Cerebral blood flow and intracranial pressure in the dog during intravenous infusion of nitroglycerin alone and in combination with dopamine.

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Abstract
Nitroglycerin, long known as a safe and effective dilator of the large coronary arteries, has recently been shown to dilate the basilar artery of the dog after experimentally induced vasospasm. In this study we have evaluated the effects of intravenous nitroglycerin on local cerebral blood flow (H2 clearance technique) and intracranial pressure (intracisternal needle monitor) in normal beagle dogs (Group 1). In each of 7 dogs, infusion of nitroglycerin at rates of 3, 5, and 10 microgram/kg/min did not change blood flow in the right and left caudate nucleus, thalamus, frontal and parietal cortex. Autoregulation of cerebral blood flow remained unimpaired and intracranial pressure remained stable during nitroglycerin infusion. The effects of a combination of intravenous nitroglycerin and dopamine on local cerebral blood flow was evaluated in another group of normal beagle dogs (Group 2). Local cerebral blood flow decreased or remained unchanged in response to intravenous infusion of dopamine at low rates, increased in response to moderate rates and again decreased in response to high infusion rates. These dopamine induced changes in blood flow occurred whether or not nitroglycerin was infused simultaneously. When the vasoconstrictor activity of dopamine was blocked by phentolamine or methysergide, local cerebral blood flow increased at moderate and high infusion rates, again whether or not nitroglycerin was infused simultaneously. Our data suggest that nitroglycerin affects mainly the extracerebral capacitance arteries while dopamine affects the smaller intraparenchymal resistance vessels. Nitroglycerin has little effect on cerebral blood flow even when used in combination with dopamine.