Overview

Many competing modalities may be useful in the diagnosis of Pulmonary Embolism. What does the physician do when faced with the patient complaining of shortness of breath, chest pain and other non-specific symptoms? The consequence of an incorrect diagnosis can be dire.

This leaflet, whilst not exhaustive, is the first in a coordinated series designed to provide a logical rationale for considering Technegas as the most appropriate scanning tool in the diagnosis of P.E.

In this issue we address how big a problem P.E. is and highlight the supporting evidence for considering the Ventilation/Perfusion scan, specifically with Technegas, as the optimal test to deliver the most accurate diagnosis.

Defining The Problem

"VTE [ venous thrombo-embolism ] is an uncompromising predator that can gnaw at the very strands of life itself. Yet it develops slowly, usually unseen and unheralded, to burst upon clinical consciousness with frightening rapidity". So wrote Henry W Gray as part of a dramatic personalised introduction to his excellent review 'The Natural History of Venous Thromboembolism: Impact on Ventilation / PerfusionScan Reporting' published in 2002 (1). It is in this context of a stealthy, potentially fatal condition, where the statistics of clinical examinations alone to make a diagnosis generally point to about a 65% accuracy, that there is such an ongoing need for a reliable, non-invasive screening test that is available at any time. And there is no place for error: false positives and false negatives both carry severe risks for the patient.

Pulmonary Embolism (P.E.) is a major health problem which is more common than might be thought and more lethal than usually appreciated. The incidence of venous thromboembolic diseases in the U.S. is increasing as the population increases and ages. Morbidity is generally determined by early and accurate diagnosis and immediate treatment. Unfortunately, the diagnosis is frequently missed because the symptoms of P.E. are vague and non-specific.

The embolus is usually a thrombus but bone marrow, fat and air may be involved. In 60% of cases the source of the thrombus is the venous system of the lower legs highlighting the importance of Deep Vein Thrombosis (DVT) in the equation.

In the U.S. about 600,000 patients per year have clinically P.E. and 120,000 (20%) of them die annually without being diagnosed. P.E. is present in about 80% of patients with DVT and more than half are asymptomatic.

Making The Diagnosis

Attempts to diagnose P.E. using the traditional "gatekeeper" of emergency medicine imaging, Radiology, with the latest tomographic x-ray contrast equipment and procedures, generally known as CTPA, have been supported by an extensive literature and a growing referral rate. But apart from the high radiation dose especially to the female breast, there seems to be minimal recognition that up to 15% of the potential patients are unable to have the test for various reasons. By definition, this should eliminate its status as a screening test.

Other interpretive pitfalls include high interpreter variability, questionable sensitivity at the subsegmental level, breathhold and high false negatives.

In practice what usually happens is that the Nuclear Medicine V/Q procedure is then called upon to determine the diagnosis. Yet from its inception in 1963 (2), the sensitivity of the perfusion imaging test, whereby up to half a million radio-labelled micro-emboli, usually aggregates of human serum albumin, are injected, has never been surpassed. The clinical problem has always been vascular obstructions of non embolic origin which may confound the diagnosis, and in our urbanised industrialised societies, at least 40% of all adult lung imaging show such perfusion defects. Physiologically, the body's attempts to maintain high oxygen saturation in the blood leads to an extremely rapid closure (milliseconds) in localised perfusion (Q) if the congruent region of airways is obstructed. But the converse does not occur, and it can take days for airways patency to be affected by perfusion defects. Hence the potential value of a test for P.E. that is both highly sensitive and specific when a perfusion defect is shown still to have ventilation patency-the V/Q mismatch. (Bill Burch, newsletter Cyclomedica, issue 1, October 2006)