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Contrast media are agents used to improve the image contrast of anatomical structures which are not normally easily visualized. Contrast techniques provide detail of organ size, shape, position and internal detail.
The patient's full cooperation shoul be obtained. Communication with patient before examination and during injection is required to reduce the risk of contrast medium extravasation. Intravenous contrast media should be administered by power injector through a flexible plastic cannula. Use of metal needle in power injection should be avoided.
22 gauge catheters able to tolerate flow rates up to 5ml/sec. Before injection, position of catheter tip should be checked for venous backflow. Monitoring of the site during injection is required. Contrast media should not be administered by power injector through small bone, peripheral access central venous catheters because of the risk of catheter breakage.
Principles of contrast
Contrast on the radiograph is the difference in optical density (OD) between area of the radiograph. The density produced on a radiograph at 50-70kV is proportion to the atomic number squared of the tissue under examination.
Contrast media can be divided into positive contrast media and negative contrast media.
In general, positive contrast media are those which have an increased absorption of x rays and show up as white/grey. Positive contrast media - iodinated compounds.
Advantages - Insoluble, inert, and gives good mucosal details.
Disadvantages - Aspiration pneumonia if aspirated, can cause granulomatous reaction if leaks into body cavity.
Negative contrast media is which have less absorption and show up as dark/grey. Negative contrast media gaseous - air or carbon dioxide.
Advantages - Readily available and cheap
Disadvantages - Do not give as much mucosal detail, fatal air embolus reported following pneumocystography and urethrography, oxygen is a fire risk.
Although contrast media can cause adverse reaction, the actual reason for the adverse cause is still unknown. There are TWO common theories that used to describe adverse reaction to intravenous iodinated contrast media. Adverse reaction can be classified into physiochemotoxic and idiosynchratic in nature. Physiochemotoxic is related to adverse reaction to the chemotoxicity of the contrast media. Physiochemotoxic reaction are believed to result from contrast media's ability to upset the homeostasis of the body especially the blood and blood circulation. The term hemodynamics is used to describe blood and the factors that effects the blood circulation within the body. Injected contrast media always have the potential to alter blood cells and the circulation of the blood as well as the other organ system in the body. The body system most commonly effected by physiological changes that can be produced by contrast media are the cardiovascular, respiratory, urinary, gastrointestinal, neurologic and the integumentary system. The integumentary system include the skin and its attachment including the hand and nails which are dermal derivatives. Integumentary system is sometimes referred to as the cutaneous system. Under certain circumstances, the system of the body may not be able to compensate for the physiological changes brought on by the introduction of contrast media.
Physiochemotoxic reaction are commonly related to the following :-
Physical properties of contrast media
Iodine concentration of contrast media
Total dose or volume of contrast media
Rate or speed of injection
1. The Physical properties
The physical properties of contrast media include the ions or particles associated to the chemical breakdown of the contrast media when it enters a solution, the number and size of the iodine molecules and the number and size of the molecules of any chemical additives. The chemical composition of ionic and non-ionic contrast media contains iodine. One of the primary differences between ionic and non-ionic contrast media is that an ionic compound dissociates or dissolves into charged particles when it enters a solution such as blood.
Non-ionic contrast media do not dissolve into charged particles when it enters a solution. The osmolality of a solution is the measurement of the number of molecules and particles in a solution per kilogram of water. Osmolality can be described as a measurement of the number of the molecules that can crowd out or displace water molecules in a kilogram of water. The radiographic significance of the osmolality value of contrast media is that it is higher than the osmolality value of blood plasma. Any solutions that has an osmolality value greater than the blood plasma is said to be a hyperosmolar solution. Therefore, ionic and non-ionic contrast media are hyperosmolar solutions when compared to blood plasma. The following are examples of approximate average osmolality values of blood plasma, cerebrospinal fluid, ionic and non-ionic contrast media.
Blood plasma and cerebrospinal fluid - 300
Ionic - CM 1400-1800
Non Ionic - CM 400-750
Contrast media is primarily divided into TWO categories, high osmolar contrast media (HOCM) and low osmolar contrast media (LOCM). Most non-ionic agents are placed into the LOCM category while all ionic agents are in the HOCM category. An injection of contrast media, especially ionic HOCM, results in a big increase in the number of particles contained in the vascular system. The injection of contrast media in the vascular system cause water from body tissue to move into the vascular system an attempt to equalize concentrations.
The contrast media particles draw plasma water towards them therefore, water from body tissue (cells) rapidly moves into the vascular system via the capillary membranes to balance or equalize the situation. This process is known as osmosis. The blood vessels dilate in an attempt to compensate from the increased fluid volume. Sometimes the fluid shift may be too dramatic for the vessels to handle therefore, fluid actually extravasates into the surrounding tissues. Hypervolemia is the term that is used to describe an abnormal increase in the volume of circulating fluids or bloods. The rapid fluid movement, especially water, throughout the vascular system is believed to contribute to pain associated with vessel dilation, flushing, damage to the vascular endothelium, red cell changes, nausea, vomiting and dehydration. The osmotic effets can cause arteries of the kidneys to expand. When the arteries expand vasoconstrictors are released to compensate for the artery expansion. The vasoconstrictors constrict the arteries result in a rapid opening and closing actions of the arteries. The result of this action is a diminished blood supply to kidneys which can lead to total shut down of the kidneys. There is also a chance that the arteries may be constricted enough and totally closed. The body must attempt to regulate the fluid overload in the vascular system. If the kidneys are non functional, the fluid is forced to seek other avenues of escape causing fluid overload to occur in other body systems. One of the major results of the event is pulmonary edema.
2.The iodine concentration.
The iodine concentration is determined by the number of iodine molecules in milligrams present in a mililitre of a solution (mg/ml). The iodine concentration of an individual contrast agents determines how radiopaque an agent will be. The higher the iodine concentration, the better the chance that more x ray photon will be absorbed therefore, that particular contrast agents may be more radiopaque than a comparative low iodine concentrated agent. The iodine concentration have an effect on the severity of an adverse reaction. The higher te iodine concentration, the greater the risk of an adverse reaction.
3.The Total Volume
The total volume or dose is dependent upon several factors. Iodine concentration of the contrast media, type of contrast media being injected, body weight, anatomical structures of regions and speed of the injection and age or disease process that could increase the risk of an adverse reaction. The acute lethal dose (LD50) is the dose of a contrast medium required to cause a mortality rate of 50% following an intravenous injection of 1 minute duration. Increasing the injected volume increases the possibility of an adverse reaction occurring. An injection of a large amount of contrast media usually is accompanied by the use of a low iodine concentrated agent.
4.The rate or Speed of Injection.
It is an important factor associated with physiochemotoxic reaction. Increasing the speed or rate of injection may increase risk of adverse reactions. Viscosity is the term that associated with the thickness of contrast agent. It describes about the thickness or resistance to flow of a contrast agents. The thickness of contrast agent is related to concentration, the size of the molecules in a specific contrast agents and the temperature of contrast agent. The thickness or viscosity effects the rate that the contrast media can be injected. Contrast media with higher viscosity values will have to be injected at a slower rate. Heating the contrast media to body temperature reduces viscosity. All power injection devices are equipped with a "warming pad" covering a portion of the syringe. The warming devices heats the filled syringe to a temperature approximately to human body temperature.
The warming of the contrast media reduces the viscosity which
decreases the risk of impeding the flow. Iodine concentration, viscosity, temperature of the contrast media, catheter inner diameter, catheter length and the number of catheter holes are all factors that influence contrast media flow.
The second theory classifies adverse reaction to contrast media as idiosynchratic reactions. Idiosynchratic reactions may not be influenced by the iodine concentration, chemical properties and rate or volume of the injected contrast media.
Idiosynchratic reactions resemble allergic reactions or a hypersensitivity to a particular substance, in this case the substance is iodinated contrast media. The reason for this type of reaction remains unknown. A comparison has been made to allergic reaction that occur
In hypersensitivity to grains and pollen. The common term for allergy type reaction is "anaphylactoid reaction". Antibodies are formed in response to antigens which produces the symptoms of an allergy. The antibody-antigen response stimulates the release of histamine. Histamine is a naturally occurring chemical in the body and its release in the body is the most important mediator in allergy reactions. Histamines is found in most tissues and in specific cells of the lungs, gastrointestinal mucosa, gastrointestinal tract, lining of blood vessels and part of central nervous system. Histamine primarily effects the cardiovascular, gastrointestinal, respiratory and central nervous systems. Histamine constrict smooth muscle, dilates arterioles, constrict venules, produces localize edema, accelerates heart rate, lowers blood pressure, increase gastric secretion and increase mucous secretion. When an excessive amount of histamine is released in the body and vascular system become overloaded, histamine extravasates outside the vessel in the surrounding tissue. The extravasation can cause inflammation, swelling and reddening of the surrounding tissue which is urticaria or hives. "Watery eyes", "runny nose", migraine headaches, nausea, vomiting, laryngospasm and bronchospasm are all effects that related to an excessive amount of histamine release in the body. Idiosynchratic and anaphylactoid reactions to contrast media are categorized either mild, moderate or severe.
Contrast media reactions are classified in proportion to the type of treatment that is utilized to treat the reactions. Anytime contrast media is introduced into the body, there exists a possibility that an adverse reaction can occur. The radiologic technologist's immediate recognition and response to an adverse reaction is very important because it can safe a patient's life. Technologists must become familiar with radiology department protocols regarding adverse reactions to contrast media as well as all emergency situations that may arise.