Pulmonary Hypertension

Overview

Normal pulmonary circulation = high-flow, low-pressure (systolic ≤30/15mmHg even on exercise; venous 3-8mmHg). PHT = pulmonary arterial systolic >30mmHg. Primary (idiopathic, uncommon) vs Secondary (commoner).

Primary (idiopathic)

Unknown cause, diagnosed by EXCLUDING secondary causes. Young females 20-40yr, or children ~5yr.

Proposed mechanisms:

1. Neurohumoral vasoconstriction (chronic)
2. Occult thromboembolism/amniotic fluid embolism (explains female/pregnancy link)
3. Collagen vascular disease — Raynaud’s phenomenon precedes onset by years; SLE/scleroderma/RA association
4. Pulmonary veno-occlusive disease — fibrous obliteration of small veins (esp. children), from thrombosis/vasculitis
5. Ingested substances — bush tea, OCPs, appetite depressants (aminorex)
6. Familial occurrence

Secondary

Commoner, any age but >50yr more frequent. 3 mechanistic groups:

A. Passive (COMMONEST) — ↑pulmonary VENOUS pressure backs up: mitral stenosis, chronic LVF (severe HTN, aortic stenosis, myocardial fibrosis)

B. Hyperkinetic (reactive) — ↑volume/pressure into pulmonary arteries: PDA, ASD/VSD

C. Vaso-occlusive — progressive ↓vascular bed, 3 subtypes:

1. Obstructive — physical block: multiple emboli/thrombi, sickle cell disease, schistosomiasis
2. Obliterative — chronic parenchymal disease reduces bed: emphysema, chronic bronchitis, bronchiectasis, TB, pneumoconiosis
3. Vasoconstrictive — sustained hypoxic vasoconstriction+alveolar hypoventilation: high altitude, Pickwickian obesity, upper airway obstruction (tonsillar hypertrophy), neuromuscular disease (polio), severe kyphoscoliosis

Morphology

ALL types → chronic → COR PULMONALE. Right heart: RVH + RA dilatation. Vascular changes (main PA → arterioles):

• Arterioles/small arteries (MOST conspicuous): medial hypertrophy, elastic lamina thickening/reduplication, PLEXIFORM arteriopathy (intraluminal capillary tufts in dilated thin-walled branches) — LESS marked in secondary
• Medium arteries: medial hypertrophy (less in secondary), concentric intimal thickening, adventitial fibrosis, elastic lamina changes
• Large arteries: atheromatous deposits

Clinical relevance

3-way secondary mechanism split (passive/venous, hyperkinetic/volume, vaso-occlusive/bed-reduction) = organising framework for entire cause list — sort any new cause by which route it uses. Vaso-occlusive’s own 3-way subdivision (obstructive/obliterative/vasoconstrictive) shows genuinely different lung diseases converging on same physiologic endpoint — explains why sickle cell, emphysema, high altitude all → same right-heart consequence. Plexiform arteriopathy more prominent in PRIMARY = useful histologic discriminator, consistent with primary being more severe/localised vascular insult vs downstream consequence. Uniform progression to cor pulmonale regardless of trigger = right heart’s response to sustained afterload is stereotyped — diversity is entirely upstream (what raises pressure), downstream consequence is the same hypertrophy→failure sequence.

Overview

The pulmonary circulation is normally a high-flow, low-pressure system — systolic pressure does not exceed 30/15 mmHg even during exercise (pulmonary venous pressure: 3–8 mmHg), reflecting the pulmonary arteries’ thin, elastic wall structure (readily distinguished from the thick-walled bronchial arteries supplying the airways/pleura). Pulmonary hypertension is defined as pulmonary arterial systolic pressure above 30 mmHg. It is classified as primary (idiopathic) — uncommon — or secondary — the more common form.

Primary (Idiopathic) Pulmonary Hypertension

Uncommon, cause unknown; diagnosed only after excluding the recognised causes of secondary pulmonary hypertension. Typically young females aged 20–40, or children around age 5.

Etiopathogenesis (multiple proposed mechanisms, no single confirmed cause):

1. Neurohumoral vasoconstrictor mechanism — proposed chronic vasoconstriction driving the hypertension.
2. Occult thromboembolism/amniotic fluid embolism — suggested by the disease’s occurrence in young females, potentially linked to pregnancy-related embolic events.
3. Collagen vascular disease association — supported by Raynaud’s phenomenon preceding onset by years in many patients, and association with SLE, scleroderma, rheumatoid arthritis.
4. Pulmonary veno-occlusive disease — fibrous obliteration of small pulmonary veins, thought to underlie some cases (especially in children), itself considered a consequence of thrombosis or vasculitis.
5. Ingested substances — “bush tea,” oral contraceptives, appetite depressants (e.g. aminorex).
6. Familial occurrence reported in some cases.

Secondary Pulmonary Hypertension

More common; occurs at any age, more frequent over 50. Divided into three mechanistic groups:

A. Passive pulmonary hypertension — the commonest mechanism; diseases that raise pulmonary venous pressure, which backs up into the arterial circulation:

1. Mitral stenosis.
2. Chronic left ventricular failure (severe systemic hypertension, aortic stenosis, myocardial fibrosis).

B. Hyperkinetic (reactive) pulmonary hypertension — blood enters the pulmonary arteries in greater volume or at higher pressure:

1. Patent ductus arteriosus.
2. Atrial/ventricular septal defects.

C. Vaso-occlusive pulmonary hypertension — progressive diminution of the pulmonary vascular bed, itself subdivided by mechanism:

1. Obstructive type — a physical block within the circulation: multiple emboli/thrombi, sickle cell disease, schistosomiasis.
2. Obliterative type — vascular bed reduced by chronic parenchymal lung disease: chronic emphysema, chronic bronchitis, bronchiectasis, pulmonary TB, pneumoconiosis (see Emphysema, Chronic Bronchitis, Bronchiectasis, Pneumoconiosis).
3. Vasoconstrictive type — widespread, sustained hypoxic vasoconstriction and alveolar hypoventilation: high-altitude residence, pathologic obesity (Pickwickian disease), upper airway obstruction (e.g. tonsillar hypertrophy), neuromuscular disease (poliomyelitis), severe kyphoscoliosis.

Morphology

Irrespective of type, chronic pulmonary hypertension invariably leads to cor pulmonale — pathologic change confined to the right heart (right ventricular hypertrophy, right atrial dilatation) and the pulmonary arterial tree. Vascular changes are broadly similar in primary and secondary disease, spanning the entire arterial tree from main pulmonary arteries down to arterioles:

• Arterioles/small arteries (most conspicuous changes): medial hypertrophy; thickening/reduplication of elastic laminae; plexiform pulmonary arteriopathy (intraluminal capillary tuft formation within dilated thin-walled branches) — this specific lesion is less marked in secondary disease.
• Medium-sized arteries: medial hypertrophy (again less marked in secondary disease), concentric intimal thickening, adventitial fibrosis, elastic lamina thickening/reduplication.
• Large arteries: atheromatous deposits.

Clinical Relevance

• The three-way mechanistic split of secondary pulmonary hypertension (passive/venous back-pressure, hyperkinetic/volume-overload, vaso-occlusive/bed-reduction) is the organising framework for the entire long list of causes — any new cause can be sorted by asking which of these three physiological routes it uses, rather than memorised as an unstructured list.
• The vaso-occlusive group’s own three-way subdivision (obstructive block, obliterative parenchymal destruction, vasoconstrictive hypoxic response) captures three genuinely different lung diseases converging on the same physiologic endpoint — recognising the shared endpoint is what explains why such different conditions (sickle cell disease, emphysema, high-altitude residence) all produce the same right-heart consequences.
• Plexiform arteriopathy being more prominent in primary than secondary pulmonary hypertension is a useful histologic discriminator when the two are difficult to distinguish clinically, and is consistent with primary disease representing a more severe, more localised vascular insult rather than a downstream consequence of an already-identified cardiopulmonary lesion.
• Chronic pulmonary hypertension’s uniform progression to cor pulmonale, regardless of the primary trigger, illustrates that the right heart’s response to sustained afterload is stereotyped — the diversity is entirely upstream, in what raises the pulmonary pressure, while the downstream cardiac consequence is the same hypertrophy-then-failure sequence every time.

• Three mechanistic routes to secondary pulmonary hypertension: passive, hyperkinetic, and vaso-occlusive, with their causes mapped underneath
• Vaso-occlusive subtypes (obstructive/obliterative/vasoconstrictive) converging on one endpoint