Most vascular computed tomography (CT) and magnetic imaging resonance (MRI), the vast majority of angiographic procedures, and many nonvascular interventions rely on contrast media to reveal the anatomy. Contrast media can be broadly classified according to their use and also their chemical structure. X-ray contrast affects tissue X-ray attenuation, ultrasound contrast affects tissue and blood reflectivity and MRI contrast affects tissue relaxation times. The Royal College of Radiologists has issued pragmatic guidance on the use of intravascular contrast agents. Discussion in this chapter is confined to contrast media used for angiography and vascular diagnosis.
Intravascular X-ray contrast agents
The two principal categories of X-ray contrast both affect X-ray attenuation. Details of the chemical and physical properties of these agents are extensively discussed in many texts. It helps to be familiar with the different options and their indications, this will be most relevant in cases where there is kidney disease or a history of adverse reaction.
Positive contrast agents
These are liquids containing iodine or gadolinium that have greater attenuation than the patient’s soft tissues.
Negative contrast
This has lower attenuation than the patient’s tissues; at present, carbon dioxide gas is the only available option.
Basic principles for using intravascular contrast
Optimal demonstration of anatomy and pathology requires sufficient difference in attenuation between the target tissue and the surroundings, as well as X-ray equipment parameters, e.g. kV and mAs.
The aim is to adequately opacify the vessel but allow a level of grey-scale that allows branches/filling defects to be seen through the contrast. This requires the correct strength contrast in sufficient quantity (volume of contrast) delivered in the right place. If the contrast is too diluted, there will be insufficient change in attenuation, conversely too concentrated contrast can obscure lesions.
As a general principle, the contrast column should opacify the entire vessel segment. To achieve this, the total contrast dose and the duration of the bolus must be correct. Appropriate catheter positions, contrast volumes and flow rates are indicated throughout the diagnostic angiography sections. When the blood flow is slow, it may take several seconds for the opacified blood to pass through the vessel. Hence, a long contrast bolus is necessary. This is one of the reasons for increasing the volume of contrast to image the more distal vessels. Modern angiography equipment allows integration of multiple images, which has the same effect as increasing the length of the bolus, but it can reduce image quality due to minor degrees of patient movement between frames.
Iodinated contrast media
These are the most frequently used agents. Non-ionic contrast media are recommended in high-risk patients (see below).
Most diagnostic and therapeutic intervention is performed using ‘300 strength’ contrast (300 mg/mL iodine). This density of contrast is fine for pump injections into large vessels where the contrast is diluted by rapid blood flow. For selective hand injections in the vascular system and for non-vascular examinations, 300 strength contrast is diluted with saline to ‘two-thirds’ or ‘half strength’.
Contrast reactions with iodinated contrast media
There are two forms of contrast reaction: direct effects and idiosyncratic responses; these are more common in certain groups of patients and emphasis should be on identifying them, reducing risk and preventing reactions. Up to 2% of patients require treatment for adverse reactions to intravascular iodinated contrast agents. Fortunately, the majority of cases require only observation and minor supportive treatment. Less than 1% are severe but these require prompt recognition and immediate treatment.
Alarm Contrast reactions should be discussed during the consent procedure.
Direct effects
Direct effects are secondary to the osmolality and direct chemotoxicity of the contrast, and they include heat, nausea and pain. More important are the effects on organ systems.
Renal
Contrast-induced acute kidney injury (CI-AKI), defined as a rise in creatinine of 0.5–1 mg/dL or 44–88 µmol/L, is common and most likely in patients with chronic kidney disease. Strategies for preventing CI-AKI are discussed in Chapter 1 .
Cardiac
Cardiac problems are most likely to occur during coronary angiography and are usually manifest as arrhythmias or ischaemia. It is prudent to use non-ionic contrast in patients with ischaemic heart disease or heart failure.
Haematological
Significant haematological interactions are uncommon. Non-ionic iodinated contrast can induce clotting if mixed with blood; hence, scrupulous attention to catheter flushing and avoidance of contaminating syringes with blood are essential.
Neurological
Most neurological sequelae occur during carotid angiography and are related to angiographic technique. Genuine contrast-related problems are rare and are usually seen in patients with abnormalities in the blood–brain barrier.
Idiosyncratic reactions
The mechanism of these reactions is uncertain; vasoactive agents such as histamine, serotonin, bradykinin and complement have been implicated but a causal role has not been established.
Idiosyncratic reactions are classified according to severity.
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Minor: Common, ~1 : 30, e.g. metallic taste, sensation of heat, mild nausea, sneezing; these do not require treatment.
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Intermediate: Common, ~1 : 100, e.g. urticaria; not life-threatening; these respond quickly to treatment.
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Severe: Rare, ~1 : 3000, e.g. circulatory collapse, arrhythmia, bronchospasm, dyspnoea; may be life-threatening; these require prompt therapy. Remember the A, B, C, D approach and do not hesitate to call for help.
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Death: Rare, ~1 : 40,000, mostly caused by cardiac arrhythmia, pulmonary oedema, respiratory arrest or convulsions.
Assessing the risk
The risk of a contrast reaction varies depending on the circumstances of individual patients; however, the following are associated with an increased risk of a severe idiosyncratic reaction:
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Previous allergic reaction to iodine-containing contrast and shellfish allergy: 10×
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Cardiac disease: 5×
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Asthma: 5×
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General allergic responses: 3×
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Drugs: β-blockers, interleukin-2: 3×
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Age >50 years: 2× risk of death.
Remember that these factors increase the relative risk; the absolute risk remains very low.
Reducing the risk
The vast majority of severe and fatal contrast reactions occur within 20 min of administration and therefore it is vital that patients are kept under constant supervision with a cannula in situ during this period. There have been a few isolated reports of delayed hypotensive reactions hours after contrast injection.
The ideal method of reducing risk is to avoid iodine-containing contrast examinations by using other imaging modalities, such as ultrasound or MRI. When this is not possible:
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Prepare for reaction. Ensure that resuscitation equipment and drugs are immediately available every time contrast is injected. Make sure that you are familiar with the management of the reaction; most catheter laboratories have charts on the wall to remind you in times of need (note: it is much less stressful to check it out in advance).
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Use non-ionic contrast agents. Non-ionic contrast agents certainly reduce the risk of minor reactions and may reduce the risk of more significant reactions.
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Reassure the patient. Explain that contrast reactions are unlikely and that the situation is under control. In severe anxiety, short-acting anxiolytic agents may be warranted.
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Consider steroid pre-medication. There is no conclusive evidence that oral steroid premedication reduces the risk of moderate–severe reactions but it pays to check whether your department has guidelines on steroid administration.
In the rare patient with a previously documented severe reaction:
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Try to identify which agent was responsible (and make certain you do not use it again)!
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Avoid iodinated contrast; use CO 2 or gadolinium or another imaging modality.
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If this is impossible and the examination is essential:
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If there is time, seek the opinion of an allergy specialist.
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Ensure resuscitation personnel, equipment and drugs are immediately available. You may need expert assistance maintaining the airway, so consider enrolling anaesthetic assistance.
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Monitor the patient carefully.
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Use non-ionic contrast.
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Consider steroid premedication in accordance with local guidelines.
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Reassure the patient.
Treatment of contrast reactions
Warn patients that a sense of warmth, a metallic taste and transient nausea are all common after rapid IV injection of contrast and these effects wear off after a few minutes.
Minor reactions
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Nausea and vomiting. Active treatment rarely required. Reassure and monitor patient.
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Urticaria. One of the commonest contrast reactions. Localized patches of urticaria do not require treatment. Simply observe and monitor the patient (pulse, BP). Generalized urticaria or localized urticaria in sensitive areas, e.g. periorbital, should be treated by chlorphenamine 20 mg, given slowly by IV injection.
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Vasovagal/syncope. Monitor the patient’s pulse, BP, oxygen saturation and ECG. Elevate the legs. Establish IV access. Give atropine 0.6–1.2 mg by IV injection for symptomatic bradycardia. Volume expansion with IV fluids for persistent hypotension. If hypotension persists, seek medical support.
Intermediate–severe reactions
Bronchospasm
Monitor the patient’s pulse, BP, oxygen saturation and ECG.
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Give 100% O 2 .
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Treat initially with β-agonist inhaler, e.g. salbutamol.
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Give IV corticosteroids, e.g. 100 mg hydrocortisone. In acute reactions, steroids may work surprisingly quickly, although it can take a few hours for them to achieve full effect.
If there is continuing bronchospasm seek assistance.
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Consider intramuscular epinephrine: 0.3–0.5 mL of 1 : 1000 solution.
Laryngeal oedema/angioneurotic oedema
Seek anaesthetic assistance. Monitor the patient’s pulse, BP, oxygen saturation and ECG. Give:
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100% O 2 and watch the oxygen saturation closely.
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Chlorphenamine 20 mg by slow intravenous injection.
Consider:
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Epinephrine intramuscularly (IM) 0.3–0.5 mL of 1 : 1000 solution.
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Get an anaesthetist to assess the airway. Tracheostomy may be required in severe cases.
Severe hypotension
Call for support. Hypotension accompanied by tachycardia may indicate vasodilation and increased capillary permeability. Monitor the patient’s pulse, BP, oxygen saturation and ECG.
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