Method:
(1) A dose of ICG (20 mg) is suspended in sterile water (4 mL).
(2) This is injected as a bolus, followed by 20 mL Ringer's lactate to flush the line.
(3) Optical density of a well-perfused site is measured for 10 minutes after the bolus injection at wavelengths of 805 nm (peak absorption for ICG) and 890 nm.
(4) The nostril or finger is typically used for monitoring.
(5) The apparatus has 2 light-emitting diodes, one at 805 nm and the second at 890, with a photodiode detector.
(6) The pulse spectrophotometer uses the ratio of pulsatile to total light transmitted at the wavelengths to calculate the ratio of arterial ICG concentration to arterial hemoglobin concentration.
(7) It takes several seconds for the ICG to distribute. The plasma concentration then rises to a peak after which it falls. The time to the initial peak is the mean transit time (MTT).
(8) After the initial peak, the dye concentration decreases as a first-order process, with
concentration at time t =
= (hypothetical initial concentration at t=0) * EXP ((-1) * k * (time t))
LN(concentration at time t) =
= LN(hypothetical concentration) - (k * (time t))
Calculation of Blood Volume
Measure ICG concentration at about 3 (Y1) and 5 (Y2) minutes after injection.
Calculate the number of minutes from the peak (mean transit time or MTT).
• X1 in minutes = 3 - (time to peak from time of injection)
• X2 in minutes = 5 - (time to peak from time of injection)
slope of line =
= (Y2 - Y1) / (X2 - X1)
Y axis intercept =
= Y2 - ((slope of line) * X2)
initial concentration in mg/L =
= (injected dose of ICG in mg) / (blood volume in L)
= EXP(Y axis intercept)
blood volume in liters =
= (injected dose of ICG in mg) / (initial concentration in mg/L)
Plasma Level to Whole Blood Conversion
Indocyanine green is distributed in the plasma. If the plasma concentration is known, the whole blood concentration can be estimated using the hematocrit.
ICG concentration in whole blood in mg/L =
= ((100 - (hematocrit as a percent from 0 to 100)) / 100) * (plasma ICG concentration in mg/L)
