الفهرس | Only 14 pages are availabe for public view |
Abstract The autonomic nervous system may be considered as having a dual function. The first function is to maintain the internal environ- ment of the body in a state that encourages optimal function of the various organ systems. The second function is to prepare the body to undertake extra-ordinary efforts in situations that threaten the body. Classically, two divisions of autonomic nervous system have been described. The parasympathetic nervous system which can be conceived as restorative system either before or after the stress& the sympathetic nervous system which prepare the body forfight or flight. Most of the sympathomimetic drugs in use today have mixed cc and B stimulating properties. Only the B-agonist isoproterenal and the cc agonist phenylephrine and methoxamine are relatively pure drugs. The endogenous catecholamines are a perfect exam pIe. Dopamine has ~1 - B1-, and dopamine receptor mediated effects. The dopamine receptors are specifically activated by low doses of dopamine. Higher doses of dopamine agonize first B-receptors and then, at higher doses still, cc receptors. The other sympathomimetic arm nes all have mixed actions in two respectsdoses still, a:: receptors. They have both a direct action on the receptor as well as the affect noted for dopamine of releasing endogenous norepinephrine. The three most commonly used synthetic sympathomimetic, mephentermine (Wyamine), ephedrine and metaraminol (Ar ami ne) all have prominent indirect effect, although the former two agents are also direct-acting drugs. A major prolbem in modern cardiovascular intensive care is the support of the failing or ischemic heart without producing either marked tachycardia or peripheral vasodilation. |