Hi christinejroberts, Thank you for your query. It sounds like the test performed on your grand daughter was a blood gas. It is usually performed to asses the acidity of the blood, and the likely causes. It is difficult to intpret just one abnormality in isolation but I shall do my best. In this test, carbon dioxide (CO2) blood levels are a measure of how effective a person is breathing, they are often low when someone is hyperventilating and therefore blowing out a lot of CO2 with the frequent breaths. Children tend to hyperventilate if they are hot (with fever or environmental) anxious or over excited. CO2 levels are not a direct measurement of blood acid levels, but because of the way many systems in the body interact, an abnormally low CO2 level can reflect a abnormality elsewhere. I believe this may be what the nurse was referring to. When children are dehydrated the concentrations of acids in their tissues and build stream build up. These are acids that normally accumulate during normal cellular activity but because the child is dehydrated, the concentrations are high. The body tries to correct this imbalance by hyperventillating. This rapid breathing is called Kussmal s respiration and it through this hyperventillation that the CO2 drops. If your grand child is drinking a lot and still having this rapid breathing I would suggest having her blood sugar checked as these are symptoms of diabetes which can occur in children of any age. You may have also noticed some weight loss and frequent urination. If so, it is important that she is seen at pediatric hospital. If you are able to resubmit more of the results this would be very helpful in giving you advice about your grand daughters situation.