 ### Description

The circulating blood volume can be estimated by bolus injection of the dye indocyanine green (ICG) and monitoring blood levels over the next several minutes using pulse spectrophotometry. Pulse spectrophotometry is similar to pulse oximetry and allows the ratio of the arterial concentration of hemoglobin to ICG dye to be measured. From this ratio the concentration of ICG dye can be calculated. Based on the concentration of dye over time and the amount injected the blood volume can be calculated.

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)