Effect of sildenafil on haemodynamic response to exercise and exercise capacity in Fontan patients

Giardini et al.1 reported on the increase of peak oxygen uptake at a cardiopulmonary exercise test in Fontan patients after a single dose of sildenafil. This is an excellent study to outline that there are pulmonary vascular changes in these patients. These small but important changes cause an impaired inflow pattern of the single ventricle and curtail the whole cardiovascular performance. Secondly, concerning the same formula, ‘the values of pulmonary vein saturations obtained in room air during the most recent cardiac catheterization were used in the analysis’. We are not sure that pulmonary vein saturation can really substitute pulmonary endcapillary saturation in Fontan patients. This assumes that there were no intrapulmonary or veno-venous shunts at the catheter assessment. However, veno-venous collateral connections are a common problem in Fontan patients.3 Furthermore, we all assume and the study even showed that there is a pulmonary vascular dysfunction in Fontan patients. However, this might cause a V̇/Q̇ mismatch with functional shunting of desoxyHb in the lungs at exercise. It is hard to predict whether and how PBF measurement with nitrous oxide dilution is affected by this V̇/Q̇ mismatch. Therefore, it is questionable whether this functional right-to-left shunt of desoxyHb caused by the V̇/Q̇ mismatch during exercise should provoke a correction for cardiac output like the authors did. In summary, there are some doubts on the validity of the cardiac output calculation in that study. Nevertheless, this does not curtail the primary study conclusion that a single dose of sildenafil improves exercise capacity in Fontan patients.