Pocket Psych Drugs: Point-of-Care Clinical Guide 2nd Edition

Book Preface

Pharmacokinetics (PK) can be defined as “how the body processes a drug” resulting in a drug’s concentration in the body. This is done through absorption, distribution, metabolism, and excretion (ADME).
Absorption: Describes the drug’s movement from point of administration (oral, injection, skin) until it reaches the bloodstream. In oral administration, first-pass metabolism refers to how much of the drug is metabolized by the liver and therefore is not available to the bloodstream (bioavailability of drug). This determines the dose needed for oral administration or the need for an alternative route of entry (such as parenteral).
Distribution: Movement of drug from the bloodstream to the rest of the body.
Concerned with movement over the blood-brain barrier (may affect the brain) or crossing the placenta (may affect the fetus). Also concerns highly protein-bound drugs that may cause drug interactions.
Metabolism and Excretion: The primary organ of metabolism is the liver, and excretion of drugs takes place through the kidneys. Dosing considerations are based on how well the liver and kidneys are functioning. Half-life is also a factor as drugs with long half-lives may accumulate, resulting in overdose or toxicity.
Half-life is the time (hours) that it takes for 50% of a drug to be eliminated from the body. Time to total elimination involves halving the remaining 50%, and so forth, until total elimination. Half-life is considered in determining dosing frequency and in determining time to steady state. The rule of thumb for steady state (stable concentration/manufacture effect) attainment is 4–5 half-lives. Because of fluoxetine’s long half-life, a 5-week washout is recommended after stopping fluoxetine and before starting an MAOI to avoid a serious and possibly fatal reaction.
Protein Binding is the amount of drug that binds to the blood’s plasma proteins; the remainder circulates unbound. It is important to understand this concept when prescribing two or more highly protein-bound drugs as one drug may be displaced, causing increased blood levels and adverse effects.
Pharmacodynamics is usually defined as “what the drug does to the body” and therefore the effect the drug has on the body (positive effects and side effects)

Cytochrome P-450 Enzyme System is involved in drug biotransformation and metabolism. It is important to develop a knowledge of this system to understand drug metabolism and especially drug interactions. Over 30 P-450 isoenzymes have been identified. The major isoenzymes include CYP1A2/2A6/2B6/2C8/2C9/2C18/2C19/2D6/2E1/3A4/3A5-7.

Medication and the Elderly
● Relevant drug guides provide data about dosing for the elderly and debilitated clients. (See also Drug/tabs.)
● Elderly and debilitated clients are started at lower doses, often half the recommended adult dose. This is due to:
• Decreases in GI absorption
• Decrease in total body water (decreased plasma volume)
• Decreased lean muscle and increased adipose tissue

SYMPATHETIC AND PARASYMPATHETIC EFFECTS—cont’d
Structure Sympathetic Parasympathetic
Kidney
Bladder
Sweat glands
(From Pedersen: PsychNotes, 5e, 2018, with permission)
Increased urine
Relaxation of sphincter
No change
Decreased urine
Contraction of sphincter
↑ Sweating
NEUROTRANSMITTER FUNCTIONS AND EFFECTS
Neurotransmitter Function Effect
Dopamine
Serotonin
Norepinephrine
GABA
Acetylcholine
(From Pedersen: PsychNotes, 5e, 2018, with permission)
Fine movement, emotional behavior.
↑ in schizophrenia and ↓ Parkinson’s.
Implicated in addiction.
Sleep, mood, eating behavior.
Implicated in mood disorders, anxiety,
and violence.
Arousal, wakefulness, learning.
Implicated in anxiety, ADHD, and
“fight or flight reaction.”
Anxiety states.
Arousal, attention, movement.
Increase = spasms and decrease =
paralysis.
Inhibitory
Inhibitory
Excitatory
Inhibitory
Excitatory
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• Reduced first-pass effect in the liver and cardiac output
• Decreased serum albumin
• Decreased glomerular filtration and renal tubular secretion
• Time to steady state is prolonged
Because of decrease in lean muscle mass and increase in fat (retains lipophilic drugs [fat-storing]), reduced first-pass metabolism, and decreased renal function, drugs may remain in the body longer and produce an additive effect.
Clinical Tips/Alert: With the elderly, start doses low and titrate slowly. Drugs that result in postural hypotension, confusion, or sedation should be used cautiously or not at all.
● Poor Drug Choices for the Elderly – Drugs that cause postural hypotension or anticholinergic side effects (sedation).
• TCAs – anticholinergic (confusion, constipation, visual blurring); cardiac (conduction delay; tachycardia); alpha-1 adrenergic (orthostatic hypotension [falls])
• Benzodiazepines – longer the half-life, the greater the risk of falls. May also increase agitation and confusion. Choose a shorter half-life. Lorazepam (T 1/2 12-15 hr) is a better choice than diazepam (T 1/2 20-70 hr; metabolites up to 200 hr).
• Sedatives/hypnotics – Always start with a lower dose and monitor for sedation, confusion, cognitive impairment, amnesia, and daytime drowsiness. Caution against next day driving and until adequate adjustment to medication has been observed. Increased visits to EDs have happened due to falls and adverse reactions.
• Lithium – use cautiously in elderly, especially if debilitated. Risk of dehydration may yield lithium toxicity.
• Diphenhydramine (Benadryl; Canada: Allerdryl) – ↑ susceptibility to adverse reactions/anticholinergic effects (delirium, dizziness, confusion).
• Consider age, weight, mental state, and medical disorders and compare with side-effect profile in selecting medications