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The time-varying elastance concept provides a comprehensive description of the intrinsic mechanical properties of the left ventricle that are assumed to be load independent. Based on pressure-volume measurements obtained with combined pressure conductance catheterization in six open-chest anesthetized sheep, we show that the time to reach end systole (defined as maximal elastance) is progressively prolonged for increasing ventricle pressures, which challenges the original (load-independent) time-varying elastance concept. Therefore, we developed a method that takes into account load dependency by normalization of time course of the four cardiac phases (isovolumic contraction, ejection, isovolumic relaxation, filling) individually. With this normalization, isophase lines are obtained that connect points in pressurevolume loops of different beats at the same relative time in each of the four cardiac phases, instead of isochrones that share points at the same time in a cardiac cycle. The results demonstrate that pressure curves can be predicted with higher accuracy, if elastance curves are estimated using isophase lines instead of using isochrones [root-meansquare error (RMSE): 3.8 [+ or -] 1.0 vs. 14.0 [+ or -] 7.4 mmHg (P < 0.001), and variance accounted for (VAF): 94.8 + 1.3 vs. 78.6 [+ or -] 14.8% (P < 0.001)]. Similar results were found when the intercept volume was assumed to be time varying [RMSE: 1.7 [+ or -] 0.3 vs. 13.4 [+ or -] 7.4 mmHg (P < 0.001), and VAF: 97.4 + 0.5 vs. 81.8 [+ or -] 15.5% (P < 0.001)]. In conclusion, phase-dependent time normalization reduces cardiac load dependency of timing and increases accuracy in estimating time-varying elastance. ventricular loading; cardiac time course; isochrones; isophase lines

Right ventricular (RV) afterload is commonly defined as pulmonary vascular resistance, but this does not reflect the afterload to pulsatile flow. The purpose of this study was to quantify RV afterload more completely in patients with and without pulmonary hypertension (PH) using a three-element windkessel model. The model consists of peripheral resistance (R), pulmonary arterial compliance (C), and characteristic impedance (Z). Using pulmonary artery pressure from right-heart catheterization and pulmonary artery flow from MRI velocity quantification, we estimated the windkessel parameters in patients with chronic thromboembolic PH (CTEPH; n = 10) and idiopathic pulmonary arterial hypertension (IPAH; n = 9). Patients suspected of PH but in whom PH was not found served as controls (NONPH; n = 10). R and Z were significantly lower and C significantly higher in the NONPH group than in both the CTEPH and IPAH groups (P < 0.001). R and Z were significantly lower in the CTEPH group than in the IPAH group (P < 0.05). The parameters R and C of all patients obeyed the relationship C = 0.75/R ([R.sup.2] = 0.77), equivalent to a similar RC time in all patients. Mean pulmonary artery pressure P and C fitted well to C = 69.7/P (i.e., similar pressure dependence in all patients). Our results show that differences in RV afterload among groups with different forms of PH can be quantified with a windkessel model. Furthermore, the data suggest that the RC time and the elastic properties of the large pulmonary arteries remain unchanged in PH. pulmonary circulation; windkessel model; pulmonary blood flow; pulmonary artery pressure

The aim of this study was to investigate the contribution of direct right-to-left ventricular interaction to left ventricular filling and stroke volume in 46 patients with pulmonary arterial hypertension (PAH) and 18 control subjects. Stroke volume, right and left ventricular volumes, left ventricular filling rate, and interventricular septum curvature were measured by magnetic resonance imaging and left atrial filling by transesophageal echocardiography. Stroke volume, left ventricular end-diastolic volume, and left ventricular peak filling rate were decreased in PAH patients compared with control subjects: 28 [+ or -] 13 vs. 41 [+ or -] 10 ml/[m.sup.2] (P < 0.001), 46 [+ or -] 14 vs. 61 [+ or -] 14 ml/[m.sup.2] (P < 0.001), and 216 [+ or -] 90 vs. 541 [+ or -] 248 ml/s (P < 0.001), respectively. Among PAH patients, stroke volume did not correlate to right ventricular end-diastolic volume or mean pulmonary arterial pressure but did correlate to left ventricular end-diastolic volume (r = 0.62, P < 0.001). Leftward interventricular septum curvature was correlated to left ventricular filling rate (r = 0.64, P < 0.001) and left ventricular end-diastolic volume (r = 0.65, P < 0.001). In contrast, left atrial filling was normal and not correlated to left ventricular end-diastolic volume. In PAH patients, ventricular interaction mediated by the interventricular septum impairs left ventricular filling, contributing to decreased stroke volume. ventricular interdependence; stroke volume

Presents a letter to the editor in response to the article "A Computed Method for Noninvasive MRI Assessment of Pulmonary Arterial Hypertension," by Eric Laffon, Christophe Vallet, Virginie Bernard, Michel Montaudon, Dominique Ducassou, François Laurent and Roger Marthan published in the August 2003 issue.