Pulmonary hypertension

Pulmonary hypertension is defined as a chronic increase in pulmonary artery systolic pressure above 30 mm Hg or a mean pressure greater than 20-25 mm Hg. From a consideration of Poiseuille's equation (page 187) it is apparent that pulmonary hypertension may be expected to occur in the following circumstances:

1 When there is a reduction in the number of vessels perfused.

2 When there is a narrowing of the vessels as a result of intimai thickening, muscle hypertrophy, vascular spasm, or a decrease in the transmural pressure holding them open.

3 When there is an increase in the pulmonary venous pressure.

4 When there is an increase in blood viscosity (for example, as a result of polycythaemia).

The clinical conditions that may cause pulmonary hypertension are listed in the box. For convenience these are grouped under five main headings:

• those predominantly associated with a reduction in the size of the vascular bed

• those associated with a narrowing of the vessels

• those associated with an increase in pulmonary venous pressure

• primary or idiopathic (cause unknown)70

• diverse aetiology.

It will, however, become apparent that such a categorisation is somewhat artificial for there are often a number of factors contributing to the hypertension in each patient.

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