pulmonary artery pressure. See Table 1 and 2 legends for expansion of other abbreviations.

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Tedford RJ. Determinants of right ventricular afterload (2013 Grover Conference series). Pulm Circ. 2014;4(2):211-219. [CrossRef] [PubMed]
 
Chemla D, Creuzé N, Hoette S, et al. Relationship between the pulsatile and steady component of right ventricular aftertload in patients with precapillary pulmonary hypertension. A1899. Paper presented at: American Thoracic Society International Conference; May 16-21, 2014; San Diego, CA.
 
Hadinnapola C, Pepke-Zaba J, Toshner M. The time constant of the pulmonary circulation is not so constant. A4717. Paper presented at: American Thoracic Society International Conference; 2014; May 16-21, San Diego, CA.
 
Naeije R. Pulmonary vascular resistance. A meaningless variable? Intensive Care Med. 2003;29(4):526-529. [PubMed]
 
Chemla D, Castelain V, Zhu K, et al. Estimating right ventricular stroke work and the pulsatile work fraction in pulmonary hypertension. Chest. 2013;143(5):1343-1350. [CrossRef] [PubMed]
 
Chemla D, Hébert JL, Coirault C, Salmeron S, Zamani K, Lecarpentier Y. Matching dicrotic notch and mean pulmonary artery pressures: implications for effective arterial elastance. Am J Physiol. 1996;271(4 pt 2):H1287-H1295. [PubMed]
 
Schroff SG, Weber KT, Janicki JS. Coupling of the left ventricle with the systemic arterial circulation.. In:Nichols WW, O’Rourke MF., eds. McDonald’s Blood Flow in Arteries. London, England: Arnold; 1990:343-359.
 
Hoeper MM, Bogaard HJ, Condliffe R, et al. Definitions and diagnosis of pulmonary hypertension. J Am Coll Cardiol. 2013;62(suppl 25):D42-D50. [CrossRef] [PubMed]
 
Rich S, D’Alonzo GE, Dantzker DR, Levy PS. Magnitude and implications of spontaneous hemodynamic variability in primary pulmonary hypertension. Am J Cardiol. 1985;55(1):159-163. [CrossRef] [PubMed]
 
Kovacs G, Berghold A, Scheidl S, Olschewski H. Pulmonary arterial pressure during rest and exercise in healthy subjects: a systematic review. Eur Respir J. 2009;34(4):888-894. [CrossRef] [PubMed]
 
McQuillan BM, Picard MH, Leavitt M, Weyman AE. Clinical correlates and reference intervals for pulmonary artery systolic pressure among echocardiographically normal subjects. Circulation. 2001;104(23):2797-2802. [CrossRef] [PubMed]
 
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