**9. Pulmonary vascular reactivity testing and vasodilator therapy**

Diagnostic catheterization followed by pharmacological testing of vasodilator therapy response is required to test the pulmonary vasoreactivity in patients with IPAH before prescribing a vasodilator. The most commonly used drugs are: iv prostacyclin, iv adenosine, inhaled nitric oxide and inhaled iloprost. Oxygen, nitroprusside, and hydralazine should not be used as pulmonary vasodilator testing agents. A complete right heart catheterization and an invasive monitoring of the systemic pressures are mandatory. The increased pulmonary vascular resistance results from extensive vascular changes and vasoconstriction. Therefore, in pulmonary hypertension true pulmonary vasodilation is only present if, in addition to a decreased pulmonary vascular resistance, reductions in the transpulmonary gradient and the mean pulmonary artery pressure are achieved.

A positive test or 'responder to vasodilator' is defined as a drop in mPAP of ≥ 10 mmHg to an absolute level < 40 mmHg. A positive test is observed in 10-15% of patients with IPAH. However half of these patients will have a long-term response to calcium channel blockers (CCB). [39]

Only patients with an acute vasodilator response to an intravenous or inhaled pulmonary vasodilator challenge (eg, with adenosine, epoprostenol, nitric oxide) derive any long-term benefit from CCBs. Such patients constitute less than 15% of patients with IPAH and probably less than 3% of patients with other forms of PAH. [24], [35], [39]

Patients who do not have an acute vasodilator response to a vasodilator challenge have a worse prognosis on long-term oral vasodilator therapy compared with those who have an initial response. These non-responders are those who have no significant change of the mean pulmonary vascular pressure or symptomatic systemic hypotension and no change or a reduction of cardiac index (by more than 10 %), possibly accompanied by an increase in right atrial pressure (by more than 20 – 25 %). However, the absence of an acute response to intravenous or inhaled vasodilators does not preclude the use of intravenous vasodilator therapy. In fact, continuous intravenous vasodilator therapy is strongly suggested for these patients because CCBs are contraindicated. [40]
