Pulmonary Artery Obstruction by Aortic Aneurysm Mimicking Pulmonary Embolism

The combination of tachypnea, cyanosis, right heart dysfunction, a normal chest radiograph, and a unilateral segmental defect on perfusion scan is highly suggestive of pulmonary embolism (PE). Therapy is usually directed initially at empirical anticoagulation with heparin. However, in the setting of shock due to right heart dysfunction, thrombolysis with streptokinase or surgical embolectomy, if facilities are available, are potentially life-saving therapies. A broader differential diagnosis, however, should be considered based on the underlying pathophysiology of pulmonary artery (PA) obstruction. Not all potential causes are remediable by surgery or thrombolysis, and definitive diagnosis should be sought before starting therapy [1-4]. A case of unusual PA obstruction is presented. Case Report A 75-yr-old male was admitted to a peripheral hospital 1 mo previously with symptoms of intractable upper gastrointestinal tract bleeding due to gastric ulcer. The ulcer was oversewn urgently, and the postoperative course was complicated by congestive heart failure requiring mechanical ventilation. Transthoracic echocardiogram at that time demonstrated aortic sclerosis, mild left ventricular diastolic dysfunction, mild tricuspid and mitral insufficiency, and increased right ventricular systolic pressure of 50 mm Hg. The patient was discharged in satisfactory condition, but complained of dyspnea on initial followup 1 wk later. The patient was readmitted 1 mo later with symptoms of dyspnea at rest. He had no orthopnea, chest pain, or cough, but had developed ankle swelling bilaterally. His arterial blood pressure was 140/80 mm Hg, respirations were 30 breaths/min, and heart rate was 110 bpm and regular. Jugular venous pressure was visible 5 cm above the sternal angle, with a prominent V wave. A right-sided S3 was audible as well as a Grade 3/6 pansystolic, nonradiating murmur heard best at the base of the heart. His chest was clear. His liver was pulsatile and palpable 8 cm below the costal margin. Pedal edema was present. Electrocardiogram showed a right bundle-branch block, and the chest radiograph was normal. Ventilation/perfusion scan showed a large perfusion defect involving essentially the entire right lung, but no other unmatched perfusion defects. The ventilation scan was essentially normal. The patient rapidly became more tachypneic and distressed, requiring tracheal intubation and mechanical ventilation with positive end-expiratory pressure of 5 cm H2 O. Arterial blood gases on 100% O2 after intubation were pHa 7.45, PaCO2 26 mm Hg, PaO2 46 mm Hg, and HCO3- 18 mEq/L. The presumed diagnosis at this time was PE, and the patient was heparinized. Confirmatory evidence was sought, initially with bilateral venous duplex ultrasound, which was negative for distal venous thrombosis. Flotation PA catheterization via the right internal jugular vein was attempted, but the PA catheter could not be advanced beyond the right ventricle. PA angiography was subsequently attempted fluoroscopically via the femoral route. This showed a right ventricular (RV) pressure of 60/8 mm Hg, but the PA still could not be cannulated due to apparent extrinsic compression of the main PA. After contrast injection into the RV, flow was visibly less in the right PA than the left PA. Subsequent computed tomography (CT) scanning demonstrated dilation of the ascending aorta, a site of probable rupture of the posterior ascending aorta, and associated aortic and mediastinal thrombus. Transesophageal echocardiography (TEE) with a 5.0 MHz biplane transducer demonstrated aneurysmal dilation of the ascending aorta 2 cm distal to the aortic valve Figure 1, A and B. There was mild left ventricular dysfunction, aortic sclerosis, and trivial aortic regurgitation. The RV was severely hypokinetic, and the tricuspid leaflets failed to coapt Figure 2, with severe tricuspid regurgitation. The right atrium and the superior and inferior venae cava were dilated, with systolic flow reversal in the inferior vena cava. The interatrial septum bulged into the left atrium, with right atrial to left atrial flow through a patent foramen ovale (PFO) Figure 3. The estimated RV systolic pressure was 60 mm Hg.Figure 1: A, Transesophageal echocardiography, horizontal plane through the base of the heart. The ascending aorta is aneurysmal, and obliterates the pulmonary artery. LA = left atrium; SVC = superior vena cava. B, The same view in a normal study. The pulmonary artery is evident. PA = pulmonary artery.Figure 2: Transesophageal echocardiography, horizontal plane. In systole, the tricuspid leaflets fail to coapt (arrow), due to right ventricular failure. RA = right atrium; TV = tricuspid valve; RV = right ventricle.Figure 3: Transesophageal echocardiography, longitudinal plane. The speckled flow within the right atrium represents severe tricuspid regurgitation. FO = foramen ovale in the interatrial septum (arrow); RA = right atrium; SVC = superior vena cava; LA = left atrium.The patient was taken to the operating room for resection of the ascending aortic aneurysm. This was performed using a combination of femoral-femoral cardiopulmonary bypass (CPB) and deep hypothermic circulatory arrest. No attempt was made to operatively close the PFO. Postoperatively, the A-a gradient had improved; PaO2 was 85 mm Hg with a FIO2 of 0.5, and the patient was hemodynamically stable. Follow-up TEE showed significant diminution in the PFO flow, but underlying RV function was comparable to that of the previous study. The patient's recovery was complicated by his inability to satisfactorily clear pulmonary secretions, which necessitated tracheostomy. He remained neurologically intact, and was discharged home in good condition. Discussion This patient's clinical presentation was classic for PA obstruction. The finding of hypoxemia despite 100% O2 and a normal chest radiograph was suggestive of anatomic right-to-left shunt. Reports of similar clinical scenarios have attributed the finding of hypoxemia purely to ventilation-perfusion inequality [2,4]. TEE confirmed the presence of an anatomic right-to-left shunt through a PFO; this shunt undoubtedly contributed to the refractory hypoxemia seen in our patient. The PFO probably represented an otherwise functionally closed foramen ovale, opened only in response to the increased right atrial pressures. Since PE represented the most likely diagnosis, empirical anticoagulation was warranted while proceeding to definitive diagnosis. Furthermore, the degree of shunt and the clinical findings of pulmonary hyper-tension suggested the incipient development of right heart failure and shock. This mandated aggressive intervention, either by thrombolysis or surgical relief of the PA obstruction. Thrombolysis in clinical situations mimicking myocardial ischemia, which have subsequently been found due to other etiologies, has resulted in very high mortality rates, however [1]. Such mimickers include aortic dissection and expanding aortic aneurysm, and could be anticipated to be worsened by anticoagulation [3]. The lethality of mistaken diagnosis warrants definitive diagnosis prior to aggressive intervention. In the case presented, pulmonary angiography suggested the diagnosis of extrinsic compression, and supported the view that thrombolysis would be best avoided. Subsequent CT scanning confirmed the presence of the mass, extrinsic to the PA, compromising its lumen. When assessing PA obstruction by TEE, there are several images which must be obtained. The PA can be visualized with horizontal sections through the base of the heart. As the probe is withdrawn, the PA can be followed from its origin Figure 1, B cephalad to and including its bifurcation. The RV outflow tract, the pulmonic valve, and PA can be imaged using a longitudinal plane, again at the base of the heart. Images of the RV, tricuspid valve, right atrium, interatrial septum, and fossa ovalis complete the assessment of right heart function. TEE provides better and faster information, and exposes the patient to less risk than either pulmonary angiogram or CT scan. In circumstances of severe PA obstruction coming to surgery, it has been suggested that CPB be instituted via the femoral approach before inducting anesthesia [2,3]. The problem to be averted by this preemptive maneuver is sudden complete PA obstruction and cardiac arrest associated with the loss of spontaneous respiration. The mechanism responsible for this is believed to begin with cephalad shift of the diaphragm and inward retraction of the chest wall associated with the loss of consciousness and institution of neuromuscular blockade. The subsequent loss of intrathoracic volume forces the "solid" (less compliant) mediastinal structures closer together. A relatively solid mediastinal mass compresses the PA more readily than the aorta, since the PA is a more compliant structure and, in fact, compression of the right PA is the more frequently reported scenario [4]. Although preinduction femoral CPB would have been considered in this patient, positive pressure ventilation had already been required due to acute respiratory failure. TEE has evolved to become a first line diagnostic tool in the assessment of aortic dissection, with a sensitivity and specificity respectively of 96% and 86% in the ascending aorta, 95% and 94% in the aortic arch, and 97% and 94% in the descending aorta [5]. Its use in the diagnosis of aortic trauma is not as extensive, but preliminary reports indicate that it may be a suitable alternative to aortography in selected cases [6-8]. Our case presentation, while uncommon, illustrates another situation where TEE can be used to delineate both the extent of an aneurysm and its secondary effects on cardiac function and intracardiac shunting. Benefits of TEE, compared to transthoracic echocardiography, include superior image quality, especially in patients with underlying lung disease and those receiving mechanical ventilation [9]. In summary, this case describes an unusual presentation of PA obstruction. The most common diagnosis in this situation is PE, and would warrant anticoagulation pending definitive diagnosis. However, this clinical scenario includes a broader differential diagnosis, for some of which anticoagulation may be disastrous. TEE expediently provided a definitive diagnosis of the cause of the PA obstruction, as well as delineating an otherwise unsuspected anatomic shunt. We wish to thank Valerie Oxorn for her illustrations.