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CHAPTER 18 Gastrointestinal System 1

Antihistamine

Use Provides symptomatic Half-life: Onset: Peaks: Duration:

relief of allergic symptoms; 715 h PO: 20 min 2 h PO/IMsedative/antiemetic in IM: 20 min Rectal/IV:

surgery/labor; decreases Rectal: 20 min 28 h

post op nausea/vomiting; IV: 35 min

adjunct to analgesics

in control of pain;

management of

motion sickness.

Example Promethazine Route: Pregnancy Pharmacokinetic:

hydrochloride PO/IM/ category: C Well absorbed from GI tract,

(Phenergan) Rectal/IV after IM administration:

Widely distributed;metabolized in liver;

excreted in urine;

PB:6090%.

How it works Antihistamine: Inhibits histamine at histamine receptor sites,preventing, antagonizing most allergic effects (e.g., urticaria,

pruritus).

Antiemetic: Diminishes vestibular stimulation, depresseslabyrinthine function, acts on chemoreceptor trigger zone

(CTZ), producing antiemetic effect.

Sedative-hypnotic: Decreases stimulation to brain stem reticular

formation producing CNS depression.

Adult dose Allergic symptoms: PO: 25 mg at bedtime of 12.5 mg4 times/day; Rectal/IM/IV: 25 mg; may repeat in 2 hours.

Motion sickness: PO: 25 mg 3060 minutes before departure;may repeat in 812 h, then every morning on arising and before

evening meal.

Antiemetic: PO/IM/IV/Rectal: 12.525 mg q46 h as needed.

Pre- and postop sedation; adjunct to analgesics: IM/IV: 2550 mg.

Before administration Assess BP and pulse for bradycardia/tachycardia if patient isgiven parenteral form.

If used as an antiemetic assess for dehydration (poor skinturgor, dry mucous membranes, longitudinal furrows in tongue).

Obtain baseline blood work to include electrolyte levels.

Assess medical and drug history.

If used for pre-op sedation, have patient void beforeadministering medication and raise side rails to provide a

safe environment.

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Pharmacology Demystified2

Antihistamine (continued)

Administration PO: Give without regard to meals.

Scored tablets may be crushed.

IM: Note: Significant tissue necrosis may occur if given SubQ.Inadvertent intra-arterial injection may produce severe

arteriospasm, resulting in severe circulation impairment.

Inject deep IM.

IV: May be given undiluted or dilute with 0.9% NaCl.Final dilution should not exceed 25 mg/mL.

Administer at 25 mg/min rate through IV infusion tube.A too rapid rate of infusion may result in transient fall in B/P,

producing orthostatic hypotension, reflex tachycardia; if patient

complains of pain at IV site, stop injection immediately(possibility of intra-arterial needle placement/perivascular

extravasation).

Rectal: Refrigerate suppository; moisten with cold water beforeinserting well into rectum.

After administration Monitor vital signs.

Monitor serum electrolytes in patients with severe vomiting.

Assist with ambulation if drowsiness, lightheadedness occurs.

Maintain a safe environment.

Contraindications Comatose, those receiving large doses of other CNSdepressants, acutely ill/dehydrated children, acute asthmatic

attack, vomiting of unknown etiology in children, Reyes

syndrome, those receiving MAO inhibitors.

Extreme caution: History of sleep apnea, young children,family history of sudden infant death syndrome (SIDS), those

difficult to arouse from sleep.

Cautions: Narrow-angle glaucoma, peptic ulcer, prostatichypertrophy, pylorduodenal/ bladder neck obstruction, asthma,

COPD, increased intraocular pressure, cardiovascular disease,

hyperthyroidism, hypertension, seizure disorders.

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CHAPTER 18 Gastrointestinal System 3

Antihistamine (continued)

Side effects/ High incidence: drowsiness, disorientation. Hypotension,

adverse reaction confusion, syncope more likely in the elderly.

Frequent: Dry mouth, urinary retention, thickening of bronchialsecretions.

Occasional: Epigastric distress, flushing, visual disturbances,hearing disturbances, wheezing, paresthesia, sweating, chills.

Rare: Dizziness, urticaria, photosensitivity, nightmares.Fixed-combination form with pseudoephedrine may produce

mild CNS stimulation.

Adverse/toxic:

Paradoxical reaction (particularly in children) manifested asexcitation, nervousness, tremors, hyperactive reflexes,convulsions. CNS depression has occurred in infants and

young children (respiratory depression, sleep apnea, SIDS).

Long-term therapy may produce extrapyramidal symptoms

noted as dystonia (abnormal movements), pronounced motor

restlessness (most frequently occurs in children), and

Parkinsonian symptoms (esp. noted in elderly). Blood

dyscrasias, particularly agranulocytosis, have occurred.

Patient education Drowsiness and dry mouth may be an expected response todrug. Sugarless gum, sips of tepid water may relieve dry

mouth. Coffee/tea may help reduce drowsiness.

Report visual disturbances.

Avoid tasks that require alertness and motor skills untilresponse to drug is established.

Avoid alcohol and other CNS depressants.