The overall goal of measuring pressure volume loops in the mouse is to obtain a detailed assessment of cardiac function. This method is useful for the physiological characterization of mice harboring specific genetic manipulations, which in turn can produce insights into the role of specific proteins in cardiac function. The main advantage of this technique is that it provides a detailed assessment of contractile function that is not available through other methodologies.
Once the mouse is prepped for surgery, begin by making an incision at the sternal notch extending from about five millimeters right of the midline to about five millimeters left of the midline. Make a second incision extending along the right edge of the first rostrally to about two millimeters coddled to the end of the mandible. Then make a third incision extending from the rostral end of the second incision to about five millimeters right of midline.
And retract the resulting skin flap to the right to expose the underlined tissues. Next, under a dissecting microscope, blunt dissect the parotide and sub-mandibular salivary glands at the midline to expose the underlining musculature overlining the trachea. Then bluntly separate the right and left sternal hyoideus muscles.
When the trachea is visible, pass a ten centimeter piece of three-zero silk suture under the throat tissue, taking care not to include the esophagus. Using a 20 gauge needle, make a wide incision in the gap just coddle to the larynx before the first tracheal ring. Then immediately remove the anesthesia mask from the animal, and carefully insert the endotracheal tube into the trachea.
Once the tube has been secured, extend the left edge of the original skin incision down to the xiphoid process and across the midline to about one point five centimeters right of the midline. Next, use blunt dissection to retract the skin flap laterally exposing the underline musculature. And use vessel dilating forceps to isolate the insertion of the pectoralis major on the right side near the coddle aspect of the sternum.
Cauterize and cut the muscle. Then cut through the pectoralis major along its attachment to the sternum. And undermine the latissimus dorsi on the same side.
Retract the cut end cranially to reveal the ribs. And use a pair of sharp forceps to carefully dissect down through the intercostal muscle layers to enter the chest. Once the plural space has been opened, carefully insert the blunt tip vessel dilators and lift the chest wall with gentle upward force.
Now, cut laterally along the intercostal muscles taking care not to cut the lung lobe and extend the incision medially, keeping three to four millimeters lateral of the midline to avoid the internal mammary artery running parallel to the sternum. Then place a saline-soaked small cotton-tipped applicator through the incision toward the midline and provide a gentle upward traction to pull the chest wall away from the underline structures. Carefully use a cautery to cut through the chest wall beginning at the medial edge of the incision and ending about one centimeter lateral of the midline on the left side.
Once the tissue has been thoroughly cauterized, use scissors to carefully cut through the sternum. The apex of the heart should be clearly visible. Then, using blunt dissection, disrupt the pericardium and identify the caudal vena cava.
To place the jugular vein catheter, orient a 30-gauge needle catheter attached to a syringe containing ten percent albumin such that the needle lies upon the jugular vein on its own, bevel side up. To visualize the jugular vein, grasp the needle with forceps in one hand and retract the salivary gland with tissue forceps in the other. Apply gentle traction to the tissue surrounding the diesel jugular vein to create tension on the vessel wall.
Then, using a shallow angle of approach, carefully insert the needle into the vein and advance the tip three to four millimeters into the vessel. When the catheter is in position, secure the needle to the underline salivary glands using surgical glue and begin infusing the albumin solution. To place the pressure volume catheter, first, move a syringe containing equilibrated saline solution and the pressure volume catheter next to the mouse with the catheter tip at roughly the same height as the heart and zero the pressure reading.
Next, using a saline-soaked small cotton-tipped applicator, maneuver the heart to visualize the apex and use a 25-gauge needle to make a stab incision as close to the center of the apex as possible. Following the removal of the needle, quickly but gently insert the catheter through the incision. The ventricular pressure volume loops can then be measured.
Following the placement of the catheter, turn the anesthesia down to illuminate the cardio dipressive effects that the anesthesia may be having. If there was excessive blood loss during the procedure, you can simply increase the volume of albumin infused. By convention, the volume is plotted on the X-axis and the pressure on the y-axis.
Pressure volume loops resulting from plotting the pressure against the volume should resemble a rectangle with the vertical edges representing the isovolumic changes in the pressure. The bottom horizontal side representing the ventricular filling through the mitral valve and the upper horizontal side representing the ventricular emptying through the aortic valve. In a healthy wild type mouse, left ventricular pressures of 90 to 110 millimeters of mercury are expected with a maximal rate of a maximum derivative of pressure of 8000 to 12000 millimeters of mercury per second.
Further, one of the most common artifacts of measurement that can complicate the analysis of pressure volume data is catheter entrapment which is evident as a spike in pressure at the end of systole likely resulting from the direct compression of the pressure transducer by a papillary muscle or other dynamic structure within the ventricle. While performing this procedure, it is important to remember to continuously monitor the mouse to ensure a sufficient anesthetic depth. Once mastered, this technique can be completed in roughly an hour depending on the protocol used after the catheter placement.
Following the completion of this procedure, infusions of experimental compounds can be used to assess their affects on cardiac function. After watching this video, you should have a good understanding of how to surgically place a pressure volume catheter in the left ventricle of the mouse and obtain high quality pressure volume loops for cardiac analysis.
