Terbutaline is a medication that is used to treat serious asthma attacks (status asthmaticus). The purpose of this study is to determine if terbutaline is effective and safe when given as a continuous intravenous infusion to children ages 2 years to 18 years of age. Children participating in this study are inpatients in the Intensive Care Unit with a diagnosis of status asthmaticus and are receiving terbutaline as part of their routine care. Blood samples and physiological assessments will be used to determine how the body is using terbutaline and how well it is working.