Why are my heart rates extremely low when recording PV data even though my pressure and volume data are within range?

Your research animal may be over-anesthetized and/or the body temperature may have dropped below normal physiologic level (37°C).

Check the anesthesia concentration and dosage. Perform the surgeries on a temperature-controlled blanket or heated operating table. If temperature-regulating equipment is being used, check the functionality of the temperature probe and temperature control hardware.