Pulmonary Pathology Online
Pathology of Pulmonary Hypertension
In fetal life the pulmonary arterial walls are thick, but after the third day of life they become thin , mainly because of dilatation.
The pressure within the pulmonary arterial system may be increased for one of two reasons - increased flow or increased resistance within the pulmonary circulation.
Whatever the cause, similar morphologic abnormalities result from increased pulmonary arterial pressure.
Pulmonary atherosclerosis is seen in the major vessels.
It should be noted that even mild degrees of atherosclerosis are uncommon when pulmonary arterial pressure is normal.
The walls of the small pulmonary vessels thickened, owing to an increase in muscle in the media and an increase in fibrous tissue in the intima.
An additional change is the development of muscle in vessels that are normally nonmuscular or partially muscular.
If the pressure is high and sustained, the arteries dilate and may exhibit curious angiomatoid or plexiform lesions.
These nodular lesions, composed of irregular interlacing blood channels, impose a further obstruction in the pulmonary circulation.
Necrosis of the arterial wall and pulmonary hemorrhage occur in severe , sustained pulmonary hypertension.
Mild structural changes of the pulmonary vasculature are reversible.
However, severe lesions (plexiform lesions, necrosis, and hemorrhage) indicate that pulmonary arterial hypertension is not correctable.
As a result of the increased pressure in the lesser circulation, hypertrophy of the right ventricle of the heart occurs, which lead to cor pulmonale.
Increased Flow -
A shunt from systemic circulation (including the heart) to the pulmonary circulation results in increased flow through the lungs.
The great majority of cases represents left-to-right shunts in congenital heart disease, although occasional acquired conditions occur, such as rupture of the interventricular septum or an aortic-pulmonary fistula.
The severity of the lesions in the pulmonary arteries depends on the severity of the pulmonary hypertension.
An additional lesion is present when hypertension exists from birth.
At this time pulmonary artery and the aorta have about the same number of elastic lamellae in their media. In normal infants there is a loss of elastic lamellae in the pulmonary artery after birth, but when pulmonary hypertension is present the fetal pattern persists.
Increased Resistance to Flow-
Increased resistance to flow may be caused by obstruction proximal to the lung capillary bed (precapillary hypertension), destruction of the capillary bed, or obstruction distal to the capillary bed (postcapillary hypertension).
- Precapillary Pulmonary Hypertension
-Primary Pulmonary Hypertension:
Primary pulmonary hypertension, of unknown etiology, is caused by increased tone within the pulmonary arteries.
It occurs at all ages but is most common in young women in their 20s and 30s.
The disorder presents with an insidious onset of dyspnea, and physical signs and radiologic abnormalities are initially slight.
As time passes, clinical and radiologic abnormalities become more apparent.
Severe morphologic changes of pulmonary hypertension eventually ensue and the patients die from cor pulmonale and right-sided heart failure.
Medical treatment is generally ineffective, and this disease is an indication for heart-lung transplantation.
-Multiple Recurrent Pulmonary Emboli:
Multiple thromboemboli in the smaller pulmonary vessels gradually restrict the pulmonary circulation.
The presenting symptoms are often the same as in primary pulmonary hypertension, but some patients have evidence of peripheral venous thrombosis, unusually in the leg veins, or a history of circumstances predisposing to peripheral venous thrombosis.
The lesions in the pulmonary vascular system are those of pulmonary hypertension, often severe.
Organized thromboemboli are present.
Characteristically, these form "webs" in the small pulmonary arteries composed of fibrous bands that extend across the lumen of the vessel.
Clinically, it is difficult to distinguished multiple recurrent pulmonary emboli from primary pulmonary hypertension.
Lung biopsy is not always helpful, since the lesions in the smallest vessels are identical and vessels of the size that commonly have webs are not included in the biopsy.
If the condition is diagnosed during life, plication of, or placement of filters in, the inferior vena cava may prevent further emboli.
-Occlusion of Major Vessels:
Larger pulmonary arteries than those involved in multiple recurrent pulmonary emboli can also be occluded.
Occlusion may be due to thromboemboli in larger pulmonary arteries or thrombosis of the pulmonary arteries.
In these instances the onset is more acute, and specific evidence of classic embolization is often present.
Acute pulmonary hypertension is a concomitant of large emboli but these may be resorbed and incorporated into the wall, after which the pulmonary artery pressure return to normal.
The pulmonary arteries may be narrowed and finally occluded by rare tumors of the pulmonary artery.
Distal to the occluded large vessel the small pulmonary arteries are normal, but the classic changes of pulmonary hypertension may be seen in other parts of lung.
-Functional Resistance to Flow:
The pulmonary circulation is sensitive to hypoxemia and any condition that produce hypoxemia results in pulmonary hypertension.
This is the most common form of pulmonary hypertension and results from constriction of the small pulmonary arteries.
Its causes include chronic airflow obstruction due to any condition that is accompanied by hypoxemia - for instance infiltrative lung disease, living at high altitude, and alveolar hypoventilation.
Alveolar hypoventilation may be due to abnormalities of the chest wall that interfere with mechanics of ventilation, such as severe kyphoscoliosis and extreme obesity.
Idiopathic hypoventilation is another cause of pulmonary hypertension and in patients with extreme obesity is referred to as the "Pickwickian syndrome".
The situation in these subjects is complex.
Severe obesity by itself may lead to hypoventilation and abnormal ventilation/perfusion ratios.
Obesity may produce obstruction to the upper airway (pharynx) at night, thereby leading to the sleep apnea syndrome.
This episodic occlusion during the night results in profound intermittent hypoxemia and disturbances in the pattern of breathing.
The disturbance of sleep at night leads to daytime somnolence.
Rarely, obesity may be due to hypothalamic dysfunction and may also be associated with a disturbance of the respiratory center and diminished hypoxic drive.
In other instances, upper airway obstruction may not be associated with obesity.
Such cases are often difficult to recognize, although the characteristic feature is loud and prolonged snoring.
All forms of hypoventilation are more common in men than women.
Surgery to the pharynx, pharyngoplasty (in effect, a radical tonsillectomy), may be curative of upper airway obstruction, although a permanent tracheostomy is sometimes required.
In all forms of reactive (functional) pulmonary hypertension the histologic changes in the vessels are slight.
They consist of muscularization of the nonmuscular and partially muscular pulmonary arteries and slight medial thickening of the smallest muscular pulmonary arteries.
The mildness of the condition is related to the modest degree of hypertension and the often intermittent nature of the hypoxemia.
- Capillary Causes of Pulmonary Hypertension:
Experimentally, it has been shown that three fourths of the lung can be removed without ensuing pulmonary hypertension.
Pneumonectomy in man does not result in pulmonary hypertension, although pulmonary artery pressure may be slightly increased on exercise.
Emphysema is commonly cited as a cause of pulmonary hypertension, and it is claimed by some that this is due to the loss of capillary bed.
Pulmonary hypertension in emphysema is mainly due to hypoxemia caused by ventilation/perfusion abnormalities, but it may be that this is compounded by a loss of capillary bed.
-Post Capillary Causes of Pulmonary Hypertension:
Postcapillary hypertension differs from the other forms of pulmonary hypertension in that there are venous as well as arterial changes and the parenchyma (alveolar walls) may also be abnormal.
The parenchyma is generally normal in precapillary hypertension except in the most severe grades, when pulmonary hemorrhage occurs.
In capillary hypertension the parenchyma is by definition abnormal, but not as a consequence of venous hypertension.
Venous changes include intimal fibrosis and thickening, or "arterialization", of veins.
Veins normally have little elastic tissue and muscle, but with severe venous hypertension extra elastic laminae and new smooth muscle develop.
- Cardiac Causes of Pulmonary Hypertension:
Left ventricular failure from any cause increases pulmonary venous pressure and hence pulmonary arterial pressure, but the increase is generally quite small.
By contrast, mitral stenosis produces severe venous hypertension and significant pulmonary artery hypertension.
Although some have argued that pulmonary arterial hypertension reflects reactive vasoconstriction, it is now thought that increased venous resistance is responsible.
Changes in the venous system and parenchyma depend on the severity of the venous hypertension.
When severe, as in longstanding mitral stenosis, obvious lesions are present, and mild changes are present in the pulmonary arterial system.
The capillaries are dilated and because of intermittent hemorrhage, red blood cells are seen in alveolar spaces and in the interstitium.
Hemosiderosis, resulting from the breakbown of the red blood cells and phagocytosis of their debris, is prominent.
Large macrophages loaded with hemosiderin, so-called heart failure cells, often pack the alveoli.
In addition, hemosiderin is seen in the interstitium of the lung and may coat blood vessels.
Calcium also accumulates in the elastic fibers of the alveolar walls, which stain blue with hematoxylin, leading to so-called iron and calcium encrustation.
Elastic fibers may fragment and elicit a giant cell reaction.
There is usually thickening of alveolar walls, with some mild fibrosis.
On gross examination, the lungs are brown and firm, an appearance that gives rise to the term brown induration of the lung.
Elevated pulmonary artery pressure is caused by increased pulmonary vascular resistance.
This is secondary to:
(1) Chronic obstructive or interstitial lung disease ;
(3) Recurrent pulmonary emboli.
The cause is unknown but current hypothesis postulate that endothelial dysfunction and injury (may be due to some chemical and dietary agents), lead to persistent vasoconstriction .
There is subsequent intimal and medial hypertrophy and resultant increase in vascular resistance.
Primary (or idiopathic) pulmonary hypertension is rare and is seen in children or in women aged 20 to 49 years.
It usually progressed to severe pulmonary insufficiency, cor pulmonale and death over several years.
Therapies include vasodilators and occasionally, heart-lung transplantation.
Vascular lesions : Atheroma in large elastic arteries and intimal fibrosis or medial hypertrophy in medium seized muscular arteries and smaller arterioles.
Presence of numerous organized thrombi suggests recurrent pulmonary thromboembolism.
So called "plexogenic arteriopathy" (tufts within capillary channels creating a vascular plexus) is seen in severe primary pulmonary hypertension or in some congenital cardiovascular anomalies.
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