1 Massachusetts General Hospital, Boston, Massachusetts 02114
We would like to respond to Dr. Brady's questions. First, we chose a heart rate exceeding 180 beats per minute as an arbitrary limit since it is likely that cardiac output falls at rates above that level due to inadequate filling time. As mentioned in our article, there were no other signs of toxicity except for sporadic vomiting. The toxic level of 20µg/ml cited by Dr. Brady seems to be reliable for older children and adults treated for bronchospasm but isn't necessarily applicable to preterm infants.