How can asthma affect vital capacity?

Slightly reduced. Asthma can cause a slight drop in vital capacity but a greater reduction in flow rates (such as fev1). Hence the ratio of fev1 to forced vital capacity is also reduced, suggestive of the characteristic pattern of obstruction. It is important to review the technique of pft to ensure adequate effort was put in while testing as that can also lead to falsely reduced vital capacity. Talk 2 yr lung doc.