Situation: A 52-year-old woman taking estrogen-containing hormone therapy is admitted to the intensive care unit (ICU) 3 days after a 14-hour flight. Computed tomography pulmonary angiography confirms pulmonary embolism (PE). She weighs 75 kg and has no history of bleeding, stroke, recent surgery, or head injury. Because advanced therapy may still be needed, intravenous unfractionated heparin is ordered by weight-based protocol: bolus 80 units/kg, then an infusion of 18 units/kg/h. The bag contains 25,000 units in 500 mL. After giving the bolus, at what rate should the nurse set the pump? Round to one decimal place.

Infusion dose: 18 units/kg/h × 75 kg = 1,350 units/h. Concentration: 25,000 units ÷ 500 mL = 50 units/mL. Rate: 1,350 ÷ 50 = 27.0 mL/h. Heparin infusions are double-checked with a second nurse.

Clinical context
A pulmonary embolism confirmed by CT pulmonary angiography requires immediate therapeutic anticoagulation. When advanced therapy such as catheter-directed interventions or systemic thrombolysis may still be needed, intravenous unfractionated heparin is preferred because it has a short half-life and can be stopped or reversed more quickly than low-molecular-weight heparin or direct oral anticoagulants. Weight-based dosing is used to reach a therapeutic activated partial thromboplastin time rapidly, which matters because the first 72 hours of anticoagulation strongly influence outcomes in intermediate-risk PE ^[1].

Step-by-step calculation
The bolus is given first and is not part of the infusion rate calculation. The maintenance dose is 18 units/kg/h.

Infusion dose = 18 units/kg/h × 75 kg = 1,350 units/h.

The bag concentration is 25,000 units in 500 mL, which equals 50 units/mL.

Pump rate = 1,350 units/h ÷ 50 units/mL = 27.0 mL/h.

The correct infusion rate is 27.0 mL/h, which corresponds to option 3.

Why this calculation matters clinically
Unfractionated heparin has unpredictable bioavailability, so dosing is adjusted to a target aPTT range. In a prospective cohort of patients with intermediate-risk PE treated with UFH, the time spent within the therapeutic aPTT range during the first 72 hours correlated with prognosis, meaning that both under-anticoagulation and over-anticoagulation can worsen outcomes ^[1]. Setting the pump accurately from the start is the first step in achieving that therapeutic range.

Watch out! A common error is including the bolus in the infusion calculation or using the wrong concentration. The bolus is a separate one-time dose; only the maintenance dose determines the continuous infusion rate.

Weight-based dosing and special populations
Weight-based UFH protocols are standard for venous thromboembolism, but their applicability in obese patients has been questioned because actual body weight may overestimate the dose when fat mass is high . In this case, the patient weighs 75 kg, which is within a range where standard weight-based dosing is appropriate. For patients with extreme obesity, many institutions cap the initial infusion rate or use adjusted body weight, because dosing based on total body weight can produce supratherapeutic aPTT values and increase bleeding risk .

Why UFH instead of thrombolysis here
Systemic thrombolysis is indicated for high-risk PE with hemodynamic instability, but it carries substantial bleeding risk from thrombolysis-induced coagulopathy . This patient has no signs of hemodynamic collapse in the stem, so anticoagulation with UFH is the appropriate first-line strategy. The statement that advanced therapy may still be needed explains the choice of UFH: if the patient deteriorates and requires catheter-directed thrombolysis or embolectomy, the heparin infusion can be stopped quickly, and its anticoagulant effect dissipates within hours because of the short half-life.

Double-check and safety
Heparin is a high-alert medication. The infusion rate must be independently double-checked by a second nurse before the pump is started, and the aPTT is typically rechecked 6 hours after initiation or any dose change. The pump should also be programmed with the correct concentration, because heparin bags may be prepared in different dilutions depending on institutional protocol.

Key point! For weight-based UFH, calculate the dose in units/h first, then convert to mL/h using the specific bag concentration. Never assume a standard concentration across all heparin infusions.