British Journal of Radiology 74 (2001),86-88 © 2001 The British Institute of Radiology
Pulmonary hypertension secondary to left-sided heart disease: a cause for ventilation–perfusion mismatch mimicking pulmonary embolism
V W K Au, FRCR, FRANZCR,
D N Jones, MBBS, FRANZCR and
J P Slavotinek, MBBS, FRANZCR
Division of Medical Imaging, Flinders Medical Centre, Bedford Park, Adelaide SA 5042, Australia
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Abstract
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Ventilation–perfusion (V/Q) scans are commonly performed in patients with suspected pulmonary thromboembolism (PE). V/Q mismatch is typically attributed to PE. We describe a case in which a V/Q scan performed on a patient with advanced hypertrophic obstructive cardiomyopathy showed large areas of V/Q mismatch not due to PE. The mismatch was due to pulmonary hypertension secondary to left-sided heart disease. The pathophysiology is briefly reviewed.
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Case report
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A 31-year-old woman was referred for a ventilation–perfusion (V/Q) lung scan because of suspected pulmonary thromboembolism (PE). Cardiomyopathy had been diagnosed 10 years previously. She was becoming progressively debilitated by her cardiac problem and was being considered for cardiac transplantation. There was a history of PE 3 years previously, following the delivery of a child. This diagnosis had been based on CT pulmonary angiography in another hospital and she had been treated with anticoagulation. She presented with recent worsening of dyspnoea and chest pain.
A V/Q scan showed no perfusion in the middle and lower zones bilaterally, ventilation being relatively normal (Figure 1
). The perfusion defects were symmetrical and did not correspond to any lobar or segmental distribution, which was considered unusual for PE. A chest radiograph on the same day did not show any pleural effusion, collapse or consolidation, but there was the impression of upper zone vascular distension (Figure 2).
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Figure 1. Ventilation–perfusion lung scan shows symmetrical, large, non-segmental perfusion defects in the lower zones, with relatively normal ventilation. Anterior, posterior, and right and left lateral images are presented. Ventilation study: 99Tcm technegas; perfusion study: 99Tcm MAA.
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Figure 2. Erect posteroanterior chest radiograph shows gross cardiomegaly with distended upper lobe vessels. There is no evidence of pleural effusion.
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In view of the atypical V/Q findings, CT pulmonary angiography was performed within 1 h of the V/Q scan. No evidence of PE was detected in the central pulmonary arteries as far as the segmental branches peripherally (Figure 3). Given the size and extent of perfusion abnormalities on the V/Q scan, we expected to identify thrombus in the central pulmonary arteries, or at least in the proximal segmental branches, if PE had been the cause of the V/Q mismatch. There was no evidence of residual, organizing thrombus along the vessel, and no abrupt change of calibre of the pulmonary arteries to suggest chronic PE. Moreover, a previous CT pulmonary angiogram obtained 2 months prior to the present admission showed no evidence of PE. The negative CT pulmonary angiogram result was therefore considered to be adequate in excluding PE in this case.
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Figure 3. CT pulmonary angiogram (a) at the level of the aortic arch, showing enlarged upper lobe arteries, and (b) at the level of the left pulmonary artery, where no filling defect is seen in the central pulmonary arteries.
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Results from cardiac catheterization performed 1 month prior to this admission were reviewed and were consistent with the imaging findings. The mean pulmonary arterial pressure (39 mmHg) and pulmonary arterial wedge pressure (35 mmHg) had been elevated, there being a low transpulmonary gradient (normal range for mean pulmonary arterial pressure=10–20 mmHg; normal range for pulmonary artery wedge pressure=5–14 mmHg). Echocardiography showed a markedly dilated left atrium with restrictive dynamics and moderate impairment in left ventricular contraction. Endomyocardial biopsy demonstrated cardiomyopathy with fibrosis, indistinguishable from "burnt-out" hypertrophic cardiomyopathy. The overall picture was consistent with pulmonary arterial hypertension secondary to pulmonary venous congestion due to left-sided cardiac failure.
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Discussion
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The V/Q lung scan has been the commonest investigation requested for patients with suspected PE. Although unmatched perfusion defects are usually indicative of acute PE, there are other causes (Table 1) of this abnormality that potentially mimick acute PE. Chronic PE with unresolved thrombus in the pulmonary arterial tree is a common cause for V/Q mismatch. Follow-up V/Q scan after completion of anticoagulation is important in order that the extent of any persistent perfusion defect is documented. Another possible aetiology for V/Q mismatch is a mass lesion compressing the pulmonary arteries or pulmonary veins [1]. Plain chest radiography will provide a clue, as these masses are often readily detected, and CT is very helpful for further evaluation of the mass in relation to the pulmonary vessels. Vasculitis, caused by radiotherapy or associated with connective tissue diseases such as systemic lupus erythematosus or Takayasu's arteritis, can lead to perfusion defects [2, 3]. A careful review of the clinical history is important in these cases and further imaging is often required to document the diagnosis of PE. Other rare causes for V/Q mismatch are included in Table 1. This particular case illustrates that altered haemodynamics in left ventricular failure and pulmonary hypertension can cause V/Q mismatch.
The perfusion defects seen on the V/Q scan can be explained by the pathophysiology of pulmonary hypertension. In the erect position, pulmonary arterial blood flow is normally less in the upper lung zones than in the lower lung zones. Pulmonary venous pressure increases in patients with left ventricular failure. Vasoconstriction of pulmonary arteries will occur when the mean pulmonary capillary pressure is constantly above 20 mmHg. Post-capillary pulmonary hypertension will follow. Pulmonary vascular resistance also rises and typically increases first in the lower zones. Reflex vasoconstriction occurs in the lower lobe arteries and pulmonary blood flow is therefore diverted to the upper lung zones where the resistance is initially unchanged. This results in the "upper lobe diversion" phenomenon observed on the erect chest radiograph. Medial hypertrophy and intimal fibrosis of the pulmonary arteries will eventually occur [4]. Previous studies have also shown that preferential blood flow to the upper lung zones is positively correlated to mean pulmonary wedge pressure, the degree of pulmonary arterial hypertension and pulmonary vascular resistance [5, 6]. This redistribution of flow has also been demonstrated on pulmonary angiography in patients with mitral stenosis and pulmonary arterial hypertension. Constriction of the lower lobes' vessels with preferential flow to the upper lobes occurs with increased pulmonary arterial pressure and the appearance may mimic PE [6].
This phenomenon of redistribution of pulmonary blood flow leads to perfusion defects in the lower zones on the V/Q scan, unmatched by the ventilation scan. The bilateral, symmetrical, non-segmental, lower zone distribution of the unmatched perfusion defects should raise the suspicion that these defects are not due to PE. Further imaging may be necessary to exclude PE in these cases. CT pulmonary angiography is a non-invasive investigation and is sensitive (sensitivity 91%, specificity 78%) in the detection of pulmonary emboli in the central pulmonary arteries up to the segmental branches [7]. With a negative CT pulmonary angiogram, the imaging findings could then be interpreted as "upper lobe redistribution of blood flow" phenomenon, supported by the documented elevated pulmonary wedge pressure and pulmonary arterial pressure. Although this appearance has been described in patients with mitral stenosis and pulmonary hypertension [5], it is an uncommon phenomenon. This probably reflects earlier treatment for mitral stenosis and fewer patients with substantial pulmonary hypertension secondary to pulmonary venous congestion.
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Acknowledgments
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The authors would like to thank the Cardiac Transplant Unit, Flinders Medical Centre, for their assistance in the preparation of the case report, and Dr Robert Minson for his review of the manuscript.
Received for publication August 3, 2000.
Revision received August 20, 2000.
Accepted for publication October 5, 2000.
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Case report
