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Question 448
Pharmacology → General
Classification
Question
SSBAQuestion Header
A beta-sympathomimetic is administered at a therapeutic dose. Heart rate and myocardial contractility increase substantially, yet mean arterial pressure changes little. Which accompanying effect best accounts for this observation?
Question Stem
Select the single best answer.
Options
A
Marked reduction in cardiac output
B
Increased total peripheral resistance in skeletal muscle
C
Reduced total peripheral resistance due to skeletal-muscle vascular dilatation
D
Reflex inhibition of myocardial contractility
E
Direct blockade of cardiac beta-1 receptors
Explanation
Beta-sympathomimetic drugs have direct positive chronotropic and inotropic cardiac effects. Mean blood pressure may change little because the pressure-raising effect of increased heart rate and contractility is counteracted by a reduction in total peripheral resistance caused by vasodilatation in vessels perfusing skeletal muscle.
Option Validity
A) Beta-sympathomimetics increase cardiac stimulation rather than producing a marked reduction in cardiac output.
B) Total peripheral resistance is reduced rather than increased due to vasodilatation in skeletal-muscle vessels.
D) These drugs directly stimulate rather than reflexly inhibit myocardial activity.
E) Beta-1 blockade would oppose the positive chronotropic and inotropic effects of beta-sympathomimetic stimulation.
Further Reading
Saxena R. Textbook for MRCOG-1. Chapter 12: Pharmacology. Adrenergic Agents, p.345.