Although the classical notion is that CBF is maintained over a range of BP, it has more recently been established that CBF is also dependent on Q̇ (85, 142, 205; for review see Ref. 177). Thus, because SNA and heart rate (HR) are altered during CO2 reactivity testing, subsequent changes in Q̇ may theoretically influence CBF. It should be acknowledged, however, that it is not clear whether these changes in Q̇ would have direct effects on CBF independent of changes in cerebral perfusion pressure. Although this idea is supported indirectly, it remains counterintuitive, because, according to Poiseuille's law (Eq. 1) (122), blood flow is the quotient of perfusion pressure and vascular resistance, and changes in Q̇ are not variables of the equation (202)