The Effects of Changes in Pa CO 2 Cerebral Blood Volume, Blood Flow, and Vascular Mean Transit Time

Abstract

The relationships between cerebral blood volume (CBV), cerebral blood flow (CBF), and the cerebral vascular mean transit time (t_v) during acute changes in the PaCO₂ over a range of 15 to 76 torr were investigated in vivo in rhesus monkeys by serially determining the mean transit time of a vascular tracer, ¹⁵O-labeled carboxyhemoglobin, and the mean transit time of a diffusible tracer, ¹⁵O-labeled water. Over this range of PaCO₂, a significant linear relationship of CBV = 0.041 PaCO₂ + 2.0 was found. For each one torr change in PaCO₂, there is a change in CBV of 0.041 ml/100 gm of perfused tissue. At a normocarbic value of PaCO₂ (37 torr), an average value of 3.5 ml/100 gm was found. A nonlinear relationship of CBV and CBF was found. This relationship is expressed in the equation, CBV = 0.80 CBF^0.38. A significant linear relationship was found between CBF and PaCO₂. This was described by the equation, CBF = 1.8 PaCO₂ - 16.75. For each one torr change in the PaCO₂, there is a 1.8 ml/100 gm per minute change in the CBF. At a normocarbic value of PaCO₂(37 torr), an average value of CBF of 50 ml/100 gm per minute was found. The relationship of CBV and t_v was nonlinear and was expressed in the equation, t_C15O = 41 CBF^-0.62.