Simultaneous Electrocardiography Recording and Invasive Blood Pressure Measurement in Rats

Studies focused on cardiovascular disease models, cardiovascular therapeutic targets, or pharmaceutical agents require the assessment of systemic arterial pressure and heart rhythm changes. Femoral artery cannulation is an economic method for recording intraarterial pressure waveforms that can be performed with standard equipment in an animal facility. Demonstrating the procedure will be Hsin-Ting Lin, a technician from my laboratory.

Before beginning the experiment, place the anesthetized rat in a supine position on a surgical board and confirm the lack of response to toe pinch. Fix the legs with rubber bands to immobilize the body, and set up the blood pressure and electrocardiogram instruments for recording, including an analog input unit to acquire the signals, a pressure transducer with a compatible hub, three bipolar needle-tipped electrocardiogram leads, and a computer with suitable software. Before initiation of the blood pressure recording, remove the pressure cuff from a standard sphygmomanometer and connect the three-way stopcock on the inflation tube to the pressure transducer of the data acquisition system.

Screw the air release valve clockwise, keeping an eye on the gauge while pumping the inflation bulb. When the gauge shows 100 millimeters of mercury, open the three-way stopcock to connect to the pressure transducer. The conversion factor for calculating the blood pressure will be determined automatically.

To release the pressure, screw the air release valve counterclockwise until the pressure of the sphygmomanometer is back to zero. Repeat the calibration with 200 millimeters of mercury pressure, then detach the pressure transducer from the mercury sphygmomanometer. Connect the pressure transducer to the three-way stopcock of the PE catheter and check the connection junctions of the three-way stopcock.

For surface landmark identification, locate the inguinal crease at the junction between the abdomen and thigh and lift the full layer of skin at the center of the crease. Use scissors to cut the lifted skin at an orientation approximately parallel to the ipsilateral thigh. The femoral nerve and vessels are located beneath the exposed subcutaneous tissue.

Using fine-tipped forceps, dissect the tissue layer by layer, stopping the dissection at the level of the femoral vessels. Carefully clear out the soft tissue along the femoral nerve and vessels to obtain a good view of the fiber-like nerve. The vein is dark purple and the artery is pulsatile.

When the tissue dissection reaches to femoral vessels, switch and go change forceps to prevent damaged vessels. Then use forceps with angled tips to extend the exposed length of femoral artery and vein. To cannulate the femoral artery, use the forceps to separate the femoral vein from the artery and apply a bulldog clamp at the femoral artery as cranially as possible.

Make a loose tie of the two silk strings, one just below the bulldog clamp and the other at the caudal terminal of the exposed femoral artery. Use microscissors to make a small hole over the ventral side of the femoral artery and insert the tip of a polyethylene or PE catheter through the small hole. Advance the catheter cranially.

A back flush of blood into the catheter after removing the clamp indicates that the catheter tip has reached the femoral artery lumen. Tighten the upper silk to secure the position of the catheter and tighten the lower silk to prevent bleeding from the caudal side of the femoral artery. To confirm the success of the cannulation, use a one-milliliter syringe to inject 0.1 to 0.2 milliliters of heparinized saline into the femoral artery.

Then check for oozing around the cannulation site and cover the site with a wet cotton ball. To perform a blood pressure recording, attach the PE catheter and three-way stopcock to the pressure transducer stopcock and make sure that there are no air bubbles in the cannulation system. Then check the connection junctions of the three-way stopcock.

When the equipment is ready, start the data acquisition system with a sampling frequency of 1000 hertz. Arterial pressure waves will be displayed. Allow the whole set-up to stabilize for at least three to five minutes, checking the cannulation site periodically to confirm that there is no bleeding.

To perform a surface electrocardiogram, first check the three leads of the bipolar ECG to make sure that the positive, negative, and reference platinum electrodes are intact, and insert the leads subcutaneously at the left foreleg, right foreleg, and right hind leg. Then attach the electrode hubs to a custom-built ECG amplifier with a sampling frequency of 1000 hertz and a filter frequency of three to 500 hertz. Simultaneous blood pressure and ECG recordings in six 47-week-old normotensive and hypertensive rats revealed a significantly higher systolic and mean blood pressures in spontaneously hypertensive animals compared to normal rats.

P wave, PR intervals, QRS width, and QT interval parameters measured from the ECG recordings did not demonstrate any differences between the two groups. You may perform the pressure system calibration and set up the pressure transducer and the data acquisition system before anesthetizing the animals. The acquired raw arterial pressure waveform and electrocardiogram data can be analyzed on the software.

Radiotelemetry is considered a gold standard technique for monitoring BP and heart rate but the technique is costly. Intraarterial cannulation with simultaneous ECG is an economical alternative. Anesthesia by isoflurane inhalation is easy to perform but can be hazardous if a substantial leak occurs.

Therefore, always check the equipment and tubing before use.