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Pharmacology

Shannon Brindle

INTRODUCTION

Test Taking Tip

The most commonly tested mechanism of action of drugs are succinylcholine (vs. other paralytics), imatinib, fluoroquinolones, warfarin, and corticosteroids. Don’t overmemorize intricate details of exotic drug classes.

MECHANICS

What are some basic drug properties?

  • Absorption, distribution, metabolism, elimination

What is first-order kinetics?

  • Drug dose determines quantity eliminated (dose-dependent elimination)

What is zero-order kinetics?

  • Also known as Hoffman elimination: constant drug quantity eliminated per unit time

What is the primary role of the P-450 system?

  • Primary drug oxidizers

What are some of the inducers of the P-450 system?

  • Cigarette smoke, phenobarbital, rifampin, ethanol, INH, phenytoin, etc.

What are some of the inhibitors of the P-450 system?

  • Grapefruit, erythromycin, nelfinavir, itraconazole, etc.

FIGURE 5-1

Drug administration by continuous intravenous infusion (upper panel) or intermittent intravenous bolus (lower panel). Attainment of steady-state plasma concentration (Cpss) occurs after 3 to 5 half-lives, regardless of the dosing regimen. Peak and trough fluctuations around Cpss are aimed to each be within the therapeutic range (therapeutic but subtoxic plasma levels). (Reproduced with permission from Hall JB, Schmidt GA, Wood LDH. Principles of Critical Care, 3rd ed. New York, NY: McGraw Hill; 2005.)

Two graphs show the drug administration by continuous intravenous infusion and intermittent intravenous bolus. The horizontal axes represent the time, the number of half-lives ranging from 0 to 10 in increments of 1 The vertical axes represent the concentration, percent of the steady-state ranging from 0 to 100 in increments of 20. The first graph shows a line for steady-state plasma concentration that begins at the origin and passes through the points (1, 50), (2, 70), (3, 90), (4, 98), (5, 100, (6, 100), (7, 100), (8, 100), (9, 100) and ends at (10, 100). The second graph shows a line for steady-state plasma concentration that begins at the origin and passes through the points (1, 50), (2, 70), (3, 90), (4, 98), (5, 100, (6, 100), (7, 100), (8, 100), (9, 100) and ends at (10, 100). A frequency curve that begins from the origin is drawn along the line. The high-points are the steady-state plasma concentration peak and the lower-points are labeled as steady-state plasma concentration trough from 0 to 10 half-lives. All the data are approximate.
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What is the half-life of a drug?

  • Time it takes for half of the drug to be eliminated from the system. To achieve either complete elimination or steady state takes approximately 5 half-lives.

What is a phase I reaction?

  • Also known as modification. Nonsynthetic reaction, for example, oxidation/reduction/hydrolysis, NOT conjugation. Catalyzed mainly by cytochrome P-450.

What is a phase II reaction?

  • Synthetic reaction, for example, conjugation, methylation, sulfation, etc.

GASTROINTESTINAL

ESOPHAGUS

What are some treatments for achalasia?

  • Calcium channel blockers, long-acting nitrates, botulinum toxin injections

What is the management for esophageal varices bleeding?

  1. Vasoactive medications: vasopressin (splanchnic vasoconstrictor at V1 receptors, increases factor VIII and VWF via extrarenal V2 receptors), somatostatin/octreotide (inhibits release of vasodilator hormones—indirect cause of splanchnic vasoconstriction and decreased portal flow)

  2. Prophylaxis: nonselective β blockade: Propranolol/nadolol (preventative role; inhibits B2 adrenergic splanchnic vasodilation). Carvedilol (β blocker with mild anti α-1 activity) and alternative to nonselective blockade. Must watch for significant MAP drops, which won’t be tolerated in cirrhotic patients. Nitrates (not recommended as primary prophylaxis).

What are some treatment options for reflux disease?

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