He was commenting on the decision to euthanise the Kentucky Derby winner after serious complications developed in the four-year-old colt.
"Clearly this was a difficult decision to make," he told reporters.
The decision, he said, was based on what his caregivers and owners had said all along: that the horse not be allowed to suffer, and the likelihood of recovery.
"This happened very quickly in the last couple of days and probably the thing that pushed us over the top ... last night ... for the first time ever, he really struggled with what he was doing.
"He did not feel comfortable enough to lie down. He was not comfortable standing up. We had him in and out of the sling several times trying to get him down and up. The bottom line is he was a completely different horse," he said.
"Our goal from the beginning was to do what was right for the horse. After discussing the complications that existed with the Jacksons, I really did not think it appropriate to continue with the treatment because the probable outcome was just so poor, and he would have to go through an unmanageable amount of discomfort," Dr Richardson said.
"He developed fairly severe laminitis in both front feet. It left him with not a good leg to stand on, and that was just not going to work out in the long run.
"It was very difficult to get to this point," he said of Barbaro's last day. He said he was fortunate to have a long and complicated surgery after Babaro was euthanised, which allowed him to take his mind off the case. However, it would be impossible to forget.
Dr Richardson said Barbaro was clearly uncomfortable with the turn in his health. "He was just a different horse. I could see he was upset. That was the difference. It was more than we wanted to put him through."
He said he and Barbaro's owners, Roy and Gretchen Jackson, had a discussion during the morning about the colt.
"The Jacksons and I have had a close relationship for a long time. None of this was a news flash to them. We were all going through the same things... Knowing, from the beginning, that there were lots of bad things that could happen ... knowing that this day could come.
"They knew, and I knew, that we were going to have this conversation.
"We got through it and made the decision that I am comfortable with ... very comfortable that we made the right decision. You have to do what is best for the patient."
Responding to questions, Dr Richardson said that he knew that if the day came it would be very difficult to get through, but added: "It's not the first horse I have cried over."
He said many people played a part in Barbaro's care.
Knowledge was gained from Barbaro's treatment, but he described it as incremental learning. "If you asked me if I had exactly the same fracture, the same injury came in tomorrow, I honestly believe I would have a better chance of saving the horse's life.
"I would probably not make the same mistakes. I am sure I made mistakes. Everyone makes mistakes. There are things that you could have done slightly better."
You have to believe that you can get better at what you do, he said.
Dr Richardson said the medical team had managed to handle the laminitis in Barbaro's uninjured hind leg, keeping him moving and weight-bearing on both hind legs.
However, complications began arising as Barbaro began bearing too much weight on his previously fractured leg. The fractures, he said, had healed, but this was not the cause of the complications.
Barbaro developed a bruise on the heel of his previously fractured leg. That deep bruise meant he developed an abscess in the heel region, well below the original injury.
"His fractures were completely, totally healed. That was not the problem."
He went on to develop laminitis in his front feet because he was uncomfortable behind. "Then he started overloading his front feet," he said.
"This type of situation is like a deck of cards. Sometimes if things are that tenuous, if one thing starts to go you have a lot of other parts that start to go on you as well."
Dr Richardson said Barbaro had his regular morning grass while in his sling, with the weight off his feet. Although sedated, he remained alert and recognised the Jacksons and his caregivers.
He was given a heavy dose of tranquiliser before the overdose of anaesthetic through an existing catheter that ended his life.
The end was very peaceful. "We were all there," he said.
Barbaro had been a happy horse for the vast majority of the nine months he was under care. "I assure there have been many cases I have had in the past where I know I waited too long. I do not believe that is the case here."
People appreciated that Babaro was a great athlete.
"People love greatness ... this was the story of his greatness and bravery."