Sample size was calculated according to different variables. We selected the biggest one calculated for a difference in nitric oxide of 25% after treatment between groups and a standard deviation (SD) value of 10µmol/L. Whit this information we obtained a sample size of 31 patients per group for an 80% statistical power with α= 0.05. Assuming a 15% patient lost, a sample size of 35 per group was considered.