Oxygen supplementation before and after anesthesia is always intended to be helpful and usually is not harmful as long as a short safety check list is implemented. This post will set aside free radical concerns for now. Each practice should make there own checklist, and I have a few suggestions on what the checklist should include.
Is the patient able to exhale carbon dioxide in a manner that allows him/her to not re-breathe the exhaled carbon dioxide? Typical veterinary anesthetic induction masks are supposed to fit with low dead space to avoid rebreathing exhaled gases. This is not a comfortable feeling on a patient who is awake. Try it sometime: have someone place a plastic mask over you and hold you down. Probably you're able to visualize all the unpleasant noxious stimulus without this dramatic role-playing. This leads to number 2.
Is the patient struggling? This means there will likely be a more difficult induction, more medications may be required and the early anesthesia may be more challenging. There will also be more physiological stress and more consumption of oxygen. There is also a subtle undermining of the anesthetic plan. Here's what I think happens: an induction with struggling, leads to catecholamine release which stimulates a cascade that likely probably further compromises in immunity in the long run, but in the short run can lead to a dangerous depth. Initially, with the struggling, more medications or inhalant is added, and then as the patient loses that cardiovascular excitement vital start dropping before inhalant can be reduced.
Is the use of oxygen actually increasing the fraction of inspired oxygen? Sometimes I watch well-intended anesthetists hold a single port of oxygen a few inches away from a patient. This choice is often the result of number 1 in my suggestions. I believe this is doing very little except taking extra effort on the part of the anesthesia team. In the ICU we place patients in oxygen cages, or place nasal cannulas.
Is the use of oxygen supplementation potentially dangerous? Recently, and this is second hand, I was told of a patient for whom the oxygen supplementation was placed inside the ET tube and obstructed the gases and caused barotrauma. If this happened as described, I can believe it. I have removed a few of these lines out of ET tubes placed b well-intervened caregivers with patients in recovery. If preoxygenation is the concern, the patient needs to be sedated to the point of not resenting the mask. That has some problems too. Or the mask can be placed as the animal is being induced, feeling unaware or unable to struggle (another post on that new information), and let the last few breathes before unconsciousness sip the oxygen rich "air" deep into the lungs as a protection against hypoxemia from apnea during intubation.
If supplementing oxygen, it is best to use an anesthesia mask connected to an anesthesia circuit so there is a bag and a valve for excess. If using a separate systems, such as in recovery, be sure it is a bag mask valve, so the oxygen can be provided and the excess can be discarded. Examples are oxygen masks with two valves in the side and the Ambu bag. Both increase the work of breathing but allow safer oxygen supplementation in recovery.
Jennifer Hess is a board-certified anesthesiologist who has a life-long interest in helping high-risk patients survive and thrive after anesthesia.