Data from the main study allow estimations of anticipated results and statistical power. If the average of maximal RVSP during exercise in our group is 50 mmHg and sildenafil causes a 20% decrease in average to 40 mmHg (SD 10.0) a sample size of 13 patients leads to statistical power of 81.7% with 5% confidence level (calculation: DSS research 10.7.2008, http://www.dssresearch.com/toolkit/spcalc/power_a2.asp