What is Indocyanine Green used for?

Indocyanine Green (ICG) is a water-soluble, tricarbocyanine dye that has been employed in medical diagnostics and imaging for several decades. Originally approved by the FDA in 1956, ICG is marketed under trade names such as IC-Green and Cardiogreen. It is primarily used in liver function tests, ophthalmic angiography, and cardiovascular diagnostics. Renowned research institutions and hospitals like the Mayo Clinic and Johns Hopkins have been pivotal in advancing the applications of ICG. Over the years, its versatility has expanded to include applications in neurosurgery, cancer diagnostics, and lymphatic mapping. Given its unique properties and safety profile, ICG continues to be an invaluable tool in modern medicine, with ongoing research aimed at exploring new therapeutic and diagnostic avenues.

Indocyanine Green Mechanism of Action
The mechanism of action of Indocyanine Green is rooted in its unique optical properties. When injected into the bloodstream, ICG binds almost exclusively to plasma proteins, particularly lipoproteins. It absorbs light in the near-infrared spectrum (around 800 nm) and emits fluorescence when exposed to light of a specific wavelength. This characteristic makes ICG an excellent agent for imaging tissues and organs.

In liver function tests, ICG's uptake and subsequent clearance by the hepatic parenchymal cells reflect the liver's ability to process and excrete substances, thereby providing a measure of hepatic function. In ophthalmic angiography, ICG fluoresces under near-infrared light, allowing detailed visualization of the retinal and choroidal vasculature. Moreover, in oncological applications, the dye's accumulation in tumor tissues due to the Enhanced Permeability and Retention (EPR) effect enables surgeons to delineate cancerous tissues from normal ones during surgery.

How to Use Indocyanine Green
Indocyanine Green is administered intravenously, typically in a controlled clinical setting. The dosage and method of administration vary depending on the diagnostic or therapeutic purpose. For liver function tests, a typical dose ranges from 0.5 mg/kg to 1 mg/kg of body weight. The dye is injected intravenously, and blood samples are taken at specific intervals to measure its clearance.

In ophthalmic angiography, ICG is administered as a bolus injection, usually at a dose of 25 mg. The onset of action is rapid, with fluorescence detectable within seconds of administration. For surgical applications such as tumor margin delineation or lymphatic mapping, ICG is often injected at a site proximal to the target tissue, followed by near-infrared imaging to visualize the dye's distribution.

The onset time of ICG's action is generally rapid, with peak plasma concentrations occurring within minutes of injection. The half-life of ICG in the bloodstream is relatively short, approximately 3 to 4 minutes, making it suitable for procedures requiring real-time imaging. The dye is primarily eliminated through the liver, with minimal renal excretion.

What are Indocyanine Green Side Effects
While Indocyanine Green is generally safe when used appropriately, it is not without potential side effects and contraindications. Common side effects include mild reactions such as nausea, flushing, and a transient metallic taste in the mouth. These reactions are typically short-lived and resolve without intervention.

More severe allergic reactions, although rare, can occur. These may include anaphylactic shock, urticaria (hives), and hypotension. Patients with a history of iodine allergy should be monitored closely, as ICG contains iodine. In such cases, premedication with antihistamines or corticosteroids may be advisable to mitigate the risk of allergic reactions.

ICG is contraindicated in patients with known hypersensitivity to the dye. It is also not recommended in pregnant or breastfeeding women due to the lack of sufficient safety data. Caution is advised in patients with severe hepatic impairment, as the dye's clearance is primarily dependent on liver function. In these patients, prolonged retention of ICG may interfere with diagnostic accuracy and potentially exacerbate liver dysfunction.

What Other Drugs Will Affect Indocyanine Green
The interaction of Indocyanine Green with other drugs is relatively minimal, given its rapid clearance from the bloodstream and its exclusive binding to plasma proteins. However, certain medications and conditions could potentially affect the pharmacokinetics and dynamics of ICG.

Drugs that alter liver function can impact the clearance of ICG. For instance, medications such as rifampicin, which induce hepatic enzymes, may accelerate the metabolism and clearance of ICG, potentially reducing its diagnostic utility. Conversely, drugs that impair liver function, such as hepatotoxic agents or those causing cholestasis, can prolong the retention of ICG in the bloodstream.

Patients on anticoagulant therapy, particularly warfarin, need to be monitored closely, as ICG binds to plasma proteins that could theoretically displace warfarin, altering its anticoagulant effect. However, clinically significant interactions are rare.

In conclusion, Indocyanine Green is a versatile and invaluable tool in modern medicine, offering a broad range of diagnostic and therapeutic applications. Its unique optical properties, rapid onset of action, and relatively safe profile make it a preferred choice for various imaging procedures. However, like all medical interventions, it is essential to be aware of its potential side effects, contraindications, and interactions with other drugs to ensure its safe and effective use. Ongoing research continues to uncover new applications for this remarkable dye, promising even greater utility in the future.