Based on the details provided below, how many milliliters of the potassium chloride injection should be added by the pharmacist?

Question

A patient is receiving an intravenous infusion containing 40 mEq of potassium chloride in 1000 mL of D5 ½NS (dextrose 5% in half normal saline). The infusion has been running at a rate of 80 mL per hour for the past 6.5 hours. Following a lab report showing the patient’s serum potassium level to be 3.5 mEq/L, the physician decides to increase the potassium dose while slowing the infusion flow rate to 40 mL per hour. The physician prescribes a potassium chloride injection (14.9% KCl) to be added to the remaining IV solution, such that the patient will receive 80 mEq of potassium over the remaining time for the completion of the infusion. How many milliliters of the potassium chloride injection should be added by the pharmacist?

Answer 1

To determine how many milliliters of the potassium chloride injection should be added, follow these steps:

### Step-by-Step Solution

#### Step 1: Determine the Remaining Volume in the IV Bag

1. **Calculate the volume already infused:**

- Infusion rate: 80 mL per hour

- Duration: 6.5 hours

- Volume infused = 80 mL/hour * 6.5 hours = 520 mL

2. **Calculate the remaining volume in the IV bag:**

- Initial volume = 1000 mL

- Remaining volume = 1000 mL - 520 mL = 480 mL

#### Step 2: Determine the Remaining Time for the Infusion

1. **New infusion rate:** 40 mL per hour

2. **Remaining time to infuse the remaining 480 mL:**

- Remaining volume = 480 mL

- Infusion rate = 40 mL per hour

- Remaining time = 480 mL / 40 mL per hour = 12 hours

#### Step 3: Calculate the Total Potassium Required for the Remaining Time

1. **Total potassium needed:** 80 mEq over the remaining time (12 hours)

#### Step 4: Calculate the Concentration and Volume of the Potassium Chloride Injection

1. **Potassium chloride injection concentration:** 14.9% KCl

2. **Convert percentage to mEq/mL:**

- 14.9% KCl means 14.9 grams of KCl per 100 mL of solution

- Molar mass of KCl = 74.55 g/mol

- 14.9 grams KCl is approximately 14.9 g / 74.55 g/mol = 0.2 mol

- Since 1 mol of KCl provides 1 mol of K+ ions, 0.2 mol of KCl = 0.2 mol of K+

- 1 mol of K+ = 1000 mEq, so 0.2 mol = 0.2 * 1000 mEq = 200 mEq

- Therefore, 100 mL of 14.9% KCl provides 200 mEq of K+

- Concentration = 200 mEq / 100 mL = 2 mEq/mL

#### Step 5: Calculate the Volume of Potassium Chloride Injection Needed

1. **Total potassium required:** 80 mEq

2. **Concentration of the injection:** 2 mEq/mL

3. **Volume needed:**

- Volume (mL) = Total mEq required / Concentration (mEq/mL)

- Volume = 80 mEq / 2 mEq/mL = 40 mL

### Final Answer

The pharmacist should add 40 milliliters of the potassium chloride injection to the remaining IV solution.