Assessment of segmental wall motion abnormalities using contrast two-dimensional echocardiography in awake mice

Echocardiographic assessment of the right heart in mice

Transgenic and toxic models of pulmonary arterial hypertension (PAH) are widely used to study the pathophysiology of PAH and to investigate potential therapies. Given the expense and time involved in creating animal models of disease, it is critical that researchers have tools to accurately assess phenotypic expression of disease. Right ventricular dysfunction is the major manifestation of pulmonary hypertension. Echocardiography is the mainstay of the noninvasive assessment of right ventricular function in rodent models and has the advantage of clear translation to humans in whom the same tool is used. Published echocardiography protocols in murine models of PAH are lacking. In this article, we describe a protocol for assessing RV and pulmonary vascular function in a mouse model of PAH with a dominant negative BMPRII mutation; however, this protocol is applicable to any diseases affecting the pulmonary vasculature or right heart. We provide a detailed description of animal preparation, image acquisition and hemodynamic calculation of stroke volume, cardiac output and an estimate of pulmonary artery pressure.

Anesthetic regimen for cardiac function evaluation by echocardiography in mice: comparison between ketamine, etomidate and isoflurane versus conscious state

Mice with genetic alterations are used in heart research for the extrapolation of human diseases. Echocardiography is an essential tool for evaluating cardiac and hemodynamic functions in small animals. The purpose of this study was to compare the effect of different anesthetic regimens and the conscious state on the evaluation of cardiac function by echocardiography. Mice were examined in the conscious state after three days of training, and then for a 7 min period after a single intraperitoneal injection of ketamine at 100 mg/kg, etomidate at 10, 20 or 30 mg/kg, or after inhalation of isoflurane at 1.5% with or without a short period of induction with isoflurane 3%. Intra- and inter-observer variabilities were assessed. The operator's comfort was also assessed. Heart rate, left ventricular end diastolic diameter, fraction shortening and cardiac output were measured using echocardiography. Ketamine at 5 and 7 min after induction and isoflurane at 3, 5 and 7 min after induction provided good anesthetic conditions and a quick awakening time, and did not influence cardiac performance, whereas the conscious state was associated with a non-physiological sympathetic activation and other anesthetic drugs induced a significant decrease in heart rate. Etomidate 10 mg/kg and 20 mg/kg were not enough to provide adequate anesthesia. Etomidate 30 mg/kg induced a good anesthetic condition but influenced cardiac performance and had a long awakening time. Our results indicate that ketamine and isoflurane with a short induction period are better anesthetic drugs than isoflurane without induction or etomidate for evaluating cardiac function in healthy mice.

Ultrasound- and liposome microbubble-mediated targeted gene transfer to cardiomyocytes in vivo accompanied by polyethylenimine

OBJECTIVES

Gene transfer to cardiomyocytes in vivo has received much research attention in the last decade but remains a substantial hurdle. Gene transfer using ultrasound-targeted microbubble destruction is a promising tool for gene therapy. Little data have shown the feasibility and optimization of this method for primary myocardial disease. In this study, we sought to determine the feasibility and efficiency of in vivo gene transfer to the myocardium mediated by ultrasound-targeted microbubble destruction accompanied by polyethylenimine.

METHODS

Three plasmids (luciferase reporter, red fluorescent protein reporter, and enhanced green fluorescent protein reporter) were used in this study. The ultrasound parameters were also optimized. A solution containing phosphate-buffered saline, a plasmid, plasmid complex, or polyethylenimine/plasmid, and liposome microbubbles was injected via a tail vein with (study) or without (control) transthoracic ultrasound irradiation. The efficiency of reporter gene transfer was determined by detection of luciferase activity or microscopy, and histologic investigations of the tissue specimens were performed.

RESULTS

Ultrasound-targeted microbubble destruction significantly increased luciferase activity in vivo compared to plasmids and microbubbles alone (P < .001). More importantly, the increase in transgene expression was significantly related to ultrasound-targeted microbubble destruction in the presence of polyethylenimine (P < .001). In addition, fluorescein expression was present in all sections that received ultrasound-targeted microbubble destruction. The fluorescent reporter genes and luciferase plasmid all had similar results. Regardless of ultrasound exposure, expression in other organs was close to a background level except for the liver and lung. Hematoxylin-eosin staining showed no notable myocardial injury or death in control and treated mice.

CONCLUSIONS

An atraumatic targeted gene delivery technique based on ultrasound-targeted microbubble destruction and polyethylenimine has been developed to transfect cardiomyocytes in vivo. If a suitable target gene is added, the novel technique could be highly effective in many kinds of heart disease.

Simvastatin reverses cardiac hypertrophy caused by disruption of the bradykinin 2 receptor

Bradykinin 2 receptor (B2R) deficiency predisposes to cardiac hypertrophy and hypertension. The pathways mediating these effects are not known. Two-month-old B2R knockout (KO) and wild-type (WT) mice were assigned to 4 treatment groups (n = 12-14/group): control (vehicle); nitro-L-arginine methyl ester (L-NAME) an NO synthase inhibitor; simvastatin (SIM), an NO synthase activator; and SIM+L-NAME. Serial echocardiography was performed and blood pressure (BP) at 6 weeks was recorded using a micromanometer. Myocardial eNOS and mitogen-activated protein kinase (MAPK, including ERK, p38, and JNK) protein expression were measured. Results showed that (i) B2RKO mice had significantly lower ejection fraction than did WT mice (61% +/- 1% vs. 73% +/- 1%), lower myocardial eNOS and phospho-eNOS, normal systolic BP, and higher LV mass, phospho-p38, and JNK; (ii) L-NAME increased systolic BP in KO mice (117 +/- 19 mm Hg) but not in WT mice and exacerbated LV hypertrophy and dysfunction; and (iii) in KO mice, SIM decreased hypertrophy, p38, and JNK, improved function, increased capillary eNOS and phospho-eNOS, and prevented L-NAME-induced LV hypertrophy without lowering BP. We conclude that disruption of the B2R causes maladaptive cardiac hypertrophy with myocardial eNOS downregulation and MAPK upregulation. SIM reverses these abnormalities and prevents the development of primary cardiac hypertrophy as well as hypertrophy secondary to L-NAME-induced hypertension.

Early uptake of 99mTc-C2A in the acute phase of myocardial infarction as a prognostic indicator for follow-up cardiac dysfunction

OBJECTIVE

The C2A domain of Synaptotagmin I is a molecular probe for the specific imaging of cell death. Here we test the hypothesis that the uptake of 99mTc-C2A in the acute phase of an infarction is associated with cardiac dysfunction in follow-ups.

METHODS

The left coronary artery was occluded in Sprague-Dawley rats for 0, 10, 20, and 30 min. 99mTc-C2A was injected intravenously at 2 h of reperfusion. Anterior planar images were acquired with one million counts on a gamma camera 3 h after injection. 99mTc-C2A uptake was calculated as the total counts in the left ventricle region minus blood pool signal. The in-vivo signal detected was correlated with wall motion score index at 1 and 3 weeks follow-ups measured by echocardiography.

RESULTS

99mTc-C2A uptake was higher with increased ischemic time (2244+/-852, 4054+/-1223, and 6178+/-1451 for 10, 20, and 30 min ischemia, analysis of variance P<0.001). A significant correlation was found between 99mTc-C2A uptake and wall motion score index at 1 week (R=0.800, P=0.0006) and 3 weeks (R=0.810, P=0.0008).

CONCLUSION

In this ischemia/reperfusion model, 99mTc-C2A uptake in the acute phase was associated with functional abnormality at 1 and 3 weeks. This demonstrates the potential diagnostic and prognostic value of 99mTc-C2A as a novel imaging agent.

Hypertrophy signaling during peripartum cardiac remodeling

Molecular signaling pathways that regulate peripartum cardiac remodeling are not well understood. Our objectives were to study the role of mitogen-activated protein kinases (MAPKs), protein kinase B (Akt), and endothelial nitric oxide synthase (eNOS) in mediating pregnancy and postpartum (PP) cardiac remodeling. Methods: Adult female Sprague-Dawley rats were divided into nonpregnant ( n = 5), 18 days pregnant ( n = 5), 0 days PP ( n = 7), and 14 days PP ( n = 8). Rats underwent echocardiography under sedation to measure left ventricle (LV) size and function, and Western blots were performed to measure myocardial protein expression of MAPKs (p38, JNK, ERK), Akt, and eNOS. Results: 1) During pregnancy, there was an increase in LV mass (0.62 ± 0.03 to 1.1 ± 0.04 g, P < 0.001), mass/volume ratio (0.7 ± 0.02 to 1.28 ± 0.02 g/ml, P < 0.0001), and ejection fraction (EF) (64 ± 3 to 74 ± 2%). Whereas LV mass and mass/volume ratio returned to prepregnancy values in the PP period, EF remained below normal range (53 ± 3%, P < 0.05). 2) The expression of anti-hypertrophic factors (p38, JNK, Akt) decreased during pregnancy and normalized PP, except JNK, which increased to higher than normal levels. eNOS also increased to higher than baseline levels PP. 3) Activation of p38 and JNK was directly correlated with lower LV mass/volume ratio ( r = −0.81 and −0.71, respectively; P < 0.05). Conclusion: Pregnancy is associated with physiological cardiac hypertrophy. There is rapid reversal of hypertrophy in the PP period while recovery of cardiac function is delayed, possibly related to PP upregulation of JNK. A dysregulation of MAPK signaling may be an important determinant of PP cardiac dysfunction.

Comparative Study of High-resolution Microimaging with 30-MHz Scanner for Evaluating Cardiac Function in Mice

BACKGROUND

The accurate assessment of cardiac function in mice is challenging because of their small heart size and rapid heart rate.

METHODS

We examined the usefulness of novel high-resolution echocardiography (HRE) with a 30-MHz transducer in evaluating cardiac function in 20 mice compared with conventional echocardiography (CE) with a 13-MHz transducer. The left ventricular (LV) regional wall motion (RWM), LV end-diastolic dimension, fractional shortening, anterior LV wall thickness, E/A, and myocardial performance index were assessed.

RESULTS

RWM analysis was more feasible by HRE than by CE (P < .05). Interobserver agreement in RWM analysis and correlation in LV end-diastolic dimension, fractional shortening, anterior LV wall thickness, E/A, and myocardial performance index were all better with HRE than CE.

CONCLUSIONS

HRE is superior to CE in assessing LV function in mice. HRE is potentially a useful method for accurate assessment of cardiac function in various mice models.

Echocardiographic evaluation of ventricular function in mice

Ventricular dysfunction remains a hallmark of most cardiac disease. The mouse has become an essential model system for cardiovascular biology, and echocardiography an established tool in the study of normal and genetically altered mice. This review describes the measurement of ventricular function, most often left ventricular function, by echocardiographic methods in mice. Technical limitations related to the small size and rapid heart rate in the mouse initially argued for the performance of echocardiography under anesthesia. More recently, higher frame rates and smaller probes operating at higher frequencies have facilitated imaging of conscious mice in some, but not all, experimental protocols and conditions. Ventricular function may be qualitatively and quantitatively evaluated under both conditions. Particular detail is provided for measurement under conscious conditions, and measurement under conscious and sedated or anesthestized conditions are contrasted. Normal values for echocardiographic indices for the common C57BL/6 strain are provided. Diastolic dysfunction is a critical pathophysiologic component of many disease states, and progress in the echocardiographic evaluation of diastolic function is discussed. Finally, echocardiography exists among several competing imaging technologies, and these alternatives are compared.

Validation of the wall motion score and myocardial performance indexes as novel techniques to assess cardiac function in mice after myocardial infarction

The aim of this study was to determine the feasibility and accuracy of wall motion score index (WMSI) and myocardial performance index (MPI) for measuring regional and global left ventricular (LV) function with use of high-resolution echocardiography after myocardial infarction (MI) in mice. In 48 mice, myocardial infarction was induced by ligation in the middle of the left anterior descending coronary artery. Echocardiography was performed under anesthesia at baseline and 1 mo after MI. WMSI was analyzed by a 16-segment model on short-axis views, and wall motion was scored as 1 for normal, 2 for hypokinetic, 3 for akinetic, 4 for dyskinetic, and 5 for aneurysmal. WMSI was calculated as the sum of scores divided by the total number of segments. MPI was calculated on the basis of isovolumetric contraction time (IVCT), isovolumetric relaxation time (IVRT), and ejection time (ET): MPI = (IVCT + IVRT)/ET. We measured LV ejection fraction (LVEF), end-systolic and end-diastolic volumes (ESV and EDV), fractional shortening (FS), and infarct size (IS). LVEF at 4 wk after MI was reduced at 32.8 +/- 9.0%. Linear correlation analyses showed that WMSI (1.6 +/- 0.3) correlated with LVEF (r = -0.84, P < 0.0005), FS (r = -0.43, P = 0.003), and IS (34.3 +/- 15.3%, r = 0.86, P < 0.0005). MPI (0.67 +/- 0.09) correlated with LVEF (r = -0.67, P < 0.0005) and IS (r = 0.72, P < 0.0005). MPI also correlated with mitral inflow velocity (r = -0.68, P < 0.0005) and deceleration time (r = -0.42, P = 0.003). Stepwise regression analysis revealed that WMSI was independently associated with IS. IS, FS, mitral inflow velocity, and deceleration time were independent determinants of MPI. In conclusion, echocardiographic assessments of WMSI and MPI in mice are feasible and correlate strongly with two-dimensional measurement of LV function and IS. These novel parameters provide additional noninvasive assessment of regional and global LV function in mice after MI.

Murine Echocardiography: A Practical Approach for Phenotyping Genetically Manipulated and Surgically Modeled Mice

There have now been literally hundreds of genetically manipulated mouse models developed during the past decade of cardiac research. Echocardiography is considered an extremely important tool to noninvasively assess and serially follow the phenotype of genetically and surgically altered mice. This review describes in detail the technical considerations, various routinely used methods to assess cardiac function, and some emerging techniques in the assessment of cardiac function in experimental mouse models of cardiac disease.

Influence of scanning frequency and ultrasonic contrast agent on reproducibility of left ventricular measurements in the mouse

BACKGROUND

Mice are now widely used as models of cardiovascular disease. Their small size and fast heart rates are technically challenging to echocardiography. This study examined the influence of different scanning frequencies and ultrasonic contrast agent (UCA) on the accuracy and reproducibility of measurements of left ventricular (LV) structure and function.

METHODS

Normal mouse hearts (C57BL6) were imaged at 3 different scanning frequencies before and after intravenous injection of the UCA, Optison. Coronary artery ligation mice and sham-operated controls were scanned at 10-22 MHz with and without UCA.

RESULTS

Scanning frequency had no significant effect on intraobserver or interobserver variation of LV measurements in normal mice under baseline conditions. Use of UCA significantly reduced estimated ejection fraction at 10-22 MHz compared with baseline (baseline 50.8 +/- 7.6% vs UCA 39.7 +/- 7.6%; P = .03) and significantly increased values for LV cavity dimensions (eg, LV area diastole 20.74 +/- 1.20 vs 23.23 +/- 0.98 mm 2 ; P = .002). UCA significantly reduced intraobserver and interobserver variation in LV ejection fraction.

CONCLUSIONS

Scanning frequency had no significant effect on reproducibility of LV measurements in the mouse but UCA significantly reduced interobserver variation. Use of UCA could reduce the number of mice required in any given experiment to observe a statistically significant change in LV function.

Relationship of systolic dysfunction to area at risk and infarction size after ischemia-reperfusion in mice

Whereas the extent of wall-motion abnormality (WMA) correlates well with the area at risk (AAR) and infarction size in murine models of nonreperfused myocardial infarction, this relationship is less clear in the setting of ischemia-reperfusion injury. Echocardiography was performed in mice at baseline, after left anterior descending coronary artery ligation (30 minutes) followed by reperfusion (24 hours), and after religation of the left anterior descending coronary artery. The extent of WMA before and after religation was compared with the initial infarction size measured by triphenyltetrazolium chloride and the AAR measured by fluorescent microspheres, respectively. Echocardiography showed left ventricular dilation and dysfunction after ischemia and reperfusion. WMA after religation correlated well with AAR (r2 = 0.70, P <.0001). The correlation between WMA and infarction size was incomplete (r2 = 0.59, P <.0002) in part because of underestimation of nontransmural infarcts. Echocardiography can reliably assess AAR after ischemia-reperfusion in mice; however, it does not allow for precise quantification of the small areas of necrosis that often occur in this setting.

Simvastatin preserves cardiac function in genetically determined cardiomyopathy

Endothelial dysfunction characterizes heart failure (HF). Simvastatin (Sim) increases endothelial nitric oxide (NO) independent of lipid-lowering. We evaluated the effect of Sim on cardiac function, apoptosis, and NO availability in HF. Five-month-old cardiomyopathic (CM) hamsters were divided into 2 groups: Sim (20 mg/kg, 6 weeks, n = 6) and Untreated (n = 6). Age-matched normal hamsters served as controls (n = 6). Serial echocardiograms were performed to measure LV function. Myocardial apoptosis, eNOS, and capillary density were measured at 6 weeks. Cardiomyopathic hamsters had lower LV shortening fraction (SF) compared with controls (17 +/- 3% vs 59 +/- 2%), higher LV end-diastolic volume (30 +/- 3 vs 6 +/- 2 mL/m2), and lower LV mass/volume ratio (0.5 +/- 0.04 vs 0.72 +/- 0.02 mg/ml, P < 0.001). During follow-up, SF decreased (9 +/- 2%) and LV volume increased (38 +/- 1 mL/m2) in untreated hamsters (P < 0.05 from baseline) but did not change significantly in the Sim group (P < 0.05 vs untreated). Myocardial caspase-3 activity was higher and apoptotic nuclear density was lower in Sim compared with untreated CM hamsters (0.072 +/- 0.02% vs 0.107 +/- 0.03%, P < 0.01). Myocardial capillary density was highest in the Sim group (P < 0.05). eNOS expression was not different between groups. Sim retards the progression of HF in CM hamsters. This may be related to an increase in coronary microvasculature, increase in NO availability, and decreased apoptosis.

Assessment of infarct size and myocardial function in mice using transesophageal echocardiography

BACKGROUND

Because transthoracic echocardiography (TTE) has significant limitations in assessing changes consequent to myocardial infarction (MI) in mice, we studied two novel methods to characterize such infarcts.

METHODS

Large MIs were produced by proximal left coronary artery ligation, and small MIs by distal left coronary artery ligation. Serum cardiac troponin I levels were measured 24 hours postoperatively. At 2 weeks, mice underwent transesophageal echocardiography (TEE) and TTE. Infarct sizes were determined histologically.

RESULTS

Surviving mice were classified according to infarct size. TEE identified all histologically proven large infarcts, and 4 of 5 small infarcts. TTE identified 4 of 5 large infarcts, but only 1 of 5 small infarcts. TEE-derived fractional area change, but not TTE-estimated left ventricular fractional shortening, was significantly different among large, small, and sham infarcts. Cardiac troponin I showed excellent correlation with infarct size and mortality.

CONCLUSIONS

Cardiac troponin I was found to predict infarct size and mortality, whereas TEE proved superior to TTE in determining infarct size and/or myocardial function in a murine MI model. These tools should provide more accurate assessments in preclinical studies of ischemic cardiomyopathy.

Use of echocardiography for the phenotypic assessment of genetically altered mice

Transgenic mice displaying abnormalities in cardiac development and function represent a powerful new tool for understanding molecular mechanisms underlying normal cardiovascular function and the pathophysiological bases of human cardiovascular disease. Complete cardiac evaluation of phenotypic changes in mice requires the ability to noninvasively assess cardiovascular structure and function in a serial manner. However, the small mouse heart beating at rates in excess of 500 beats/min presents unique methodological challenges. Two-dimensional and Doppler echocardiography have been recently used as effective, noninvasive tools for murine imaging, because quality images of cardiac structures and valvular flows can be obtained with newer high-frequency transthoracic transducers. We will discuss the use of echocardiography for the assessment of 1) left ventricular (LV) chamber dimensions and wall thicknesses, 2) LV mass, 3) improved endocardial border delineation using contrast echocardiography, 4) LV contractility using ejection phase indices and load-independent indices, 5) vascular properties, and 6) LV diastolic performance. Evaluation of cardiovascular performance in closed chest mice is feasible in a variety of murine models using Doppler echocardiographic imaging.

Ultrafast multiplanar determination of left ventricular hypertrophy in spontaneously hypertensive rats with single-shot spin-echo nuclear magnetic resonance imaging

OBJECTIVE

To determine whether left ventricular hypertrophy can be correctly evaluated in hypertensive rats with a new nuclear magnetic resonance (NMR) imaging modality that is relatively simple to operate and provides results of constant quality while offering a high signal-to-noise ratio. DESIGN Left ventricular mass as calculated from the NMR imaging analysis was compared with the actual left ventricular mass measured by gravimetry.

METHODS

Single-shot ultrafast spin-echo (SSFSE) imaging of hearts of Wistar-Kyoto rats and spontaneously hypertensive rats was performed at 4 T. Left ventricular mass was determined by using Simpson's rule on stacks of images acquired in systole and diastole.

RESULTS

SSFSE NMR imaging performed in systole or in diastole evaluated and quantified left ventricular hypertrophy in hearts of spontaneously hypertensive rats very similarly to gravimetry. The left ventricular mass as determined by NMR was in good accordance with the actual left ventricular weight (SEE: 30.39 and 35.86 mg for the systolic and diastolic NMR acquisitions, respectively).

CONCLUSION

Using an SSFSE sequence, high-quality NMR images of the rat heart can be generated very reliably with sufficient contrast and temporal and spatial resolution, and allow precise, non-invasive and fast characterization of left ventricular hypertrophy in a hypertensive rat model.