Differentiating Precapillary From Postcapillary Pulmonary Hypertension

Abstract

ulmonary hypertension (PH) may be postcapillary, a result of an increase in pulmonary venous pressure in left-sided heart diseases, or precapillary, caused by pulmonary vascular remodeling leading to increased pulmonary vascular resistance. Differentiation between these 2 conditions is based on whether pulmonary artery wedge pressure (PAWP) or left ventricular (LV) end-diastolic pressure (LVEDP) is elevated >15 mm Hg. 1 Accurate measurement of PAWP or LVEDP is thus crucial to avoid misclassification of patients and unnecessary or contraindicated treatments. This was discussed extensively at the World Symposium on Pulmonary Hypertension held in 2018 in Nice, France. A predominant opinion emerged that PAWP measurements should be performed at the end-diastolic and end-expiratory phases and that LVEDP should be measured directly when the accuracy of PAWP is uncertain. 2 We wonder whether this is the final word. PAWP and LVEDP are generally assumed to be interchangeable. In the absence of mitral valve disease, this should be the case because downstream from a wedged pulmonary artery catheter, blood flow is stopped, and because of the structure of pulmonary vascular tree, the fluid lumen of the catheter is de facto extended to the pulmonary veins, left atrium, and LV in diastole. However, major discrepancies between PAWP and LVEDP measurements are still reported. In a study that included 2270 patients referred for right-and left-sided heart catheterization, there was a mean difference between PAWP and LVEDP of −1.6 mm Hg (interquartile range, −15 to 12 mm Hg), and the 2 measurements were only moderately correlated. 3 Furthermore, atrial fibrillation, a history of rheumatic valve disease, and larger left atrial diameter were associated with PAWP exceeding LVEDP. As discussed by the authors, some of these discrepancies could have been related to the use of electronic mean PAWP. LVEDP is considered the gold-standard measure of LV preload. Because LVEDP is by definition end-diastolic, it would appear reasonable to measure PAWP nearest to this point in the cardiac cycle rather than averaged over the cardiac cycle. 2,4 For this purpose, PAWP can be measured as a mean A wave, just before the C wave (when visible), or alternatively QRS gated with a 130-to 200-millisecond interval summing up the phase delay between LVEDP and PAWP and electromechanical delay between depolarization and contraction 4 (Figure). QRS gating has the advantage of being useful in nonsinus rhythm as well as when the A wave is poorly visualized. 4 However, neither LVEDP nor QRS-gated PAWP measurements integrate increased V waves, which occur during systole. Because V-wave pressure is transmitted upstream to PAP during systole, assessment of PAWP only at the end of diastole will underestimate mean pulmonary venous pressure and thereby falsely increase the numerator of the pulmonary vascular resistance calculation. This is of particular concern in settings in which large V waves occur, such