Nuclear ventilation/perfusion scans are often used to diagnose a PE. A normal perfusion scan usually excludes a PE but can be found in a minority of patients with a PE. Perfusion defects are non-specific; about one third of those with defects actually have a PE. The probability that a perfusion defect is a PE increases with the size, shape and number of defects as well as the presence of a normal ventilation scan. Mismatched perfusion defects (normal ventilation scan), which are segmental in size or larger are “high probability” defects and are associated with an approximately 80% prevalence of PE. Three or more mismatched defects are associated with a prevalence of approximately 90%. Patients should be treated if presenting with a positive V/Q scan and high clinical suspicion of a PE.
PIOPED (prospective investigation of pulmonary embolism diagnosis) investigators demonstrated that a low-probability or normal ventilation-perfusion scan with a low clinical suspicion of pulmonary embolism essentially excludes the diagnosis of pulmonary embolism (negative predictive values of 96% and 98%, respectively; Gill & Nahum 2000; PIOPED Investigators 1990). When clinical suspicion is high and the scan indicates a high probability of pulmonary embolism, the positive predictive value is 96% (Gill & Nahum 2000; PIOPED Investigators 1990).