The overall goal of this procedure is to assess cardiac structure and function using echocardiographic imaging in a mouse model of heart disease induced by tranverse aortic constriction, or TAC. This method can help answer key questions in the cardiovascular research field regarding the images of heart disease in animal models. The main advantage of this technique is that it can be used to reliably monitor the progression of heart disease over time in a non-invasive manner.
Generally, introducing this method is a real struggle, because the echocardiographic imaging and measurement acquisition are challenging without real demonstration. Before beginning the imaging, apply electrode gel to the paws of the transverse aortic constricted mouse and tape the paws to an electrode pad. Then insert a rectal probe to monitor the body temperature during the procedure.
When the animal is ready, set the imager to the B mode and tilt the left side of the platform up as far as possible to rotate the mouse into the left decubitis position. Holding the ultrasound transducer by the stand in the vertical position place the probe on the animal's chest along the right parasternal line with the notch pointing towards the chin. Next, tilt the transducer up to the scapula and rotate the probe slightly clockwise until the aortic arch comes into view.
Observe the transverse aortic constriction site located between the branching of the innominate artery and the left common carotid artery. When the constriction site has been identified, click the color doppler button on the work station to switch to the color doppler mode. And acquire images of the directionality and velocity of the blood flow across the constriction site.
Click cine store to store the images. Then click the pulse wave doppler button to switch to the pulse wave doppler mode. Place the dashed cursor sample volume box immediately distal to the constriction site to find the stenotic jet with the highest velocity and click the pulse wave doppler button again to obtain wave forms of the aortic flow then measure the peak velocity and use the modified Bernoulli's equation to calculate the pressure gradient across the constriction site.
To assess the cardiac dimensions and contractility, place a mouse subjected to TAC surgery onto the platform in the supine position and position the transducer vertically with the notch pointing toward the mouse's head. Lower the transducer onto the thorax parallel to the left parasternal line and rotate the probe 30 degrees counter clockwise. Set the imager to the B mode.
Adjust the angle of the transducer and focus the depth to visualize the left ventricle, the interventricular septal wall, and a slight portion of the right ventricular wall. Click cine store to save the images for a later measurement of the cardiac wall thickness and chamber dimension. Using the cardiac package in the appropriate imaging software, select the parameters of interest, and draw the corresponding lines for each perameter in the desired echocardiographic image to obtain the cardiac wall thickness and chamber dimension measurements.
Then observe the cardiac wall movement patterns to check for any possible motion abnormalities. Next, switch to M mode and place the M mode cursor perpendicular to the left ventricle walls at the papillary muscle. Then click cine store to acquire images for later measurement of the cardiac dimensions and fractional shortening.
From the parasternal long axis view, rotate the transducer 90 degrees clockwise to obtain a parasternal short axis view. Adjust the transducer to give a horizontal cross sectional tranverse view of the heart in B mode with the papillary muscles visible in the two and four o'clock positions. Next, switch to the M mode and place the M mode axis at the middle of the left ventricle.
Click cine store to save the images. Then, using the cardiac package, select the appropriate short axis parameters and click on the image to draw the corresponding lines for each parameter to obtain the measurements. To visualize both the left and right ventricles as well as the atria from the short axis view in the B mode tilt the upper left corner of the platform to angle the mouse's head down and orient the transducer toward the right shoulder.
A coronal view of the heart will be achieved. Visualize the mitral valve. Then switch to the color doppler mode and place the dashed cursor box sample volume at the tip of the mitral valve.
To assess the flow patterns across the mitral valve, switch to the pulse wave doppler mode and align the doppler probe cursor parallel to the direction of mitral blood flow using a less than 20 degree angle to determine the peak velocity. After saving the images, choose the cardiac package and choose mitral valve flow. Select the parameters of interest and draw the corresponding lines to obtain the appropriate measurements.
Then calculate the myocardial performance index. As demonstrated in these representative B mode images, the aortic arch, innominate artery, left common carotid artery, and left subclavian artery can be observed in both Sham and TAC surgery hearts. But the aortic constriction is visible only in the TAC hearts.
Color and pulse wave doppler imaging of aortic flow across the constriction site demonstrates that a successful TAC leads to a significantly increased flow velocity down stream from the constriction site. Further, the M mode tracings of several cardiac cycles from Sham and TAC operated hearts demonstrate a significantly increased wall thickness and chamber dilation in TAC subjected hearts compared to control Sham surgery hearts. In the parasternal short axis view an increased wall thickness, ventricular dilatation, decreased contractility, and increased left ventricular mass are further evident in the TAC hearts.
More, in pathological conditions with diastolic or systolic cardiac disfunction, as in mice subjected to TAC, a decreased E and A velocity ratio and and increased myocardial performance index value are also typically observed within the apical four chamber view. Once mastered, this technique can be completed in less than thirty minutes, if it is performed properly. While attempting this procedure it is important to remember to follow the institutional guidelines on animal studies.
Following this procedure, other methods like systematic analysis can be performed to further assess the pressure volume relationship. After its development, this technique paves the way for researchers in the field of cardiovascular research to study cardiac hypertrophy and heart failure in the animal models. After watching this video, we should have a good understanding of how to assess cardiac structure and functional changes in the animal models of heart beats using endocardiographic imaging.
Don't forget that working with Isoflurane can be hazardous and that proper engineering controls and personal protective equipment should always be used while performing this procedure.
