How to treat a patient with pulmonary hypertension but have systemic hypotension?

Pulmonary hypertension is increased pressure in the pulmonary circulation. It has many secondary causes; when the cause is unknown it is called primary pulmonary hypertension (PPH). In PPH, pulmonary vessels become constricted, hypertrophied, and fibrosed. Pulmonary hypertension leads to right ventricular overload and failure. Symptoms are fatigue, exertional dyspnea, and, occasionally, chest discomfort and syncope. Diagnosis is by measuring pulmonary artery pressure. Treatment is with vasodilators and, in selected advanced cases, lung transplantation. Prognosis is poor overall if a treatable secondary cause is not found.(Merck)
There is no known cure. The goal of treatment is to control symptoms.
Medicines used to treat pulmonary hypertension include:
* Calcium channel blockers
* Diuretics
* Prostacyclin
* Bosentan
Your doctor will decide which medicine is best for you.
Persons with advanced cases of pulmonary hypertension may need oxygen. If treatment with medications fails, suitable candidates may be helped by lung or heart-lung transplant. (MedlinePlus).

The above respondant gave you a nice overview concern PRIMARY pulmonary hypertension.

Secondary pulmonary hypertension is more common, however. A very common secondary source is sleep apnea. An overnight sleep study is required to make the diagnosis. After the diagnosis is established there is a variety of equipment that can used with the goal of keeping the back of the throat open to keep air flow moving while asleep. The good news? If the sleep apnea is treated, the pulmonary hypertension will largely resolve. Treating sleep apnea affords other health benefits, by the way.

To address your specific question...if the patient is NOT taking long-acting nitrates, then there is some data that supports using Viagra for pulmonary hypertension.
http://www.medscape.com/viewarticle/4418...
Bosetan also is helpful without lowering systemic hypertension.

You are right that most other agents will exacerbate systemic hypotension.

I hope this is helpful. Good luck.