**4.1.6 Natural history**

The natural history studies (Nugent et al 1977) have classified the degree of pulmonary valve obstruction based on peak-to-peak catheter-measured pulmonary valvar gradient: trivial = gradient 25 mmHg; mild = gradient 25-49 mmHg; moderate = gradient 50 to 79 mmHg and severe = gradient > 80 mmHg. Patients with trivial and mild (gradients 50 mmHg) pulmonary stenosis generally remain mild at follow-up. Patients with moderate stenosis (gradients of 50 to 79 mmHg) in contradistinction to trivial and mild stenosis, progressively increase their gradient
