Mitral regurgitation is a common valve disease that leads to heart failure. This rat MR model can help to further our understanding of the underlying mechanisms of heart failure. This model can be used to induce MR in a beating heart mimicking the hemodynamics observed in patients with this valve lesion and making this model clinically relevant.
Microsurgery and handling experience will help with replicating this protocol and familiarity with echocardiographic imaging and image interpretation are key aspects for successful needle placement. As this is an image-guided technique, visual demonstration of the methods can aid in understanding the procedure. After performing a left thoracotomy according to standard protocols in an anesthetized 350 to 400 gram adult Sprague-Dawley rat, use a 6-0 PROLENE suture and microneedle holder to place a pursestring suture on the apex of the left ventricle.
Gently tether the apical suture to stabilize the apex and insert a 23 gauge needle flushed with saline and with a stopcock at the distal end into the center of the pursestring suture and into the left ventricular cavity. Using one hand to stably guide the needle, use the other hand to simultaneously manipulate the transesophageal echo probe to achieve an optimal echo view for visualizing the needle. Using real-time ultrasound guidance, advance the needle toward the ventricular side of the anterior mitral leaflet.
Once the needle position is confirmed, advance the needle in one fine motion through the valve leaflet. Once through the leaflet, retract the needle into the left ventricular chamber away from the mitral valve and have an assistant turn on color Doppler imaging to confirm MR adjusting the echo probe as necessary to obtain a better view. Once the MR is confirmed, fully retract the needle from the left ventricular cavity and gently tie the pursestring suture.
Then use a sterile gauze to absorb any blood on the apex and in the thoracic cavity. After five to 10 minutes of stable cardiac function, use an interrupted 4-0 Vicryl suture to approximate the ribs while reducing the isoflurane to 2%Next, insert a chest tube into the sixth intercostal space and secure the tube to the sterile drapes to avoid inadvertent advancement of the tube into the thoracic cavity, then finish approximating the ribs. Use a continuous suture to close the muscle layer with the isoflurane at 1.5%and the skin layer with the isoflurane at 1%Connect a 10 milliliter Luer lock valve tipped syringe to the chest tube and drain 10 to 12 milliliters of air from the chest cavity before removing the tube.
Once the tube is removed, administer a final subcutaneous dose of Carprofen. After turning off the isoflurane, continue the mechanical ventilation while the rat weans from the anesthesia monitoring the vital signs. At the onset of spontaneous breathing, turn off the ventilation to test the ability of the rat to maintain such breathing and a good peripheral cavity oxygen saturation.
Once the rat is able to maintain peripheral capillary oxygen saturation levels without the ventilation, cut the anchoring sutures to the endotracheal tube and extubate the animal. When the rat is ambulatory, transfer the animal to a clean cage with minimal bedding and continue to monitor the vital signs with a handheld peripheral capillary oxygen saturation monitor. Within three hours of waking, administer buprenorphine.
Two weeks after surgery, place the anesthetized rat in the right decubitus position and insert an eight megahertz 8-French intracardiac ultrasound probe with a small amount of gel applied to the tip into the esophagus of the animal. Using color Doppler imaging in the two-chamber view, visualize the left ventricle and left atrium and measure the area of the left atrium and MR jet to allow calculation of the MR jet area fraction as indicated. To approximate the area of the regurgitant orifice, use the outer diameter of the needle to calculate the area of 23 gauge needle using the formula.
Obtain continuous wave Doppler imaging with the Doppler gate at the orifice of the regurgitant jet and trace the waveform to compute the velocity time integral of the regurgitant jet. Use the formula to estimate the MR volume. Then rotate the echo probe laterally clockwise to obtain pulse wave Doppler imaging of the pulmonary vein and measure the systolic and diastolic wave velocities using the equation to calculate the ratio.
In these images, the direction of the needle as it is inserted into the mitral valve can be observed. Here, the hole left in the mitral valve leaflet in a representative rat explanted at two weeks after the procedure is shown. At two weeks post-surgery, the hearts from rats with MR are spherical and severely dilated and a vivid MR jet is apparent with an average area of about 21 square millimeters and a mean velocity time integral of about 40 centimeters.
The normalized MR fraction is typically about 42%which is considered severe according to the guidelines of the American Society of Echocardiography. The severity of the MR is adequate to induce pulmonary flow reversal with a decrease in the systolic/diastolic ratio from about 0.9 at baseline to approximately minus 0.7 at two weeks. Real-time imaging with transesophageal echocardiography and the introduction of a needle into the beating heart to puncture the mitral leaflet is a feasible surgical procedure that can be taught.
Our model can be combined with ventricular cardiomyopathies to study cardiac remodeling and facilitates investigating of the impact of early onset versus late onset of MR on cardiac remodeling.
