Ratio between right ventricular strain and systolic pulmonary artery pressure as a surrogate for right ventricular to pulmonary arterial coupling: validation against the gold standard

We have read with interest the article from Unlu and co-workers exploring right ventricular (RV) mechanics using the ratio between RV strain and systolic pulmonary artery pressure (PASP) as a surrogate of RV to pulmonary arterial (PA) coupling.1 The gold-standard measure of RV-PA coupling is the ratio of RV contractility [end-systolic elastance (Ees)] to its load [arterial elastance (Ea)], obtained by conductance catheterization.2 Recently, the ratio of tricuspidal annular plane excursion (TAPSE) to PASP was validated as a non-invasive surrogate3 and has proved its prognostic relevance.4 However, this index is not without limitation and especially TAPSE is influenced by various condition and might not be the most robust or sensitive parameter to reflect RV contractility. The idea of Unlo and co-workers to replace TAPSE with a more sensitive echocardiographic parameter, such as RV free-wall longitudinal strain (FWLS), appears plausible, since the ratio of RV strain/PASP might also represent the length/force relationship and overcome certain limitations. However, the relationship of RV strain/PASP has not yet been validated. It is debatable whether proposed surrogates should be presented as measures of RV-PA coupling without any validation against the gold standard. Therefore, we think that it is of crucial importance to validate the proposed method and to assess its relation to gold standard method.