Pfizer plans to sell a children’s form of Viagra to treat a rare lung disorder as a humanitarian gesture — in exchange for getting a six-month extension of its patent on the adult version.

The Food and Drug Administration approached Pfizer about using its drug to treat a lung ailment that affects about 600 kids a year.

Viagra, which modifies blood flows, could reduce unusually high blood pressure in a child’s lungs, or pulmonary arterial hypertension. Symptoms include dizziness, chest pain and fatigue.

A panel of FDA advisers will determine on Thursday whether to green light the kid’s version.

If Pfizer’s drug meets FDA requirements for children’s treatment, Pfizer would get an extra six months of exclusive Viagra sales without generic competition. Viagra sales in 2009 were $1.89 billion. Its patents expire in 2012.

Doctors already had been using Viagra’s principal ingredient in lower doses to treat children with the lung ailment, under a different name, Revatio. The principal drug for both Viagra and the softer sounding Revatio is the same, Sildenafil.

The FDA began asking Pfizer in 2001 to develop ways to use Viagra to treat children suffering from the lung ailment, but the pool of just 600 patients offered limited revenue opportunities.

Doctors who have been using the drug on children say it is well-tolerated and doesn’t have as many side effects as other treatments.

Pfizer shares rose 25 cents, or 1.7 percent, to $15.27 in NYSE trading.