Cerebral vascular responses to hypocapnia during nitroglycerin-induced hypotension.

For many neurosurgical procedures, elective hypotension is used to reduce the risk of cerebral vessel rupture and hypocapnia is used to constrict cerebral vessels, thereby reducing cerebral blood volume. Although nitroglycerin (NTG) often is used to produce hypotension during neurological surgery, it is not known whether NTG-induced cerebral vasodilation interferes with the cerebral vasoconstrictor response to hypocapnia. This study examined cerebral vascular responses to hypocapnia during NTG-induced hypotension in eight dogs that were lightly anesthetized with halothane and had an open cranium. Cerebral vascular resistance (CVR) and cerebral blood flow (CBF) at PaCO2 = 40 mm Hg and at PaCO2 = 20 mm Hg were examined first at normal mean arterial pressure (MAP) and then at MAP = 50 mm Hg. CO2 responsiveness, as indicated by increased CVR and decreased CBF, was intact at normal MAP but absent during hypotension. These results suggest that the cerebral vasodilation that accompanies NTG-induced hypotension exerts a greater influence on cerebral vessels than the cerebral vasoconstricting influence of hypocapnia. It is concluded that, during NTG-induced hypotension and craniotomy, hypocapnia will not reduce cerebral blood volume or further decrease CBF to cause ischemia.