Drugs and the Sympathetic Nervous System

Question 1

True or false:

Stimulation of α1 receptors elicits the contraction of smooth muscle in all locations

Answer 1

False. In the gastrointestinal (GI) tract, the stimulation of α1 receptors produces smooth muscle relaxation.

Question 2

True or false:

The most obvious effect of α1 stimulation results from its effect on the tone of vascular smooth muscle

Answer 2

True. The most significant effect is on the tone of vascular smooth muscle, where constriction of large veins, arteries and arterioles occurs, particularly in the skin and the splanchnic vessels. The resulting increase in blood pressure is usually associated with a decrease in cardiac output because of the increased afterload and activation of the baroreflex, which may cause bradycardia.

Question 3

True or false:

The distribution of α1 receptors determines the degree of regional vasoconstriction

Answer 3

True. The distribution of α1 receptors determines the degree of regional vasoconstriction, with minimal effect on the vasculature of the heart and brain. The density of α1 receptors is much greater in the radial artery than the internal mammary artery, which has obvious clinical relevance to postoperative management of some cardiac surgical patients.

Question 4

True or false:

Long-term α1-receptor stimulation produces a trophic response

Answer 4

True. Long-term α1-receptor stimulation produces a trophic response, involving smooth muscle proliferation. In particular, this occurs in blood vessels, receptor subtype α1B, and in the prostate gland, receptor subtype α1A. Tamsulosin is used in the treatment of benign prostatic hypertrophy, relieving obstruction and thus improving urinary flow, because of its selectivity for the receptor subtype α1A.

Question 5

True or false:

Alpha receptors have no effect on heart function

Answer 5

False. Changes in afterload have an indirect effect of cardiac function, but alpha receptors make up 10% of the total adrenoceptor population of the normal human heart. They are inotropic but have little effect on coronary flow.

Question 6

Stimulation of α receptors results in an increase in systemic vascular resistance

Answer 6

True

Question 7

Stimulation of α1 receptors results in the contraction of all smooth muscle (supplied by α1 receptors) except GI tract muscle.

Answer 7

True

Question 8

Circulating catecholamines are the main source of stimulation of α2 receptors

Answer 8

True

Question 9

Stimulation of β1 receptors stimulates the heart

Answer 9

True

Question 10

Stimulation of β2 receptors results in constriction of skeletal, bronchial and uterine smooth muscle

Answer 10

False. Stimulation of β2 receptors results in relaxation of skeletal, bronchial and uterine smooth muscle.

Question 11

A 42-year-old woman, previously fit and well, is anaesthetized for a laparotomy for peritonitis secondary to a perforated appendical abscess. She becomes hypotensive despite generous IV fluids. Her heart rate is 130 beats per minute (bpm), and central venous pressure (CVP) is 15 mmHg. Agents that would be suitable for this patient include:

A. Metaraminol
B. Phenylephrine
C. Ephedrine
D. Clonidine
E. Salbutamol
F. Aminophylline

Answer 11

Metaraminol or phenylephrine are the logical choices here, acting directly via α-adrenergic receptors, to elicit peripheral vasoconstriction. They are a good choice in sepsis, when cardiac function is likely to be relatively normal.

Question 12

A 28-year-old woman is under spinal anaesthesia for elective Caesarean section. Her initial heart rate of 84 bpm and blood pressure of 120/60 mmHg fall to 55 bpm and 80/45 mmHg respectively, following spinal injection. IV fluids are running freely. In these circumstances, you would administer:

A. Metaraminol
B. Phenylephrine‏
C. Ephedrine
D. Clonidine
E. Salbutamol
F. Aminophylline

Answer 12

Ephedrine is the correct agent for this situation, as it stimulates α-adrenergic receptors and β-adrenergic receptors to provide a desirable combination of peripheral vasoconstriction (α receptors) and positive chronotropy (increased heart rate and β receptors). The bradycardia associated with both metaraminol and phenylephrine make these less satisfactory options.

Question 13

A 19-year-old man and known asthmatic is post-induction for operative reduction of a fracture.

He develops increased airway pressures and a prolonged, expiratory wheeze on auscultation. Agents that would be suitable for this patient include:

A. Metaraminol
B. Phenylephrine‏
C. Ephedrine
D. Clonidine
E. Salbutamol
F. Aminophylline

Answer 13

Salbutamol elicits brochodilatation by stimulating β2 receptors. Aminophylline is usually a second-line treatment.

Question 14

An 80-year-old man with known ischaemic heart disease is undergoing a right hemicolectomy. He has a general anaesthetic plus epidural in situ and generous IV fluids to a CVP of 16 mmHg. However, his blood pressure falls from 150/85 to 95/65 mmHg, with a heart rate of 50-55 bpm throughout. An initial agent that you would choose for this patient is:

A. Metaraminol
B. Phenylephrine‏
C. Ephedrine
D. Clonidine
E. Salbutamol
F. Aminophylline

Answer 14

Ephedrine. Metaraminol and phenylephrine‏ produce a beneficial peripheral vasoconstriction, but cardiac output is likely to be reduced further and a reflex bradycardia may also occur. These are arguable disadvantages, but some anaesthetists would choose to use these drugs in this situation.