4.3 Contrast Agents, Indications & Contraindications
Key Takeaways
- Modern coronary work uses nonionic LOCM such as iohexol or iopamidol, or nonionic IOCM such as iodixanol; high-osmolar ionic agents are essentially unused because of arrhythmia and hemodynamic instability.
- Osmolality, iodine concentration (mg I/mL), and viscosity are different properties; warming lowers viscosity, and IOCM matches plasma osmolality.
- Iodixanol is often chosen in CKD, but volume minimization still matters more than the brand switch alone.
- CO2 is negative contrast for venous or infra-diaphragmatic arterial work and must NEVER be injected into coronary or cerebral arteries.
- High-yield stops for elective iodinated work include prior severe anaphylactoid reaction without prep, anuria without a dialysis plan, and pregnancy as a documented risk-benefit decision; power-inject ventriculograms, not coronaries at LV settings.
Contrast Agents, Indications & Contraindications
Outline Patient Care 1.D asks for types and properties, then indications and contraindications. In a cardiac lab the working menu is short: nonionic low-osmolar contrast media (LOCM) such as iohexol and iopamidol, nonionic iso-osmolar contrast media (IOCM) such as iodixanol, and carbon dioxide (CO2) as a special-purpose negative contrast. High-osmolar ionic agents have no modern coronary role. The CI technologist's job is to know which bottle is on the injector, why it was chosen, how much has gone in, and which vascular beds are forbidden territory.
Quick Answer: Coronary work uses warmed nonionic LOCM or IOCM. Iodixanol is often chosen in chronic kidney disease (CKD). Never inject CO2 into coronary or cerebral arteries. Minimize volume with dilution, biplane, and intravascular imaging. Treat prior severe anaphylactoid reaction without prep, anuria without a dialysis plan, and pregnancy as stop-the-line conversations.
Osmolality, iodine concentration, and viscosity are three different numbers
Osmolality is particles per kilogram of water. Plasma is about 290 mOsm/kg. Older high-osmolar contrast media (HOCM) such as ionic diatrizoate sit around 1500–2000 mOsm/kg. Typical LOCM monomers (iohexol, iopamidol, ioversol) used at coronary strength sit roughly 600–800 mOsm/kg — still hyperosmolar to blood, but far closer than HOCM. Iodixanol is a nonionic dimer formulated to be iso-osmolar to plasma (~290 mOsm/kg) at clinical iodine strength.
Iodine concentration is milligrams of iodine per milliliter (mg I/mL) — 300, 320, 350, 370. That number, not osmolality, is what makes the coronary lumen white. You can match iodine concentration while changing osmolality by changing molecular design (monomer versus dimer).
Viscosity is resistance to flow. It rises with iodine concentration and falls when contrast is warmed. Cold, viscous 370 mg I/mL through a small diagnostic catheter produces catheter kick, inadequate mixing, and a miserable injection. Warming is a safety and image-quality step, not a comfort gimmick.
Why high-osmolar ionic agents are essentially unused in coronary work
HOCM in a coronary artery is a recipe for bradycardia, VF, pain, and hemodynamic swings, with a higher historical rate of acute reactions. Modern diagnostic catheterization and PCI use nonionic LOCM or IOCM. If a question stem offers diatrizoate as the default coronary agent in 2026, it is a trap.
LOCM versus IOCM: who gets which
Iohexol and iopamidol (and similar LOCM) are the workhorse coronary agents: diagnostic angiography, PCI, ventriculography, and aortography when iodinated contrast is indicated.
Iodixanol (IOCM) is often selected when estimated glomerular filtration rate (eGFR) is reduced because iso-osmolality may be better tolerated by the kidney and by unstable hemodynamics. Journals still debate the size of the renal benefit; the CI-exam skill is the practice pattern — IOCM is the CKD-leaning choice — plus the more important fact that volume still drives contrast-associated acute kidney injury. Switching to iodixanol and then injecting 400 mL is not a renal-protection strategy.
Indications, stated positively: iodinated LOCM or IOCM when you need positive arterial opacification of coronaries, grafts, chambers, or aorta and the patient can receive iodine. IOCM when the physician wants iso-osmolality (CKD, tenuous hemodynamics). CO2 when iodinated contrast is unsafe and the vascular bed is an accepted CO2 territory.
Carbon dioxide: negative contrast with hard geographic limits
CO2 is radiolucent (negative contrast). It is used conceptually for venography and some infra-diaphragmatic arterial work when iodinated contrast is contraindicated. It is buoyant and can trap in non-dependent structures. Delivery systems are closed and dedicated; room air is not a substitute.
Never inject CO2 into the arterial cerebral or coronary circulation. Coronary or cerebral arterial CO2 is gas embolism: ischemia, arrest, stroke. That contraindication is absolute in CI teaching. Do not “try a little CO2” in the left coronary because the creatinine is 4.0. Use intravascular ultrasound (IVUS) or optical coherence tomography (OCT), minimize iodinated volume, stage the case, or choose another strategy the physician orders.
A right-to-left shunt is another reason venous CO2 can reach systemic arteries — flag it before anyone treats CO2 as harmless gas.
Volume minimization
Contrast nephropathy and the contrast load for a failing left ventricle are volume problems as much as molecule problems.
- Dilute ventriculograms or some peripheral runs when the operator agrees.
- Biplane angiography can cut duplicate single-plane runs.
- IVUS and OCT can replace some angiographic acquisitions once the vessel is wired.
- Skip a “routine” left ventriculogram if a recent echocardiogram already gave the ejection fraction and the LVEDP is already high.
- Announce cumulative milliliters at planned checkpoints, especially in CKD.
Contraindications
Prior severe anaphylactoid reaction to iodinated contrast (bronchospasm, hypotension, anaphylaxis) without a premedication or prep plan is a stop for elective work. Premedication and a different agent are physician decisions; they are not a technologist workaround of skipping the history.
Anuria without a dialysis plan means there is no exit for the osmotic load. Elective iodinated angiography waits for nephrology's plan. Anuria is not the same as a reduced but functioning eGFR.
Pregnancy is risk-benefit: iodinated contrast crosses the placenta, and fluoroscopy adds fetal dose. Necessary, life-saving PCI can proceed with documentation; elective diagnostic work is deferred when possible.
Relative issues the physician weighs include ongoing unstable CKD and a recent contrast load. The exam-high-yield three remain severe unprepared reaction, anuria without dialysis, and pregnancy.
Power injector versus hand injection
Selective coronary arteries are typically hand-injected or delivered with a controlled coronary injector at low rate and small volume (often on the order of 4–10 mL). The operator feels pressure and can stop instantly.
Power injectors are programmed for left ventriculography, aortography, and some graft or peripheral runs: volume, flow rate, pressure limit, and rise time. Purge air until the syringe and tubing are bubble-free. A 30–40 mL LV-gram recipe into a coronary catheter is a dissection and VF pathway.
Confirm sheath or catheter patency and position before the injection. Do not power-inject against a damped, ventricularized coronary waveform.
LOCM vs IOCM vs CO2
| Property | Nonionic LOCM (iohexol, iopamidol) | Nonionic IOCM (iodixanol) | CO2 |
|---|---|---|---|
| Appearance | Positive (radiopaque iodine) | Positive (radiopaque iodine) | Negative (radiolucent gas) |
| Osmolality vs plasma | Low-osmolar but still hyperosmolar (~600–800 mOsm/kg at coronary strength) | Iso-osmolar (~290 mOsm/kg) | Not an iodine osmole load |
| Typical CI use | Workhorse coronaries, PCI, LV, aorta | Often preferred in CKD or tenuous hemodynamics | Venous or infra-diaphragmatic arterial work when iodine is unsafe |
| Coronary / cerebral arteries | Indicated when iodine is acceptable | Indicated when iodine is acceptable | Contraindicated — never |
| Main cautions | Volume, allergy, viscosity if unwarmed | Still nephrotoxic if the volume is huge | Gas embolism if used above the diaphragm arterially; buoyancy; right-to-left shunt |
Exam traps
- Treating CO2 as a universal iodine substitute, including coronaries and cerebral arteries.
- Equating osmolality with iodine concentration or assuming iodixanol contains no iodine.
- Power-injecting coronaries with an LV-gram setting.
- Calling a remote shellfish allergy an absolute ban while ignoring documented prior contrast anaphylaxis.
- Believing an IOCM label authorizes unlimited milliliters in CKD.
A patient with eGFR 28 mL/min/1.73 m² needs coronary angiography. Why might the physician choose iodixanol rather than iohexol?
Which statement about carbon dioxide as a contrast agent is CORRECT?
Which situation is a high-yield contraindication or stop-the-line issue for elective iodinated coronary angiography?