Rapid estimation of serum theophylline clearance in children with acute asthma

Evaluation of three theophylline dosing equations for use in infants up to one year of age

To develop appropriate theophylline dosage recommendations for infants < or = 1 year of age, we evaluated the Nassif, Hendeles, and Hatzopoulos dosing equations in 75 infants who were receiving theophylline intravenously by continuous infusion. Postnatal age ranged from 1 to 52 weeks and postconceptional age from 33 to 93.8 weeks. Using each patient's measured theophylline clearance at steady state and the dose recommended by each of the three equations, we predicted a steady-state serum theophylline concentration for the three equations. The recommended theophylline dose and resultant serum concentrations differed significantly for the three equations. The Nassif, Hendeles, and Hatzopoulos equations resulted in 78.7%, 97.3%, and 93.8%, respectively, of serum theophylline concentrations between 5 and 15 mg/L. The Hendeles and Hatzopoulos equations tended to produce concentrations between 5 and 10 mg/L in the majority of infants; the Nassif equation generally resulted in values between 10 and 20 mg/L. Four percent of the calculated serum concentrations with the Nassif equation were > 20 mg/L. When a desired concentration of 10 mg/L was used in the Hatzopoulos equation, predictions of theophylline concentrations were consistently inflated. We conclude that the Hendeles equation ((0.008 x Postnatal age in weeks) + 0.21 = mg/kg per hour or mg/kg daily = (0.2 x Postnatal age in weeks) + 5) is preferred when intravenous theophylline therapy for apnea and bradycardia or for asthma is initiated. Regardless of the equation used to estimate an initial theophylline dose in infants < or = 1 year of age, serum theophylline concentrations should be monitored within 6 to 12 hours after the start of therapy.

Aminophylline in the emergency department. Maximizing safety and efficacy

An experimental technique designed to predict theophylline doses needed to attain therapeutic theophylline concentrations in 43 emergency department (ED) patients was compared with a standard conventional regimen in 46 ED patients. The experimental protocol utilized a computer-assisted dosage prediction program that incorporated baseline theophylline concentration rapidly obtained using a bedside assay. The standard protocol used conventional loading and infusion rates, as well as an estimate of time of last theophylline dose based on patient history. Plasma theophylline concentrations, estimated 1 and 6 hours after commencement of aminophylline therapy in each regimen, were compared. The experimental protocol was equally rapid but much more accurate in achieving targeted theophylline concentrations. Experimental dosage prediction was associated with a higher proportion of theophylline concentrations in the therapeutic range at 1 (81 percent vs 26 percent; p less than 0.001) and 6 hours (91 percent vs 37 percent; p less than 0.001). There was a trend toward fewer toxic concentrations recorded at 1 (0 percent vs 7 percent; p = 0.27) and 6 hours (0 percent vs 10 percent; p = 0.08). This protocol, which was performed quickly and without difficulty by residents in a busy hospital ED, offers an opportunity to improve the efficacy and decrease the toxicity of theophylline use in asthma emergencies.

An updated comparison of drug dosing methods. Part II: Theophylline

This article updates the previous review in the Journal regarding theophylline dosing methods. Among the predictive algorithms evaluated, the dose-titration scheme of Weinberger and Hendeles was extensively tested in 1073 asthmatic patients. When the scheme was followed appropriately, 78% of initial serum concentrations and 66% of repeat serum concentrations were within the therapeutic range of 10 to 20 mg/L. Several authors have also demonstrated that the 'condition correction factor' method for estimating theophylline clearance is of limited value. The individualised methods of Chiou, Koup and Vozeh have been evaluated in over 300 patients. In addition, numerous authors have reported the relative predictive performance of Bayesian dosing programs for theophylline. All methods continue to be a rapid and accurate means of individualizing dosing requirements for patients with a diverse range of theophylline disposition characteristics. Overall, the Bayesian predictions have been less biased and slightly more precise than the pharmacokinetics-based dosing methods. The most recent cost-effectiveness data has shown that a pharmacokinetic dosing program resulted in fewer toxic serum concentrations (18.9% vs 37.8%), a shorter mean duration of hospital stay (6.3 vs 8.7 days) and more therapeutic concentrations with subsequent oral therapy (71.1% vs 44.4%) than among control patients receiving dosages prescribed by physicians.

Evaluation of three theophylline dosing methods in pediatric patients

Three methods used for individualizing theophylline dosing were prospectively evaluated in 34 pediatric patients to compare the methods' ability to accurately predict steady-state serum theophylline concentrations (STCs) from non-steady-state data. Methods evaluated included a Bayesian weighted sum of squares regression program, an algebraic method developed by Chiou, and a commonly used population-based pediatric dosing algorithm. An expanded retrospective evaluation was also done to further compare the Bayesian and Chiou methods. Patients (aged 1-11 y) admitted to the hospital for acute bronchospasm refractory to inhaled beta agonists and with physician's orders to receive intravenous aminophylline were eligible for participation. Study patients received a 6-mg/kg loading dose aminophylline, followed by a 0.8-1.0 mg/kg/h constant aminophylline infusion. STCs were obtained 0.5 and 5.5 hours postinfusion. Subjects were randomized to one of three dosing method groups (Bayesian, Chiou, or pediatric algorithm). Doses were calculated using the assigned method to attain a target steady-state STC. Predictions from each dosing method were compared with actual serum concentrations for bias and precision. Additionally, analysis of fit-to-the-line-of-identity for predicted versus observed STCs was evaluated for each method. Precision of methods was also compared with regard to their ability to predict within 20 percent of their observed steady-state STC. Results from the prospective evaluation showed no significant difference between the three methods tested. Predicted STCs fell within 20 percent of their observed steady-state concentrations for 18 percent (2/11) of patients in the algorithm group, and 45 percent (5/11) of patients in the Chiou group and 17 percent (2/12) of the patients in the Bayesian group met this criterion. Retrospective analysis of all 34 patients demonstrated that the Bayesian and Chiou methods had similar bias and precision and no statistical difference was found between them. The results from this evaluation suggest that the pediatric dosing algorithm is equivalent in predictive bias and precision to the Bayesian and Chiou methods as well as in its ability to identify doses that result in steady-state STCs within 20 percent of their target values. Given the relative inaccuracy of these methods, cautious use of these techniques is recommended when evaluating non-steady-state STCs obtained from children during the acute stages of reactive airway disease.