Unasyn Dosing for Orbital Cellulitis in a 6-Month-Old Infant
For a 6-month-old infant with orbital cellulitis, administer intravenous Unasyn (ampicillin-sulbactam) at 200-300 mg/kg/day of the ampicillin component, divided into doses every 6 hours. 1, 2
Standard Dosing Regimen
- Dose: 200-300 mg/kg/day (based on ampicillin component) divided every 6 hours IV 1, 2
- Practical calculation: For a typical 6-month-old weighing approximately 7-8 kg, this translates to roughly 350-600 mg of the ampicillin component per dose (given as Unasyn 1.5g vials contain 1g ampicillin + 0.5g sulbactam) 3
- Administration: Give as IV bolus or 30-minute infusion every 6 hours 3, 4
Pharmacokinetic Considerations in This Age Group
- Peak serum concentrations achieved are 177-200 mcg/mL for ampicillin and 82-102 mcg/mL for sulbactam in pediatric patients, providing excellent coverage for typical orbital cellulitis pathogens 3
- The half-life in children is approximately 0.77 hours for ampicillin and 0.81 hours for sulbactam, supporting the every-6-hour dosing interval 3
- The combination is well-tolerated in infants and young children, with minimal side effects 3, 5, 6
Clinical Context for Orbital Cellulitis
- Orbital cellulitis in infants typically involves Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes, and Haemophilus influenzae 6
- Unasyn provides excellent coverage for these pathogens, including beta-lactamase producing strains (88% of isolates in pediatric studies) 5, 6
- Clinical studies demonstrate 93.9-96.4% cure rates for soft tissue and serious infections in pediatric patients 5, 6
Duration of Therapy
- Continue IV therapy until clinical improvement is evident (typically 48-72 hours), with fever resolution and decreased periorbital swelling 2, 5
- Total treatment duration should be 7-14 days, with consideration for step-down to oral therapy once clinically stable 2, 5
- Mean duration in pediatric infection studies was 7.4 days, with significant resolution occurring within 48 hours 5
Important Monitoring Parameters
- Obtain blood cultures before initiating therapy if not already done, even though antibiotics may have been started 2
- Reassess clinical status at 24-48 hours; if no improvement or worsening occurs, consider imaging (CT/MRI) to evaluate for abscess or intracranial extension 2
- Monitor for side effects, though these are rare in infants (diarrhea in 2-4% of cases) 5, 6
Common Pitfalls to Avoid
- Do not underdose: The 200-300 mg/kg/day range is necessary for serious infections like orbital cellulitis; lower doses may be insufficient 1, 2
- Do not delay cultures: Even if antibiotics have been started, cultures remain diagnostically valuable and should be obtained immediately 2
- Do not extend therapy unnecessarily: If cultures are negative and clinical improvement is rapid, consider stopping antibiotics rather than completing an arbitrary course to avoid promoting resistance 2
- Do not use every-8-hour dosing: The pharmacokinetics in children support every-6-hour administration for optimal efficacy 3, 4