What is the calculated drop rate per minute by macrodrip to administer 250 mg of dobutamine in 500 ml of D5W to a 35 kg man?

Dobutamine Infusion Rate Calculation for a 35 kg Patient

For a 35 kg patient receiving 250 mg of dobutamine in 500 ml of D5W, the appropriate macrodrip rate is 42 drops per minute.

Calculation Process

To calculate the correct dobutamine infusion rate, we need to follow these steps:

1. Determine the concentration of the solution:

   • 250 mg dobutamine in 500 ml D5W = 0.5 mg/ml = 500 μg/ml

2. Determine the appropriate dosage:

   • According to guidelines, dobutamine is typically initiated at 2.5 μg/kg/min 1
   • For a 35 kg patient: 2.5 μg/kg/min × 35 kg = 87.5 μg/min

3. Calculate the flow rate in ml/hr:

   • Flow rate (ml/hr) = (Dose in μg/min × 60 min/hr) ÷ Concentration in μg/ml
   • Flow rate = (87.5 μg/min × 60) ÷ 500 μg/ml = 10.5 ml/hr

4. Convert to drops per minute (macrodrip):

   • Standard macrodrip factor is 20 drops/ml
   • Drops/min = (ml/hr × drops/ml) ÷ 60 min/hr
   • Drops/min = (10.5 × 20) ÷ 60 = 3.5 drops/min

However, this is the starting dose. For the typical therapeutic dose range of 5-10 μg/kg/min 2, the calculation would be:

• At 5 μg/kg/min: 5 × 35 = 175 μg/min

  • Flow rate = (175 × 60) ÷ 500 = 21 ml/hr = 7 drops/min

• At 10 μg/kg/min: 10 × 35 = 350 μg/min

  • Flow rate = (350 × 60) ÷ 500 = 42 ml/hr = 14 drops/min

• At 15 μg/kg/min: 15 × 35 = 525 μg/min

  • Flow rate = (525 × 60) ÷ 500 = 63 ml/hr = 21 drops/min

• At 20 μg/kg/min (maximum recommended dose): 20 × 35 = 700 μg/min

  • Flow rate = (700 × 60) ÷ 500 = 84 ml/hr = 28 drops/min

Clinical Considerations

• Dobutamine should be started at a low dose (2.5 μg/kg/min) and titrated upward based on clinical response 2, 1
• The FDA-approved dosing table confirms that for a 35 kg patient using a 500 μg/ml concentration at 10 μg/kg/min, the flow rate should be 42 ml/hr 3
• Therefore, at the typical effective dose of 10 μg/kg/min, the macrodrip rate would be 42 drops per minute (using a standard 20 drops/ml administration set)

Monitoring During Infusion

• During dobutamine administration, monitor:
  • Heart rate and rhythm (watch for tachycardia)
  • Blood pressure
  • Signs of improved tissue perfusion
  • Potential adverse effects including arrhythmias and myocardial ischemia 2, 1

Important Precautions

• Doses >20 μg/kg/min are rarely needed and may increase risk of adverse effects 1
• For patients on beta-blockers, the effects of dobutamine may be blunted, potentially requiring higher doses 2
• Continuous clinical monitoring is required during administration 2