Fig. 13. Effects of adenosine on human cerebral blood flow (CBF). Adenosine is rapidly metabolized by the vascular endothelium and the erythrocytes with an elimination half-life of 1–10 s. An intracarotid bolus of 1–2 mg adenosine results in significant change in shape of the ¹³³xenon washout curve. Compare the washouts during injection of adenosine, nicardipine, and saline (  A ). The effects of adenosine bolus seem to last for less than 30 s. The conventional CBF measurements (standard initial slope analysis [standard ISI]) that are measured between 20 and 80 s after ¹³³xenon injection does not show an increase in CBF (  solid circles , B and  C ) with intracarotid adenosine. However, early initial slope analysis (early ISI) that is measured between 5 and 25 s after ¹³³xenon (  solid squares , B and  C ) seem to have a dramatic effect on CBF. In case of nicardipine, a longer-acting drug, the early ISI and standard ISI yield similar results. Effects of intracarotid adenosine on CBF are exceedingly transient, whereas nicardipine has a sustained effect. From Joshi  et al. 58; used with permission.