One phenomenon associated with prazosin is known as the "first dose response", in which the side effects of the drug –  specifically orthostatic hypotension, dizziness, and drowsiness –  are especially pronounced in the first dose.Orthostatic hypotension and syncope are associated with the bodys poor ability to control blood pressure without active α-adrenergic receptors. People on prazosin should be told to rise to stand up slowly, since their poor baroreflex may cause them to faint if their blood pressure is not adequately maintained during standing. The nasal congestion is due to dilation of vessels in the nasal mucosa. Pharmacology
Pharmacodynamics
Prazosin is an α1-blocker that acts as a non-selective inverse agonist at α1-adrenergic receptors, including of the α1A-, α1B-, and α1D-adrenergic receptor subtypes. It binds to these receptors with affinity (Ki) values of 0.13 to 1.0 nM for the α1Α-adrenergic receptor, 0.06 to 0.62 nM for the α1B-adrenergic receptor, and 0.06 to 0.38 nM for the α1D-adrenergic receptor. It has much lower affinity for the α2-adrenergic receptors (Ki = 210–5,012 nM for the α2A-adrenergic receptor, 13–398 nM for the α2B-adrenergic receptor, and 10–200 nM for the α2C-adrenergic receptor). The α1-adrenergic receptors are found in vascular smooth muscle, where they are responsible for the vasoconstrictive action of norepinephrine. They are also found throughout the central nervous system. α1-Adrenergic receptors have additionally been found on immune cells, where catecholamine binding can stimulate and enhance cytokine production.
A papillary fibroelastoma is a primary tumor of the heart that typically involves one of the valves of the heart. Papillary fibroelastomas, while considered generally rare, make up about 10 percent of all primary tumors of the heart. They are the third most common type of primary tumor of the heart, behind cardiac myxomas and cardiac lipomas. Signs and symptoms
A papillary fibroelastoma is generally considered pathologically benign, however outflow obstruction or embolism can be associated with syncope, chest pain, heart attack, stroke and sudden cardiac death. Symptoms due to papillary fibroelastomas are generally due to either mechanical effects of the tumor or due to embolization of a portion of the tumor to a distal organ. In particular, chest pain or syncope may be due to transient occlusion of the left main coronary artery by the tumor, while a heart attack or sudden cardiac death may be due to embolization of a portion of the tumor into a coronary artery. Diagnosis
Papillary fibroelastoma are typically found and accurately diagnosed by imaging. The diagnosis is confirmed by pathology. Histologically, papillary fibroelastomas have branching avascular papillae, composed of collagen, that are covered by endothelium. Treatment
If the tumor is found incidentally in an asymptomatic person, the treatment approach is controversial. Certainly a conservative approach is warranted in certain individuals. If the tumor is large, greater than 1 cm in asymptomatic patients, and pedunculated, a case may be made for surgical excision prior to symptoms developing due to the higher risk of embolism.