Metabolic Response of the Immature Right Ventricle to Acute Pressure Overloading

Right ventricular energy metabolism in a porcine model of acute right ventricular pressure overload after weaning from cardiopulmonary bypass

Acute right ventricular pressure overload (RVPO) occurs following congenital heart surgery and often results in low cardiac output syndrome. We tested the hypothesis that the RV exhibits limited ability to modify substrate utilization in response to increasing energy requirements during acute RVPO after cardiopulmonary bypass (CPB). We assessed the RV fractional contributions (Fc) of substrates to the citric acid cycle in juvenile pigs exposed to acute RVPO by pulmonary artery banding (PAB) and CPB. Sixteen Yorkshire male pigs (median 38 days old, 12.2 kg of body weight) were randomized to SHAM (Ctrl, n = 5), 2-h CPB (CPB, n = 5) or CPB with PAB (PAB-CPB, n = 6). Carbon-13 (13 C)-labeled lactate, medium-chain, and mixed long-chain fatty acids (MCFA and LCFAs) were infused as metabolic tracers for energy substrates. After weaning from CPB, RV systolic pressure (RVSP) doubled baseline in PAB-CPB while piglets in CPB group maintained normal RVSP. Fc-LCFAs decreased significantly in order PAB-CPB > CPB > Ctrl groups by 13 C-NMR. Fc-lactate and Fc-MCFA were similar among the three groups. Intragroup analysis for PAB-CPB showed that the limited Fc-LCFAs appeared prominently in piglets exposed to high RVSP-to-left ventricular systolic pressure ratio and high RV rate-pressure product, an indicator of myocardial oxygen demand. Acute RVPO after CPB strongly inhibits LCFA oxidation without compensation by lactate oxidation, resulting in energy deficiency as determined by lower (phosphocreatine)/(adenosine triphosphate) in PAB-CPB. Adequate energy supply but also metabolic interventions may be required to circumvent these RV energy metabolic abnormalities during RVPO after CPB.

Physiology of the Right Ventricle Across the Lifespan

The most common cause of heart failure in the United States is ischemic left heart disease; accordingly, a vast amount of work has been done to elucidate the molecular mechanisms underlying pathologies of the left ventricle (LV) as a general model of heart failure. Until recently, little attention has been paid to the right ventricle (RV) and it has commonly been thought that the mechanical and biochemical properties of the RV are similar to those of the LV. However, therapies used to treat LV failure often fail to improve ventricular function in RV failure underscoring, the need to better understand the unique physiologic and pathophysiologic properties of the RV. Importantly, hemodynamic stresses (such as pressure overload) often underlie right heart failure further differentiating RV failure as unique from LV failure. There are significant structural, mechanical, and biochemical properties distinctive to the RV that influences its function and it is likely that adaptations of the RV occur uniquely across the lifespan. We have previously reviewed the adult RV compared to the LV but there is little known about differences in the pediatric or aged RV. Accordingly, in this mini-review, we will examine the subtle distinctions between the RV and LV that are maintained physiologically across the lifespan and will highlight significant knowledge gaps in our understanding of pediatric and aging RV. Consideration of how RV function is altered in different disease states in an age-specific manner may enable us to define RV function in health and importantly, in response to pathology.

Increasing carbohydrate oxidation improves contractile reserves and prevents hypertrophy in porcine right heart failure

In heart failure, myocardial overload causes vast metabolic changes that impair cardiac energy production and contribute to deterioration of contractile function. However, metabolic therapy is not used in heart failure care. We aimed to investigate the interplay between cardiac function and myocardial carbohydrate metabolism in a large animal heart failure model. Using magnetic resonance spectroscopy with hyperpolarized pyruvate and magnetic resonance imaging at rest and during pharmacological stress, we investigated the in-vivo cardiac pyruvate metabolism and contractility in a porcine model of chronic pulmonary insufficiency causing right ventricular volume overload. To assess if increasing the carbohydrate metabolic reserve improves the contractile reserve, a group of animals were fed dichloroacetate, an activator of pyruvate oxidation. Volume overload caused heart failure with decreased pyruvate dehydrogenase flux and poor ejection fraction reserve. The animals treated with dichloroacetate had a larger contractile response to dobutamine stress than non-treated animals. Further, dichloroacetate prevented myocardial hypertrophy. The in-vivo metabolic data were validated by mitochondrial respirometry, enzyme activity assays and gene expression analyses. Our results show that pyruvate dehydrogenase kinase inhibition improves the contractile reserve and decreases hypertrophy by augmenting carbohydrate metabolism in porcine heart failure. The approach is promising for metabolic heart failure therapy.

Heart failure in single right ventricle congenital heart disease: physiological and molecular considerations

Because of remarkable surgical and medical advances over the past several decades, there are growing numbers of infants and children living with single ventricle congenital heart disease (SV), where there is only one functional cardiac pumping chamber. Nevertheless, cardiac dysfunction (and ultimately heart failure) is a common complication in the SV population, and pharmacological heart failure therapies have largely been ineffective in mitigating the need for heart transplantation. Given that there are several inherent risk factors for ventricular dysfunction in the setting of SV in addition to probable differences in molecular adaptations to heart failure between children and adults, it is perhaps not surprising that extrapolated adult heart failure medications have had limited benefit in children with SV heart failure. Further investigations into the molecular mechanisms involved in pediatric SV heart failure may assist with risk stratification as well as development of targeted, efficacious therapies specific to this patient population. In this review, we present a brief overview of SV anatomy and physiology, with a focus on patients with a single morphological right ventricle requiring staged surgical palliation. Additionally, we discuss outcomes in the current era, risk factors associated with the progression to heart failure, present state of knowledge regarding molecular alterations in end-stage SV heart failure, and current therapeutic interventions. Potential avenues for improving SV outcomes, including identification of biomarkers of heart failure progression, implications of personalized medicine and stem cell-derived therapies, and applications of novel models of SV disease, are proposed as future directions.

Metabolic Response to Stress by the Immature Right Ventricle Exposed to Chronic Pressure Overload

Background

The right ventricle exposed to chronic pressure overload exhibits hypertrophy and decompensates when exposed to stress. We hypothesize that impaired ability to increase myocardial oxidative flux through pyruvate dehydrogenase leads to hypertrophied right ventricular (RV) dysfunction when exposed to hemodynamic stress, and pyruvate dehydrogenase stimulation can improve RV function.

Methods and Results

Infant male Yorkshire piglets (13.5±0.6 kg weight, n=19) were used to assess substrate fractional contribution to the citric acid cycle after sustained pulmonary artery banding (PAB). Carbon 13–labeled glucose, lactate, and leucine, oxidative substrate tracers for the citric acid cycle, were infused into the right coronary artery on 7 to 10 days after PAB. RV systolic pressure, RV free wall thickness, and individual cardiomyocyte cell size after PAB were significantly elevated compared with the sham group. Both fractional glucose and lactate oxidations in the PAB group were >2‐fold higher than in the sham group. Pigs with overdrive atrial pacing (≈80% increase in heart rate) stress after PAB showed only a 22% increase in rate‐pressure product from baseline before atrial pacing and limited carbohydrate oxidation rate in the right ventricle. Intracoronary infusion of dichloroacetate, a pyruvate dehydrogenase agonist, produced higher rate‐pressure product (59% increase) in response to increased workload by atrial pacing in association with a marked increase in lactate oxidation.

Conclusions

The immature hypertrophied right ventricle shows limited ability to increase carbohydrate oxidation in response to tachycardia stress leading to energy supply/utilization imbalance and decreased systolic function. Enhanced pyruvate dehydrogenase activation by dichloroacetate increases energy supply and preserves hypertrophied RV contractile function during hemodynamic stress.

Pathophysiology, adaptation, and imaging of the right ventricle in Fontan circulation

The Fontan procedure, which creates a total cavopulmonary anastomosis and represents the final stage of palliation for hypoplastic left heart syndrome, generates a unique circulation relying on a functionally single right ventricle (RV). The RV pumps blood in series around the systemic and pulmonary circulation, which requires adaptations to the abnormal volume and pressure loads. Here, we provide a complete review of RV adaptations as the RV assumes the role of the systemic ventricle, the progression of RV dysfunction to a distinct pattern of heart failure unique to this disease process, and the assessment and management strategies used to protect and rehabilitate the failing RV of Fontan circulation.