Pharmacokinetics and pharmacodynamics of ranitidine in critically ill children

Objective: To determine the pharmacokinetics and pharmacodynamics of raniti dine in critically ill children and to design a dosage regimen that achieve s a gastric ph greater than or equal to4. Design: Prospective, open-label, pharmacokinetic-pharmacodynamic study. Setting: Pediatric intensive care unit in a tertiary care children's hospit al. Patients: Mechanically ventilated, critically ill children greater than or equal to 10 kg who required intravenous ranitidine for stress ulcer prophyl axis. Interventions: Ranitidine pharmacokinetics were determined after a single i ntravenous dose. Gastric ph was monitored hourly via nasogastric ph probe. After the last blood sample, patients received an intravenous bolus of rani tidine (0.5 mglkg) followed by a continuous infusion (0.1 mg . kg(-1). hr(- 1)). The infusion was increased incrementally (0.05 mg . kg(-1). hr(-1)) un til reaching gastric pH greater than or equal to4 far greater than or equal to 75% of a 24-hr period, after which steady-state plasma concentrations w ere measured. Plasma concentrations were analysed by high-pressure liquid c hromatography. Measurements and Main Results: Twenty-three children (ranging in age from 1 .4 to 17.1 yrs) were studied. Pharmacokinetic variables included a clearanc e of 511.7 +/- 219.7 mL . kg(-1). hr(-1), volume of distribution of 1.53 +/ - 0.99 L/kg, and half-life of 3.01 +/- 1.35 hrs, After the single intraveno us dose (1.52 +/- 0.47 mg/kg), gastric ph increased from 1.6 +/- 1.0 to 5.1 +/- 1.1 (p < .001), which was associated with a plasma concentration of 37 3 <plus/minus> 257 ng/mL, Based on the pharmacokinetic variables, the dose of intravenous ranitidine required to target 373 ng/mL as the average stead y-state concentration is 1.5 mg/kg administered every 8 hrs, During the con tinuous infusion, the mean steady-state ranitidine concentration associated with gastric pH greater than or equal to4 was 287 greater than or equal to 133 ng/mL, This concentration may be achieved with an intravenous loading dose of 0.45 mg/kg followed by a continuous infusion of 0.15 mg kg(-1). hr( -1). Conclusions: The pharmacokinetics of ranitidine in critically ill children are variable. The description of ranitidine's pharmacokinetics and pharmaco dynamics in this study may used to design an initial ranitidine dosage regi men that targets a gastric ph greater than or equal to4. Thereafter, gastri c ph should be monitored and the dose of ranitidine adjusted accordingly.