Ronaldo blamed the high altitude after a goal from the Crystal Palace striker Ivan Kaviedes allowed Ecuador to end Brazil's 10-match unbeaten run in World Cup qualifiers on Wednesday night.
"I had a big headache. I was short of air and I needed oxygen to breathe," the Brazil striker said following Ecuador's 1-0 win.
Despite his problems, Ronaldo played the full 90 minutes and wasted an excellent late chance to equalise for the world champions. Ronaldo was due to fly straight back to Spain to prepare for Real Madrid's crucial game against the leaders of La Liga, Barcelona, on Saturday.
Brazil's defeat, which took place 2.8km above sea level, means they lost first place in the South American qualifying group. That honour went to Argentina who defeated Venezuela 3-2, with goals from Juan Roman Riquelme, Javier Saviola and Jose Manuel Rey, who put through his own net.
No one expects Brazil to fail to qualify - as they almost did in 2002, guaranteeing their berth only in the final match against Venezuela. They remain comfortably on course for a top-four finish and a place in the 2006 finals in Germany.
"The important thing is to qualify," said the coach of Brazil, Carlos Alberto Parreira. "Losing first place and losing the leadership doesn't mean anything. We knew this was one of our high-risk games. They improved in the second half. We lost the game. We didn't want to, but it happened."
The coach of Ecuador, Luis Fernando Suarez, was delighted with the result. "It's good, from any point of view, to beat the world champions. This team has got a lot of good things, a lot of organisation. It was very tense but, when we made mistakes, Brazil didn't take advantage."
Argentina moved to 22 points, two ahead of Brazil. Paraguay and Ecuador are level in third with 16 points. The top four South American teams qualify for the World Cup finals and the fifth-placed team faces the Oceania champion for another place.Reuse content