In depth echocardiographic evaluation of the right ventricular structure and function is a valuable tool to screen the effectiveness of novel therapeutic targets in animal models of pulmonary hypertension. The main advantage of this protocol is that it provides a detailed description of all the echocardiographic views and measurements needed to assess the right atria and right ventricular morphology, as well as to characterize the right ventricle systolic and diastolic functions. This technique describes the echocardiographic characterization of a rat model of pulmonary hypertension induced by Monocrotaline.
It is important that the echocardiographer spends enough time practicing and becoming proficient in the right ventricle imaging even if they had previous experience with the left ventricle. Begin by removing the rats from the induction chamber after they lose consciousness and transfer them to the imaging station animal platform in a dorsal decubitus position. Administer isoflurane using a nose cone connected to a vaporizer.
Apply electrode gel to each paw and secure the paws in the electrocardiogram lead plates of the animal platform. Remove the fur by shaving the chest and using a depilating agent. Secure a rectal temperature probe in place.
Place cotton rolls on the right and left side of the animal and secure them with tape to maintain the position when the platform is tilted. To acquire images, tilt the platform to the right by 10 to 15 degrees and bring it caudally down approximately five degrees. Move the transducer to point to the right parasternal line of the rat, and lower the transducer.
Rotate the transducer counterclockwise to ensure the aorta and mitral valve are visible. Place the M-mode sample volume line at the region where the right ventricle is widest and adjust the gate to encompass the right and left ventricle. Lift the transducer and reposition it such that it is slightly inclined toward the right parasternal line of the rat.
Move the platform to a position that is slightly tilted to the right. Lower the transducer until it is in contact with the gel. Move the platform caudally into the right or left until the right ventricular outflow track is in view and the pulmonary valve is in focus and clearly visible.
Then press Sign Store to record the B-Mode image of the pulmonary artery valve. Maintaining the same B-Mode image location, press Color to aid the identification of flow through the pulmonary valve. Adjust the velocity so that the highest velocity point is visible.
Press PW or Pulsed Wave to quantify the blood flow spectrum. Align the PW angle parallel to the direction of the flow through the pulmonary valve. Press update to view the pulmonary velocities.
Tilt the platform to the left corner and down cranially as far as possible. Rotate the transducer counterclockwise by 30 to 45 degrees and move the transducer to point at the right shoulder or ear of the animal. Lower the transducer until it is in contact with the gel.
Ensure a typical four chamber view where the left ventricle and left atria are visible, but the sternum shadow is over the right ventricle free wall. Move the transducer for adjusting the applicable four chamber view acquiring a focused view of the right ventricle. Move the platform slightly caudally if required until the maximal plane is obtained.
Ensure that the right ventricle is not foreshortened and that the left ventricle outflow tract is not opened. Place the M-mode cursor through the tricuspid annulus at the right ventricular free wall to visualize TAPSE and press Update and Sign Store to record the TAPSE data. Press B-mode and then press Color to aid the identification of flow through the TV.Press PW to quantify the blood flow spectrum.
Return to B-mode, press tissue, and place the Tissue Doppler sample volume gate at the tricuspid annulus at the RV free wall. Press Update to view the Tissue Doppler image. Using PLAX views, initial assessment of the position of the heart and left ventricular morphology was performed.
Assessment of the right ventricle using a modified PLAX view showed that the monocrotaline or MCT-treated rats had an enlarged right ventricle and displaced left ventricle compared to the control rats. A significant increase in RVIDd, RVIDs, and RVFWT was observed in the MCT-treated rats indicating dilation of the right ventricle and thickening of the right ventricular free wall. The modified PLAX view was also used to visualize the pulmonary artery and measure the pulmonary valve diameter.
Measurement of pulmonary artery flow velocities using doppler imaging showed that in control rats, pulmonary flow exhibited a symmetrical V shape with a peak velocity in mid-systole. Whereas an MCT-treated rats, peak velocity was slower and occurred earlier in the systole and displayed a notch in late systole. The right ventricle-focused apical four chamber view was used to measure right ventricular and systolic area and right atrial area.
MCT-treated rats exhibited right atrial dilation because of increased pulmonary artery pressure. Moreover, the right ventricular pressure was increased resulting in loss of normal curvature of the left ventricle compared to controls. The intraventricular septum also appeared flattened.
The bright ventricle-focused apical four chamber view was also used to measure TAPSE from the M-mode interrogation of the tricuspid annulus. TAPSE was significantly reduced in MCT-treated rats suggesting compromised function. The most challenging part of this protocol is obtaining an optimal right ventricle-focused apical four chamber view.
We suggest that the echocardiographer ensures that the entire right ventricle is in the view during systole and diastole. Images are obtained with a right ventricle-focused apical four chamber view can be later used for a strain analysis, a powerful tool that may be used to evaluate right ventricle systolic dysfunction at the initial stages.
