To begin, place the anesthetized animal in the surgical area, ensuring that the surgical site is adequately shaved and disinfected. Next, properly place the conductance catheter through standard surgical procedures. To record and analyze the baseline pressure volume or PV loop, in the lab chart software, select the baseline parameters, then click the Analyze Now button.
Select the baseline option. Open the hemodynamic table and select the average data. Perform inferior vena cava or IVC occlusion by compressing the IVC using forceps.
In the software, manually select the point of occlusion of IVC where both left ventricular pressure and volume decrease. Then record the PV loop for at least 20 cardiac cycles while the IVC remains occluded by selecting the occlusion option and then clicking the Analyze Now button. To perform transesophageal pacing, insert a 2-French tetrapolar electrode catheter into the esophagus.
Connect the catheter to a pulse stimulator, then proceed to determine the atrial capture threshold. To slow the heart rate below 400 beats per minute, administer 20 milligrams per kilogram of ivabradine intraperitoneally. Following stabilization, acquire 20 continuous cardiac cycles of PV loops while pacing the heart at different rates from 400 to 700 beats per minute.
The baseline PV loop data showed no significant differences in diastolic parameters, such as the relaxation time constant, the minimum rate of pressure change, and the end diastolic pressure volume relationship, or EDPVR, between the control and HFpEF mice. However, the HFpEF mice exhibited higher blood pressure and arterial elastins, demonstrating a typical mountain-shaped PV loop during ventricular systole. This PV loop was distinguishable from a spike caused by direct contact of the ventricular muscle on the pressure transducer.
Notably, using atrial pacing, the diastolic function could be clearly distinguished between the control and HFpEF mice. In the control group, both the relaxation time constant and EDPVR improved as the pacing rate increased. Whereas in the HFpEF group, both the relaxation time constant and EDPVR worsened as the heart rate increased with atrial pacing.
