This silverback gorilla is terrifying and awe-inspiring even when he’s unconscious.

Reddit user atodaso explains that the giant was getting a routine “heart work up,” since gorillas are prone to cardiac disease when they live in captivity. Atodaso, then a veterinary student, was assisting with the procedure. He said being in the animal’s presence was “a little terrifying,” despite knowing all the precautions; the animal was under plenty of anesthetic, his feet were strapped together, and both arms were tethered.

Recovery from such a procedure is tricky if only because of the silverback’s capabilities. Atodaso explains, “We kept him intubated for as long as possible. Before we took him off of gas, we administered a good dose of an opiod to slow the recovery. We also made sure to pad his recovery room if there was any dysphoria.”