Theophylline formulas

The theophylline model is not hard-coded into the program.  The parameters are found in the drug model database and are fully user-editable.  You can tailor each drug model to fit your patient population, or you can create your own models.  See the Edit drug models section of the help file for further information.

Initial dosing

1.    Determine loading dose of IV aminophylline.

2.    Determine initial maintenance dose of IV aminophylline

ko = 0.5 x LBW x Smoking x Age factor x CHF/Liver failure x Drug factor

where factors are:

• Smoking = 1.5

• Age > 70 = 0.8

• Acute CHF or Liver failure = 0.6

• Interacting drugs = 0.75

• If theophylline is selected, multiply by 0.8

Dosage adjustment before steady state

1.    Calculate Clearance (CL) with Chiou Method

Two serum levels are required, the first level drawn one hour after the constant infusion started, and the second drawn at least one half-life later. The rate of infusion must be constant during the time between the two levels and the patient should not have received any oral theophylline 6 to 12 hours prior to the IV dose.

CL = [(2 x ko) / (Cp1 + Cp2)] + [(2 x Vd x (Cp1 - Cp2)) / (Cp1 + Cp2) x (t2 -t1)]

 where ko = administration rate (mg/hr)

Cp1 = level (mg/l) drawn 1 hour after start of infusion

Cp2 = level (mg/l) 6 hours later

Vd = 0.5 l/kg LBW

t1 = time Cp1 drawn

t2 = time Cp2 drawn

2.    Calculate Incremental Loading Dose or Temporary Interruption

 ILD = (Cpdes - Cpmeas) x Vd

 TI = -[ln (Cpdes / Cpmeas )] / Ke

3.    Determine new infusion rate (ko')

Ko' = Cpdes x CL

where Cpdes = desired level (mg/l)

CL = clearance (l/hr)

Dosage adjustment at steady state

1.   Calculate Clearance (CL)

CL = ko / Cpss

where ko = administration rate (mg/hr)

Cpss = steady state level (mg/l)

2.   Calculate Incremental Loading Dose (ILD) OR Temporary Interruption (TI)

Same as above

3.   Determine new infusion rate (ko')

Same as above

See also:

Introduction

Monitoring parameters

Precautions

Bibliography