Allergy to radiopaque contrast agents

When using modern radiocontrast agents (RCA), the overall incidence of intolerance reactions is 5-8%. They can be divided into two groups: allergic and chemotactic. Chemotactic reactions are caused by the physical properties of the RCA (osmolarity, viscosity, ability to bind blood calcium) and, as a rule, are clinically manifested by hypotension, bradyarrhythmia and the development of pulmonary congestion. Allergy to radiocontrast agents is associated with the response of various parts of the patient's immune system to the chemical structure of the RCA and includes a wide range of clinical conditions - from minor to fatal.

In the general population, the incidence of allergy to radiocontrast agents is about 1%. Severe allergic reactions are rare - in 0.1% of patients.

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Why does an allergy to radiocontrast agents develop?

The main mechanism of allergy to radiocontrast agents is degranulation of basophils and mast cells due to direct activation of the complement system. The release of histamine and other active substances from granules causes clinical manifestations of allergy (cough, sneezing, bronchospasm, rash and, in severe cases, collapse due to excessive systemic vasodilation). In any patient who develops hypotension during PCI or CAG, a severe allergic reaction should be excluded. Differential diagnosis should be made with vasovagal reactions. A distinctive feature of an allergic reaction is the development of tachycardia, which, however, may be absent in patients receiving beta-blockers or with an implanted pacemaker.

Most allergic reactions occur within the first 20 minutes after contact with RVC. A serious or fatal allergic reaction develops earlier in 64% of cases - within the first 5 minutes after contact. Severe allergic reactions may begin as minor reactions with subsequent rapid progression within a few minutes. There are two categories of patients with an increased risk of developing an allergic reaction to RVC. If the patient has previously had an allergy to radiocontrast agents, then with its subsequent introduction, the risk of its development increases to 15-35%. The second risk group consists of patients with atopic diseases, asthma and allergy to penicillin. The risk of developing an allergic reaction in these patients increases by 2 times. There are indications of an increased risk in patients with an allergy to shellfish and other seafood in the anamnesis.

Symptoms of an allergy to radiocontrast agents

Allergic reactions include a wide range of clinical manifestations - from mild (in the form of itching and local urticaria) to severe (shock, respiratory arrest, asystole).

Classification of the severity of allergy to radiocontrast agents

Easy	Moderate severity	Heavy
Limited urticaria Itching Erythema	Diffuse urticaria Kiinke's edema Laryngeal edema Bronchospasm	Shock Respiratory arrest Cardiac arrest

Treatment of allergy to radiocontrast agents

In the treatment of an allergic reaction to the introduction of RCA, 5 classes of pharmacological drugs are used: H1 blockers, H2 blockers, corticosteroids, adrenaline and saline. Treatment tactics depend on the severity of the allergic reaction and the patient's condition. In mild cases (urticaria, itching), diphenhydramine is used at a dose of 25-50 mg intravenously. If there is no effect, adrenaline is administered subcutaneously (0.3 ml of a solution diluted 1:1000 every 15 minutes up to a dose of 1 ml). In this case, cimetidine diluted in 20 ml of saline solution can be additionally administered within 15 minutes at a dose of 300 mg intravenously or ranitidine at a dose of 50 mg intravenously.

If bronchospasm develops, the following sequence of actions is recommended:

• oxygen through a mask, oximetry;
• in mild cases - albuterol inhalation; in moderate cases - adrenaline subcutaneously (0.3 ml of a solution diluted 1:1000 every 15 minutes up to a dose of 1 ml); in severe cases - adrenaline 10 mcg intravenously as a bolus over a minute, then infusion of 1-4 mcg/min (under the control of blood pressure and ECG);
• diphenhydramine 50 mg intravenously;
• hydrocortisone 200-400 mg intravenously;
• H2 blocker.

For swelling of the face and larynx:

• call a resuscitator;
• airway patency assessment:
  • additional oxygen via mask;
  • intubation;
  • preparing a tracheostomy kit;
• in milder cases - adrenaline subcutaneously (0.3 ml of solution diluted 1:1000 every 15 minutes up to a dose of 1 ml), in moderate and severe reactions - adrenaline intravenously bolus 10 mcg over 1 min, then infusion 1-4 mcg/min (under the control of blood pressure and ECG);
• diphenhydramine 50 mg intravenously;
• oximetry;
• H2 blocker.

For hypotension and shock:

• simultaneously - intravenous adrenaline bolus 10 mcg every minute until an acceptable blood pressure level is achieved, then infusion 1-4 mcg/min + large volumes of isotonic solution (up to 1-3 l in the first hour);
• supplemental oxygen via mask or intubation;
• diphenhydramine 50-100 mg intravenously;
• hydrocortisone 400 mg intravenously;
• central venous pressure control;
• oximetry. If ineffective:
• intravenous dopamine at a rate of 2-15 mcg/kg/min;
• H2 blocker;
• resuscitation measures.

Prevention of allergy to radiocontrast agents

The basis for the prevention of allergic reactions to RVC is premedication with a combination of corticosteroids and H1 blockers. A number of studies have shown the benefit of adding H2 blockers, which are believed to additionally block the IgE-mediated component of the allergic reaction. There are several regimens for the prevention of allergic reactions, which use different doses and routes of administration of drugs from these groups. The following regimen has the greatest evidence base: prednisolone 50 mg orally 13, 7, and 1 hour before the procedure (150 mg in total) + diphenhydramine 50 mg orally 1 hour before the procedure. In one study, the use of this regimen in patients with a history of allergy to radiocontrast agents reduced the overall incidence of recurrent allergic reactions to 11%. At the same time, hypotension developed in only 0.7% of patients. A simpler regimen is often used: taking prednisolone orally at a dose of 60 mg in the evening before the procedure, and in the morning of the procedure taking prednisolone orally at a dose of 60 mg + 50 mg diphenhydramine. There is also an alternative regimen: taking 40 mg of prednisolone every 6 hours for 24 hours + diphenhydramine 50 mg intravenously + cimetidine 300 mg intravenously once.

In the presence of an allergic reaction to ionic RCA, if a repeat procedure is required in the future, non-ionic RCA should be used, since the risk of a severe cross-allergic reaction in this case is less than 1%.