Chronic Thromboembolic Pulmonary Hypertension

By Peter F. Fedullo, MD

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Diagnosis

Once the diagnosis of pulmonary hypertension has been considered, the diagnostic pathway is straightforward but must be undertaken in a sequential fashion. Transthoracic echocardiography, when performed with a contrast study, commonly provides the initial objective evidence that pulmonary hypertension is present and that it is not the result of primary left ventricular dysfunction, valvular disease, or an intracardiac shunt. Typical findings in all forms of pulmonary arterial hypertension include enlargement of the right cardiac chambers, an increased velocity of the tricuspid regurgitant envelope from which the pulmonary artery systolic pressure can be estimated, flattening or paradoxical motion of the interventricular septum, and encroachment of an enlarged right ventricle on the left ventricular cavity.

Once the diagnosis of pulmonary hypertension has been confirmed, distinguishing between the multiple etiologic possibilities is the next critical step (Table 1). Depending on the clinical circumstances, pulmonary function testing, chest CT, sleep testing, serologic studies, or abdominal ultrasound may be required. However, once the differential possibility has been narrowed to a primary problem of the pulmonary vasculature, ventilation-perfusion lung scanning represents a simple, noninvasive means of differentiating disorders of the peripheral pulmonary vascular bed from those of the central. In chronic thromboembolic disease, at least one segmental or larger mismatched perfusion defect is present (more commonly, several are noted).⁹ In disorders of the distal pulmonary vascular bed, perfusion scans either are normal or exhibit a ÒmottledÓ appearance characterized by subsegmental defects. It should be recognized that mismatched segmental or larger defects in patients with pulmonary hypertension may also arise from other processes that result in obstruction of the central pulmonary arteries or veins, such as pulmonary artery sarcoma, large-vessel pulmonary vasculitides, extrinsic vascular compression by mediastinal adenopathy or fibrosis, and pulmonary veno-occlusive disease.¹⁰

Table 1ÑClassification of Pulmonary Hypertension

1. Decreased cross-sectional area of the pulmonary vascular bed

A. Parenchymal lung diseases

B. Pulmonary resection

C. Congenital hypoplasia

2. Increased flow through the pulmonary arteries

A. Systemic to pulmonary shunts

3. Increased resistance to flow through large pulmonary arteries

A. Chronic thromboembolic disease

B. TakayasuÕs arteritis

C. Congenital pulmonary artery stenosis

D. Mediastinal processes (fibrosis, tumors)

E. Pulmonary artery tumors

4. Increased resistance to flow through small pulmonary arteries

A. Primary pulmonary arterial hypertension

B. Pulmonary vasculitides

C. Autoimmune diseases

D. Chemical/toxic damage

5. Increased resistance to pulmonary venous drainage

A. Elevated left ventricular diastolic pressure

B. Elevated left atrial pressure

C. Pulmonary venous obstruction

6. Chronic alveolar hypoxia

A. Obesity-hypoventilation syndrome

B. Chest wall disorders

C. Neuromuscular disorders

D. Parenchymal lung disease

7. Miscellaneous conditions

A. High altitude

B. Portopulmonary hypertension

C. HIV infection

D. Sickle hemoglobinopathies

E. Pulmonary capillary hemangiomatosis

The magnitude of the perfusion defects in chronic thromboembolic disease often understates to a considerable extent the actual degree of pulmonary vascular obstruction determined angiographically or at surgery.^11,12 Therefore, the presence of even a single, mismatched, segmental ventilation-perfusion scan defect in a patient with pulmonary hypertension should raise concerns regarding a potential thromboembolic basis.

In the evaluation of a patient with pulmonary hypertension, CT scanning is invaluable in detecting disorders of the pulmonary parenchyma, interstitium, chest wall, and mediastinum. However, it has a limited role in the diagnosis of chronic thromboembolic disease. Although a variety of CT abnormalities have been described in patients with chronic thromboembolic disease, the absence of these findings does not preclude the possibility of surgically accessible chronic thromboembolic disease. Furthermore, central thrombi have been described in primary pulmonary hypertension and other forms of chronic lung disease.¹³

The alveolar-arterial oxygen gradient is typically widened and the majority of patients have a decrease in the arterial Po₂ with exercise.¹⁴ Profound hypoxemia, however, is not a usual component of the disease unless a large right-to-left shunt develops through a patent foramen ovale. Results of pulmonary function testing are usually within normal limits but may demonstrate a mild to moderate restrictive impairment, caused to a large extent by parenchymal scarring related to prior infarcts.¹⁵ Although a mild to moderate reduction in single-breath diffusing capacity for carbon monoxide can be observed, a normal value does not exclude the diagnosis. Chest radiography, although often normal, may demonstrate one or more findings that suggest the diagnosis of pulmonary hypertension, such as right ventricular prominence, asymmetry in the size of the central pulmonary arteries, or enlargement of both pulmonary arteries. Areas of mosaic oligemia also may be present (Fig 1). Results of routine hematologic and blood chemistry studies are usually unremarkable. A prolonged activated partial thromboplastin time in the absence of heparin therapy or a decreased platelet count may suggest the presence of a lupus anticoagulant or anticardiolipin antibody.

Figure 1. Chest radiograph in a patient with CTEPH. Note markedly enlarged right pulmonary artery, absence of descending left pulmonary artery, oligemic left lower lobe, and nodule representing infarct in the peripheral left mid-lung field.

 

Right-heart catheterization with pulmonary angiography remains the gold standard in terms of both diagnosis and surgical referral.¹² Symptom-limited exercise hemodynamic measurements are obtained when the level of pulmonary hypertension at rest is only modest. In patients in whom the central pulmonary vascular obstruction has abolished the normal compensatory mechanisms of recruitment and dilation, exercise-related increases in cardiac output are associated with an almost linear elevation of the pulmonary artery pressure.

The angiographic appearance of chronic thromboembolic disease bears little resemblance to that of acute pulmonary embolism. Well-defined, intraluminal filling defects found in acute disease are not present. Instead, the angiographic patterns encountered in chronic thromboembolic disease are reflective of the complex patterns of organization and recanalization that occur following an acute thromboembolic event (Fig 2). Five angiographic patterns have been described in chronic thromboembolic disease that correlate with findings at the time of surgery.¹² These include (1) pouch defects; (2) pulmonary artery webs or bands; (3) intimal irregularities; (4) abrupt, often angular narrowing of the major pulmonary arteries; and (5) complete obstruction of main, lobar, or segmental vessels at their point of origin. In most patients with extensive chronic thrombembolic disease, two or more of these angiographic findings are present and the findings are present bilaterally.

Figure 2. Left, right anteroposterior angiogram, and right, lateral pulmonary angiogram in a patient with chronic thromboembolic disease. Interlobar artery is markedly irregular. Lateral view demonstrates abrupt cut-off of descending pulmonary artery with complete absence of flow to the right lower lobe. Right middle lobe artery is dilated and tortuous.

 

Even in the presence of severe pulmonary hypertension, pulmonary angiography can be performed safely if simple precautions are taken.¹⁶ In terms of technique, multiple selective injections are not required. A single injection of nonionic contrast into both proximal pulmonary arteries, with the volume and injection rate adjusted based on cardiac output, appears to be sufficient. The patient is also carefully monitored and supplemental oxygen is provided during the procedure. Biplane acquisition provides optimal anatomic detail, the lateral projection providing more detailed definition of lobar and segmental anatomy than can be achieved with an anterior-posterior view alone. If the findings of pulmonary angiography are not conclusive, fiberoptic pulmonary angioscopy has proven valuable in confirming the presence of chronic thromboembolic obstruction and in determining whether it is amenable to surgical intervention.¹⁷

If the patient is considered to be an operative candidate, several other interventions must be undertaken before surgery. Given the risk of embolic recurrenceÑboth over the long term and especially during the high-risk perioperative period when bleeding complications may contraindicate the administration of even prophylactic doses of anticoagulationÑan inferior vena caval filter is routinely placed. For those at risk of coronary artery disease, coronary angiography is routinely performed before surgery, usually at the time of the right-heart catheterization and pulmonary angiogram. Coronary artery bypass grafting, if necessary, can be performed at the time of the thromboendarterectomy.

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