vasopressin
Pitressin Synthetic

Available forms
Available by prescription only
Injection: 0.5-ml and 1-ml ampules, 20 units/ml; 0.5-ml, 1-ml vials, 20 units/ml

Indications and dosages
 Nonnephrogenic, nonpsychogenic diabetes insipidus. Adults: 5 to 10 units I.M. or S.C. b.i.d. to q.i.d., p.r.n.
Children: 2.5 to 10 units I.M. or S.C. b.i.d. to q.i.d., p.r.n.
 Postoperative abdominal distention. Adults: 5 units I.M. initially; then q 3 to 4 hours, increasing dose to 10 units, if needed. Reduce dosage for children proportionately.
 To expel gas before abdominal X-ray. Adults: Inject 5 to 15 units S.C. at 2 hours; then again at 30 minutes before X-ray. Enema before first dose may also help to eliminate gas.
 Upper GI tract hemorrhage. Adults: 0.2 to 0.4 units/minute I.V. or 0.1 to 0.5 units/minute intra-arterially.

Pharmacodynamics
Antidiuretic action: Used as an antidiuretic to control or prevent signs and complications of neurogenic diabetes insipidus. Acting primarily at the renal tubular level, drug increases cAMP, which increases water permeability at the renal tubule and collecting duct, resulting in increased urine osmolality and decreased urine flow rate.
Peristaltic stimulant action: Used to treat postoperative abdominal distention and to facilitate abdominal radiographic procedures, vasopressin induces peristalsis by directly stimulating contraction of smooth muscle in the GI tract.
Hemostatic action: In patients with GI hemorrhage, vasopressin, administered I.V. or intra-arterially into the superior mesenteric artery, controls bleeding of esophageal varices by directly stimulating vasoconstriction of capillaries and small arterioles.

Pharmacokinetics
Absorption: Destroyed by trypsin in GI tract; must be given intranasally or parenterally.
Distribution: Distributed throughout extracellular fluid, with no evidence of protein binding.
Metabolism: Most of dose destroyed rapidly in liver and kidneys.
Excretion: About 5% of S.C. dose excreted unchanged in urine after 4 hours. Duration of action after I.M. or S.C. administration is 2 to 8 hours. Half-life, 10 to 20 minutes.

Contraindications and precautions
Contraindicated in patients hypersensitive to vasopressin or its components and in patients with anaphylactoid reactions to them. Also contraindicated in patients with chronic nephritis accompanied by nitrogen retention. Use cautiously in children, pregnant women, elderly patients, preoperative or postoperative patients who are polyuric, and patients with seizure disorders, migraine headache, asthma, CV or renal disease, heart failure, goiter with cardiac complications, arteriosclerosis, or fluid overload.

Interactions
Drug-drug. Carbamazepine, chlorpropamide, clofibrate, fludrocortisone, phenformin urea, tricyclic antidepressants: May potentiate antidiuretic effect. Monitor therapeutic effect.
Demeclocycline, epinephrine, heparin, lithium, norepinephrine: Decreases antidiuretic effect. Monitor therapeutic effect.
Drug-lifestyle. Alcohol use: Reduces antidiuretic activity. Discourage alcohol use.

Adverse reactions
CNS: tremor, headache, vertigo.
CV: angina in patients with vascular disease, vasoconstriction, arrhythmias, cardiac arrest, myocardial ischemia, circumoral pallor, decreased cardiac output.
GI: abdominal cramps, nausea, vomiting, flatulence.
Skin: cutaneous gangrene, diaphoresis.
Other: water intoxication (drowsiness, listlessness, headache, confusion, weight gain, seizures, coma), hypersensitivity reactions (urticaria, angioedema, bronchospasm, anaphylaxis).

Effects on lab test results
None reported.

Overdose and treatment
Signs and symptoms of overdose include drowsiness, listlessness, headache, confusion, anuria, and weight gain (water intoxication).
 Treatment requires water restriction and temporary withdrawal of vasopressin until polyuria occurs. Severe water intoxication may require osmotic diuresis with mannitol, hypertonic dextrose, or urea, either alone or with furosemide.

Special considerations
 ALERT Never inject during first stage of labor; this may cause uterine rupture.
 ALERT Use extreme caution to avoid extravasation because of risk of necrosis and gangrene.
• Establish baseline vital signs and intake and output ratio at start of therapy.
• Monitor patient’s blood pressure twice daily. Watch for excessively elevated blood pressure or lack of response to drug, which may be indicated by hypotension.
• Monitor fluid intake and output and daily weight.
• Observe patient for signs of early water intoxication-drowsiness, listlessness, headache, confusion, and weight gain-to prevent seizures, coma, and death.
• Monitor abdominal distention and GI function; a rectal tube will facilitate gas expulsion after vasopressin injection.
Breast-feeding patients
• It isn’t known if drug appears in breast milk. Use cautiously in breast-feeding women.
Pediatric patients
• Children show increased sensitivity to drug’s effects. Use cautiously.
Geriatric patients
• Elderly patients show increased sensitivity to drug’s effects. Use cautiously.

Patient education
• Advise patient to rotate injection sites.
• Tell patient to drink one or two glasses of water with each dose of vasopressin to reduce adverse reactions such as unusual paleness, nausea, abdominal cramps, and vomiting.
• Tell patient to call immediately if he develops chest pain, confusion, fever, hives, rash, headache, problems with urination, seizures, weight gain, unusual drowsiness, wheezing, trouble with breathing, or swelling of face, hands, feet, or mouth.

Reactions may be common, uncommon, life-threatening, or COMMON AND LIFE THREATENING.
◆ Canada only
◇ Unlabeled clinical use