Vasodilators used for acute management of
hypertensive crisis or malignant hypertension
include sodium nitroprusside and diazoxide.
Sodium nitroprusside is the agent of choice.
Administered by a continuously variable
rate i.v. infusion pump, precise blood pressure
control can be obtained.
Nitroprusside, a nitrovasodilator, is
metabolized by smooth muscle cells to nitric
oxide which dilates both arterioles and venules.
Diazoxide is infrequently used unless
accurate infusion pumps are unavailable.
The mechanism of action involves
activation of ATP-sensitive potassium
channels, depolarization of arteriolar
smooth muscle, relaxation and dilation.
Adverse effects include salt
and water retention and hyperglycemia.
Diazoxide inhibits insulin release.
Side effects are mainly due to excessive
Much less commonly, toxicity may result
from conversion of nitroprusside to cyanide and
thiocyanate. Risk of toxicity due to thiocyanate
increases after 24 to 48 hours.
Nitroprusside can worsen arterial
hypoxemia in patients with obstructive
pulmonary airway disease
since nitroprusside will interfere with hypoxic
pulmonary vasoconstriction. A result is
increasing ventilation-perfusion mismatching.