Autonomic Agents and Vasopressors

Autonomic agents and vasopressors are drugs that act on the sympathetic and parasympathetic nervous systems and on vascular and cardiac receptors to control blood pressure, heart rate and perfusion. In anesthesia they are used to counteract the cardiovascular effects of anesthetic drugs and surgical stress, and they include vasopressors that raise blood pressure by constricting blood vessels as well as agents that adjust cardiac output and heart rate.

Definition

Autonomic agents are drugs that modulate the sympathetic or parasympathetic nervous system through adrenergic and cholinergic receptors; vasopressors are the subset that raise arterial pressure, chiefly by constricting blood vessels, and are used to support the circulation during anesthesia.

Scope

This topic covers the adrenergic and cholinergic receptor systems through which these drugs act, the main classes of sympathomimetic vasopressors and inotropes and of agents acting on the parasympathetic system, and the physiological reasoning that links a drug's receptor profile to its haemodynamic effect. It is a reference and educational account of how autonomic and vasoactive drugs work and are classified, not a guide to their administration or dosing.

Core questions

• Through which receptors do autonomic agents and vasopressors act, and how does the receptor profile determine the haemodynamic effect?
• How do vasopressors that primarily constrict vessels differ from agents that primarily change cardiac output or heart rate?
• How do these drugs counteract the cardiovascular effects of anesthesia and surgery?

Key concepts

• Sympathetic and parasympathetic nervous systems
• Adrenergic receptors (alpha-1, alpha-2, beta-1, beta-2)
• Muscarinic and nicotinic cholinergic receptors
• Vasopressors and vasoconstriction
• Inotropes and chronotropes
• Direct versus indirect sympathomimetics
• Anticholinergic and antimuscarinic agents

Key theories

Receptor-based classification of autonomic drugs

The haemodynamic effect of an autonomic agent follows from the adrenergic and cholinergic receptors it engages: alpha-1 activation constricts vessels and raises blood pressure, beta-1 activation increases cardiac rate and contractility, beta-2 and muscarinic actions have their own effects, so a drug's receptor selectivity predicts whether it acts mainly as a vasoconstrictor, an inotrope or a chronotrope.

Mechanisms

Autonomic agents act at adrenergic and cholinergic receptors. Sympathomimetic vasopressors stimulate alpha-1 adrenergic receptors on vascular smooth muscle to cause vasoconstriction and raise arterial pressure, while beta-1 stimulation increases the rate and force of cardiac contraction; some agents act directly on receptors and others indirectly by releasing endogenous noradrenaline. Parasympathetic and antimuscarinic drugs act at cholinergic receptors to slow or accelerate heart rate and to modify secretions. A drug's particular combination of receptor actions therefore predicts its cardiovascular profile — for example, a relatively pure alpha-1 agonist raises blood pressure chiefly by vasoconstriction, whereas a mixed agent also alters heart rate and cardiac output. In anesthesia these actions are used to oppose the vasodilation and cardiac depression produced by anesthetic drugs and by neuraxial blockade.

Clinical relevance

Autonomic agents and vasopressors are used to maintain blood pressure and perfusion during anesthesia, and understanding their receptor pharmacology underlies the rational matching of a drug to a haemodynamic problem. This entry is descriptive and educational; it does not provide dosing, administration thresholds or individualized treatment guidance.

Evidence & guidelines

The receptor pharmacology of adrenergic and cholinergic agents is established in pharmacology references such as Westfall and colleagues (2018), and the physiology and use of vasopressors and inotropes are summarised in reviews such as Overgaard and Dzavik (2008). Specific perioperative comparisons have been tested in randomized trials, for example Cooper and colleagues (2002) comparing phenylephrine and ephedrine during spinal anesthesia for cesarean delivery. Detailed clinical recommendations are treatment-oriented and beyond the educational scope of this node.

History

The autonomic basis of cardiovascular control was established through the receptor pharmacology of the twentieth century, which distinguished adrenergic from cholinergic transmission and subdivided the adrenergic receptors into alpha and beta types. This framework, set out in pharmacology references such as Westfall and colleagues (2018), allowed vasopressors and other autonomic agents to be selected by their receptor profile. In anesthesia, controlled trials such as Cooper and colleagues (2002) have refined the comparison of specific vasopressors in particular settings.

Key figures

• David W. Cooper
• Thomas C. Westfall
• Christopher B. Overgaard

Seminal works

• cooper-2002
• westfall-westfall-2018

Frequently asked questions

What is a vasopressor?

A vasopressor is a drug that raises arterial blood pressure, chiefly by constricting blood vessels through alpha-1 adrenergic receptor activation; in anesthesia vasopressors are used to oppose the fall in blood pressure caused by anesthetic drugs and neuraxial blockade.

Why does a drug's receptor profile matter for its cardiovascular effect?

Because each adrenergic and cholinergic receptor produces a characteristic response — alpha-1 vasoconstriction, beta-1 increased cardiac rate and force, and so on — the combination of receptors a drug activates predicts whether it acts mainly as a vasoconstrictor, an inotrope or a chronotrope.