Preoperative Assessment Particular emphasis should be placed on the following aspects of the patient’s history:AllergiesDangerous reactions can occur with radiologic contrast media, and protamine. Contrast media are associated with a 5% to 8% incidence of untoward systemic reactions that can range from nausea to anaphylaxis and death.(32) Reactions can be due to the hypertonicity of the substance, direct cardiac depression, or idiosyncratic anaphylactoid reactions.(33) Older hypertonic, hyperosmolar, ionic contrast media are especially associated with intravascular volume problems, such as initial hypervolemia followed by diuresis and dehydration. The newer nonionic low osmolarity agents, although more expensive, seem to be safer and can be used in greater volumes,(34) but the incidence of serious idiosyncratic reactions ( 2 to 3 percent) is no different when compared with older agents. Because these substances contain iodine to increase their density, allergies to iodine and shellfish may predispose to a contrast reaction. Patients that have a history of reactions to contrast media should be pretreated orally with steroids (e.g., prednisone 50 mg PO q6h for 18 hours prior to the study) and antihistamines (e.g., diphemhydramine 50 mg PO or IM 1 hour prior to the study), or with intravenous infusions of steroids and antihistamines the night before and the morning of the study.(35)Severe reactions can occur when administering protamine. These can be due to rapid administration, immunologically mediated anaphylaxis, or anaphylactoid reactions, and range from rash and urticaria to severe pulmonary hypertension and death.(36) (37) A diabetic on protamine insulin may have an increased risk of developing a protamine reaction.Other weakly predisposing factors reported include prior vasectomy, a history of fish allergy, and previous protamine administration.(38) (39) (40)Treatment of contrast media reactions Adrenergic agonists Epinephrine 3-5 mg/Kg IV bolus Epinephrine 1-4 mg/min IV infusion Methylxanthines Aminophyline 5-6 mg/kg/20 min.,initial dose Aminophyline 0.5 - 0.9 mg/kg/h., maintenance Anticholinergics Atropine 0.5 - 2 mg IV Antihistamines Diphenhydramine 25 - 50 mg IV Steroids Methylprednisone 100 - 1000 mg IV Dexamethasone 4-20 mg IV From Goldberg M: Systemic reactions to intravascular contrast media: a guide for the anesthesiologist. Anesthesiology 60:46-56, 11984.;Premedication The primary goals of premedication are anxiolysis, sedation, amnesia, and a decrease in autonomic adrenergic discharge. Additional goals are a reduction of gastric acidity and volume, an antisialogogue effect, and a reduction in nausea and vomiting.(51) The primary goals can usually be met by benzodiazepines alone. These agents produce relatively little respiratory or cardiac depression(52), and can be safely used in most patients undergoing INR procedures. Midazolam, when used in doses of 0.05 to 0.1 mg/Kg IM, or titrated intravenously, provides close to ideal sedation of short duration. Benzodiazepines must be used cautiously in patients with elevated ICP since they can cause respiratory depression in this setting. Unless a patient is experiencing significant preoperative pain, narcotic premedication is not needed, and is contraindicated in cases with elevated ICP.(53)In patients with subarachnoid hemorrhage, the obese, and those with gastroesophageal reflux, H2 receptor antagonists, such as cimetidine or ranitidine are used in order to decrease the risk of pulmonary aspiration.(54) Glycopyrrolate is an effective and frequently used antisialogogue, but it does not alter gastric pH or volume, and is seldom used for this indication.(55) Metoclopramide, which decreases gastric volume due to a gastrokinetic action and has antiemetic effects, is used most effectively when combined with H2-receptor antagonists, to decrease the risk of aspiration.(56)
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