New federal guidelines recommend checking children for possible heart and blood vessel damage if they have high blood pressure - a hazard increasing among the very young as Americans put on more and more weight.

The new guidelines, like earlier ones issued eight years ago, urge doctors to begin checking children for high blood pressure at age 3 during routine office visits, just as they do for adults.

“I think there is still a large proportion of pediatricians and family practitioners who are not routinely measuring blood pressure,” said Ronald Portman of the University of Texas at Houston, a member of the committee that drew up the new guidelines.

The updated guidelines are being released Thursday at a meeting in New York City of the American Society of Hypertension and will be published in the July issue of the journal Pediatrics. They were written by the National High Blood Pressure Education Program.

“The real problem is obesity,” said Dr. Barbara Alving, acting director of the National Heart, Lung and Blood Institute. “We are setting the stage for our children to develop into really unhealthy young adults.”

The government now estimates that 16 percent of U.S. children are overweight. The risk of both high blood pressure and the adult form of diabetes increase as children get heavier.

The latest nationwide health statistics, gathered in the late 1990s, show that children’s blood pressures have edged up slightly but significantly in a decade. Average systolic pressure (the higher number in a blood pressure reading) has risen from 105 to 106, and diastolic has gone from 58 to 62.

Much of this can be explained by children’s increasing weight, although doctors think that less physical activity and changes in diet also play a role.

Unlike adult blood pressure, healthy readings for children vary according to their size and age. Doctors consider any reading over the 95th percentile to be hypertension. Somewhat less than 5 percent of the population - perhaps 1 percent to 3 percent - is in this category, because readings that are initially high tend to drop with repeated measurements.

Children’s readings between the 90th and 95th percentiles are now considered to be prehypertension. Earlier guidelines called this category high normal.

Dr. Bonita Falkner of Thomas Jefferson University, who chaired the guidelines committee, said that in recent years, doctors have learned that even very young children with high blood pressure can have resulting organ damage. So patients should be routinely checked for such conditions as heart enlargement and thickening of the carotid artery.

Even without signs of damage, doctors should work to help young patients get their blood pressure under control. “The first step is lifestyle changes, since the most common issue is that they are overweight,” Falkner said.

If this does not work, then children should be put on blood pressure-lowering medicines, just as adults are.