Since inhaled nitric oxide (iNO) dilates only the ventilated areas of the lung, it enhances the matching of ventilation and perfusion. Whereas, other pulmonary vasodilators - Sildenafil, Prostacycline, etc. dilate all the pulmonary vessels leading to worsening V/Q mismatch.
If there is deterioration in oxygenation after starting one of those latter pulmonary vasodilators, this phenomenon should be considered. Also consider this phenomenon when conversion of iNO to one of these dilators.