Anesthesia Keywords Defined - 2006
Close Window.

AS026          PROPHYLAXIS: IV CONTRAST ALLERGY

Prophylactic medications
Methylprednisolone, 2 oral doses of 32 mg each administered 12 and 2 hours before IV contrast media (ICM) administration, can reduce the incidence of all adverse reactions to ionic ICM from 9% to 6.4%. A single dose of 32 mg of methylprednisolone administered 2 hours before ICM administration has no effect.

Premedication with a single 100-mg tablet of hydroxyzine 12 hours before the IV injection of the ionic contrast agent reduces the incidence of adverse reactions.

Studies of the potential role of H2 blockers, such as cimetidine, have shown a beneficial effect, no effect, or even adverse effects with the addition of H2 blockers to the premedication regimen.

Some investigators have incorporated ephedrine into their premedication regimens; however, its sympathomimetic and cardiac effects may limit its usefulness.

Recommended prophylactic regimens
If the patient had a previous moderate or severe reaction or one that included a respiratory component, an alternate study, such as sonography or MRI, should be considered. Otherwise, the following may be used: H1 antihistamines; diphenhydramine, one 50-mg tablet orally administered 1 hour before the study; H2-histamine receptor blockers, which is optional; cimetidine, 300 mg orally administered 1 hour before study; and/or ranitidine 50 mg orally administered 1 hour before the study. Methylprednisolone, one 32-mg tablet, may be orally administered 12 and 2 hours before the study, or prednisone, one 50-mg tablet, may be orally administered 13 hours, 7 hours, and 1 hour before the study.

Most restrict corticosteroid pretreatment to patients in whom previous idiosyncratic adverse reactions to ICM were moderate or severe. Usually, corticosteroids are well tolerated and cause no adverse effects when only a few doses are administered.

Although the utility of H2-receptor blockers is questionable, these agents are well tolerated and might be of benefit, particularly because they are effective in the treatment of at least some allergic cutaneous reactions to agents other than ICM. However, H2 blockers should not be used without H1 blockers.

Prophylaxis in nonvascular studies
Although rare, systemic reactions are reported after extravascular instillation of ICM, for example, during retrograde pyelography.

When patients have had previous severe idiosyncratic or anaphylactic reactions to IV ICM, pre-medication with corticosteroids should be considered, even in nonvascular studies.

1. Laude EA, Emery CJ, et al: The effect of antihistamine, endothelin antagonist and corticosteroid prophylaxis on contrast media induced bronchospasm. The British Journal of Radiology 1999;72:1058-63.

DS023          MASSIVE TRANSFUSION AND COAGULOPATHY

Massive transfusion may be defined as transfusion of = one blood volume in 24 hours (e.g., 10 U of whole blood in a 70 kg adult). When a patient receives stored blood in such large volumes, the patient's own blood may be in effect "washed out," with only about 1/3 of original blood components remaining; hemodilution may thus occur.

In circumstances not complicated by prolonged hypotension or DIC, dilutional thrombocytopenia is the most likely complication. Stored blood does not contain fully functional platelets. Microvascular bleeding (abnormal oozing and continued bleeding from raw and cut surfaces) may result. Six to eight platelet concentrates are usually enough to correct such bleeding in an adult. Because clotting factors are not significantly decreased, FFP is not needed. A similar complication caused by dysfunctional platelets rather than thrombocytopenia can occur in patients maintained on extracorporeal circulation for > 2 hours; if microvascular bleeding occurs, platelets should not be given until the pump has been discontinued.

Hypothermia due to rapid transfusion of large amounts of cold blood can cause a worsening coagulopathy, arrhythmias or cardiac arrest. Hypothermia is avoided by using an IV set with a heat exchange device specifically designed to warm blood gently. Other means of warming blood are contraindicated because of potential RBC damage and hemolysis.

Complications of massive transfusion:

• Dilutional thrombocytopenia (coagulopathy)
• Coagulation factor (especially 5 and 8) dilution or consumption
• DIC (possibly from mismatched blood or patient’s underlying disease
• Hyperkalemia (due to storage; rarely a problem in adults- negative cardiovascular effect is antagonized by calcium; washing RBCs gets rid of K)
• Hypomagnesemia (occurs as it binds to citrate)
• Citrate toxicity (citrate binds to Ca and Mg)
• Acidosis/Alkalosis - stored blood is acidic (pH 6.6-6.9) due to citric acid in the anticoagulant, CO2 and lactic acid from RBC metabolism, however, these acids are converted to citrate and lactate by the liver and the patient will become alkalemic with large transfusions. However, with transfusion rates faster than 100ml/min, patients will become transiently academic. Patients with little hepatic reserve may remain severely acidemic.
• Impaired O2 delivery with left shift of the O2 dissociation curve (from decreased 2,3 DPG in storage). This problem is auto-corrected within 8 to 24 hours.
• Hypothermia from lack of warming of blood.

1. ASA Taskforce: Practice idelines for Perioperative Blood Transfusion and Adjuvant Therapies. Anesthesiology 1996; 84: 732-47.

PS019          THORACIC ANEURYSM REPAIR: COMPLICATIONS

Aortic aneurysmal disease occurs most commonly in patients with hypertension or other risk factors for atherosclerotic disease. The risk of rupture increases dramatically with the size of the aneurysm, with aneurysms measuring greater than 5 cm in diameter having a 20% chance of rupture over a six year period. Perioperative mortality from aortic reconstructive surgery ranges from 5% to 14%, and the larger the aneurysm, the greater the chance of morbid events. Complications commonly seen with aortic reconstruction are related to interruptions in perfusion due to aortic occlusion during the repair, uncontrolled rupture, or the usual postoperative concerns in a vascular surgery patient.

One of the most feared complications of thoracic aneurysm repair is lower extremity paralysis as a result of an interruption in perfusion to the distal spinal cord. The risk of spinal cord ischemia depends on the size and the location of the aneurysm, duration of aortic cross-clamping, presence of preoperative rupture, the extent of dissection, and the use of protective measures, with an incidence approaching 11% in the repair of distal descending thoracic aorta.

Acute renal failure has a reported incidence of up to 30% of patients undergoing repair of thoracoabdominal aneurysm. Associated mortality exceeds 30%, with the risk factors for this complication being similar to those listed for spinal cord ischemia.

Pulmonary complications are the most commonly reported morbidities of thoracic aneurysm repair, including atelectasis, pneumonia, respiratory failure, and ARDS. The incidence of postoperative respiratory failure is greater than 50%, with up to 14% of these patients requiring tracheostomy.

As with other major vascular procedures, congestive heart failure, myocardial ischemia, stroke, and death are all possible complications.

1. Norris EJ: Anesthesia for Vascular Surgery, Miller’s Anesthesia, 6th edition. Edited by Miller R. Philadelphia, Elsevier, 2005:2051-2126.

2. Scubas N, Lichtman AD, Sharma A, Thomas SJ: Anesthesia for Vascular Surgery, Clinical Anesthesia, Fifth edition. Edited by Barash P, Cullen BF, Stoelting R. Philadelphia, Lippincott Williams & Wilkins, 2006:886-932.

Close Window.