Assertion: viagra's use | /r/CapableOf | produce variable efficacy results
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text: Note: Use of preoperative sildenafil therapy for the prevention of pulmonary hypertension following congenital heart surgery has produced variable efficacy results (Palma, 2011); a double-blind, placebo-controlled trial showed a negative impact on oxygenation and ventricular function (Vassalos, 2011).