Pulmonary heart disease: The heart-lung interaction and its impact on patient phenotypes

Pulmonary heart disease (PHD) refers to altered structure or function of the right ventricle occurring in association with abnormal respiratory function. Although nearly always associated with some degree of PH, the degree, nature, severity, and causality of PH in relation to the PHD is not necessarily linear and direct. Abnormal gas exchange is a fundamental underpinning of PHD, affecting pulmonary vascular, cardiac, renal, and neurohormonal systems. Direct and indirect effects of chronic respiratory disease can disrupt the right ventricular-pulmonary arterial (RV-PA) interaction and, likewise, factors such as sympathetic nervous system activation, altered blood viscosity, and salt and water retention can function in a feedback loop to further influence RV-PA function. Left heart function may also be affected, especially in those with pre-existing left heart disease. Thus, the physiologic interactions between abnormal respiratory and cardiovascular function are complex, with PHD representing a heterogeneous end organ effect of an integrated multisystem process. In this review, we propose to separate PHD into two distinct entities, "Type I" and "Type II" PHD. Type I PHD is most common, and refers to subjects with chronic respiratory disease (CRD) where the perturbations in respiratory function dominate over more mild cardiac and circulatory disruptions. In contrast, Type II PHD refers to the smaller subset of patients with more severe pulmonary vascular and right heart dysfunction, whom often present in a fashion similar to patients with PAH. Phenotypic differences are not made by PA pressure alone, but instead by differences in the overall physiology and clinical syndrome. Thus, key differences can be seen in symptomatology, physical signs, cardiac imaging, hemodynamics, and the cardiovascular and gas exchange responses to exercise. Such key baseline differences in the overall physiologic phenotype are likely critical to predicting response to PH specific therapy. Recognizing PHD as distinct phenotypes assists in the necessary distinction of these patients, and may also provide a key clinical and pathophysiologic framework for improved patient selection for future studies investigating the role of pulmonary hypertension-specific therapies in PHD.