What is the drop rate per minute for dobutamine (Dobutrex) in 500 ml of D5W (Dextrose 5% in Water) administered via microdrip to a 35 kg man?

Dobutamine Administration Calculation for a 35 kg Patient

For a 35 kg patient, dobutamine in 500 ml of D5W administered via microdrip should be infused at 42 drops per minute when using a standard starting dose of 2.5 μg/kg/min.

Calculation Process

To determine the correct drop rate for dobutamine administration, we need to follow these steps:

1. Determine the appropriate concentration of dobutamine
2. Calculate the infusion rate in mL/hr
3. Convert to drops per minute using the microdrip factor (60 drops/mL)

Step 1: Preparation of Dobutamine Solution

According to the FDA drug label, dobutamine must be diluted in an IV solution such as 5% Dextrose (D5W) 1. For this patient, we're using 500 mL of D5W as the diluent.

Step 2: Determining the Appropriate Dose

The European Society of Cardiology guidelines recommend starting dobutamine at 2.5 μg/kg/min for patients with heart failure 2. This is an appropriate starting dose that can be titrated based on clinical response.

Step 3: Calculating the Infusion Rate

For a 35 kg patient at 2.5 μg/kg/min:

• Dose = 2.5 μg/kg/min × 35 kg = 87.5 μg/min

If we assume a standard concentration of 1000 μg/mL (1 mg/mL) in the 500 mL solution:

• Flow rate (mL/hr) = (87.5 μg/min × 60 min/hr) ÷ 1000 μg/mL = 5.25 mL/hr

Step 4: Converting to Microdrip Rate

With a microdrip administration set (60 drops/mL):

• Drop rate = 5.25 mL/hr × 60 drops/mL ÷ 60 min/hr = 5.25 drops/min

However, this calculation assumes a standard concentration. Let's calculate using the FDA-provided table for a more accurate result:

For a 35 kg patient at 2.5 μg/kg/min using a 500 μg/mL concentration:

• According to the FDA table, a 30 kg patient would receive 9 mL/hr and a 40 kg patient would receive 12 mL/hr 1
• For a 35 kg patient: 9 + (35-30)/(40-30) × (12-9) = 9 + 0.5 × 3 = 10.5 mL/hr

Converting to drops/min with microdrip (60 drops/mL):

• 10.5 mL/hr × 60 drops/mL ÷ 60 min/hr = 10.5 drops/min

Titration Protocol

The dose should be titrated based on the patient's response:

• Start at 2.5 μg/kg/min (10.5 mL/hr or 10.5 drops/min)
• Double the dose every 15 minutes according to response or tolerability 2
• For a 35 kg patient at 5 μg/kg/min: 21 drops/min
• For a 35 kg patient at 10 μg/kg/min: 42 drops/min

Important Monitoring Parameters

During dobutamine infusion, monitor:

1. Heart rate and rhythm
2. Blood pressure
3. Oxygen saturation
4. Urine output
5. Signs of improved tissue perfusion (skin temperature, capillary refill)

Precautions and Considerations

• Doses >20 μg/kg/min are rarely needed and may increase risk of adverse effects 2
• Watch for excessive tachycardia, arrhythmias, or myocardial ischemia 2
• Dobutamine may have mild vasodilator activity due to β2-adrenergic stimulation 2
• For patients on beta-blockers, the hemodynamic effects of dobutamine may be blunted 3
• The patient should be kept under regular observation until stabilized 2

Alternative Approach for Higher Concentration

If using a higher concentration of 1000 μg/mL:

• For a 35 kg patient at 2.5 μg/kg/min: 5.25 mL/hr or 5.25 drops/min
• For a 35 kg patient at 5 μg/kg/min: 10.5 mL/hr or 10.5 drops/min
• For a 35 kg patient at 10 μg/kg/min: 21 mL/hr or 21 drops/min