INDICATOR-DILUTION STUDIES: IC-GREEN™ permits recording of the
indicator-dilution curves for both diagnostic and research purpose independently
of fluctuations in oxygen saturation. In the performance of dye dilution curves,
a known amount of dye is usually injected as a single bolus as rapidly as possible
via a cardiac catheter into selected sites in the vascular system. A recording
instrument (oximeter or densitometer) is attached to a needle or catheter for
sampling of the dye-blood mixture from a systemic arterial sampling site.
Under sterile conditions, the IC-GREEN™ powder should be dissolved with
the Aqueous Solvent provided for this product, and the solution used within
6 hours after it is prepared. If a precipitate is present, discard the solution.
The amount of solvent to be used can be calculated from the dosage form which
follows. It is recommended that the syringe used for injection of the dye be
rinsed with this diluent. Saline is used in all other parts of the catheterization
procedure.
This matter of rinsing the dye syringe with distilled water may not be critical,
since it is known that an amount of sodium chloride sufficient to make an isotonic
solution may be added to dye that has first been dissolved in distilled water.
This procedure has been used for constant-rate injection techniques without
precipitation of the dye.
The usual doses of IC-GREEN™ which have been used for dilution curves
are as follows:
Adults -5.0 mg
Children -2.5 mg
Infants -1.25 mg
These doses of the dye are usually injected in a mL volume. An average of five
dilution curves are required in the performance of a diagnostic cardiac catheterization.
The total dose of dye injected should be kept below 2 mg/kg.
Calibrating Dye Curves: To quantitate the dilution curves, standard
dilutions of IC-GREEN™ in whole blood are made as follows. It is strongly
recommended that the same dye that was used for the injections be used in the
preparation of these standard dilutions. The most concentrated dye solution
is made by accurately diluting 1 mL of the 5 mg/ mL dye with 7 mL of distilled
water. This concentration is then successively halved by diluting 4 mL of the
previous concentration with 4 mL of distilled water. (If a 2.5 mg/ mL concentration
was used for the dilution curves, 1 mL of the 2.5 mg/ mL dye is added to 3 mL
of distilled water to make the most concentrated "standard" solution. This concentration
is then successively halved by diluting 2 mL of the previous concentration with
2 mL of distilled water.) Then 0.2 mL portions (accurately measured from a calibrated
syringe) of these dye solutions are added to 5 mL aliquots of the subject's
blood, giving final concentrations of the dye in blood beginning with 24.0 mg/liter,
approximately (actual concentration depends on the exact volume of dye added).
This concentration is, of course, successively halved in the succeeding aliquots
of the subject's blood. These aliquots of blood containing known amounts of
dye, as well as a blank sample of which 0.2 mL of saline containing no dye has
been added, are then passed through the detecting instrument and a calibration
curve is constructed from the deflections recorded.
HEPATIC FUNCTION STUDIES: Due to its absorption spectrum, changing concentrations
of IC-GREEN™ (indocyanine green for injection) in the blood can be monitored
by ear densitometry or by obtaining blood specimens at timed intervals. The
technique for both methods is as follows.
The patient should be studied in a fasting, basal state. The patient should
be weighed and the dosage calculated on the basis of 0.5 mg/kg of body weight.
Under sterile conditions, the IC-GREEN™ powder should be dissolved with
the Aqueous Solvent provided. Exactly 5 mL of aqueous solvent should be added
to the 25 mg vial giving 5 mg of dye per mL of solution.
Inject the correct amount of dye into the lumen of an arm vein as rapidly as
possible, without allowing the dye to escape outside the vein. (If the photometric
method is used, prior to injecting IC-GREEN™, withdraw 6 mL of venous
blood from the patient's arm for serum blank and standard curve construction,
and through the same needle, inject the correct amount of dye)
Ear Densitometry: Ear oximetry has also been used and makes it
possible to monitor the appearance and disappearance of IC-GREEN™ without
the necessity of withdrawal and spectrophotometric analysis of blood samples
for calibration. An ear densitometer which has a compensatory photo- electric
cell to correct for changes in blood volume and hematocrit, and a detection
photocell which registers levels has been described. This device permits simultaneous
measurement of cardiac output, blood volume and hepatic clearance of IC-GREEN™
and was found to provide a reliable index of plasma removal kinetics after single
injections or continuous intrusions of IC-GREEN™. This technique was employed
in newborn infants, healthy adults and in children and adults with liver disease.
The normal subject has a removal rate of 18-24% per minute. Due to the absence
of extra-hepatic removal, IC-GREEN™ was found to be ideally suited for
serial study of severe chronic liver disease and to provide a stable measurement
of hepatic blood flow. In larger doses, IC-GREEN™ has proven to be particularly
valuable in detecting drug-induced alterations of hepatic function and in the
detection of mild liver injury.
Using the ear densitometer, a dosage of 0.5 mg/kg in normal subjects gives
the following clearance pattern.
Photometric Method
Determination Using Percentage Retention of Dye: A typical curve
obtained by plotting dye concentration versus optical density is shown below.
Percent retention can be read from this plot.
If more accurate results are desired, a curve using the patient's blood and the vial of IC-GREEN™ being used in the determination can be constructed as follows:
- Take 6 mL of non-dye-containing venous blood from the patient's arm. Place
in a test tube and allow the blood to clot. The serum is separated by centrifugation.
- Pipette 1 mLof the serum into a microcuvette.
- Add 1 lambda ((lambda)) of the 5 mg/mL aqueous IC-GREEN™ (sterile
indocyanine green) solution to the serum, giving a dilution of 5 mg/liter,
the standard for 50% retention. (The addition of 2 lambda (lambda) of the
5 mg/mL IC-GREEN™ solution would give 100% retention; however, this
concentration cannot be read on the spectrophotometer.)
- The optical density of this solution is read at 805 nm, using normal serum
as the blank.
- Plot the 50% figure obtained in Step 4, and draw a line connecting this
point with the zero coordinates.
Percentage Retention: A single 20-minute sample (withdrawn from
a vein in the opposite arm to that injected) is allowed to clot, centrifuged
and its optical density is determined at 805 nm using the patient's normal serum
as the blank. Dye concentration is read from the curve above. A single 20-minute
sample of serum in healthy subjects should contain no more than 4% of the initial
concentration of the dye. The use of percentage retention is less accurate than
percentage disappearance rate, but provides reproducible results. Hemolysis
does not interfere with a reading.
Determination Using Disappearance Rate of Dye: To calculate the
percentage disappearance rate, obtain samples at 5, 10, 15 and 20 minutes after
injecting the dye. Prepare the sample as in the previous section and measure
the optical densities at 805 nm, using the patient's normal serum as the blank.
The IC-GREEN™ concentration in each timed specimen can be determined by
using the concentration curve illustrated. Plot values on semilogarithmic paper.
Specimens containing IC-GREEN™ should be read at the same temperature
since its optical density is influenced by temperature variations.
Normal Values: Percentage disappearance rate in healthy subjects
is 18-24% per minute. Normal biological half-time is 2.5-3.0 minutes.
OPHTHALMIC ANGIOGRAPHY STUDIES: The excitation and emission spectra
(Figure 1) and the absorption spectra (Figure 2) of IC-GREEN™ make it
useful in ophthalmic angiography. The peak absorption and emission of IC-GREEN™
lie in a region (800-850 nm) where transmission of energy by the pigment epithelium
is more efficient than in the region of visible light energy. IC-GREEN™
also has the property of being nearly 98% bound to blood protein, and therefore,
excessive dye extravasation does not take place in the highly fenestrated choroidal
vasculature It is, therefore, useful in both absorption and fluorescence infrared
angiography of the choroidal vasculature when using appropriate filters and
film in a fundus camera.
Dosages up to 40 mg IC-GREEN™ dye in 2 mL of aqueous solvent have been found to give optimal angiograms, depending on the imaging equipment and technique used. The antecubital vein injected IC-GREEN™ dye bolus should immediately be followed by a 5 mL bolus of normal saline.
Clinically, angiograms of uniformly good quality can be assured only after
taking care to optimize the contributions of all possible factors such as, patient
cooperation and dye injection. The foregoing injection regimen is designed to
provide delivery of a spatially limited dye bolus of optimal concentration to
the choroidal vasculature following intravenous injection.