Absent Recruitment of Capillaries in Brain Tissue Recovering from Stroke

Abstract

The density of perfused capillaries (d_CAP), defined as capillaries that transport glucose, as well as the volume fraction of these capillaries in the vascular bed (f_CAP), and the mean transit time of blood through the capillaries (t_CAP), were calculated from hemodynamic variables obtained in vivo by positron tomography of brains of six patients affected by stroke. Each patient was studied twice, within 38 hrs of the insult, and one week later. 38 ischemic and 38 contralateral mirror regions were compared. The metabolic rate for glucose (CMR_glc) was determined on the basis of regional calculations of the lumped constant. No significant change of the lumped constant was observed in any region.

In normal regions, no significant differences of any variables existed between the first and second studies. In the infarct regions of the first study, CMR_glc and CMR_O2 (cerebral metabolic rate for oxygen) were 30–50% of control (deactivation) and CBF (cerebral blood flow), capillary density, and the capillary diffusion capacity for fluorodeoxyglucose (K₁) were similarly reduced, although the oxygen/glucose ratio was only 3.75 in the ischemic regions. While f_CAP decreased, t_CAP doubled. One week after the first study, blood flow returned to normal in the infarct regions despite continued depression of metabolism. Capillary density and diffusion capacity remained low, indicating absent recruitment of nutrition vessels (perfusion capillaries).