Stressing the stepchild: assessing right ventricular contractile reserve in pulmonary arterial hypertension

Right Ventricular Contractile Reserve: A Key Metric to Identifying when Cardiorespiratory Fitness will Improve with Pulmonary Vasodilators

Cardiorespiratory fitness (CRF) has been proposed as a vital sign for the past several years, supported by a wealth of evidence demonstrating its significance as a predictor of health trajectory, exercise/functional capacity, and quality of life. According to the Fick equation, oxygen consumption (VO₂) is the product of cardiac output (CO) and arterial-venous oxygen difference, with the former being a primary driver of one's aerobic capacity. In terms of the dependence of aerobic capacity on a robust augmentation of CO from rest to maximal exercise, left ventricular (LV) CO has been the historic focal point. Patients with pulmonary arterial hypertension (PAH) or secondary pulmonary hypertension (PH) present with a significantly compromised CRF; as pathophysiology worsens, so too does CRF. Interventions to improve pulmonary hemodynamics continue to emerge and are now a standard of clinical care in several patient populations with increased pulmonary pressures; new pharmacologic options continue to be explored. Improvement in CRF/aerobic capacity has been and continues to be a primary or leading secondary endpoint in clinical trials examining the effectiveness of pulmonary vasodilators. A central premise for including CRF/aerobic capacity as an endpoint is that pulmonary vasodilation will lead to a significant downstream increase in LV CO and therefore peak VO₂. However, the importance of right ventricular (RV) CO to the peak VO₂ response continues to be overlooked. The current review provides an overview of relevant principles of exercise physiology, approaches to assessing RV contractile reserve and proposals for clinical trial design and subject phenotyping.

Reduced exercise capacity in patients with systemic sclerosis is associated with lower peak tissue oxygen extraction: a cardiovascular magnetic resonance-augmented cardiopulmonary exercise study

BACKGROUND

Exercise intolerance in systemic sclerosis (SSc) is typically attributed to cardiopulmonary limitations. However, problems with skeletal muscle oxygen extraction have not been fully investigated. This study used cardiovascular magnetic resonance (CMR)-augmented cardiopulmonary exercise testing (CMR-CPET) to simultaneously measure oxygen consumption and cardiac output. This allowed calculation of arteriovenous oxygen content gradient, a recognized marker of oxygen extraction. We performed CMR-CPET in 4 groups: systemic sclerosis (SSc); systemic sclerosis-associated pulmonary arterial hypertension (SSc-PAH); non-connective tissue disease pulmonary hypertension (NC-PAH); and healthy controls.

METHODS

We performed CMR-CPET in 60 subjects (15 in each group) using a supine ergometer following a ramped exercise protocol until exhaustion. Values for oxygen consumption, cardiac output and oxygen content gradient, as well as ventricular volumes, were obtained at rest and peak-exercise for all subjects. In addition, T1 and T2 maps were acquired at rest, and the most recent clinical measures (hemoglobin, lung function, 6-min walk, cardiac and catheterization) were collected.

RESULTS

All patient groups had reduced peak oxygen consumption compared to healthy controls (p < 0.022). The SSc and SSc-PAH groups had reduced peak oxygen content gradient compared to healthy controls (p < 0.03). Conversely, the SSc-PAH and NC-PH patients had reduced peak cardiac output compared to healthy controls and SSc patients (p < 0.006). Higher hemoglobin was associated with higher peak oxygen content gradient (p = 0.025) and higher myocardial T1 was associated with lower peak stroke volume (p = 0.011).

CONCLUSIONS

Reduced peak oxygen consumption in SSc patients is predominantly driven by reduced oxygen content gradient and in SSc-PAH patients this was amplified by reduced peak cardiac output. Trial registration The study is registered with ClinicalTrials.gov Protocol Registration and Results System (ClinicalTrials.gov ID: 100358).

Exercise cardiac MRI unmasks right ventricular dysfunction in acute hypoxia and chronic pulmonary arterial hypertension

Coupling of right ventricular (RV) contractility to afterload is maintained at rest in the early stages of pulmonary arterial hypertension (PAH), but exercise may unmask depleted contractile reserves. We assessed whether elevated afterload reduces RV contractile reserve despite compensated resting function using noninvasive exercise imaging. Fourteen patients with PAH (mean age: 39.1 yr, 10 women and 4 men) and 34 healthy control subjects (mean ageL 35.6 yr, 17 women and 17 men) completed real-time cardiac magnetic resonance imaging during submaximal exercise breathing room air. Control subjects were then also exercised during acute normobaric hypoxia (fraction of inspired O₂: 12%). RV contractile reserve was assessed by the effect of exercise on ejection fraction. In control subjects, the increase in RV ejection fraction on exercise was less during hypoxia (P = 0.017), but the response of left ventricular ejection fraction to exercise did not change. Patients with PAH had an impaired RV reserve, with half demonstrating a fall in RV ejection fraction on exercise despite comparable resting function to controls (PAH: rest 53.6 ± 4.3% vs. exercise 51.4 ± 10.7%; controls: rest 57.1 ± 5.2% vs. exercise 69.6 ± 6.1%, P < 0.0001). In control subjects, the increase in stroke volume index on exercise was driven by reduced RV end-systolic volume, whereas patients with PAH did not augment the stroke volume index, with increases in both end-diastolic and end-systolic volumes. From baseline hemodynamic and exercise capacity variables, only the minute ventilation-to-CO₂ output ratio was an independent predictor of RV functional reserve (P = 0.021). In conclusion, noninvasive cardiac imaging during exercise unmasks depleted RV contractile reserves in healthy adults under hypoxic conditions and patients with PAH under normoxic conditions despite preserved ejection fraction at rest.

NEW & NOTEWORTHY Right ventricular (RV) reserve was assessed using real-time cardiac magnetic resonance imaging in patients with pulmonary arterial hypertension and in healthy control subjects under normobaric hypoxia, which has been previously associated with acute pulmonary hypertension. Hypoxia caused a mild reduction in RV reserve, whereas chronic pulmonary arterial hypertension was associated with a marked reduction in RV reserve.

Right Ventricular Functional Reserve in Pulmonary Arterial Hypertension

BACKGROUND

Right ventricular (RV) functional reserve affects functional capacity and prognosis in patients with pulmonary arterial hypertension (PAH). PAH associated with systemic sclerosis (SSc-PAH) has a substantially worse prognosis than idiopathic PAH (IPAH), even though many measures of resting RV function and pulmonary vascular load are similar. We therefore tested the hypothesis that RV functional reserve is depressed in SSc-PAH patients.

METHODS AND RESULTS

RV pressure-volume relations were prospectively measured in IPAH (n=9) and SSc-PAH (n=15) patients at rest and during incremental atrial pacing or supine bicycle ergometry. Systolic and lusitropic function increased at faster heart rates in IPAH patients, but were markedly blunted in SSc-PAH. The recirculation fraction, which indexes intracellular calcium recycling, was also depressed in SSc-PAH (0.32±0.05 versus 0.50±0.05; P=0.039). At matched exercise (25 W), SSc-PAH patients did not augment contractility (end-systolic elastance) whereas IPAH did (P<0.001). RV afterload assessed by effective arterial elastance rose similarly in both groups; thus, ventricular-vascular coupling declined in SSc-PAH. Both end-systolic and end-diastolic RV volumes increased in SSc-PAH patients to offset contractile deficits, whereas chamber dilation was absent in IPAH (+37±10% versus +1±8%, P=0.004, and +19±4% versus -1±6%, P<0.001, respectively). Exercise-associated RV dilation also strongly correlated with resting ventricular-vascular coupling in a larger cohort.

CONCLUSIONS

RV contractile reserve is depressed in SSc-PAH versus IPAH subjects, associated with reduced calcium recycling. During exercise, this results in ventricular-pulmonary vascular uncoupling and acute RV dilation. RV dilation during exercise can predict adverse ventricular-vascular coupling in PAH patients.