The pulmonary circulation usually contains around 10% of the total blood volume. Pulmonary congestion refers to the condition where the pulmonary circulation contains significantly more blood, up to 20% of the total blood volume.
The condition can either be active, where the body intentionally increases the blood volume in the pulmonary circulation due to an inflammatory process in the lungs, like pneumonia. As part of the inflammatory process will the capillary permeability increase, which allows leukocytes to enter the interstitium. However, that’s not what we usually mean when we talk about congestion.
The more common situation is passive congestion, where the left heart has problems with ejecting enough blood, or the mitral valve is stenotic. The right heart however still pumps happily, meaning that the pulmonary circulation becomes overfilled with blood and therefore congested. The pressure inside the pulmonary circulation increases, which increases the capillary hydrostatic pressure. This forces fluid out of the capillaries.
We can distinguish two types of pulmonary oedema, haemodynamic type and microvascular type.
Haemodynamic oedema occurs when the capillary hydrostatic pressure increases, or when the intravascular oncotic pressure decreases. The former occurs in acute left ventricular failure (not chronic!) or mitral stenosis, while the latter occurs in any condition that causes hypoproteinaemia, like nephrotic syndrome or cirrhosis.
Microvascular oedema occurs when there is damage to the microvascular epithelium due to an acute pulmonary infection, gastric content aspiration or radiation.
It’s important to note that only acute left ventricular failure will cause pulmonary oedema. Chronic left ventricular failure only causes pulmonary congestion.
Clots that occlude pulmonary arteries are almost always thromboemboli and rarely thrombi. 95% of all pulmonary emboli originate from deep vein thrombi (DVT). Therefore are many of the factors that predispose to pulmonary embolism the same factors that predispose to DVT, such as:
- Prolonged bedrest
- Varicose veins
- Congestive heart failure
- Estrogen-containing birth control
- Advanced cancer
We distinguish four types of pulmonary embolism, based on the artery affected:
|Total embolization||Pulmonary trunk||Death|
|Subtotal embolization||Pulmonary artery||Most commonly death|
|Partial embolization||Segmental branch of pulmonary artery||Rarely causes death|
Large emboli usually get stuck in the bifurcation of the pulmonary artery as a saddle-shaped embolus.
Pulmonary embolism has two consequences: The downstream pulmonary parenchyme becomes ischaemic, and the pressure inside the blocked artery increases. This sudden pressure increase can cause acute right-sided heart failure, so-called acute cor pulmonale.
Hypoxaemia often occurs too. The ischaemic part of the lung produces less surfactant, so these alveoli become atelectic. They still receive perfusion however (despite the embolism), so shunts will form. Also, the pressure increase causes the pressure in the right atrium to increase. Considering that 20-30% of all people have a patent foramen ovale can blood be shunted from the right atrium to the left atrium, contributing to the hypoxaemia.
If only a small artery is occluded is it common that no symptoms can be seen. This occurs in 60-80% of all emboli.
Because the lungs are oxygenated from the bronchial circulation as well as from alveolar air and the pulmonary circulation is it rare that embolism causes haemorrhagic infarct. It only occurs if there is already pulmonary congestion, because of the increased blood volume in the lung.
In some edge cases can emboli other than thromboemboli occur in the lung, like:
- Fat emboli
- Tumour cell emboli
- Bone marrow emboli
- Air emboli
The pulmonary circulation usually has 1/8 of the pressure of the systemic circulation; the normal MAP is 18 mmHg. Pulmonary hypertension means that the pulmonary mean arterial pressure is above 25 mmHg and is almost always secondary to pulmonary vasoconstriction or pulmonary congestion. We distinguish three different types of pulmonary hypertension, depending on which site of the circulation is abnormal. They, and their causes, are:
- Precapillary pulmonary hypertension
- Primary pulmonary hypertension
- Hypoxic vasoconstriction
- Sleep apnoea syndrome
- Capillary pulmonary hypertension
- Pulmonary fibrosis, due to capillaries being destroyed by fibrosis
- Postcapillary pulmonary hypertension
- Left-sided heart failure
- Mitral stenosis
If a person has pulmonary hypertension but the all possible causes are ruled out, is that person said to have primary or idiopathic pulmonary hypertension. 80% of primary pulmonary hypertension is caused by bad genetic background.
The consequences of pulmonary hypertension include dyspnoea, attenuation of already present respiratory failure, and increased strain on the right heart, possibly causing chronic cor pulmonale.
Atherosclerosis is an extremely rare occurrence in the pulmonary arteries due to the low pressure. In pulmonary hypertension is it possible, however.
Diffuse pulmonary haemorrhage syndromes
Syndromes belonging to this category are syndromes that cause primary pulmonary haemorrhage, without any infection or congestion.
The most characteristic type is Goodpasture syndrome, a hypersensitivity type II reaction where the immune system produces antibodies against collagen IV. Collagen IV is found in basement membranes in the lungs and glomeruli, which is why haemorrhagic pneumonitis and glomerulonephritis are characteristic consequences.
Idiopathic pulmonary haemosiderosis is a rare disease where Goodpasture antibodies can’t be detected in the blood but the symptoms are similar as in Goodpasture syndrome.
No matter the reason will the lungs become heavy and reddish-brownish macroscopically. With histology we can see haemosiderin-laden macrophages in the alveoli. The most common symptoms are haemoptysis and anaemia.
Vasculitis haunts everyone forever
Granulomatosis with polyangiitis, also known as Wegener granulomatosis can also affect the lungs. The disease has three components:
- Necrotizing granulomatosis of the respiratory tract
The lung manifestations are:
- Haemorrhagic, necrotic nodules in the lung
- Pulmonary haemorrhage
- Cavity-forming lesions
80. Chronic restrictive lung diseases
82. Lung tumors