Pulmonary hypertension is defined as a pulmonary arterial pressure > 25 mm Hg at rest or > 30 mm Hg on exercise, although pulmonary hypertension in childhood is usually associated with considerably higher pressures. Pulmonary hypertension can be described as either primary, being of unknown aetiology, or secondary resulting from cardiac or parenchymal lung disease. This description is unsatisfactory, however, since it takes no account of the similarities in pathobiology and response to treatment between primary and certain other types of pulmonary hypertension. It narrows our perspective. A new classification was proposed at a World Health Organization symposium in 1998, based on anatomy, clinical features, and an appreciation of the commonality of at least some of the underlying mechanisms.1 PPH and pulmonary hypertension related to congenital heart disease, PPHN, connective tissue disease, HIV infection, drugs, and toxins were grouped together as "pulmonary arterial hypertension". This new classification encourages the extension of therapeutic modalities known to be effective in PPH to other forms of hypertension, in both adults and children.
Further clarification is necessary in children with congenital heart disease. In the majority of children pulmonary arterial hypertension is usually caused and driven by a cardiac abnormality which leads to the development of the Eisenmenger syndrome. However, in some children the abnormality is, and always has been, haemodynamically insignificant. Clinically these children behave as though they have PPH, and can be treated as such.
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