I know the signs of respiratory distress and all three times that Morgan was admitted to the hospital for pneumonia and supplemental oxygen she did not have ANY signs of respiratory distress-- no labored breathing, no retractions, no loss of color to her lips, etc. She had a fever, cough and low oxygen levels. On the other hand, my son who has asthma can have all the signs of respiratory distress but still be around 90% on his oxygen saturations. Even though his oxygen isn't low he still needs to go to the ER, his body is able to compensate with increased respirations, labored breathing and retractions but only for so long.
My point is that a pulse oximeter is helpful when there aren't other signs of respiratory distress. It is an additional piece of information but not the only one. I know that Morgan would have gone several days with low oxygen levels before showing any signs of respiratory distress. And I wouldn't have ever known without the pulse oximeter.
One more thing--if you are considering buying a pulse oximeter, look for one that is recommended for pediatrics. And be sure to read the directions--nothing like taking in your child when you didn't do the oximeter right and their oxygen was just fine. :) It can take some time to get a good, accurate reading on little, wiggly fingers. Make sure you have a good signal and the pulse rate looks correct. I usually try to get a good reading for a minute to make sure the oxygen levels are accurate.
P.S. I still love my pulse oximeter!