Cardiovascular magnetic resonance-derived left ventricular mechanics-strain, cardiac power and end-systolic elastance under various inotropic states in swine

BACKGROUND

Cardiovascular magnetic resonance (CMR) strain imaging is an established technique to quantify myocardial deformation. However, to what extent left ventricular (LV) systolic strain, and therefore LV mechanics, reflects classical hemodynamic parameters under various inotropic states is still not completely clear. Therefore, the aim of this study was to investigate the correlation of LV global strain parameters measured via CMR feature tracking (CMR-FT, based on conventional cine balanced steady state free precession (bSSFP) images) with hemodynamic parameters such as cardiac index (CI), cardiac power output (CPO) and end-systolic elastance (Ees) under various inotropic states.

METHODS

Ten anaesthetized, healthy Landrace swine were acutely instrumented closed-chest and transported to the CMR facility for measurements. After baseline measurements, two steps were performed: (1) dobutamine-stress (Dobutamine) and (2) verapamil-induced cardiovascular depression (Verapamil). During each protocol, CMR images were acquired in the short axisand apical 2Ch, 3Ch and 4Ch views. MEDIS software was utilized to analyze global longitudinal (GLS), global circumferential (GCS), and global radial strain (GRS).

RESULTS

Dobutamine significantly increased heart rate, CI, CPO and Ees, while Verapamil decreased them. Absolute values of GLS, GCS and GRS accordingly increased during Dobutamine infusion, while GLS and GCS decreased during Verapamil. Linear regression analysis showed a moderate correlation between GLS, GCS and LV hemodynamic parameters, while GRS correlated poorly. Indexing global strain parameters for indirect measures of afterload, such as mean aortic pressure or wall stress, significantly improved these correlations, with GLS indexed for wall stress reflecting LV contractility as the clinically widespread LV ejection fraction.

CONCLUSION

GLS and GCS correlate accordingly with LV hemodynamics under various inotropic states in swine. Indexing strain parameters for indirect measures of afterload substantially improves this correlation, with GLS being as good as LV ejection fraction in reflecting LV contractility. CMR-FT-strain imaging may be a quick and promising tool to characterize LV hemodynamics in patients with varying degrees of LV dysfunction.

Cardiovascular magnetic resonance feature tracking in pigs: a reproducibility and sample size calculation study

Cardiovascular magnetic resonance feature tracking (CMR-FT) is a novel technique for non-invasive assessment of myocardial motion and deformation. Although CMR-FT is standardized in humans, literature on comparative analysis from animal models is scarce. In this study, we measured the reproducibility of global strain under various inotropic states and the sample size needed to test its relative changes in pigs. Ten anesthetized healthy Landrace pigs were investigated. After baseline (BL), two further steps were performed: (I) dobutamine-induced hyper-contractility (Dob) and (II) verapamil-induced hypocontractility (Ver). Global longitudinal (GLS), circumferential (GCS) and radial strain (GRS) were assessed. This study shows a good to excellent inter- and intra-observer reproducibility of CMR-FT in pigs under various inotropic states. The highest inter-observer reproducibility was observed for GLS at both BL (ICC 0.88) and Ver (ICC 0.79). According to the sample size calculation for GLS, a small number of animals could be used for future trials.

P5265Magnetic resonance multilayer assessment of strain in patients with heart failure

Introduction

Muscular architecture of the heart is three dimensionally complex and contractility parameters vary widely. Cardiac magnetic resonance (CMR) feature tracking is a largely available and facile method to assess myocardial strain at different layers of the myocardium.

Purpose

Assessing and compare the myocardial longitudinal (GLS) and circumferential strain (GCS) at three distinct layers of the myocardium in patients with heart failure (HF).

Methods

59 patients with a clinical diagnosis of HF who were post-hoc subdivided according to the measured EF and echo assessment of diastolic impairment into 3 groups, following ESC guidelines, were included: (1) patients with HF with preserved ejection fraction (HFpEF) where EF >50% and diastolic dysfunction (E/e' ratio) is present and plasma levels of natriuretic peptides are elevated, (2) patients with HF with mid-range ejection fraction (HFmrEF), where EF = 40–49% and similar additional criterias are present, (3) patients with HF with reduced ejection fraction (HFrEF) where EF <40%. Exclusion criteria: valvulopathy, arrhythmias, insufficient acquisition and artefacts.

Results

Strain values are the highest in the Endo− and progressively lower in the Myo− and Epi− layers with a gradient present in all groups but decreasing in HFmEF and further in HFrEF. GLS decrease with the severity of the disease in all 3 layers Normal > HFpEF > HFmrEF > HFrEF (Endo−: −23.0±3.5 vs −20.0±3.3 vs −16.4±2.2 vs −11.0±3.2, p<0.001, Myo−: −20.7±2.4 vs −17.5.0±2.6 vs −14.5±2.1 vs −9.6±2.7, p<0.001, Epi−: −15.7±1.9 vs −12.2±2.1 vs −10.6±2.3 vs −7.7±2.3, p<0.001) (Figure A), GCS is not different between the Normal and HFpEF (Endo−: −34.5±6.2 vs −33.9±5.7, p=0.51; Myo−: −21.9±3.8 vs −21.3±2.2, p=0.39; Epi−: −11.4±2.0 vs −10.9±2.3, p=0.54) but markedly lower in systolic HF groups Normal > HFmrEF > HFrEF (Endo−: −34.5±6.2 vs −20.0±4.2 vs −12.3±4.2, p<0.001; Myo−: −21.9±3.8 vs −13.0±3.4 vs −8.0±2.7, p<0.001; Epi−: −11.4±2.0 vs −7.9±2.3 vs −4.5±1.9) (Figure B). ROC analysis renders Endo− GCS (AUC=0.89) and respectively Endo− GLS (AUC=0.74) as optimal to detect contractile impairment in HF with Youden's thresholds of −20.2 for Endo− GLS and, respectively, −28.1 for Endo− GCS. Endo− GCS is not different between control and HFpEF and GLS impairment is present only inconstantly in HFpEF.

Conclusions

Feature tracking CMR successfully assess layer-specific myocardial strain and emerges as a powerful tool in functional stratification of patients with HF. Strain amplitude varies consistently throughout the myocardium and its quantification warrants careful standardization. Sub-endocardial strain values of strain are comparatively the highest and show most predictive power to detect contractile impairment. Underlying systolic impairment is present only in a subgroup of patients with HFpEF and only GLS and not the GCS is for this purpose a useful diagnostic tool.