Echocardiographic Assessment of Cardiac Anatomy and Function in Adult Rats

This protocol allows the assessment of cardiac anatomy and function in adult rats to facilitate the testing of hypotheses derived from the clinical setting for innovative therapy development. This non-invasive protocol allows the position of multiple offline measurements from a single animal in longitudinal studies that can be subsequently revised upon the integration of new variables. Cardiovascular disease is the leading cause of death in Europe.

A technological evolution in biomedical imaging, including echocardiography, has contributed to recent progress in cardiovascular patient care. The heart chamber size and left ventricular systolic, diastolic, and valvular function can be acquired by different echo views through the left parasternal, apical, and suprasternal windows. A combination of ultrasound imaging, Doppler flow, and tissue Doppler assessment can be used to study heart chambers and their dilatation, left ventricle hypertrophy, and left and right ventricle function.

After confirming a lack of response to pedal reflect in an anesthetized experimental adult rat, use conventional clinical echocardiographic equipment with a 12-megahertz cardiac probe to position the probe on the left side of the sternum with the index mark turned to the right shoulder. Record M-mode images at the aortic valve, mitral valve leaflets, and left ventricular mid-cavity with the M-mode cursor perpendicular to the structure of interest. Record a 2D loop at the left ventricular outflow tract.

Then, record a 2D loop with color Doppler imaging simultaneously at the aortic and mitral valves. To obtain an image at the aortic valve level, place the probe on the left side of the sternum with the index mark rotated to the left shoulder, and tilt the probe slightly cranially. Acquire a spectral pulsed Doppler image at the pulmonary artery with the cursor parallel to flow, and record a 2D loop with color Doppler imaging simultaneously at the aortic and pulmonary valves.

To obtain an image of the left ventricle at the papillary muscle level, tilt the probe slightly downward, and record a 2D loop of all of the views. To obtain an apical four-chamber view, position the probe at the apical area in the anterior axillary line with the index mark turned to left shoulder, and record a 2D loop of all of the views. Then, record a loop of 2D and tissue Doppler imaging that includes all four of the chambers.

Next, focusing on the left cardiac chambers, record a 2D loop with color Doppler imaging at the mitral valve and left atrium. Record simultaneous M-mode and color Doppler images for the left ventricular propagation flow. Acquire a spectral pulsed wave Doppler at the mitral valve for left ventricular inflow, placing the sample at the mitral leaflet tips in their fully open diastolic position.

Add a continuous wave Doppler image at the mitral valve, and obtain a spectral pulsed tissue Doppler image at the mitral annulus. Record an M-mode image of the mitral annulus to obtain a mitral annular plane systolic excursion measurement. Focusing on the right cardiac chambers, record a 2D loop with color Doppler imaging at the tricuspid valve and the right atrium.

Then, obtain a spectral pulsed tissue Doppler image at the tricuspid annulus, and place the 2D cursor at the tricuspid lateral annulus to record an M-mode image of the tricuspid annular plane systolic excursion. To obtain an apical five-chamber view from the four-chamber view, tilt the probe slightly anterior to the chest. Record a 2D loop with color Doppler imaging at the aortic valve and the left ventricular outflow tract, and place the cursor parallel to the flow with the sample at the left ventricular outflow tract to obtain a spectral pulsed wave Doppler image at the left ventricular outflow tract.

Then, acquire a spectral pulsed wave Doppler image at the left ventricle mid-cavity for simultaneous left ventricular inflow and outflow wave measurement, and obtain a spectral continuous wave Doppler image at the aortic valve. For apical two-chamber viewing, return the probe to the four-chamber view position, and rotate the probe 90 degrees counterclockwise. Then, record a 2D loop of all of the views, and record a 2D loop with color Doppler imaging at the mitral valve.

To obtain an apical three-chamber view, tilt the probe slightly cranially, and record a 2D loop with color Doppler simultaneously at the aortic and mitral valves. To obtain a suprasternal window view, position the probe on the left side of the supraclavicular space with the probe directed downward, and record a 2D loop of the aortic arch. Then, acquire a spectral pulsed wave Doppler image at the ascending and descending aortas.

The parasternal window long-axis view allows accurate measurements of the left ventricle interventricular septum thickness in diastole, left ventricle internal diameter in diastole and systole, posterior wall thickness, fractional shortening, ejection fraction, left ventricle mass, and parietal thickness. The parasternal window short-axis view allows visualization of the right ventricular outflow, the aortic valve, the pulmonary artery, and the left ventricular mid-cavity size and function. In the apical four-chamber view, all four chamber dimensions and the function of the left ventricle can be assessed.

The apical two-chamber view focuses on the left atrium and ventricular size and function. The left ventricular diastolic function can be assessed by pulsed Doppler imaging at the mitral valve. Under normal conditions, the early flow coincident with the E-wave is higher than the later flow that occurs with the atrial contraction.

Spectral tissue Doppler imaging can be used to evaluate the systolic and diastolic function over a cardiac cycle and has one positive systolic peak representing the myocardial contraction and two negative diastolic peaks that can be assessed at the mitral annular level from the septal or lateral annulus. The myocardial deformation of the left ventricle can be evaluated by analysis of the longitudinal strain in the four-chamber view. The standardization of methods and measurements is critical, as obtaining more precise echocardiographic diagnosis of animal models results in a better understanding of the molecular biology of human cardiovascular diseases.