Measuring Flow Hemodynamic Indices and Oxygen Consumption in Children with Pulmonary Hypertension: A Comparison of Catheterization and Phase-Contrast MRI

Measured Oxygen Consumption During Pediatric Cardiac Catheterization is More Accurate than Assumed Oxygen Consumption

When calculating cardiac index (C.I.) by the Fick method, oxygen consumption (VO2) is often unknown, so assumed values are typically used. This practice introduces a known source of inaccuracy into the calculation. Using a measured VO2 (mVO2) from the CARESCAPE E-sCAiOVX module provides an alternative that may improve accuracy of C.I. calculations. Our aim is to validate this measurement in a general pediatric catheterization population and compare its accuracy with assumed VO2 (aVO2). mVO2 was recorded for all patients undergoing cardiac catheterization with general anesthesia and controlled ventilation during the study period. mVO2 was compared to the reference VO2 (refVO2) determined by the reverse Fick method using cardiac MRI (cMRI) or thermodilution (TD) as a reference standard for measurement of C.I. when available. 193 VO2 measurements were obtained, including 71 with a corresponding cMRI or TD measure of cardiac index for validation. mVO2 demonstrated satisfactory concordance and correlation with the TD- or cMRI-derived refVO2 (ρc = 0.73, r2 = 0.63) with a mean bias of - 3.2% (SD ± 17.3%). Assumed VO2 demonstrated much weaker concordance and correlation with refVO2 (ρc = 0.28, r2 = 0.31) with a mean bias of + 27.5% (SD ± 30.0%). Subgroup analysis of patients < 36 months of age demonstrated that error in mVO2 was not significantly different from that observed in older patients. Many previously reported prediction models for assuming VO2 performed poorly in this younger age range. Measured oxygen consumption using the E-sCAiOVX module is significantly more accurate than assumed VO2 when compared to TD- or cMRI-derived VO2 in a pediatric catheterization lab.

Characterization of pulmonary arterial stiffness using cardiac MRI

Pulmonary arterial stiffness (PAS) is a pathologic hallmark of all types of pulmonary hypertension (PH). Cardiac MRI (CMR), a gold-standard imaging modality for the evaluation of pulmonary flow, biventricular morphology and function has been historically reserved for the longitudinal clinical follow-up, PH phenotyping purposes, right ventricular evaluation, and research purposes. Over the last two decades, numerous indices combining invasive catheterization and non-invasive CMR have been utilized to phenotype the character and severity of PAS in different types of PH and to assess its clinically prognostic potential with encouraging results. Many recent studies have demonstrated a strong role of CMR derived PAS markers in predicting long-term clinical outcomes and improving currently gold standard risk assessment provided by the REVEAL calculator. With the utilization of a machine learning strategies, strong diagnostic and prognostic performance of CMR reported in multicenter studies, and ability to detect PH at early stages, the non-invasive assessment of PAS is on verge of routine clinical utilization. In this review, we focus on appraising important CMR studies interrogating PAS over the last 20 years, describing the benefits and limitations of different PAS indices, and their pathophysiologic relevance to pulmonary vascular remodeling. We also discuss the role of CMR and PAS in clinical surveillance and phenotyping of PH, and the long-term future goal to utilize PAS as a biomarker to aid with more targeted therapeutic management.

Cardiac magnetic resonance haemodynamics in paediatric heart transplant patients: fick oximetry versus cardiac magnetic resonance phase contrast

BACKGROUND

Lifetime radiation exposure for paediatric orthotopic heart transplant (OHT) patients is significant with cardiac catheterisation as the dominant source. Interventional cardiac magnetic resonance is utilised to obtain simultaneous, radiation-free haemodynamics and flow/function measurements. We sought to compare invasive haemodynamic measurements and radiation exposure in traditional cardiac catheterisation, to comprehensive interventional cardiac magnetic resonance.

METHODS

Twenty-eight OHT patients who underwent 67 interventional cardiac magnetic resonance procedures at Children's National Hospital were identified. Both invasive oximetry with peripheral oxygen saturation (Fick) and cardiac magnetic resonance phase contrast measurements of pulmonary and systemic blood flow were performed. Systemic and pulmonary blood flow from the two modalities was compared using Bland-Altman, concordance analysis, and inter-reader correlation. A mixed model was implemented to account for confounding variables and repeat encounters. Radiation dosage data were collected for a contemporaneous cohort of orthotopic heart transplant patients undergoing standard, X-ray-guided catheterisation.

RESULTS

Simultaneous cardiac magnetic resonance and Fick have poor agreement in our study based on Lin's correlation coefficient of 0.68 and 0.73 for pulmonary and systemic blood flow, respectively. Bland-Altman analysis demonstrated a consistent over estimation of cardiac magnetic resonance cardiac output by Fick. The average indexed dose area product for patients undergoing haemodynamics with endomyocardial biopsy was 0.73 (SD ±0.6) Gy*m2/kg. With coronary angiography added, the indexed dose area product was 14.6 (SD ± 7.8) Gy*m2/kg.

CONCLUSIONS

Cardiac magnetic resonancemeasurements of cardiac output/index in paediatric orthotopic heart transplant patients have poor concordance with Fick estimates; however, cardiac magnetic resonance has good internal validity and inter-reader reliability. Radiation doses are small for haemodynamics with biopsy and increase exponentially with angiography, identifying a new target for cardiac magnetic resonance imaging.

Non-Invasive Cardiac Output Determination Using Magnetic Resonance Imaging and Thermodilution in Pulmonary Hypertension

Magnetic resonance imaging (MRI) can be used to measure cardiac output (CO) non-invasively, which is a paramount parameter in pulmonary hypertension (PH) patients. We retrospectively compared stroke volume (SV) obtained with MRI (SV_MRI) in six localisations against SV measured with thermodilution (TD) (SV_TD) and against each other in 24 patients evaluated in our PH centre using Bland and Altman (BA) agreement analyses, linear correlation, and intraclass correlation (ICC). None of the six tested localisations for SV_MRI reached the predetermined criteria for interchangeability with SV_TD, with two standard deviations (2SD) of bias between 24.1 mL/beat and 31.1 mL/beat. The SV_MRI methods yielded better agreement when compared against each other than the comparison between SV_MRI and SV_TD, with the best 2SD of bias being 13.8 mL/beat. The inter-observer and intra-observer ICCs for CO_MRI were excellent (inter-observer ICC between 0.889 and 0.983 and intra-observer ICC between 0.991 and 0.999). We could not confirm the interchangeability of SV_MRI with SV_TD based on the predetermined interchangeability criteria. The lack of agreement between MRI and TD might be explained because TD is less precise than previously thought. We evaluated a new method to estimate CO through the pulmonary circulation (COp) in PH patients that may be more precise than the previously tested methods.

Metalloproteinases and their inhibitors are associated with pulmonary arterial stiffness and ventricular function in pediatric pulmonary hypertension

Disturbed balance between matrix metalloproteinases (MMPs) and their respective tissue inhibitors (TIMPs) is a well-recognized pathophysiological component of pulmonary arterial hypertension (PAH). Both classes of proteinases have been associated with clinical outcomes as well as with specific pathological features of ventricular dysfunction and pulmonary arterial remodeling. The purpose of this study was to evaluate the circulating levels of MMPs and TIMPs in children with PAH undergoing the same-day cardiac magnetic resonance imaging (MRI) and right heart catheterization. Children with PAH (n = 21) underwent a same-day catheterization, comprehensive cardiac MRI evaluation, and blood sample collection for proteomic analysis. Correlative analysis was performed between protein levels and 1) standard PAH indices from catheterization, 2) cardiac MRI hemodynamics, and 3) pulmonary arterial stiffness. MMP-8 was significantly associated with the right ventricular end-diastolic volume (R = 0.45, P = 0.04). MMP-9 levels were significantly associated with stroke volume (R = -0.49, P = 0.03) and pulmonary vascular resistance (R = 0.49, P = 0.03). MMP-9 was further associated with main pulmonary arterial stiffness evaluated by relative area change (R = -0.79, P < 0.01).TIMP-2 and TIMP-4 levels were further associated with the right pulmonary artery pulse wave velocity (R = 0.51, P = 0.03) and backward compression wave (R = 0.52, P = 0.02), respectively. MMPs and TIMPs warrant further clinically prognostic evaluation in conjunction with the conventional cardiac MRI hemodynamic indices.NEW & NOTEWORTHY Metalloproteinases have been associated with clinical outcomes in pulmonary hypertension and with specific pathological features of ventricular dysfunction and pulmonary arterial remodeling. In this study, we demonstrated that plasma circulating levels of metalloproteinases and their inhibitors are associated with standard cardiac MRI hemodynamic indices and with the markers of proximal pulmonary arterial stiffness. Particularly, MMP-9 and TIMP-2 were associated with several different markers of pulmonary arterial stiffness. These findings suggest the interplay between the extracellular matrix (ECM) remodeling and overall hemodynamic status in children with PAH might be assessed using the peripheral circulating MMP and TIMP levels.

Prognostic Value of Change in Cardiac Index After Prostacyclin Initiation in Pediatric Pulmonary Hypertension

Invasive hemodynamic assessment remains the gold standard for the diagnosis of pediatric pulmonary hypertension and for longitudinal assessment of response to therapy. This analysis sought to describe the changes in hemodynamic variables after initiation of prostacyclin therapy and determine which changes bear predictive power of adverse clinical outcomes. A retrospective chart review of established patients at Cincinnati Children's Hospital with pulmonary arterial hypertension (PAH) who required prostacyclin therapy between 2004 and 2018 was performed. The baseline hemodynamic parameters at diagnosis as well as change in those parameters between initial catheterization and post-prostacyclin initiation catheterization were independent variables. Cox proportional hazard regression and recursive partitioning analysis were used to characterize which hemodynamic factors predicted the composite adverse outcome (CAO) defined as death, lung transplantation, or reverse Pott's shunt surgery. During the study period, 29 patients met inclusion criteria in which there were 7 CAOs: 4 deaths, 3 lung transplants, and 2 reverse Pott's shunts. Median time between catheterizations was 86 days and between the initiation of prostacyclin therapy and the second catheterization was 54 days. Cox regression revealed that only baseline pulmonary artery pressure (> 51 mmHg) and a failure to increase cardiac index illustrated statistically significant hazard for occurrence of the CAO (p < 0.01). These criteria significantly dichotomized the population in a Kaplan-Meier analysis into likelihoods of experiencing the CAO. While controlling for other hemodynamic variables, the absence of augmentation of cardiac index after the initiation of prostacyclin therapy is a valuable prognostic indicator of adverse PAH outcomes in pediatrics.

Elevated Interleukin-6 Levels Predict Clinical Worsening in Pediatric Pulmonary Arterial Hypertension

OBJECTIVE

To assess whether circulating interleukin-6 (IL-6) is associated with measures of disease severity and clinical worsening in pediatric pulmonary arterial hypertension (PAH).

STUDY DESIGN

IL-6 was measured by enzyme-linked immunosorbent assay in serum samples from a cross-sectional cohort from the National Heart, Lung, and Blood Institute Pulmonary Arterial Hypertension Biobank (n = 175) and a longitudinal cohort from Children's Hospital Colorado (CHC) (n = 61). Associations between IL-6, disease severity, and outcomes were studied with regression and Kaplan-Meier analysis.

RESULTS

In analyses adjusted for age and sex, each log-unit greater IL-6 was significantly associated in the Pulmonary Arterial Hypertension Biobank cohort with greater pulmonary vascular resistance indices, lower odds of having idiopathic PAH or treatment with prostacyclin, and greater odds of having PAH associated with a repaired congenital shunt. In the CHC cohort, each log-unit greater IL-6 was significantly associated with greater mean pulmonary arterial pressure over time. Kaplan-Meier analysis in the CHC cohort revealed that IL-6 was significantly associated with clinical worsening (a composite score of mortality, transplant, or palliative surgery) (P = .037).

CONCLUSIONS

IL-6 was significantly associated with worse hemodynamics at baseline and over time and may be associated with clinical worsening. IL-6 may provide a less-invasive method for disease monitoring and prognosis in pediatric PAH as well as a potential therapeutic target.

Differences in Pulmonary and Systemic Flow Measurements by Cardiac Magnetic Resonance vs Cardiac Catheterization and Relation to Collateral Flow in Single Ventricle Patients

Both cardiac magnetic resonance (CMR) and cardiac catheterization (cath) may assess patients with single ventricle physiology prior to stage II or Fontan palliation. However, development of significant aortopulmonary collaterals may invalidate assumptions of the Fick method. We compared CMR and cath flow measurements and evaluated the relation to collateral flow. This single-center study included all pre-stage II and pre-Fontan patients between 2010 and 2017 with CMR and cath within 1 month. Pulmonary (Qp) and systemic flow (Qs) by cath were calculated by Fick method. CMR Qp was calculated by total pulmonary venous flow, and Qs by total vena caval flow. Collateral flow by CMR was the difference of pulmonary vein and pulmonary artery flow. In 26 studies (16 pre-stage II and 10 pre-Fontan) in 21 patients, collateral flow was higher in pre-Fontan patients (1.8 ± 0.6 vs 0.9 ± 0.8 L/min/m², p = 0.01). Overall, CMR and cath had good agreement for Qs and Qp:Qs, with moderate correlation (r = 0.44, p = 0.02 for Qs, r = 0.48, p = 0.02 for Qp:Qs). In pre-Fontan but not in pre-stage II patients, CMR had higher Qp (mean difference - 1.71 L/min/m²) and Qp:Qs (mean difference - 0.36). The underestimation of cath Qp correlated with amount of collateral flow (r = - 0.47, p = 0.02). Neither cath nor CMR flow measurements correlated with outcomes in this small cohort. In conclusion, collaterals lead to systematically higher Qp and Qp:Qs measurements by CMR vs cath in single ventricle patients. Measurements may not be used interchangeably, with potential clinical significance in estimating pulmonary vascular resistance. Further study is necessary to evaluate possible relation to clinical outcomes.

EXPRESS: Statement on imaging and pulmonary hypertension from the Pulmonary Vascular Research Institute (PVRI)

Pulmonary hypertension (PH) is highly heterogeneous and despite treatment advances it remains a life-shortening condition. There have been significant advances in imaging technologies, but despite evidence of their potential clinical utility, practice remains variable, dependent in part on imaging availability and expertise. This statement summarizes current and emerging imaging modalities and their potential role in the diagnosis and assessment of suspected PH. It also includes a review of commonly encountered clinical and radiological scenarios, and imaging and modeling-based biomarkers. An expert panel was formed including clinicians, radiologists, imaging scientists, and computational modelers. Section editors generated a series of summary statements based on a review of the literature and professional experience and, following consensus review, a diagnostic algorithm and 55 statements were agreed. The diagnostic algorithm and summary statements emphasize the key role and added value of imaging in the diagnosis and assessment of PH and highlight areas requiring further research.

Noninvasive wave intensity analysis predicts functional worsening in children with pulmonary arterial hypertension

The purpose of the present study was to characterize pulmonary vascular stiffness using wave intensity analysis (WIA) in children with pulmonary arterial hypertension (PAH), compare the WIA indexes with catheterization- and MRI-derived hemodynamics, and assess the prognostic ability of WIA-derived biomarkers to predict the functional worsening. WIA was performed in children with PAH ( n = 40) and healthy control subjects ( n = 15) from phase-contrast MRI-derived flow and area waveforms in the main pulmonary artery (MPA). From comprehensive WIA spectra, we collected and compared with healthy control subjects forward compression waves (FCW), backward compression waves (BCW), forward decompression waves (FDW), and wave propagation speed ( c-MPA). There was no difference in the magnitude of FCW between PAH and control groups (88 vs. 108 mm⁵·s^-1·ml^-1, P = 0.239). The magnitude of BCW was increased in patients with PAH (32 vs. 5 mm⁵·s^-1·ml^-1, P < 0.001). There was no difference in magnitude of indexed FDW (32 vs. 28 mm⁵·s^-1·ml^-1, P = 0.856). c-MPA was increased in patients with PAH (3.2 vs. 1.6 m/s, P < 0.001). BCW and FCW correlated with mean pulmonary arterial pressure, right ventricular volumes, and ejection fraction. Elevated indexed BCW [heart rate (HR) = 2.91, confidence interval (CI): 1.18-7.55, P = 0.019], reduced indexed FDW (HR = 0.34, CI: 0.11-0.90, P = 0.030), and increased c-MPA (HR = 3.67, CI: 1.47-10.20, P = 0.004) were strongly associated with functional worsening of disease severity. Our results suggest that noninvasively derived biomarkers of pulmonary vascular resistance and stiffness may be helpful for determining prognosis and monitoring disease progression in children with PAH. NEW & NOTEWORTHY Wave intensity analysis (WIA) studies are lacking in children with pulmonary arterial hypertension (PAH) partially because WIA, which is necessary to assess vascular stiffness, requires an invasive pressure-derived waveform along with simultaneous flow measurements. We analyzed vascular stiffness using WIA in children with PAH who underwent phase-contrast MRI and observed significant differences in WIA indexes between patients with PAH and control subjects. Furthermore, WIA indexes were predictive of functional worsening and were associated with standard catheterization measures.