Chronic coronary artery stenosis induces impaired function of remote myocardium: MRI and spectroscopy study in rat

Cardiac 31P MR spectroscopy: development of the past five decades and future vision-will it be of diagnostic use in clinics?

In the past five decades, the use of the magnetic resonance (MR) technique for cardiovascular diseases has engendered much attention and raised the opportunity that the technique could be useful for clinical applications. MR has two arrows in its quiver: One is magnetic resonance imaging (MRI), and the other is magnetic resonance spectroscopy (MRS). Non-invasively, highly advanced MRI provides unique and profound information about the anatomical changes of the heart. Excellently developed MRS provides irreplaceable and insightful evidence of the real-time biochemistry of cardiac metabolism of underpinning diseases. Compared to MRI, which has already been successfully applied in routine clinical practice, MRS still has a long way to travel to be incorporated into routine diagnostics. Considering the exceptional potential of ³¹P MRS to measure the real-time metabolic changes of energetic molecules qualitatively and quantitatively, how far its powerful technique should be waited before a successful transition from "bench-to-bedside" is enticing. The present review highlights the seminal studies on the chronological development of cardiac ³¹P MRS in the past five decades and the future vision and challenges to incorporating it for routine diagnostics of cardiovascular disease.

Assessing tissue metabolism by phosphorous-31 magnetic resonance spectroscopy and imaging: a methodology review

Many human diseases are caused by an imbalance between energy production and demand. Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) provide the unique opportunity for in vivo assessment of several fundamental events in tissue metabolism without the use of ionizing radiation. Of particular interest, phosphate metabolites that are involved in ATP generation and utilization can be quantified noninvasively by phosphorous-31 (31P) MRS/MRI. Furthermore, 31P magnetization transfer (MT) techniques allow in vivo measurement of metabolic fluxes via creatine kinase (CK) and ATP synthase. However, a major impediment for the clinical applications of 31P-MRS/MRI is the prohibitively long acquisition time and/or the low spatial resolution that are necessary to achieve adequate signal-to-noise ratio. In this review, current 31P-MRS/MRI techniques used in basic science and clinical research are presented. Recent advances in the development of fast 31P-MRS/MRI methods are also discussed.

Energy Deregulation Precedes Alteration in Heart Energy Balance in Young Spontaneously Hypertensive Rats: A Non Invasive In Vivo31P-MR Spectroscopy Follow-Up Study

Introduction

Gradual alterations in cardiac energy balance, as assessed by the myocardial PCr/ATP-ratio, are frequently associated with the development of cardiac disease. Despite great interest for the follow-up of myocardial PCr and ATP content, cardiac MR-spectroscopy in rat models in vivo is challenged by sensitivity issues and cross-contamination from other organs.

Methods

Here we combined MR-Imaging and MR-Spectroscopy (Bruker BioSpec 9.4T) to follow-up for the first time in vivo the cardiac energy balance in the SHR, a genetic rat model of cardiac hypertrophy known to develop early disturbances in cytosolic calcium dynamics.

Results

We obtained consistent ³¹P-spectra with high signal/noise ratio from the left ventricle in vivo by using a double-tuned (³¹P/¹H) surface coil. Reasonable acquisition time (<3.2min) allowed assessing the PCr/ATP-ratio comparatively in SHR and age-matched control rats (WKY): i) weekly from 12 to 21 weeks of age; ii) in response to a bolus injection of the ß-adrenoreceptor agonist isoproterenol at age 21 weeks.

Discussion

Along weeks, the cardiac PCr/ATP-ratio was highly reproducible, steady and similar (2.35±0.06) in SHR and WKY, in spite of detectable ventricular hypertrophy in SHR.

At the age 21 weeks, PCr/ATP dropped more markedly (-17.1%±0.8% vs. -3,5%±1.4%, P<0.001) after isoproterenol injection in SHR and recovered slowly thereafter (time constant 21.2min vs. 6.6min, P<0.05) despite similar profiles of tachycardia among rats.

Conclusion

The exacerbated PCr/ATP drop under ß-adrenergic stimulation indicates a defect in cardiac energy regulation possibly due to calcium-mediated abnormalities in the SHR heart. Of note, defects in energy regulation were present before detectable abnormalities in cardiac energy balance at rest.

Multilayer longitudinal strain at rest may help to predict significant stenosis of the left anterior descending coronary artery in patients with suspected non-ST-elevation acute coronary syndrome

Two-dimensional speckle tracking echocardiography (2D-STE) multilayer analysis of myocardial deformation is a non-invasive method that enables discrimination of transmural differences owing to myocardial ischemia or necrosis. We wished to ascertain if multilayer longitudinal strains at rest are associated with significant (≥70 %) stenosis of the left anterior descending coronary artery (LAD) in patients with suspected non-ST-elevation acute coronary syndrome (NSTE-ACS). Our cohort comprised 113 consecutive patients with suspected NSTE-ACS and preserved ejection fraction (EF). Using coronary angiography, we diagnosed 63 patients with significant stenosis of the LAD and 50 patients without significant coronary artery disease. Echocardiography was done ≤48 h before angiography. Multilayer longitudinal strains were assessed from the endocardium, mid-myocardium and epicardium by 2D-STE. Regional longitudinal strain in LAD territory (RLS_LAD) was calculated as the mean peak systolic longitudinal strain of segments subtended by the LAD for all myocardial layers. Significant differences were observed in all strain parameters between the two groups. RLS_LAD and global longitudinal strain in the endocardium showed higher accuracy than that in the mid-myocardium and epicardium, wall motion score index (WMSI), WMSI in LAD territory, and EF for detection of significant LAD stenosis (all P < 0.05), with areas under the receiver operating characteristic curve of 0.87 and 0.91, respectively. An endocardial RLS_LAD cutoff of -23.52 % showed optimal sensitivity and specificity (88.9/80.0 %). In patients with suspected NSTE-ACS, multilayer longitudinal strain analysis at rest might enable prediction of significant LAD stenosis, and could help to identify patients requiring reperfusion.

Small animal cardiovascular MR imaging and spectroscopy

The use of MR imaging and spectroscopy for studying cardiovascular disease processes in small animals has increased tremendously over the past decade. This is the result of the remarkable advances in MR technologies and the increased availability of genetically modified mice. MR techniques provide a window on the entire timeline of cardiovascular disease development, ranging from subtle early changes in myocardial metabolism that often mark disease onset to severe myocardial dysfunction associated with end-stage heart failure. MR imaging and spectroscopy techniques play an important role in basic cardiovascular research and in cardiovascular disease diagnosis and therapy follow-up. This is due to the broad range of functional, structural and metabolic parameters that can be quantified by MR under in vivo conditions non-invasively. This review describes the spectrum of MR techniques that are employed in small animal cardiovascular disease research and how the technological challenges resulting from the small dimensions of heart and blood vessels as well as high heart and respiratory rates, particularly in mice, are tackled.

Long-term left ventricular remodelling in rat model of nonreperfused myocardial infarction: sequential MR imaging using a 3T clinical scanner

Purpose. To evaluate whether 3T clinical MRI with a small-animal coil and gradient-echo (GE) sequence could be used to characterize long-term left ventricular remodelling (LVR) following nonreperfused myocardial infarction (MI) using semi-automatic segmentation software (SASS) in a rat model. Materials and Methods. 5 healthy rats were used to validate left ventricular mass (LVM) measured by MRI with postmortem values. 5 sham and 7 infarcted rats were scanned at 2 and 4 weeks after surgery to allow for functional and structural analysis of the heart. Measurements included ejection fraction (EF), end-diastolic volume (EDV), end-systolic volume (ESV), and LVM. Changes in different regions of the heart were quantified using wall thickness analyses. Results. LVM validation in healthy rats demonstrated high correlation between MR and postmortem values. Functional assessment at 4 weeks after MI revealed considerable reduction in EF, increases in ESV, EDV, and LVM, and contractile dysfunction in infarcted and noninfarcted regions. Conclusion. Clinical 3T MRI with a small animal coil and GE sequence generated images in a rat heart with adequate signal-to-noise ratio (SNR) for successful semiautomatic segmentation to accurately and rapidly evaluate long-term LVR after MI.

Effect of percutaneous coronary intervention on coronary blood flow at rest in myocardial sites remote from the intervention site in patients with stable angina pectoris

Little is known about changes in myocardial perfusion of myocardial regions supplied by angiographically normal or near-normal coronary arteries after percutaneous coronary intervention (PCI) of the target lesion. The purpose of this study was to assess the effect of PCI on coronary blood flow at rest in sites remote from the PCI. We studied 85 patients who underwent successful elective PCI for stable angina. We used the Thrombolysis In Myocardial Infarction frame count to provide a simple continuous index of coronary flow and myocardial perfusion in the target and nontarget arteries. Coronary artery diameters of nontarget vessels did not significantly differ before and after PCI and at 6 months' follow-up. At baseline, the greater the percent diameter stenosis in the target artery, the slower the flow in the target (r = 0.22, p <0.01) and nontarget arteries (r = 0.28, p <0.01). Relief of stenosis using PCI did not account for simultaneous changes in epicardial coronary blood flow of the nontarget artery. After 6 months, coronary blood flow improved in both the target (p <0.05) and nontarget arteries (p = 0.007). In conclusion, this study provided evidence of a functional link between coronary blood flow in diseased and nondiseased arteries. Relief of a significant stenosis using PCI globally improved regional and global myocardial blood flow at rest in patients with stable angina. Flow improvement was not apparent at the time of revascularization, but at 6 months' follow-up. Late upturn of the microcirculation may account for delayed recovery of myocardial perfusion.

Early improvement in cardiac tissue perfusion due to mesenchymal stem cells

The underlying mechanism(s) of improved left ventricular function (LV) due to mesenchymal stem cell (MSC) administration after myocardial infarction (MI) remains highly controversial. Myocardial regeneration and neovascularization, which leads to increased tissue perfusion, are proposed mechanisms. Here we demonstrate that delivery of MSCs 3 days after MI increased tissue perfusion in a manner that preceded improved LV function in a porcine model. MI was induced in pigs by 60-min occlusion of the left anterior descending coronary artery, followed by reperfusion. Pigs were assigned to receive intramyocardial injection of allogeneic MSCs (200 million, approximately 15 injections) (n = 10), placebo (n = 6), or no intervention (n = 8). Resting myocardial blood flow (MBF) was serially assessed by first-pass perfusion magnetic resonance imaging (MRI) over an 8-wk period. Over the first week, resting MBF in the infarct area of MSC-treated pigs increased compared with placebo-injected and untreated animals [0.17 +/- 0.03, 0.09 +/- 0.01, and 0.08 +/- 0.01, respectively, signal intensity ratio of MI to left ventricular blood pool (LVBP); P < 0.01 vs. placebo, P < 0.01 vs. nontreated]. In contrast, the signal intensity ratios of the three groups were indistinguishable at weeks 4 and 8. However, MSC-treated animals showed larger, more mature vessels and less apoptosis in the infarct zones and improved regional and global LV function at week 8. Together these findings suggest that an early increase in tissue perfusion precedes improvements in LV function and a reduction in apoptosis in MSC-treated hearts. Cardiac MRI-based measures of blood flow may be a useful tool to predict a successful myocardial regenerative process after MSC treatment.

In Vivo MR Coronary Artery Imaging in Mice

In vivo coronary artery imaging remains technically difficult in mice and only a few studies have been reported so far. 3D method is usually time-consuming and presents a practical limitation for MRI study of coronary arteries in mouse models of various cardiovascular diseases. The current study aims to develop a rapid and robust 2D procedure to visualize both LCA and RCA of mice in vivo. In addition, contrast agent of Gd-DPTA by rapid intraperitoneal (i.p.) injection was examined for its ability to improve the quality of LCA and RCA visualization. The current study demonstrated the feasibility of rapid MRI examination of coronary arteries of mice in vivo using 2D method and i.p. injection of contrast agent.

Mechanisms leading to reversible mechanical dysfunction in severe CAD: alternatives to myocardial stunning

Patients with severe chronic coronary artery disease (CAD) exhibit a highly altered myocardial pattern of perfusion, metabolism, and mechanical performance. In this context, the diagnosis of stunning remains elusive not only because of methodological and logistic considerations, but also because of the pathophysiological characteristics of the myocardium of these patients. In addition, a number of alternative pathophysiological mechanisms may act by mimicking the functional manifestations usually attributed to stunning. The present review describes three mechanisms that could theoretically lead to reversible mechanical dysfunction in these patients: myocardial wall stress, the tethering effect, and myocardial expression and release of auto- and paracrine agents. Attention is focused on the role of these mechanisms in scintigraphically “normal” regions (i.e., regions usually showing normal perfusion, glucose metabolism, and cellular integrity as assessed by nuclear imaging techniques), in which stunning is usually considered, but these mechanisms could also operate throughout the viable myocardium. We hypothesize that reversion of these three mechanisms could partially explain the unexpected functional benefit after reperfusion recently highlighted by high-spatial-resolution imaging techniques.

Optimization of cardiac cine in the rat on a clinical 1.5-T MR system

OBJECT

The overall goal was to study cardiovascular function in small animals using a clinical 1.5-T MR scanner optimizing a fast gradient-echo cine sequence to obtain high spatial and temporal resolution.

MATERIALS AND METHODS

Normal rat hearts (n = 9) were imaged using a 1.5-T MR scanner with a spiral fast gradient-echo (fast field echo for Philips scanners) sequence, three Cartesian fast gradient-echo (turbo field echo for Philips scanners) sequences with different in-plane resolution, and with and without flow compensation and half-Fourier acquisition. The hearts of four rats were then excised and left-ventricle mass was weighed. Inter- and intra-observer variability analysis was performed for magnetic resonance imaging (MRI) measurements.

RESULTS

Half-Fourier acquisition with flow compensation gave the best sequence in terms of image quality, spatial as well as temporal resolution, and suppression of flow artifact. Ejection fraction was 71 +/- 4% with less than 5% inter- and intra-observer variability. A good correlation was found between MRI-calculated left-ventricular mass and wet weight.

CONCLUSIONS

Using optimized sequences on a clinical 1.5-T MR scanner can provide accurate quantification of cardiac function in small animals and can promote cardiovascular research on small animals at 1.5-T.

Cardiopulmonary resuscitation in a rat model of chronic myocardial ischemia

Our group has developed a rat model of cardiac arrest and cardiopulmonary resuscitation (CPR). However, the current rat model uses healthy adult animals. In an effort to more closely reproduce the event of cardiac arrest and CPR in humans with chronic coronary disease, a rat model of coronary artery constriction was investigated during cardiac arrest and CPR. Left coronary artery constriction was induced surgically in anesthetized, mechanically ventilated Sprague-Dawley rats. Echocardiography was used to measure global cardiac performance before surgery and 4 wk postsurgery. Coronary constriction provoked significant decreases in ejection fraction, increases in left ventricular end-diastolic volume, and increases left ventricular end-systolic volume at 4 wk postintervention, just before induction of ventricular fibrillation (VF). After 6 min of untreated VF, CPR was initiated on three groups: 1) coronary artery constriction group, 2) sham-operated group, and 3) control group (without preceding surgery). Defibrillation was attempted after 6 min of CPR. All the animals were resuscitated. Postresuscitation myocardial function as measured by rate of left ventricular pressure increase at 40 mmHg and the rate of left ventricular pressure decline was more significantly impaired and left ventricular end-diastolic pressure was greater in the coronary artery constriction group compared with the sham-operated group and the control group. There were no differences in the total shock energy required for successful resuscitation and duration of survival among the groups. In summary, this rat model of chronic myocardial ischemia was associated with ventricular remodeling and left ventricular myocardial dysfunction 4 wk postintervention and subsequently with severe postresuscitation myocardial dysfunction. This model would suggest further clinically relevant investigation on cardiac arrest and CPR.

Left ventricular pressure-volume relationships during normal growth and development in the adult rat - studies in 8- and 50-week-old male Wistar rats

AIMS

Left ventricular (LV) pressure-volume relations provide relatively load-independent indexes of systolic and diastolic LV function, but few data are available on pressure-volume relations during growth and development in the normal adult heart. Furthermore, to quantify intrinsic ventricular function the indexes should be normalized for heart weight. However, in many studies the indexes are reported in absolute terms, or body weight-correction is used as a surrogate for heart weight-correction.

METHODS

We determined pressure-volume relations in young (8-week-old, n = 13) and middle-aged (50-week-old, n = 19) male Wistar rats in relation to their heart and body weights. The animals were anaesthetized and a 2F pressure-conductance catheter was introduced into the LV to measure pressure-volume relations.

RESULTS

Heart and body weights were significantly higher in the 50-week-old rats, whereas the heart-to-body weight ratio was significantly lower (2.74 +/- 0.32 vs. 4.41 +/- 0.37 mg g(-1), P < 0.001). Intrinsic systolic function, quantified by the slopes of the end-systolic pressure-volume relation (E(ES)), the dP/dt(MAX) vs. end-diastolic volume relation (S-dP), and the preload recruitable stroke work relation (PRSW), normalized for heart weight, was slightly decreased in the 50-week-old rats (S-dP: -6%, P < 0.004; PRSW: -3%, P < 0.06). Heart weight-corrected diastolic indexes were not significant different. The absolute indexes qualitatively showed the same results, but body-weight corrected pressure-volume indexes showed improved systolic function and significantly depressed diastolic function.

CONCLUSIONS

Intrinsic systolic function slightly decreases from the juvenile to the middle-aged period in normal male Wistar rats. Furthermore, correction of pressure-volume indexes for body weight is not an adequate surrogate for heart weight-correction in these animals.

Impaired resting perfusion in viable myocardium distal to chronic coronary stenosis in rats

Chronic coronary artery stenosis results in patchy necrosis in the dependent myocardium and impairs global and regional left ventricular (LV) function in rats in vivo. The aim of the present study was to compare regional myocardial blood flow (RMBF) and function (F) in poststenotic myocardium by using magnetic resonance imaging (MRI) and to compare MRI blood flow changes to histological alterations to assess whether RMBF in the viable poststenotic tissue remains normal. MRI was performed in 11 anesthetized Wistar rats with 2-wk stenosis of the left coronary artery. Postmortem, the extent of fibrotic tissue was quantified. Poststenotic RMBF was significantly reduced to 2.21 ± 0.30 ml·g−1·min−1 compared with RMBF in the remote myocardium (4.05 ± 0.50 ml·g−1·min−1). A significant relationship between the poststenotic RMBF (%remote area) and the poststenotic F (%remote myocardium) was calculated ( r = 0.61, P < 0.05). Assuming perfusion in scar tissue to be 32 ± 5% of perfusion of remote myocardium, as measured in five additional rats, and that in remote myocardium to be 114 ± 25% of that in normal myocardium, as assessed in five sham rats, the calculated perfusion in partially fibrotic tissue samples (35.7 ± 5.2% of analyzed area) was 2.88 ± 0.18 ml·g−1·min−1, whereas measured MRI perfusion was only 1.86 ± 0.24 ml·g−1·min−1 ( P < 0.05). These results indicate that resting perfusion in viable poststenotic myocardium is moderately reduced. Alterations in global and regional LV function are therefore secondary to both patchy fibrosis and reduced resting perfusion.

Current status of cardiac MRI in small animals

Cardiac magnetic resonance imaging (MRI) on small animals is possible but remains challenging and not well standardized. This publication aims to provide an overview of the current techniques, applications and challenges of cardiac MRI in small animals for researchers interested in moving into this field. Solutions have been developed to obtain a reliable cardiac trigger in both the rat and the mouse. Techniques to measure ventricular function and mass have been well validated and are used by several research groups. More advanced techniques like perfusion imaging, delayed enhancement or tag imaging are emerging. Regarding cardiac applications, not only coronary ischemic disease but several other pathologies or conditions including cardiopathies in transgenic animals have already benefited from these new developments. Therefore, cardiac MRI has a bright future for research in small animals.

Left ventricular pressure-volume relationship in a rat model of advanced aging-associated heart failure

Aging is associated with profound changes in the structure and function of the heart. A fundamental understanding of these processes, using relevant animal models, is required for effective prevention and treatment of cardiovascular disease in the elderly. Here, we studied cardiac performance in 4- to 5-mo-old (young) and 24- to 26-mo-old (old) Fischer 344 male rats using the Millar pressure-volume (P-V) conductance catheter system. We evaluated systolic and diastolic function in vivo at different preloads, including preload recruitable stroke work (PRSW), maximal slope of the systolic pressure increment (+dP/dt), and its relation to end-diastolic volume (+dP/dt-EDV) as well as the time constant of left ventricular pressure decay, as an index of relaxation. The slope of the end-diastolic P-V relation (EDPVR), an index of left ventricular stiffness, was also calculated. Aging was associated with decrease in left ventricular systolic pressure, +dP/dt, maximal slope of the diastolic pressure decrement, +dP/dt-EDV, PRSW, ejection fraction, stroke volume, cardiac and stroke work indexes, and efficiency. In contrast, total peripheral resistance, left ventricular end-diastolic volume, left ventricular end-diastolic pressure, and EDPVR were greater in aging than in young animals. Taken together, these data suggest that advanced aging is characterized by decreased systolic performance accompanied by delayed relaxation and increased diastolic stiffness of the heart in male Fischer 344 rats. P-V analysis is a sensitive method to determine cardiac function in rats.