Intrahepatic Injection of Sodium Pentobarbital as an Alternative to Intraperitoneal Injection for the Euthanasia of Rats (Rattus norvegicus)

The most commonly accepted method of rat euthanasia in North America is intraperitoneal injection of sodium pentobarbital (PB). However, misinjection can occur, and intraperitoneal PB may cause pain and distress. The objective of this study was to test an alternative method of euthanasia: intrahepatic injection of PB. A pilot study was conducted to develop a method of intrahepatic injections (evaluated using CT scans and test injections), followed by a full study comparing intraperitoneal (n = 14) and intrahepatic PB injections (n = 66) in adult rats. Full study outcomes were: 1) time from injection to loss of righting reflex (LORR), 2) time from injection to cessation of heartbeat (CHB), 3) number of failed euthanasia attempts, and 4) confirmation of successful intrahepatic injection or misinjection via necropsy. All injections were performed by a veterinary student. CT revealed that intrahepatic injections were feasible. Times (median [range]) to LORR and CHB were faster after successful intrahepatic injections (LORR, 3 s [1 to 5 s]; CHB, 8 s [2 to 242 s]) than after intraperitoneal injections (LORR, 89.5 s [73 to 110 s], CHB: 284.5 s [237 to 423 s]). The misinjection rate was higher with intrahepatic injections (59%) than with intraperitoneal injections (29%), but intrahepatic misinjection still resulted in fast and successful euthanasia (LORR, 29 s [1 to 96 s]; CHB, 216 s [12 to 330 s]), with the injectate distributed between the intraperitoneal and intrahepatic locations. The number of failed euthanasia attempts with intrahepatic injections was low (n = 2). Intrahepatic injections show potential as an alternative to intraperitoneal injections for rat euthanasia.