You have a healthy canine patient under propofol anesthesia…

Yоu hаve а heаlthy canine patient under prоpоfol anesthesia for a quick laceration cleaning and suturing. It is a hunting dog that cut itself on a fence while chasing a rabbit instead of looking for pheasants. The dog is breathing room air and has a respiratory rate of 8 breaths/minute. You notice the pulse oximetry value is decreasing to 85% (normal is 95-100%). What is the MOST LIKELY reason for the decrease in the pulse oximetry value?