Natural History of Pulmonary Embolism: Scintigraphic Implications

David A. Bader MD

David E. Drum MD

November 15, 1994

Case Presentation:

A 64 year old male with 3 month history of paraplegia secondary to a spinal intradural aortovenous fistula presented with shortness of breath with exertion and bilateral leg swelling. Physical exam revealed clear lungs and normal heart sounds. The right lower leg was mildly edematous. Pulse 104 bpm with normal heart sounds, respiratory rate 24 and regular, blood pressure mildly elevated. Oxygen saturation 85-90% by finger pulse oximetry. Working diagnosis was pulmonary embolism.

Findings:

CXR was normal. Lung scintigraphy (68k bytes) demonstrated near absent left lung perfusion. There were at least two full segment wedge-shaped perfusion defects in the right lung: anterior basal segment and lateral segment of the right middle lobe. Ventilation (72k bytes) was normal. The impression was high probability for pulmonary embolism.

Imaging Techniques

A ventilation/perfusion lung scan was obtained: 3.6 mCi of Tc-99m labeled MAA was injected intravenously with the patient in the supine position. Imaging was performed on a triple-headed camera in 8 projections (60 degree anterior oblique views are not shown) for 500,000 counts each. A 21 mCi Xe-133 ventilation study followed: posterior positioning (posterior and steep anterior obliques) with a 15 second first breath image followed by equilibrium and washout frames for a total of 5 minutes.

Hospital Course

The patient was started immediately on intravenous heparin and subsequently coumadin PO. He remained asymptomatic and was discharged home. A follow-up lung perfusion study (72k bytes) obtained 24 days after the initial scan showed essentially complete resolution of the perfusion abnormalities.

Discussion:

This case represents an example of near complete scintigraphic resolution of a large pulmonary embolic load. The natural and scintigraphic history of pulmonary embolism is quite variable in terms of rate and degree of resolution in both the early and late stages. There are a significant minority of patients whose scintigraphic findings never resolve. These variables have important implications regarding interpretation of, and recommendations for, ventilation/perfusion scanning.

There are approximately 630,000 cases of pulmonary embolism in the US each year, with an 11% mortality rate in the first hour. The diagnosis is missed in 71% (400,000) of the survivors (1). A vast majority of subsequent deaths from PE occur within 2 weeks of the initial episode with a 23.8% mortality at one year. Almost all of these later deaths are attributable to underlying cardiovascular, pulmonary, or malignant disease (2). It has been demonstrated angiographically that there is minimal resolution of pulmonary emboli over the first week followed by moderate to complete resolution from 10-21 days (3). However, complete resolution is uncommon and the degree of angiographic resolution continues for at least 30 days (3).

It has long been known that scintigraphic perfusion studies show a changing pattern over short periods of time (4). In fact, a fixed pattern is unlikely to represent embolic disease (5). The rate and degree of improvement is related to the size of the initial involvement (4,6). Early changes in the scintigraphic appearance are secondary to multiple other factors as well: age of the thrombi, transit from larger to smaller branches, mechanical dissolution, changes in pulmonary resistance and reflex mechanisms (7,8). Following the initial changes there is variable long term resolution of scintigraphic defects. In the Urokinase trial there was 75% complete resolution at one year (6). The two factors that have been definitively correlated with incomplete resolution are age greater than 60 and heart disease (4,6,9). The significance of unresolved PE is the decreased specificity of subsequent VQ studies. With a history of prior PE the positive predictive value of a high probability study (PIOPED criteria) decreases from 97% to 74% (1).

Summary

The natural scintigraphic history of treated pulmonary embolism will demonstrate early changes, the vast majority of which are not due to recurrent embolism. There is subsequent resolution of perfusion defects at variable rates and to variable degrees depending on age, underlying disease and initial embolic load. Early follow-up ventilation/perfusion scans for the question of recurrent pulmonary embolism or treatment breakthrough must be interpreted with caution. There may be a role for recommending a follow-up baseline study in selected populations, especially the elderly and those with cardiovascular disease.

References:

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2.Alpert JS, Dalen JE. Epidemiology and natural history of venous thromboembolism. Progress in Cardiovascular Diseases 1994; 36:417-422.

3.Dalen JE, Banas JS, Brooks HL, Evans GL, Paraskos JA, Dexter L. Resolution rate of acute pulmonary embolism in man. New Eng J Med 1969; 22;1194-1199.

4.Tow DE, Wagner HN. Recovery of pulmonary arterial blood flow in patients with pulmonary embolism. New Engl J Med 1967; 19:1053-1059.

5.McNeil BJ, Holman BL, Adelstein SJ. The scintigraphic definition of pulmonary embolism. JAMA 1974; 227:753-756.

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Additional References of Interest

Alderson PO, Dzebolo NN, Biello DR, Seldin DW, Martin EC, Siegel BA. Serial lung scintigraphy: utility in the diagnosis of pulmonary embolism. Radiology 1983;149:797-802.

Dalen JE, Alpert JS. Natural history of pulmonary embolism. Progress in Cardiovascular Diseases. 1975; 17:259-270.

Greenspan RH. Pulmonary angiography and the diagnosis of pulmonary embolism. Progress in Cardiovascular Diseases 1994; 37:93-106.

Hyung SY, Intenzo CM, Park CH. Unresolved major pulmonary embolism: importance of follow-up lung scan in diagnosis. Eur J Nucl Med 1986; 12: 252-253.

Palmer, Scott, Strauss. Lung Scanning. In: Practical Nuclear Medicine , W.B. Saunders Company. 1992: 185-240.

Prediletto R, Paoletti P, Fornai E, Perissinotto A, et al. Natural course of treated pulmonary embolism. Evaluation by perfusion lung scintigraphy, gas exchange, and chest roentgenogram. Chest 1990; 97:554-561.

Wescott JL. Radiologic-pathologic correlations of pulmonary infarcts. AFIP lecture series. June, 1991.

Yoo HS, Intenzo CM, Park CH. Unresolved major pulmonary embolism: importance of lfollow-up lung scan in diagnosis. Eur J Nucl Med 1986; 12:252-253.

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J. Anthony Parker, MD PhD, Tony_Parker@bih.harvard.edu