Page 13 - Assessing right ventricular function and the pulmonary circulation in pulmonary hypertension Onno Anthonius Spruijt

Conditions like congenital heart disease, hyperthyroidism, portal hypertension and congenital portosystemic venous shunts can also increase CO and increase mPAP [9-11]. Nevertheless, overwhelmingly patients with PH have a decreased CO as a result of the increase in PVR and RV failure.
The right ventricle in pulmonary hypertension
The increased resistance of the pulmonary vascular bed in PH patients increases the load on the RV. In order to maintain an adequate CO, the RV needs to adapt. RV adaption is a complex interplay of RV remodeling, neuro-hormonal activation, changes in myocardial metabolism and changes in coronary artery perfusion. In order to maintain cardiac output, the RV needs to generate higher pressures to overcome the increased resistance of the pulmonary vascular bed. According to Laplace law (wall stress = (pressure x radius) / (2 x wall thickness)) this increase in pressures will increase wall stress subsequently changing myocardial metabolism and activating the neurohormonal system [12]. The subsequent effects on RV remodeling are not clear-cut since also other aspects as time of onset of PH, the underlying etiology of PH and possibly genetics play a role in the process of RV adaption. Simplified, the first step in the process of remodeling is RV hypertrophy and an increase in contractility. RV hypertrophy decreases wall stress, however can increase RV diastolic stiffness [13, 14]. If, despite these adaptive changes, cardiac output cannot be maintained, the RV will dilate further increasing wall stress. Ultimately, this will lead to RV failure. Maintenance of RV systolic function is important since RV systolic function is the main predictor of survival [15-19].
Exercise intolerance in patients with pulmonary hypertension
During exercise the cardiopulmonary system is pushed to its upper limits. The increased demand for oxygen delivery to the tissues is met by an almost 4 fold increase in CO. The entire CO passes through the pulmonary circulation and the pulmonary circulation prevents a proportional increase in mPAP by vasodilatation and vessel recruitment [20].
PH patients often complain of exercise induced dyspnea and this impaired exercise tolerance is mainly due to circulatory limitations. Exercise induced dyspnea in PH patients is primarily due to the inability to increase pulmonary blood flow during exercise. This limited increase in CO is caused by an inability to increase SV and due to an abnormal chronotropic response [21, 22]. The inability to increase SV is the result of the increase in PVR and RV dysfunction [21, 23-25]. An abnormal chronotropic response is demonstrated as a decreased maximal heart rate (HR) in PH and is believed
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