Lung mechanics and exertional dyspnea in pulmonary arterial hypertension.

inspiratory capacity is often decreased in PAH, and that this is associated with a decreased aerobic exercise capacity and a shorter life expectancy. Inspiratory capacity is the difference between total lung capacity and functional residual capacity, and as such increases in the presence of expiratory airflow limitation or dynamic hyperinflation. The authors measured in 50 patients with PAH an inspiratory capacity of 87 ± 17%, which was slightly decreased compared to normal, but with a variability as indicated by large standard deviations. An inspiratory capacity above the mean value was associated with a better 5-year survival. A multivariate analysis showed that inspiratory capacity at rest like pulmonary vascular resistance, functional class, the 6-min walk distance and VO 2 max was an independent predictor of survival. However, the authors could not relate decreased resting inspiratory capacity to increased airway resistance or dynamic hyperinflation as assessed by the inspiratory capacity-to-total lung capacity ratio at rest or by the exercise-induced decrease in inspiratory capacity. Accordingly, they were hesitant about the interpretation of their results and invoked a combination of respiratory muscle weakness and insufficient diagnostic accuracy of resting inspiratory capacity. A fascinating aspect of the report by Richter et al. [8] is that it introduces inspiratory capacity as a new biomarker Shortness of breath dominates the symptomatology of pulmonary arterial hypertension (PAH). Yet these patients hyperventilate all the time: during exercise, at rest, and even during sleep. Is this related to exercise intolerance? Not clearly so. Cardiopulmonary exercise testing in PAH consistently shows an increased ventilation to CO 2 output slope but a preserved ventilatory reserve [1, 2] . The cardiopulmonary exercise test profile of PAH much resembles that of congestive heart failure (CHF) with decreased maximum (or peak) O 2 uptake (VO2 max), early lactic acidosis, decreased maximum O 2 pulse, altered chronotropic responses and depressed V O 2 kinetics, all typical of a cardiovascular limitation to aerobic exercise capacity [1, 2] . Recent studies have pointed at a decreased skeletal muscle strength in PAH [3–5] , but the clinical relevance of these observations is not clearly established [6] . Furthermore, patients with PAH generally have preserved pulmonary gas exchange and normal or near-normal lung volumes and airway resistances, which is not suggestive of a lung function limitation to exercise capacity [6, 7] . Thus aerobic exercise capacity in PAH is mainly cardiac output-limited, and thus dependent on right ventricular adaptation to increased afterload [7] . In the present issue of Respiration , Richter et al. [8] challenge this established knowledge by showing that an Published online: May 6, 2014