If the diagnosis is seriously entertained, start heparin – unless contraindicated – without waiting for the results of diagnostic studies. For an average 70 kg patient, begin with 5000–7500 unit IV bolus, followed by 1000 U/hr drip (less for smaller patients). Follow PTT and titrate drip rate for PTT 1.5 to 2 × control. The use of heparin shortly after surgery and in patients with brain tumors is controversial, and vena cava interruption may be an alternate consideration (e.g. Greenfield filter). Patients with massive PEs may be hemodynamically unstable. They usually require ICU care, often with PA catheter and pressors.