The children with the most severe apnea were excluded.

Like severe apnea, moderate and mild cases also appeared to increase the risk of cardiac events after surgery.

Patients with mild and moderate apnea were typically overweight, and people with severe apnea tended to be obese.

In analyses stratified by apnea severity, the protective effect of the D allele was most evident in those with severe apnea.

But with the exception of severe apnea, the increased risk was too small to rule out the possibility that it was due to chance.

Slightly more than one in 10 patients in the study had severe apnea, when breathing stopped and started more than 30 times a night.

Among women of all ages with hypertension, 14 percent had severe sleep apnea, and among women who were obese, 19 percent had severe apnea.

About 34 percent of patients who came in during cold weather had severe apnea, while 28 percent of patients during warm weather had severe apnea.

Among subjects with severe apnea, the odds ratios for hypertension were 0.47 (0.22-1.00) for 1 D allele and 0.57 (0.26-1.24) for 2 D alleles.