Precise measurement of cerebral blood flow in newborn piglets from the bolus passage of indocyanine green.

Indocyanine green (ICG) is a near-infrared dye that has the potential to be used as a tracer for the minimally invasive measurement of cerebral blood flow (CBF). In order to examine the technique, the arterial and cerebral concentrations of ICG were measured in newborn piglets during the bolus passage of ICG at normocapnia and two levels of mild hypercapnia. The results were analysed by applying the Fick principle in both integral and differential forms using a linear regression technique to improve the precision of calculated values of CBF. It was found that the integral method, which has been used previously, is particularly sensitive to errors in the time registration between the arterial and tissue signals whereas the differential method is less so. In addition, the differential method allows the venous outflow to be calculated which gives further information on the state of the capillary bed. CBF was 39.7 +/- 4.6 ml 100 g(-1) min(-1) at an arterial carbon dioxide tension (PaCO2) of 33.0+/-2.2 mmHg and increased to 53.7+/-9.1 and 75.4+/-15.2 ml 100 g(-1) min(-1) at a PaCO2 of 42.1 +/- 2.6 and 54.2 +/- 3.1 mmHg respectively (mean +/- SD, n = 7). There was no significant change in cerebral metabolic rate for oxygen, validating the value of blood flow to an arbitrary scaling factor. When the inspired CO2 fraction was returned to zero, calculated CBF returned to baseline with a variation of 7% of the mean, indicating that this technique is highly precise.