Chapter 12

• Adrenergic agonists can be categorized as direct or indirect. Direct agonists bind to the receptor, whereas indirect agonists increase endogenous neurotransmitter activity.
• The primary effect of phenylephrine is peripheral vasoconstriction with a concomitant rise in systemic vascular resistance and arterial blood pressure.
• Clonidine appears to decrease anesthetic and analgesic requirements and to provide sedation and anxiolysis.
• Dexmedetomidine is a novel lipophylic α-methylol derivative with a higher affinity for α2-receptors than clonidine. It has sedative, analgesic, and sympatholytic effects that blunt many of the cardiovascular responses seen during the perioperative period.
• Long-term use of these agents, particularly clonidine and dexmedetomidine, leads to supersensitization and up-regulation of receptors; with abrupt discontinuation of either drug, an acute withdrawal syndrome manifested by a hypertensive crisis can occur.
• Ephedrine is commonly used as a vasopressor during anesthesia. As such, its administration should be viewed as a temporizing measure while the cause of hypotension is determined and remedied.
• Small doses (≤ 2 μg/kg/min) of dopamine (DA) have minimal adrenergic effects but activate dopaminergic receptors. Stimulation of these nonadrenergic receptors (specifically, DA1 receptors) vasodilates the renal vasculature and promotes diuresis.
• Favorable effects on myocardial oxygen balance make dobutamine a good choice for patients with the combination of congestive heart failure and coronary artery disease, particularly if peripheral vascular resistance and heart rate are already elevated.
• Labetalol lowers blood pressure without reflex tachycardia because of its combination of α- and β-effects.
• Esmolol is an ultrashort-acting selective β1-antagonist that reduces heart rate and, to a lesser extent, blood pressure.
• Discontinuation of β-blocker therapy for 24–48 h may trigger a withdrawal syndrome characterized by hypertension, tachycardia, and angina pectoris.

The three previous chapters presented the pharmacology of drugs that affect cholinergic activity. This chapter introduces an analogous group of agents that interacts at adrenergic receptors—adrenoceptors. The clinical effects of these drugs can be deduced from an understanding of adrenoceptor physiology and a knowledge of which receptors each drug activates or blocks.

The term adrenergic originally referred to the effects of epinephrine (adrenaline), as opposed to the cholinergic effects of acetylcholine. It is now known that norepinephrine (noradrenaline) is the neurotransmitter responsible for most of the adrenergic activity of the sympathetic nervous system. With the exception of eccrine sweat glands and some blood vessels, norepinephrine is released by postganglionic sympathetic fibers at end-organ tissues (Figure 12–1). In contrast, as was explained in Chapter 10, acetylcholine is released by preganglionic sympathetic fibers and all parasympathetic fibers.

Figure 12–1.

The sympathetic nervous system. Organ innervation, receptor type, and response to stimulation. The origin of the sympathetic chain is the thoracoabdominal (T1–L3) spinal cord, in contrast to the craniosacral distribution of the parasympathetic nervous system. Another anatomic difference is the greater distance from the sympathetic ganglion to the visceral structures.

Norepinephrine is synthesized in ...

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.

Ok

Subscription Options

AccessAnesthesiology Full Site: One-Year Subscription

Connect to the full suite of AccessAnesthesiology content and resources including procedural videos, interactive self-assessment, real-life cases, 20+ textbooks, and more